[Federal Register Volume 67, Number 113 (Wednesday, June 12, 2002)]
[Notices]
[Pages 40554-40576]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 02-14761]
[[Page 40553]]
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Part VI
Environmental Protection Agency
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Standards for the Use or Disposal of Sewage Sludge; Notice
Federal Register / Vol. 67 , No. 113 / Wednesday, June 12, 2002 /
Notices
[[Page 40554]]
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ENVIRONMENTAL PROTECTION AGENCY
[FRL -7228-9]
STANDARDS FOR THE USE OR DISPOSAL OF SEWAGE SLUDGE
AGENCY: Environmental Protection Agency.
ACTION: Notice of data availability.
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SUMMARY: The Environmental Protection Agency (EPA) proposed to amend
the Standards for the Use or Disposal of Sewage Sludge to limit dioxin
and dioxin-like compounds (``dioxins'') in sewage sludge that is
applied to the land on December 23, 1999. Since that time, EPA
collected new data on the levels of dioxins in sewage sludge. EPA also
has extensively revised the risk assessment which estimates the risks
from dioxin and dioxin-like compounds associated with land application
of sewage sludge. This document summarizes the new sewage sludge data
and risk assessment. In addition, EPA is inviting comment on the effect
of applying approaches in EPA's current Draft Dioxin Reassessment
concerning non-cancer health effects of exposure to dioxins as they
relate to land application of sewage sludge. EPA also conducted a
screening analysis of the effects of dioxins in land-applied sewage
sludge on ecological species, which is addressed in this notice. EPA is
requesting comments on the new data and risk analysis, as well as
dioxin exposure information, and any impact that this may have on the
proposed rule with respect to land application of sewage sludge.
EPA is under a court-ordered deadline to take final action on the
proposed land application rule. The deadline was recently extended to
October 17, 2003 with respect to land application; EPA met the previous
court-ordered deadline of December 15, 2001 for taking final action on
the Round Two proposal concerning surface disposal and incineration in
a sewage sludge incinerator. EPA gave final notice of its determination
that numeric standards or management practices are not warranted for
dioxin and dioxin-like compounds in sewage sludge that is disposed of
in a surface disposal site or incinerated in a sewage sludge
incinerator (66 FR 66228, Dec. 21, 2001).
DATES: Your comments on this document must be submitted to EPA in
writing and must be received or postmarked on or before midnight
September 10, 2002.
ADDRESSES: Written comments and enclosures should be mailed or hand-
delivered to: W-99-18 NODA Comment Clerk, Water Docket (MC-4101),
USEPA, 1200 Pennsylvania Ave., NW., Washington, DC 20460. Hand
deliveries should be delivered to: EPA's Water Docket (MC 4101) at 401
M St., SW., Room EB57, Washington, DC 20460. Comments may also be
submitted electronically to [email protected]. Electronic
submission of comments is recommended to avoid possible delays in mail
delivery. Comments must be received or post-marked by midnight
September 10, 2002. For additional information see Additional Docket
Information section below.
FOR FURTHER INFORMATION CONTACT: Arleen Plunkett, U.S. Environmental
Protection Agency, Office of Water, Health and Ecological Criteria
Division (4304T), 1200 Pennsylvania Avenue, NW., Washington, DC 20460.
(202) 566-1119. [email protected]
SUPPLEMENTARY INFORMATION:
I. Additional Docket Information
II. Abbreviations Used
III. How Does This Document Relate to the Proposed Rule?
A. What EPA Proposed
B. Developments Since Proposal
C. Proposed Definition of Dioxins
IV. Why Did EPA Collect New Data and Revise the Land Application
Risk Assessment?
V. What Information Concerning Dioxins in Sewage Sludge Does the New
Data Provide?
A. What Data were Collected in the 2001 National Sewage Sludge
Survey?
B. What Techniques were Used to Collect Samples?
C. What Analytical Methods were Used?
D. How were the Concentrations of Dioxin Measured?
E. How were the Concentrations Reported?
F. How were the Non-Detect Measurements Handled in Developing
National Summary Statistics?
G. What were the Results of the EPA 2001 Dioxin Update of the
National Sewage Sludge Survey?
H. How do the Results of the EPA 1988 National Sewage Sludge
Survey Compare with the EPA 2001 Dioxin Update Survey?
I. Why is Temporal Variability of Dioxin in Sewage Sludge
Important?
J. What does the Variability of the Dioxin Levels Show?
K. What does Month to Month Variability in the Concentration of
Dioxins Show?
L. What Other Data did EPA Evaluate?
VI. What are the Principal Features and Assumptions of the Revised
Land Application Human Health Risk Assessment?
A. What did the Hazard Identification Analysis Conclude?
B. What did the Dose-Response Assessment Conclude?
C. How was the Exposure Analysis and Risk Assessment Conducted?
D. How did the Framework Change?
E. What are the Factors in Estimating How Much Dioxin is
Released to the Environment?
F. What are the Factors in Estimating How Much Dioxin is being
Transported in the Environment to the Individual in the Farm Family?
G. What Additional Factors are Applied to Dioxin Concentrations
to Determine How Much of the Congeners are Being Ingested or Inhaled
by a Farm Family Member?
H. How did EPA Calculate the Final Exposure Level?
I. How was Childhood and Infant Exposure Evaluated in the
Exposure Analysis?
J. How is the Risk Estimate Calculated?
K. How did EPA Analyze the Relative Importance of Inputs to the
Risk Model?
L. How does EPA Characterize the Risk?
VII. What Are the Implications of EPA's Dioxin Reassessment Process
for This Rulemaking?
A. How Would the Dioxin Cancer Risk from Land Application
Compare to Background Dioxin Cancer Risk?
B. How Would the Non-Cancer Dioxin Risk from Land Application
Compare to Background Non-Cancer Dioxin Risk?
VIII. What is EPA's Assessment of Effects on Ecological Species?
A. What Approach did EPA Use for the Screening Ecological Risk
Analysis of Dioxins in Land-Applied Sewage Sludge?
B. How did EPA Conduct the Screening Ecological Risk Analysis?
C. What are the Results of the Screening Ecological Risk
Analysis?
IX. How Might the New Data and Revised Risk Assessment Affect EPA's
Proposed Dioxin Concentration Limit for Land-Applied Sewage Sludge
and the Proposed Monitoring Requirements?
X. How Might the New Data and Revised Risk Assessment Affect EPA's
Proposal for Small Entities?
XI. How Does the New Data and Revised Risk Assessment Affect EPA's
Cost Estimates?
XII. Identification and Control of Dioxin Sources that Contribute to
Elevated Dioxin Levels in Sewage Sludge.
XIII. Request for Public Comments
XIV. List of References
I. Additional Docket Information
The record for this Notice has been established under docket number
W-99-18 and includes supporting documentation as well as the printed
paper versions of electronic materials. The record is available for
inspection from 9 a.m. to 4 p.m. Eastern Standard or Daylight time,
Monday through Friday, excluding legal holidays, at the Water Docket,
Room EB57, USEPA Headquarters, 401 M Street, SW., Washington, DC 20460.
For access to the docket materials, please call 202-260-3027 to
schedule an appointment.
For information on the existing rule in 40 CFR Part 503, you may
obtain a copy of A Plain English Guide to the EPA Part 503 Biosolids
Rule on the Internet at
[[Page 40555]]
http://www.epa.gov/owm/bio.htm or request the document (EPA publication
number EPA/832/R-93/003) from: Municipal Technology Branch, Office of
Wastewater Management (4204M), Office of Water, U.S. Environmental
Protection Agency, 1200 Pennsylvania Avenue, NW., Washington, DC 20460-
0001.
II. Abbreviations Used
AMSA--Association of Metropolitan Sewerage Agencies
CFR--Code of Federal Regulations
DL--detection limit
ED01--dose corresponding to a one percent increase in an adverse effect
relative to the control response
EPA--Environmental Protection Agency
HQ--hazard quotient
kg/m\3\--kilograms per cubic meter
LADD--lifetime average daily dose
Ln--natural logarithm
LOEL--lowest-observed-effect level
Max.--maximum
MGD--million gallons per day
mg/kg/day--milligrams per kilogram per day
MOE--margin of exposure
ng/kg--nanograms per kilogram
NOEL--no-observed-effect level
NSSS--National Sewage Sludge Survey
PCBs--polychlorinated biphenyls
PCDFs--polychlorinated dibenzofurans
PCDDs--polychlorinated dibenzo-p-dioxins
pg/kg/day--picograms per kilogram per day
pg TEQ/day--picograms toxic equivalents per day
pg TEQ/kg-d--picograms toxic equivalents per kilogram body weight per
day
POTWs--Publicly Owned Treatment Works
ppt--parts per trillion
Q1*--cancer slope factor
RfD--reference dose
SAB--Science Advisory Board
SERA--screening ecological risk analysis
Std. Dev.--standard deviation
TCDD--tetrachlorodibenzo-p-dioxin
TEF--toxicity equivalent factor
TEQ--toxic equivalent
WHO--World Health Organization
III. How Does This Document Relate to the Proposed Rule?
A. What EPA Proposed
In December 1999, EPA proposed to amend management standards for
sewage sludge by adding a numeric concentration limit for dioxins in
sewage sludge that is applied to the land (64 Fed. Reg. 72045, Dec. 23,
1999) (``Round Two proposal'').\1\ The proposed numeric limit would
prohibit land application of sewage sludge that contains greater than
300 parts per trillion (ppt) toxic equivalents (TEQ) of dioxins. EPA
based this proposed numeric limit on the results of a risk assessment
for dioxins in sewage sludge that is applied to the land.
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\1\ Section 405(d)(2)(A) of the Clean Water Act (CWA), 33 U.S.C.
Sec. 1345(d)(2)(A) required EPA to establish numeric limits and
management practices for toxic pollutants in sewage sludge
identified on the basis of available information. In 1993, EPA
promulgated the ``Round One'' rule for such toxic pollutants in
sewage sludge that is applied to the land, disposed of in surface
disposal units, and incinerated in sewage sludge incinerators. 58
Fed. Reg. 9248 (Feb. 19, 1993). Under section 405(d)(2)(B), EPA was
directed to propose and promulgate regulations for other toxic
pollutants not regulated in Round One, i.e., ``Round Two.'' The
Round Two proposal identified dioxins, and included proposed
standards for land-applied sewage sludge, but did not propose
further regulation of sewage sludge disposed of by surface disposal
or incineration.
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EPA proposed a standard for dioxins in sewage sludge that is
applied to the land in order to protect public health and the
environment from unreasonable risks of exposure to dioxins. The purpose
of this standard would be to prohibit land application of sewage sludge
containing concentrations of dioxins above the limit, and thereby
protect the health of highly exposed individuals as well as the health
of the general population.
EPA also proposed to exclude from the proposed numeric limit and
monitoring requirements treatment works with a flow rate equal to or
less than one million gallons per day (MGD) and certain sewage sludge-
only entities that receive sewage sludge for further processing prior
to land application. This exclusion was based on the relatively small
amount of sewage sludge that is prepared by these facilities and
entities and, therefore, the low probability that land application of
these materials could significantly increase risk from dioxins to human
health or the environment.
Finally, EPA proposed technical amendments to the frequency of
monitoring requirements for pollutants other than dioxin. These
amendments were intended to clarify but, with one exception, not alter
the monitoring schedule in the existing sewage sludge rule. The one
exception would require preparers of material derived from sewage
sludge to determine the appropriate monitoring schedule based on
quantity of material derived rather than quantity of sewage sludge
received for processing.
B. Developments Since Proposal
The Agency's risk assessment for land application of sewage sludge
used for the proposal estimated that sewage sludge with concentrations
of dioxins above the proposed limit may present an unreasonable cancer
risk to specific highly exposed individuals. Subsequently, for reasons
discussed below, the Agency extensively revised the land application
risk assessment. EPA also gathered new data on dioxins in sewage sludge
that was used in the revised risk assessment. This information,
however, does not change the overall technical approach for the
proposal.
The new data and the methodology of the revised risk assessment are
summarized in this notice. In addition, the results of the revised risk
assessment are described in today's notice. Also discussed in today's
notice are the possible implications of the new data and revised risk
assessment on the proposed limit, the monitoring requirements, the
small entity exclusion, and the projected cost of the proposed
regulation.
Another development since the proposal in December 1999 concerns
EPA's Dioxin Reassessment, which began in 1991. In September 2000, EPA
provided Draft Dioxin Reassessment documents to the Science Advisory
Board (SAB) for their review, and in May 2001, the SAB issued its
report. The current Draft Dioxin Reassessment (USEPA, 2000a),
``Exposure and Human Health Reassessment of 2,3,7,8-Tetrachlorodibenzo-
p-Dioxin (TCDD) and Related Compounds,'' consists of three parts. Part
I. Estimating Exposure to Dioxin-Like Compounds focuses on sources,
levels of dioxin-like compounds in environmental media, and human
exposures. Part II. Health Assessment for 2,3,7,8-Tetrachlorodibenzo-p-
Dioxin (TCDD) and Related Compounds includes information on critical
human health end points, mechanisms of toxicity, pharmacokinetics,
dose-response, and toxic equivalent factors (TEFs). Part III.
Integrated Summary and Risk Characterization for 2,3,7,8-
Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds describes key
findings pertinent to understanding the potential hazards and risks of
dioxins, including a discussion of important assumptions and
uncertainties.
The Draft Dioxin Reassessment documents do not represent Agency
policy or factual conclusions, and EPA has not yet issued final
findings or conclusions as a result of the Dioxin Reassessment process.
However, much of the information incorporated into the Draft Dioxin
Reassessment documents reflects the state of knowledge with respect to
dioxin, and scientific updates resulting from or reflected in these
[[Page 40556]]
documents are relevant to the assessment of risk from dioxins in sewage
sludge that is applied to the land. For example, the revised sewage
sludge land application risk assessment incorporates the latest science
and state of knowledge concerning characteristics of dioxin and
exposure pathways which are described in the Draft Dioxin Reassessment.
The Draft Dioxin Reassessment also presents conclusions and
findings which are still under review and which EPA has not applied to
the analysis of dioxins in sewage sludge. These aspects of the Draft
Dioxin Reassessment include, for example, a revised cancer slope factor
for calculating cancer risk from exposure to dioxins, and discussions
of various approaches to evaluating risks of non-cancer health effects
from exposure to dioxins. Although not incorporated into the revised
risk assessment, today's Notice also discusses potential implications
that these aspects of the Draft Dioxin Reassessment could have for this
rulemaking, when and if the Dioxin Reassessment is issued by EPA in
final form, and if the final version takes the same approaches and
reaches the same conclusions as the current draft.
Finally, EPA was under a consent decree deadline of December 15,
2001 to take final action on the proposed rule. Gearhart v. Whitman,
Civil No. 89-6266-HO (D. Ore.). In accordance with the consent decree,
EPA took final action on the proposal not to establish numeric limits
or management practices for dioxins in sewage sludge that is disposed
of in surface disposal units or incinerated in sewage sludge
incinerators. 66 Fed. Reg. 66228 (Dec. 21, 2001). The consent decree
deadline was extended to October 17, 2003, for EPA to take final action
on the land application portion of the proposed Round Two rule.
C. Proposed Definition of Dioxins
The proposed rule included a definition of ``dioxins'' to specify
the seven 2,3,7,8,-substituted congeners of polychlorinated dibenzo-p-
dioxins (PCDDs), the ten 2,3,7,8-substituted congeners of
polychlorinated dibenzofurans (PCDFs), and the twelve coplanar
polychlorinated biphenyl (PCB) congeners to which the numeric standard
applies. The vast majority of information on the toxicity of dioxins
relates to the congener 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).
Animals exposed to 2,3,7,8-TCDD exhibit a variety of biological
responses and adverse effects. These include both carcinogenic and non-
carcinogenic effects. These effects are primarily classified as chronic
effects and consequently they are generally associated with long term
exposure over years and decades. Relatively speaking, these exposures
and effects are observable at very low levels in the laboratory and in
the environment when compared with other environmental toxicants
(USEPA, 1994a).
Studies to elucidate the mechanism of toxicity for 2,3,7,8-TCDD in
mammalian and other species have indicated that the overall shape and
chlorine substitution of this congener are keys to its biological
potency. The fact that all of the lateral positions (the 2,3,7,8
positions) on the multi-ring system are substituted with chlorine and
that the overall molecule assumes a flat or planar configuration
apparently are essential factors that make this congener biologically
active. Other congeners with a similar structure and chlorine
substitution pattern are assumed to exhibit similar biological
properties. These include the other six 2,3,7,8-chlorinated substituted
dibenzo-p-dioxin congeners, the ten 2,3,7,8-chlorinated substituted
dibenzofuran congeners and the 12 coplanar PCB congeners. Coplanar PCB
congeners are those congeners with no more than one ortho position and
both para positions substituted with chlorine in the biphenyl ring
system. Additionally, the coplanar PCB molecule assumes a relatively
planar (i.e., flat) configuration.
The proposed TEQ numeric limit would apply to these 29 congeners in
ppt TEQ or nanograms TEQ per kilogram of dry sewage sludge. The TEQ
concentration is calculated by multiplying the concentration of each
congener in the sewage sludge by its corresponding ``toxicity
equivalent factor,'' or TEF, and then summing the resulting products
from this calculation for all 29 congeners. The TEFs (relative
potencies) are based on expert judgment about toxicity and other
biological effects for the individual compounds. The TEQs of these
compounds are summed because they are believed to act by the same
mechanism of toxicity. The December 1999 proposal specified that the
International TEF scheme described in USEPA, 1989, would be used for
the 17 2,3,7,8-substituted PCDDs and PCDFs, and the World Health
Organization's TEF scheme (Van den Berg M, et al., 1998) would be used
for the 12 coplanar PCBs, because the sewage sludge data EPA had at
that time used these TEF schemes. The World Health Organization (WHO)
has subsequently recommended and developed a single TEF scheme which
includes all relevant information on dioxins, furans and dioxin-like
(coplanar) PCBs. As part of this process, various terminologies or
definitions applicable to TEFs were reviewed and standardized.
The 2001 sewage sludge data and the revised risk assessment use the
WHO's 1998 TEF scheme (Van den Berg M, et al., 1998) for all 29 dioxin,
furan and coplanar PCB congeners. EPA intends to use the 1998 WHO TEF
scheme (or later, if the WHO adopts a revised scheme) for any final
Part 503 TEQ numeric limit.
A 1997 WHO meeting of experts concluded that an additive TEF model
remained the most feasible risk assessment method for complex mixtures
of dioxin-like compounds. The WHO panel indicated that although
uncertainties in the TEF methodology have been identified, one must
examine this method in the broader context of the need to evaluate the
public health impact of complex mixtures of persistent bioaccumulative
chemicals. On this basis, EPA has used the 1998 WHO TEF methodology for
the Agency's Draft Dioxin Reassessment, noting that it decreases the
overall uncertainties in the risk assessment process.
A Panel of EPA's Science Advisory Board has reviewed the Agency's
use of the 1998 WHO TEF scheme. The consensus of the Panel was that
this is a reasonable and widely accepted way of dealing with the joint
effects of dioxin-like compounds on human health. The majority of the
Panel noted that the TEF approach is well accepted internationally.
IV. Why Did EPA Collect New Data and Revise the Land Application Risk
Assessment?
The proposal to amend the Standards for the Use or Disposal of
Sewage Sludge to limit dioxins in sewage sludge that is applied to the
land was followed by a 90 day public comment period. During this time
the risk assessment which supported the proposed rulemaking also was
peer reviewed in accordance with EPA peer review procedures. Both the
public comments and the peer review comments raised significant issues
concerning the methodology and assumptions used for the land
application risk assessment. The public and peer review comments also
emphasized the need to collect new data on dioxins in sewage sludge.
This data is used in the risk assessment, economic analysis, and other
aspects of the rulemaking.
The data on dioxins in sewage sludge used for the proposal came
from two separate sources. The data on dioxin and furan congeners was
from the 1988 EPA National Sewage Sludge Survey
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(USEPA, 1990). Since the National Sewage Sludge Survey (NSSS) did not
include specific information on coplanar PCBs, EPA used a separate
database to estimate the amount of coplanar PCBs found in sewage sludge
(Green, et al., 1995). In addition to developing a single database
which includes information on all 29 dioxin-like congeners, EPA
developed new data on dioxins in sewage sludge to test the Agency's
assumption that dioxin levels in sewage sludge have changed over time,
and to more accurately determine dioxin levels in sewage sludge using
analytical methods with lower limits of detection. The Agency is also
using this more recent data to more reliably estimate the risk,
impacts, and costs associated with dioxins in land applied sewage
sludge. A discussion of the sewage sludge sampling and data analysis is
presented in Section V. of this Notice.
The principal comment concerning the risk assessment methodology
was that the Agency should use a probabilistic approach instead of the
deterministic approach that was used for the proposal. A probabilistic
approach uses values for certain input variables over the range of
available data, instead of the deterministic approach of determining,
or setting, certain input variables at particular values. Conducting a
risk analysis with a probabilistic approach can yield better
information about sources of variability and uncertainty in the final
risk estimates, compared to conducting a risk analysis with a
deterministic approach.
Other comments on the risk assessment recommended that the Agency
use an exposure analysis more consistent with that used in the Agency's
current Draft Dioxin Reassessment (USEPA, 2000a); that the Agency use
data from the current EPA Exposure Factors Handbook (USEPA, 1997); and
that the risk assessment include a sensitivity analysis of the critical
input variables.
The revised risk assessment is described in Section VI. of this
Notice. The revised risk assessment was submitted for peer review. The
consensus view of the peer reviewers agreed with the revised risk
assessment methodology and assumptions on input parameters. The revised
risk assessment, described below and available in the docket,
incorporates revisions made in response to the peer review.
V. What Information Concerning Dioxins in Sewage Sludge Does the New
Data Provide?
A. What Data Were Collected in the EPA 2001 Dioxin Update of the
National Sewage Sludge Survey?
The EPA 2001 dioxin update of the NSSS provides data that support
the calculation of unbiased national estimates (i.e., based on a random
selection of publicly owned treatment works) for dioxin and dioxin-like
compounds in sewage sludge (USEPA, 2002a). The publicly owned treatment
works (POTWs) sampled in the EPA 2001 dioxin update survey were
randomly selected from all POTWs in four size categories: <1 MGD, 1
MGD-10 MGD, 10 MGD-100 MGD and 100 MGD. This survey updates
the 1988 NSSS. The updated survey includes coplanar PCBs, which had not
been included in the 1988 NSSS because approved analytical methods for
these analytes were not available at that time. The updated survey also
uses the current TEFs, which have been revised since the 1988 NSSS. For
the EPA 2001 dioxin update survey, EPA collected sewage sludge samples
from 94 POTWs selected from the 174 POTWs which had been surveyed in
the 1988 NSSS. The sample of 174 POTWs included in the 1988 NSSS were
selected from the national population (as of 1988) of approximately
10,000 POTWs with secondary treatment. EPA used a survey design which
accounted for the different numbers of POTWs in different size
categories for both the 1988 NSSS and the EPA 2001 dioxin update
survey. EPA conducted the sampling at the 94 POTWs in the first
calendar quarter of 2001 and completed the laboratory analysis, data
review, and database development by mid-2001.
B. What Techniques Were Used To Collect Samples?
Sewage sludge samples were collected, documented, preserved, and
shipped to the laboratory where the analyses for dioxins were conducted
using the protocol entitled ``Sampling Procedures for the 2001 National
Sewage Sludge Survey'' (USEPA, 2001a). This document specifies the
sampling procedures used for the sewage sludge samples obtained from
the 94 POTWs that participated in the EPA 2001 dioxin update survey.
The procedures were used on a number of different types of sewage
sludge samples including liquids, samples with low solids content,
dewatered sewage sludges from filter presses and centrifuges, composted
products, and pellets. The sampling protocol specifies sample
preservation methods, collection devices and apparatus, containers,
types of labels, and label information. In accordance with the sampling
protocol used for the EPA 2001 dioxin update survey, duplicate samples
were collected for 15 percent of the samples collected for subsequent
analysis to determine the precision of the analyses. At each treatment
works sampled, a second sample aliquot was collected and archived for
potential future analyses. Chain of custody forms were completed for
the samples collected at each sampling site to ensure the integrity of
the results of the survey.
C. What Analytical Methods Were Used?
EPA used analytical methods that are considered state of the art
for the sewage sludge matrix. Dioxin and dibenzofuran congener
concentrations were determined by EPA Method 1613B (USEPA, 1994b) using
high resolution gas chromatography-mass spectrometry as the end point
system of measurement. The coplanar PCB analyte concentrations were
determined by EPA Method 1668A (USEPA, 1999a) which employs the same
type of measuring instrumentation. Method 1613B is an official EPA
analytical methodology codified at 40 CFR Part 136. EPA anticipates
that Method 1668A will be codified in Part 136 within the next two
years.
D. How Were the Concentrations of Dioxin Measured?
The sewage sludge samples were analyzed for 29 dioxin congeners
consisting of the 7 dioxin congeners, 10 dibenzofuran congeners, and 12
coplaner PCB congeners that EPA proposed for the definition of
``dioxins'' (see Section III.B. above). For the EPA 2001 dioxin update
survey, whole (wet) weight sample sizes were individually determined
for each sewage sludge sample by considering the percent solids in each
sample. Smaller whole weight sample sizes were used for the analyses
when the percent solids content of the sewage sludge sample was
greater, and vice versa. This approach led to lower and more consistent
detection limits for concentrations of target analytes for all of the
sewage sludge samples in the EPA 2001 dioxin update survey. This
procedure was a significant improvement compared to the method used for
handling the sewage sludge samples in the 1988 NSSS. For the 1988 NSSS,
equal whole weight sample sizes were used regardless of the percent
solids content of the samples. This led to higher and less consistent
detection limits for the sewage sludge samples in
[[Page 40558]]
the 1988 NSSS. In addition, other improvements in the analytical
methodology and the analytical instrumentation also contributed to
lower and more consistent detection limits than those obtained in the
1988 NSSS.
E. How Were the Concentrations Reported?
All of the individual 29 congener concentrations were converted to
TEQ concentrations by multiplying the congener concentrations by the
1998 WHO TEFs. For comparison purposes, TEQs for total dioxin and
dioxin-like compounds in the 1988 NSSS samples and the EPA 2001 dioxin
update survey samples are reported in Table 1, Table 2 and Table 3 in
nanograms per kilogram (ng/kg) dry weight basis.
F. How Were the Non-Detect Measurements Handled in Developing National
Summary Statistics?
Where congeners were not detected in sample measurements, three
different substitution methods were used in calculating national
estimates of dioxin concentrations in sewage sludge: (1) Zero was
substituted for a non-detect; (2) one-half the detection limit for the
congener was substituted for a non-detect; (3) the detection limit for
the congener was substituted for a non-detect. As a result of the small
detection limits achieved in the EPA 2001 dioxin update survey, there
were only small differences in the national summary statistics among
the three substitution methods for the EPA update survey.
G. What Were the Results of the EPA 2001 Dioxin Update of the National
Sewage Sludge Survey?
Table 1 presents the mean, standard deviation, maximum and 99th,
98th, 95th, 90th and 50th percentiles dioxin TEQ values for the sewage
sludges from the 94 POTWs in the EPA 2001 dioxin update survey. Table 1
reports summary results separately for dioxins and furans, coplanar
PCBs, and total dioxin-like compounds (i.e., 29 dioxin, furan and
coplanar PCB congeners) using the three alternative substitution values
for non-detects (i.e., zero, one-half the detection limit, and equal to
the detection limit). In Table 1, the results obtained using zero, one-
half the detection limit and the detection limit are shown in the rows
denoted by ``0'', ``\1/2\ DL'' and ``DL'', respectively. The complete
statistical analysis of the data from the EPA 2001 dioxin update survey
is presented in Statistical Support Document for the Development of
Round Two Sewage Sludge Use or Disposal Regulations (USEPA, 2002a).
Table 1.--EPA 2001 Dioxin Update Survey--National Toxic Equivalent Estimates (nanograms/kilogram dry matter basis)--Total Toxic Equivalents for POTWs
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Method Mean Std. Dev. Max. 99th % 98th % 95th % 90th % 50th %
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Total Dioxin and Furan TEQs (nanograms/kilogram dry matter basis)
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0........................................................... 21.70 47.5 682.00 100.00 54.40 33.30 31.40 15.50
\1/2\ DL.................................................... 21.70 47.5 682.00 100.00 54.40 33.30 31.60 15.50
DL.......................................................... 21.80 47.5 682.00 100.00 54.40 33.30 31.70 15.50
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Total Coplanar PCB TEQs (nanograms/kilogram dry matter basis)
--------------------------------------------------------------------------------------------------------------------------------------------------------
0........................................................... 5.22 10.3 58.30 50.60 44.80 13.10 9.66 2.05
\1/2\ DL.................................................... 9.87 14.0 58.30 55.10 54.50 49.40 19.20 6.04
DL.......................................................... 14.50 22.4 103.00 97.2 91.60 78.00 35.00 8.11
--------------------------------------------------------------------------------------------------------------------------------------------------------
Total Dioxin and Dioxin-Like TEQs (nanograms/kilogram dry matter basis)
--------------------------------------------------------------------------------------------------------------------------------------------------------
0........................................................... 26.90 49.6 718.00 114.00 76.60 59.30 42.80 19.70
\1/2\ DL.................................................... 31.60 50.0 718.00 115.00 80.10 73.50 55.10 23.40
DL.......................................................... 36.30 52.7 718.00 138.00 96.00 113.00 69.10 24.00
--------------------------------------------------------------------------------------------------------------------------------------------------------
Under the proposed rule, treatment works with a flow rate equal to
or less than one MGD and certain sewage sludge-only entities that
receive sewage sludge for further processing prior to land application
would be excluded from the proposed numeric limit and monitoring
requirements. The EPA 2001 dioxin update survey provides additional
data with respect to dioxin concentrations from POTWs that would be
excluded under the proposal. Table 2 below shows the results for dioxin
concentrations in sewage sludge for POTWs with flows of less than and
greater than one MGD. Results shown in Table 2 indicate very small
differences in the median dioxin concentrations between small and large
POTWs. At the upper percentiles, the differences between the small and
large POTW values are substantial. However, the significance of these
differences is difficult to assess due to the relatively small sample
sizes, the sensitivity of the results to the treatment of non-detect
measurements and the low precision typically associated with estimates
of upper percentiles based on small sample sizes. An additional
discussion of the proposed exclusion for small entities is presented in
Section X. of this Notice. EPA requests comments on the significance of
the differences in dioxin concentrations in sewage sludge measured at
facilities with wastewater flows greater than one MGD compared to
dioxin concentrations in sewage sludge at facilities with wastewater
flows less than one MGD.
[[Page 40559]]
Table 2.--EPA 2001 Dioxin Update Survey--Total Dioxin and Furan and Dioxin-Like PCB National TEQ (nanograms/
kilogram dry weight basis) Estimates--POTWs by Flow Groups
----------------------------------------------------------------------------------------------------------------
Method Zero for Nondetects \1/2\ DL for DL for Nondetects
------------------------------------------------------------------- Nondetects ----------------------
-----------------------
Estimate 1 MGD 1 1 MGD
MGD 1 MGD 1 MGD 1 MGD
----------------------------------------------------------------------------------------------------------------
Mean...................................... 22.10 38.50 26.50 44.10 30.80 49.60
Std. dev.................................. 16.8 86.7 18.3 86.8 24.6 88.2 dev.
Maximum................................... 78.60 718.00 78.6 718.00 118.00 718.00
99th %.................................... 71.80 401.00 76.40 403.00 109.00 406.00
98th %.................................... 65.10 265.00 74.20 269.00 101.00 276.00
95th %.................................... 46.00 62.60 67.10 94.80 77.00 134.00
90th %.................................... 37.20 54.00 46.10 64.20 46.60 86.90
50th %.................................... 19.90 18.90 22.90 22.60 23.80 25.80
----------------------------------------------------------------------------------------------------------------
H. How Do the Results of the EPA 1988 National Sewage Sludge Survey
Compare with the EPA 2001 Dioxin Update Survey?
A comparison of results for dioxin and furan congeners obtained in
the 1988 and 2001 surveys is presented in Table 3.
Table 3.--National Estimates (nanograms/kilogram dry matter basis) for Dioxin and Furan Congeners in the EPA
2001 Dioxin Update Survey and NSSS 1988
----------------------------------------------------------------------------------------------------------------
Method Zero for nondetects \1/2\ DL for DL for nondetects
--------------------------------------------------------------------- nondetects ---------------------
----------------------
Estimate 2001 1988 2001 1988 2001 1988
----------------------------------------------------------------------------------------------------------------
Mean.......................................... 21.70 46.50 21.70 67.30 21.80 88.20
Std. dev...................................... 47.5 153.0 47.5 153.0 47.5 157.00
Maximum....................................... 682.00 1870.00 682.00 1870.00 682.00 1870.00
99th %........................................ 100.00 450.00 100.00 453.00 100.00 466.00
98th %........................................ 54.40 402.00 54.40 404.00 54.40 455.00
95th %........................................ 33.30 301.00 33.30 303.00 33.30 340.00
90th %........................................ 31.40 56.70 31.60 152.00 31.70 226.00
50th %........................................ 15.50 5.68 15.50 34.20 15.50 52.40
----------------------------------------------------------------------------------------------------------------
The values obtained in the EPA 2001 dioxin update survey for the
upper percentiles are lower than those obtained in the 1988 NSSS. On
this basis, the concentrations of dioxins in sewage sludge appear to
have declined since 1988. However, the significance of these
differences between the two surveys is not certain due to changes in
the sampling procedures and analytic methods . These comparisons do not
include coplanar PCB congeners because the 1988 NSSS did not collect
coplanar PCB congener data. For the purposes of the December 1999
proposed rule, data on coplanar PCB levels in sewage sludge from a 1995
Association of Metropolitan Sewerage Agencies Survey (Green, et al.,
1995) were combined with the 1988 NSSS dioxin and furan results to
provide an estimate of total dioxin levels in sewage sludge. EPA
requests comments on the significance of the differences in dioxin
concentrations in sewage sludge measured in the EPA 2001 dioxin update
survey compared to dioxin concentrations in sewage sludge measured in
the 1988 NSSS.
VIII.Why Is Temporal Variability of Dioxin in Sewage Sludge Important?
The variability of dioxins in sewage sludge over time is important
for a number of reasons. First, understanding the temporal variability
of dioxin concentrations in sewage sludge is important for establishing
numerical limits for dioxins in sewage sludge which protect public
health and the environment with an adequate margin of safety.
Specifically, this information helps in assessing the likelihood that
individuals will be exposed to higher levels of dioxins from land
application of sewage sludge over time. A more complete discussion of
this issue is presented in the risk characterization in Section VI.L.
of this Notice. Second, information on the variability of dioxin
concentration in sewage sludge is important for determining the
appropriate frequency of monitoring for concentrations of dioxins in
sewage sludge that will ensure that any numerical limit that is
established will not be exceeded.
J. What Does the Variability of the Dioxin Levels Show?
It is not possible to draw general inferences with regard to the
variability or differences in dioxin levels observed in the two
surveys. This is due to a number of factors that include the large time
interval between the surveys (i.e., 13 years), changes that may have
occurred at the POTWs, and changes and improvements in analytical
methods. It is possible, however, to make a number of observations with
regard to changes in dioxin levels based on the data. Of the 94 POTWs
participating in both the 1988 NSSS and the EPA 2001 dioxin update
survey, a total of 14 POTWs have sewage sludge dioxin concentrations
(dioxins and furans only) equal to or greater than 93 ppt TEQ from at
least one of the surveys. These same 14 POTWs exhibited the greatest
differences in the dioxins and furans concentrations when comparing the
results of the 1988 and 2001 EPA surveys. The other 80 POTWs
[[Page 40560]]
participating in both surveys have substantially smaller differences,
as well as lower dioxin levels measured in both surveys. Of the 14
POTWs with the greatest differences between the two surveys, four had
large increases in sewage sludge dioxin concentrations and ten had
large decreases in sewage sludge dioxin concentrations from 1988 to
2001.
Based on these data, no POTWs had consistently high levels of
dioxins in sewage sludge. It appears that sewage sludge samples with
higher concentrations of dioxins may experience a greater variability
in dioxin concentrations over time and that higher dioxin levels may
not remain high for a significant period of time. Likewise, POTWs with
moderate or low levels of dioxins in their sewage sludge may experience
much less variability in dioxin concentrations over time. It is
possible that in the group of POTWs where higher concentrations of
dioxins were measured in their sewage sludge, there are unidentified
sources with relatively high levels of dioxins entering the sewers
intermittently. The second group of POTWs where lower concentrations of
dioxins were measured in both surveys appear to be experiencing typical
environmental background variation of dioxin levels. The possible
sources of dioxins which contribute to higher levels of dioxins in
sewage sludge are discussed in greater detail later in this Section and
Section XII of the Notice. EPA's assessment of the variability in
higher levels of dioxins in sewage sludge is discussed further as part
of the risk characterization in Section VI.L. of this Notice.
K. What Does Month-to-Month Variability in the Concentration of Dioxins
Show?
EPA also examined both long and short term variability in sewage
sludge dioxin concentrations in three wastewater treatment plants that
have routinely monitored for dioxins in their sewage sludge over
relatively long periods of time and voluntarily submitted their data to
EPA (USEPA, 2001b). EPA did this to better understand the extent of
variability using data collected on a relatively frequent basis.
Of the three POTWs which provided their data to EPA, one of the
POTWs provided data on two different sewage sludge products that they
produce. These data were standardized using the WHO98
standard for TEQs to provide consistency.
The December 1999 proposal specified annual monitoring for land
applied sewage sludges with dioxin concentrations between 30 ppt TEQ
and the proposed limit of 300 ppt TEQ. Sewage sludges with two
consecutive annual dioxin measurements less than 30 ppt TEQ would be
required to monitor once every five years. These less frequent
monitoring requirements were based on EPA's assumption that dioxin
concentrations in sewage sludge remained relatively constant over time.
The data for the facilities where monthly data were available
indicate that the dioxin concentrations are relatively consistent over
time on a month-to-month basis. The maximum monthly concentration was
within a factor of two to four times the average (mean) concentration
for the same facility. Similar to the comparison data from the 1988
NSSS and the 2001 update, the variability appeared the greatest for the
facility with the highest dioxin concentrations measured in its sewage
sludge. A complete analysis of the month-to-month data is presented in
the Statistical Support Document for the Development of Round Two
Sewage Sludge Use or Disposal Regulations (USEPA, 2002a).
The month-to-month variability in the dioxins concentration
observed in the sewage sludge for which the Agency had data, as well as
the longer term variability observed in the small percentage of sewage
sludge with higher concentrations of dioxins (discussed above), has led
us to re-evaluate the proposed monitoring frequency. A more complete
discussion of monitoring frequency is presented in Section IX. of this
Notice.
L. What Other Data Did EPA Evaluate?
The Association of Metropolitan Sewerage Agencies (AMSA)
voluntarily collected sewage sludge samples from 171 POTWs and analyzed
these samples for dioxins using the same methods used for the 2001 EPA
dioxin update survey. AMSA submitted the results of their survey to EPA
in a report entitled ``AMSA 2000/2001 Survey of Dioxin-Like Compounds
in Biosolids: Statistical Analyses (Final Report)'' (AMSA, 2001). The
AMSA survey began in October 2000 and was completed in July 2001. The
AMSA survey was designed to measure levels for the same 29 dioxin and
dioxin-like congeners measured in the EPA 2001 dioxin update survey.
AMSA also compared the results of their 2001 survey with the results of
their 1994/1995 survey of dioxins in sewage sludge. Participation in
AMSA's survey was on a voluntary basis.
Most participants in the AMSA survey were larger POTWs which make
up the bulk of the AMSA membership. Some non-AMSA members also
participated in the AMSA survey, including some smaller POTWs. Overall,
111 separate wastewater treatment agencies participated in the 2001
AMSA survey, providing 200 samples from 171 POTWs, located in 31
states. The sewage sludge dioxin concentrations measured in the AMSA
survey generally ranged from 7.1 ppt TEQ to 256 ppt TEQ, with one
sample measured at 3,590 ppt TEQ. The mean (average) concentration and
the median dioxin concentrations in sewage sludge from the AMSA survey
were 48.5 ppt TEQ and 21.7 ppt TEQ, respectively.
EPA has found the data from the AMSA survey to be useful in
describing dioxins in sewage sludge from larger POTWs. The results of
the AMSA survey tend to corroborate the results obtained from the EPA
2001 dioxin update survey. However, the AMSA results were not used by
EPA to establish national estimates of dioxin concentrations in sewage
sludges or for purposes of estimating risks from dioxins in land-
applied sewage sludge. EPA did not use these results because the POTWs
participating in the AMSA survey volunteered for this survey and were,
therefore, not randomly selected, as were the POTWs in the EPA 2001
dioxin update survey. The final report from the AMSA survey and
associated appendices are in the docket and can also be found on AMSA's
web site at: http://www.amsa-cleanwater.org/advocacy/dioxin/dioxin.cfm.
VI. What Are the Principal Features and Assumptions of the Revised Land
Application Human Health Risk Assessment?
The revised risk assessment is entitled ``Exposure Analysis for
Dioxins, Dibenzofurans, and CoPlanar Polychlorinated Biphenyls in
Sewage Sludge--Technical Background Document'' (USEPA, 2002b). The risk
assessment methodology, assumptions, results and characterization are
summarized below.
The revised risk assessment contains the following standard
elements used in EPA human health risk assessments: hazard
identification, dose-response assessment, exposure assessment, and risk
characterization. The revised risk assessment includes a probabilistic
methodology to determine the adult and child exposure to the 29 dioxin
and dioxin-like congeners. For the proposed rule, the risk assessment
depended on a deterministic analysis based on single value inputs and
outputs. A probabilistic analysis was well-suited for this risk
assessment because sewage
[[Page 40561]]
sludge is generated nationwide and, therefore, may be used on
agricultural fields anywhere in the United States. The probabilistic
analysis not only captures the variability in sewage sludge application
practices, it also captures the differences in the environmental
settings (e.g., soils, meteorology and agricultural practices) in which
sewage sludge may be land-applied.
In addition to a new methodology of analysis, the revised risk
assessment uses new inputs which include a redefined ``highly exposed
individual,'' new pathways and mechanisms of exposure consistent with
EPA's Draft Dioxin Reassessment (USEPA, 2000a. See Part I, Vol. 3,
Chap. 2.), a number of new exposure factors adopted from the latest EPA
Exposure Factors Handbook (USEPA, 1997), and a sensitivity analysis to
determine the relative importance of the input variables. In this
Section, EPA describes the features of the revised risk assessment with
emphasis on the new inputs used in the probabilistic analysis.
A. What Did the Hazard Identification Analysis Conclude?
The risk assessment that EPA used for the December 1999 proposal
identified cancer as the human health endpoint, i.e., as the ``hazard''
(64 FR 72051). The revised risk assessment does not change this hazard
identification and continues to assess the risk of cancer as the human
health endpoint.
B. What Did the Dose-Response Assessment Conclude?
EPA's dose-response assessment evaluated the risk of the dioxin,
dibenzofuran, and PCB congeners using cancer slope factors that are
based on the toxicity of the most highly characterized of the dioxin
congeners, 2,3,7,8-TCDD (USEPA, 2000a. See Part II, Chap. 7, Part A.).
The cancer slope factor for TCDD used by EPA in recent assessments,
including the revised sewage sludge land application risk assessment,
is 1.56 x 10-4/picograms toxic equivalents/kilogram body
weight/day (pg TEQ/kg-d) (USEPA, 1994a). The cancer slope factor (also
referred to as Q* or ``cancer potency'') is a numeric value which
relates the incremental probability of developing a cancer from
exposure to a particular substance. This cancer slope factor value is
expressed as a lifetime excess cancer risk per unit exposure, and is
usually quantified in terms of (milligrams of substance per kilogram of
body weight per day)-1. The greater the numeric value of the
cancer slope is, the greater the carcinogenic potency of the substance.
The same slope factor is used to estimate cancer risks for both
children and adults. For this analysis, only the cancer endpoint was
evaluated and a linear dose response relationship was used in the
analysis.
An extensive discussion of the dose response mechanism for TCDD is
provided in the Draft Dioxin Reassessment document (USEPA, 2000a. See
Part II, Chap. 8.). The Draft Dioxin Reassessment also includes a
revised cancer slope factor. Because the Draft Dioxin Reassessment is
preliminary and does not state EPA policy conclusions or factual
findings, the draft cancer slope factor was not used in the revised
risk assessment. However, for purposes of discussion and public
comment, this Notice includes a discussion of how the EPA Draft Dioxin
Reassessment could apply to the analysis of impacts from dioxins in
land-applied sewage sludge, including use of the revised cancer slope
factor, in Section VII.A. of this Notice. EPA is seeking comment on the
implications of this information in the event that, prior to taking
final action on the Round Two rule, EPA finalizes a cancer slope factor
or other policies or approaches currently reflected in the current
Draft Dioxin Reassessment and discussed in this Notice.
C. How Was the Exposure Analysis and Risk Assessment Conducted?
The primary methodology for the exposure analysis was to estimate
exposure to dioxins in land-applied sewage sludge using a probabilistic
approach. A probabilistic exposure analysis produces a distribution of
exposures which is then used to estimate the range of risks for the
highly exposed population being modeled. The distribution of exposure
is determined by varying parameter values where data is available over
multiple iterations of the exposure model. Values were varied for such
parameters as dioxin concentrations in sewage sludge, number of years
on the farm, and number of applications. While ranges of data were
available for the majority of input parameters, ``single point'' values
were used for some key input parameters for the exposure analysis,
including values for parameters used to define the highly exposed
population, soil ingestion rates, and number of days per year of
exposure. These assumptions are discussed in greater detail elsewhere
in this Notice.
A receptor is the entity exposed to a physical, chemical or
biological source which can cause an adverse effect. In this case the
receptors are infants, children, and adults in highly exposed farm
families living on farms where sewage sludge is applied. ``Highly
exposed'' farm families are defined as farm families whose diets
consist of 50 percent of products produced on their own farm. EPA
estimates that the maximum number of individuals in this highly exposed
population would be less than 11,000 even if all of the Nation's sewage
sludge were applied to family farms (see Section VI.L.). Since the
general population consumes only a small fraction of their diets from
products grown on farms with land-applied sewage sludge, EPA assumed
that a regulatory decision that is protective of this highly exposed
family is also protective of the general population.
The probabilistic analysis was performed using a Monte Carlo
simulation. In a Monte Carlo simulation, the model is run for a number
of iterations, each producing a single result (e.g., a single estimate
of cancer risk). For this assessment, 3,000 iterations were run in the
Monte Carlo simulation; therefore, the output of the probabilistic
analysis was a distribution of 3,000 values. This distribution
represents the distribution of possible outcomes, which reflects the
underlying variability in the data used in the analysis. These results
were then used to identify risk to the highly exposed population at
various percentile levels (e.g., 90th percentile risk value). As noted
above, the corresponding percentile risk values to the general
population would be significantly lower.
Some model input parameters used in the Monte Carlo simulation,
such as the concentrations of dioxin congeners in sewage sludge
samples, were drawn from statistical distributions. For others,
variability was associated with variable locations; thus, location
variability was explicitly considered in the setup of the data used for
the probabilistic analysis. For location-dependent parameters,
locations were first selected at random with equal probability of
occurrence \2\ based on the 41 climate regions. These regions defined a
set of related environmental conditions (e.g., soil type, hydrogeologic
environment) that characterized the environmental setting. All
location-specific parameters (e.g., rainfall) thus remained correlated,
while non-location-specific parameters were varied both within and
among locations.
---------------------------------------------------------------------------
\2\ Information was not available to allow the weighting of
these 41 climate regions based on the number of farm families in
each region.
---------------------------------------------------------------------------
D. How Did the Framework Change?
In the exposure analysis, the risk assessment evaluated a revised
scenario for exposure to sewage sludge: exposure
[[Page 40562]]
of a farm family that consumes 50% of its diet from home-produced crops
and animal products grown on their own sewage sludge-amended land. For
the December 1999 proposal, a rural family consuming a smaller
proportion of home-grown products derived from sewage sludge-amended
soil was modeled in the original risk assessment. EPA selected the new
scenario specifically to address groups of individuals who may have
high levels of exposure to dioxins in sewage sludge. EPA assumed that
the farm family lives immediately adjacent to the sewage sludge-amended
field and is exposed to a combination of agricultural products produced
on the farm, including beef and dairy products. The farm family also is
assumed to raise free-range chickens near their house (in the buffer
area). On the opposite side of the house from the field and pasture is
a fishable stream where a recreational fisher is assumed to catch fish
for personal consumption. There are four types of people who were
assumed to be representative of the individuals who would be exposed to
dioxin from sewage sludge: an infant of a farmer, a child of a farmer,
an adult farmer, and an adult recreational fisher. The exposure to the
adult fisher was combined with that of the adult farmer, when the total
exposure to the adult was calculated. Therefore, the fisher and farm
adult can be considered as the same adult. Table 4 summarizes the
exposure pathways for each type of individual.
Table 4.--Receptors and Exposure Pathways
----------------------------------------------------------------------------------------------------------------
Ingestion Ingestion
Inhalation of above- Ingestion of Ingestion
Receptor of ambient Ingestion and of beef poultry Ingestion of breast
air of soil belowground and dairy and egg of fish milk
produce products products
----------------------------------------------------------------------------------------------------------------
Adult...........................
Child...........................
Infant..........................
----------------------------------------------------------------------------------------------------------------
The new scenario includes new exposure pathways and exposure
mechanisms, incorporating updated scientific analysis for dioxin, which
is also reflected in EPA's Draft Dioxin Reassessment (USEPA, 2000a. See
Part I, Vol. 3, Chap. 2.). For the proposed rule, the risk assessment
evaluated pastured animals eating sewage sludge containing dioxins
after sewage sludge land application. The revised risk assessment
assumes tilled soil only for production of vegetables, fruits, and root
crops and untilled soil for pasturage to which sewage sludge is
applied. Half the acreage on the modeled farm is assumed to be used for
crop production (tilled) and half permanently used for pasturage
(untilled). Rather than assuming that cattle are exposed to dioxins
only by eating sewage sludge-containing soil, the Agency now assumes
that cattle are exposed to dioxins in sewage sludge by three
mechanisms: ingesting dioxins from the leaf surfaces of plants
containing dioxins which have volatilized from the top two centimeters
of the soil to which sewage sludge has been applied; ingesting dioxins
from sewage sludge particles which remain on the leaf surfaces of
plants after land application; and direct ingestion of sewage sludge-
containing soil by the grazing cattle. Of these three mechanisms of
dioxin transfer to cattle from the sewage sludge, the predominant
mechanism is ingestion of dioxins from leaf surfaces containing dioxins
which have volatilized from the sewage sludge-soil mixture. The dioxins
from land-applied sewage sludge that does not erode away from the land
application site are assumed to reside permanently in the top two
centimeters of the soil. Another new assumption reflecting the latest
science on dioxin and consistent with EPA's Draft Dioxin Reassessment
documents is that chickens will be ingesting dioxins from the buffer
area which receives dioxins from the pasture and crop fields through
erosion. EPA requests comments on the Agency's use of the farm family
scenario described for the revised risk assessment. EPA also requests
comments on the specific assumptions outlined above.
E. What Are the Factors in Estimating How Much Dioxin is Released to
the Environment?
Various inputs for sewage sludge characteristics were used in the
exposure analysis to determine how much dioxin is available for
volatilization, erosion or leaching. These included: concentrations of
each of the 29 congeners in sewage sludge (empirical distribution of
concentrations for each dioxin congener varied by sample), bulk density
of sewage sludge (single value), porosity of sewage sludge (single
value), percent moisture of sewage sludge when applied to agricultural
fields (single value), and fraction of organic carbon of sewage sludge
(single value). The use of the congener concentrations was different in
the revised exposure analysis. Rather than using point estimates for
the 29 congeners for the probabilistic analysis, all of the congener
concentrations measured in the 94 samples from the EPA 2001 dioxin
update survey were used. Specifically, for each iteration of the Monte
Carlo analysis, one of the 94 sewage sludge samples from the EPA 2001
dioxin update survey was randomly selected and the concentrations of
all congeners from that sample were considered in that iteration of the
analysis. For each iteration, the concentration of dioxins in the
sludge was assumed to remain constant for the entire period of
application since family farms would likely receive sewage sludge from
a single POTW.
When the chemical content of a substance is analyzed, the
assumption used to address non-detected chemicals can have a
significant impact on the reported results if the detection limits are
relatively large. Non-detects can be reported as zero, one-half the
detection limit, or the detection limit. Because of the excellent
sensitivity and limits of detection achieved by the analytical
procedures used in the EPA 2001 dioxin update survey, the reported
values for dioxin congeners in the samples of sewage sludge are
relatively unchanged whether non-detects are treated as zero, one-half
of the detection limit, or at full detection limit. For this risk
assessment, EPA assumed that non-detects are equal to one-half of the
detection limit. This assumption is prevalently used by EPA for risk
assessments based on data sets for non-detects, including the Draft
[[Page 40563]]
Dioxin Reassessment for calculating TEQ concentrations for dioxins in
environmental media (i.e., air, soil, water) and in exposure media
(i.e., food). Furthermore, it appears that there would be no
quantifiable difference in the estimated risk regardless of the
assumption made for non-detects for the reasons discussed above. EPA
requests comment on the treatment of non-detects in the revised risk
assessment and the effect on estimating risk.
Another sewage sludge characteristic, bulk density of sewage sludge
as it is applied to the agricultural field, was used to estimate the
loading of constituents to the soil in the model. Sewage sludge is
assumed not only to add constituents to the soil, but also to add
volume when mixed with the existing soil. Thus, bulk density is a
required parameter for the modeling scenario used in the exposure
analysis. Bulk density of the land-applied sewage sludge may be a
direct measurement or may be estimated using the dry bulk density, the
percent moisture, and the porosity of the sewage sludge.
F. What Are the Factors in Estimating How Much Dioxin Is Being
Transported in the Environment to the Individual in the Farm Family?
A conceptual site model was used to represent exposures to the
highly exposed modeled population from land application of sewage
sludge. To capture some of the variability in environmental settings
across the United States, the conceptual site model was placed in
different regions throughout the continental United States.
The risk assessment was intended to be representative of a national
distribution of environmental conditions. The 48 contiguous states
(excluding Hawaii, Alaska, and the off-shore possessions) were divided
into 41 meteorologic regions. These regions were selected to represent
the national variation of location-specific variables. Each area is
assumed to represent a single climate region (i.e., conditions within
that area can be modeled using the meteorologic data from a single
meteorologic observation station). Meteorologic and climate data were
used in air modeling, partitioning in the source model, and surface and
subsurface fate and transport modeling.
In addition, farm areas were assumed to be linked to geographic
area. Large farms are more common in the Midwest and western parts of
the United States, and smaller farms are more common in the eastern and
southern parts of the United States. Thus, a regional estimate for a
median farm size was developed and was used in this risk assessment.
The U.S. agricultural census contains estimates for the distribution of
farms within each county. These data were used to develop a median farm
size for each county. These county-wide median farm sizes were
classified according to the 41 geographic areas and the median of the
median farm sizes was estimated for each of the 41 regions. The median
area was then used in the air modeling and the erosion to surface water
modeling. This methodology was used to account for the regional
variation in agricultural practices throughout the nation, but it did
not consider variation in size within a single region.
A series of models was used to estimate concentrations of the
congeners in the environment with which a farm family may come into
contact. The revised risk assessment assumes that there are six direct
and indirect exposure pathways that the models describe:
Inhalation of ambient air;
Incidental ingestion of soil in the buffer area;
Ingestion of above- and below-ground produce grown on the
crop land;
Ingestion of beef and dairy products from the pasture;
Ingestion of home-produced poultry and eggs from the
buffer area; and
Ingestion of fish from the nearby water body.
As indicated above, a regional approach was used to define the area
surrounding the agricultural application site. A source partition model
was then used to estimate environmental releases of each constituent.
These estimated environmental releases in turn provided input to the
fate and transport models to estimate media concentrations in air,
soil, and surface water. A food chain model was used to estimate
constituent concentrations in produce, beef, dairy products, poultry,
eggs, and fish.
The source partition model determines the initial release of
congeners into the environment. Sewage sludge application to pastures
or crop land is assumed to be different and these differences affect
the behavior of constituents in the environment. The model uses
information described above on sewage sludge characteristics (e.g.,
moisture content and congener concentrations), and environmental
setting (e.g., precipitation, temperature, and soil characteristics) to
estimate environmental releases.
Fate and transport modeling procedures describe the mechanism by
which the congeners move from the source through the environment. As
described above, a source partition model was used to determine the
amount and nature of congener released from the agricultural field. A
multimedia approach was used to characterize the movement of the
dioxins through the environment. This approach considered atmospheric
concentrations, atmospheric deposition, soil concentrations, and
sediment concentrations in potentially impacted water bodies.
Air modeling procedures estimated air concentrations and deposition
of vapors and particles on the agricultural farm, onto the buffer area,
directly into the surrounding water bodies, and onto the regional
watershed. Air dispersion and deposition of vapors and particles were
modeled using the Industrial Source Complex Short Term Model. Soil
erosion comes from the crop fields and pastures, the buffer area
containing the house and chicken yard, and the remaining portion of the
watershed. Erosion was modeled using the Universal Soil Loss Equation.
All impacts in the same period of time were summed to estimate the
concentration in the stream sediment and water column.
The exposure pathways included inhalation of dioxins in ambient air
during tilling of agricultural fields, incidental ingestion of soil,
ingestion of aboveground and belowground produce (i.e., root crops),
ingestion of beef and dairy products, ingestion of eggs and poultry
products, and ingestion of fish. EPA's preliminary analysis indicated
that exposure to dioxins from the consumption of ground water was
insignificant due to the extremely low solubility of dioxins in water
and negligible leaching of dioxins to ground water (USEPA, 1999b).
With concentrations of the congeners determined for water and air,
the concentrations being delivered to humans from aboveground produce,
belowground produce, poultry, eggs, beef, dairy products, and fish were
then calculated. This was accomplished using food chain models. The
food crops (vegetables, fruits, and root vegetables) were assumed to be
grown on the sewage sludge-amended fields, and cattle (beef and dairy)
were assumed to be raised on pastures receiving sewage sludge. These
processes were modeled using a multi-pathway exposure model and the
fate and transport parameters and modeling procedures reflecting the
latest scientific knowledge on the fate and transport of dioxin. The
exposure pathways considered the transport of constituents from the
soil to plants (vegetables, fruits, roots, and pasture grass) and
ingestion of these materials by humans and animals. The transport to
plants
[[Page 40564]]
may occur through the root system, but most occurs through air-to-plant
transfer mechanisms. The contaminated plants are in turn consumed by
cattle and humans.
The latest scientific knowledge with respect to the methodology of
estimating concentration of congeners in beef and/or dairy products is
also described in the Draft Dioxin Reassessment document. This
methodology has been developed based on the transfer of congeners from
the total diet of the cattle into the fat. The method described in the
Draft Dioxin Reassessment emphasizes the importance of the differences
in diet between beef and dairy cattle in explaining different food
concentrations. While the same equation was used for all cattle,
whether they are beef cattle or dairy cattle, the differences were in
the dietary fraction assumptions. These assumptions were based on how
much of the time the cattle are pastured and how much of the time they
are confined with supplemental feed. Forage was assumed to be raised on
the sewage sludge-amended pasture where the sewage sludge was assumed
to remain on the top two centimeters of the soil and to volatilize onto
the forage. The soil was assumed to be the soil in the sewage sludge-
amended pasture. The supplemental feed for the cattle was assumed to be
grown on sewage sludge-amended crop land where the sewage sludge was
tilled into the soil. Half of the supplemental feed was assumed to be
vegetation and half was assumed to be grains. Supplemental feed was
assumed to contain a lower dioxin concentration than forage because it
was assumed to contain less volatilized dioxins (due to tilling), and
the grain portion was assumed to be free of contamination due to
stripping of the outer leaves where dioxins accumulate.
To determine the dioxin concentrations in poultry and eggs, the
risk assessment starts with the assumption that sewage sludge is not to
be applied directly to the chicken yard. The chickens are assumed to be
free range within a confined area of the buffer near the farm
residence. The chicken diet is assumed to consist of 90 percent store
bought chicken feed (uncontaminated by dioxins in sewage sludge applied
on the farm land) and 10 percent buffer soil.
As already indicated, the receptors included in the modeling are
adults and children living and working on farms where fruits,
vegetables, root crops, and farm animals are raised, and half of these
food items consumed by the adults and children living on the farm are
produced on the farm. The farm family also is assumed to be exposed to
inhalation risks from windblown and tilling emissions from the
agricultural field. Soil ingestion risks are also assessed for both
adults and children. Children are assumed to ingest soil from the
buffer area, and the adult farmer is assumed to ingest soil from the
tilled field. In addition, risks to recreational fishers who catch and
consume fish from the stream adjacent to the agricultural field is
considered and summed with the other exposure pathways on the
assumption that farmers are also recreational fishers.
EPA requests comment on the assumptions and values used in this
Section to estimate how much dioxins are being transported to
individuals in the modeled farm family (e.g., the sources (store-bought
versus farm-produced) and dioxin contamination levels of poultry
feeds).
G. What Additional Factors Are Applied to Dioxin Concentrations To
Determine How Much of the Congeners are Being Ingested or Inhaled by a
Farm Family Member?
To determine how much of the congeners adults and children are
inhaling and ingesting, exposure factors were applied to the
concentrations of the contaminants from air, produce, cattle, dairy,
poultry, eggs, and fish. The exposure factors used in this analysis
were taken from the Exposure Factors Handbook (USEPA, 1997). The
Exposure Factors Handbook summarizes data on human behaviors and
characteristics related to human exposure from relevant key studies and
provides recommendations and associated confidence estimates on the
values of exposure factors.\3\
---------------------------------------------------------------------------
\3\ EPA carefully reviewed and evaluated the quality of the data
before their inclusion in the Exposure Factors Handbook. EPA's
evaluation criteria included peer review, reproducibility,
pertinence to the United States, currency, adequacy of the data
collection period, validity of the approach, representativeness of
the population being modeled (in this case, farm families),
characterization of the variability, lack of bias in study design,
and measurement error (USEPA, 1997).
---------------------------------------------------------------------------
The proportion of home produced food commodities eaten by highly
exposed farm families was assumed to be 50% of their diet for all
iterations. This assumption defined the modeled population. Specific
distributions of other exposure factors for the general population of
farm residents were compiled from the Exposure Factors Handbook. These
include ingestion rates for adults and children for aboveground
vegetables, root vegetables, fruits, beef, dairy products, poultry, and
eggs. Distributions have been developed for adults and for three age
groups of children for these dietary categories.
Exposure factors are related to the pathways in that they describe
the rates at which dioxin doses are ingested or inhaled from the
various sources noted above (e.g., air, soil, beef, and diary, by the
highly exposed farm family adults and children). The exposure factors
used in this risk assessment are represented by a distribution or a
fixed value in the Monte Carlo probabilistic analysis.
For the probabilistic exposure analysis, probability distribution
functions were developed from the values in the Exposure Factors
Handbook. The intake factors, for which either single values or
distributions were used from the Exposure Factors Handbook, are: soil
ingestion (one value for children aged 1 to 6 and another value for all
other receptors); and fruits and vegetables ingestion, beef and dairy
ingestion, fish ingestion, and inhalation rates (all of which are
distributions of values.)
H. How Did EPA Calculate the Range of Exposure Levels?
For cancer effects, where the biological response is described in
terms of lifetime probabilities, dose is presented as a ``lifetime
average daily dose'' (LADD). Because exposure duration varies from
person to person (i.e., may not occur over the entire lifetime),
calculation of exposure produces a distribution of exposure levels (or
doses). In addition to exposure duration, the LADD takes a number of
variable factors into account, including when exposure begins, how
often and in what amounts sewage sludge is applied to the land, and the
length of time over which land application occurs. For this risk
assessment, the LADD takes into account: (1) A distribution of randomly
selected times when land application begins, i.e., either when the
highly exposed farm family begins applying sewage sludge to their land
or moves onto a farm where sewage is being or has been applied; (2) a
distribution of exposure durations ranging from one year to 70 years;
\4\ (3) a distribution of sewage sludge application duration, ranging
from a minimum of one year up to a maximum of 40 years (i.e., a minimum
of one application to a
[[Page 40565]]
maximum of 20 applications based on a fixed application frequency of
once every two years), and (4) a distribution of sewage sludge
application rates (i.e., amount of sludge applied to the land) ranging
from 5-10 metric tons per hectare per application. The LADD also
includes doses from each exposure route (i.e., inhalation and
ingestion) and body weight. A distribution of body weights for the
adult and child were taken from the Exposure Factors Handbook.
---------------------------------------------------------------------------
\4\ Exposure durations representing the residence time in the
same house were also determined using the Exposure Factors Handbook.
The lifetime of the individual was assumed to be a fixed value of 70
years. A fixed value for exposure frequency was assumed to be 350
days per year, accounting for two weeks away from the farm for
vacation (USEPA, 2002b). These single values were selected to be
protective and yet representative of realistic scenarios.
---------------------------------------------------------------------------
The purpose of the exposure assessment is to estimate the dose to
an exposed individual by combining media intake estimates with media
concentrations. Estimates of exposure are based on the potential dose
(e.g., the dose ingested or inhaled) rather than the applied dose
(e.g., the dose delivered to the gastrointestinal tract) or the
internal dose (e.g., the dose delivered to the target organ). Doses
from individual pathways (e.g., soil, exposed vegetables) were
calculated by multiplying the contaminant concentration in the food
product or other exposure media (e.g., air or soil) by the respective
intake rate on a per kilogram body weight basis. Doses received from
the various ingestion pathways (e.g., soil and food) were then summed
over the period of time in which exposure occurs, resulting in an
average daily dose received from ingestion exposure.
I. How Was Childhood and Infant Exposure Evaluated in the Exposure
Analysis?
Children are an important sub-population to consider in a risk
assessment because they may be more highly exposed than adults;
compared to adults, children may eat more food and drink more fluids
per unit of body weight. This higher intake-rate-to-body-weight ratio
can result in a higher average daily dose of dioxins than adults
experience. The risk assessment performed for sewage sludge application
to agricultural land includes an analysis of exposures to 3,000
individuals whose exposures begin in childhood. To account for intake
rates varying over different childhood age groups, parameters
characterizing exposures beginning in childhood were developed.
The first step in developing the time-weighted parameters is to
define the start age for the child and the length of exposure for that
individual. These two values then determine how long the individual is
in each age group. Four age groups were defined as follows: age group 1
(1-5 years of age); age group 2 (6-11 years of age); age group 3 (12-19
years of age); and age group 4 (over 20 years of age). After the
individual is defined, age appropriate consumption rates are chosen for
each age group which are selected from the age specific consumption
rate distribution for each item considered in the analysis. For example
if the exposure begins at age 3 and continues for 20 years, a
consumption rate for each age group was selected and weighted to
represent the number of years spent in each age group to get an average
intake rate for the entire exposure duration of 20 years (i.e., age
group 1= 3 years of exposure; age group 2 = 6 years; age group 3 = 7
years; and age group 4 = 4 years, for a total of 20 years exposure.)
This time weighted intake rate is then used with the average
concentration of dioxins for the food item over the entire exposure
duration, to yield an average daily dose.
Infants are also an important sub-population to consider in this
risk assessment because they may be exposed to dioxin-like compounds
via the ingestion of breast milk. While risks to children and adults
were integrated to incorporate individuals for whom exposure first
occurs during childhood but continues into adulthood, the lifetime
risks to infants were calculated separately from the risks to older
children (i.e., ages 1 year or older) and adults. For infants, exposure
during the first year of life was averaged over an expected lifetime of
seventy years to derive a LADD that was then used to calculate risk.
The ``lifetime'' risk to infants thus should be thought of as the
contribution to lifetime risk that occurs during the first year of life
through ingestion of breast milk for individuals born into a farm
family exposed to dioxins from land-applied sewage sludge.
J. How Was the Cancer Risk Estimate Calculated?
Cancer risk is calculated using lifetime excess cancer risk
estimates to represent the excess probability of developing cancer over
a lifetime as a result of exposure to the constituent of interest.
Lifetime excess cancer risk estimates are the product of the lifetime
average daily dose for each of the four types of individuals exposed to
dioxin and for each exposure pathway, and the corresponding cancer
slope factor.
The exposure assessment estimates delivered doses for each of the
29 congeners to a farm family individual. Each of these congener doses
were then converted to TEQ doses by multiplying each congener dose by
its TEF. These TEQ doses for each of the 29 congeners were then summed
to yield an overall TEQ dose to the individual for that exposure
pathway (e.g., inhalation or ingestion). Finally this TEQ dose was
multiplied by the cancer slope factor to estimate the excess cancer
risk to the individual for that pathway of exposure.
Using all samples from the EPA 2001 dioxin update survey, the
estimated risks and corresponding daily exposure to dioxins for the
highly exposed farm adult and child are given below in Table 5 for
various percentiles of exposure within this population. ``Adult'' means
individuals whose exposure begins when they are adults, and ``child''
means individuals whose exposure begins when they are children. In most
cases exposure which begins during childhood also ends during
childhood. However, in some instances, exposures which begin when
individuals are children continued into their adult years.
Additional risk calculations were performed to estimate the impact
on the risk if sewage sludge with 300 ppt TEQ dioxin and 100 ppt TEQ
dioxin were restricted from being land applied. Eliminating sewage
sludge samples with higher concentrations of dioxins did not change the
estimated risk. The distribution of risk estimates for scenarios
excluding samples with dioxin concentrations greater than 300 ppt TEQ
and 100 ppt TEQ are the same as the distribution below shown in Table
5, which includes data from all sewage sludge samples.
[[Page 40566]]
Table 5.--Risks and Daily Exposure for Highly Exposed Farm Adult and Child for All Exposure Pathways--(Q*=1.56 x
10-4/pg TEQ/kg-d)
----------------------------------------------------------------------------------------------------------------
Adult * Child **
-------------------------------------------------------
Percentile Daily Daily
Risk Exposure pg Risk Exposure, pg
TEQ/kg-d TEQ/kg-d
----------------------------------------------------------------------------------------------------------------
50th.................................................... 1 x 10-6 7.3 1 x 10-6 7.3
75th.................................................... 4 x 10-6 7.3 3 x 10-6 7.3
90th.................................................... 1 x 10-5 7.3 7 x 10-6 7.3
95th.................................................... 2 x 10-5 7.3 1 x 10-5 7.3
99th.................................................... 4 x 10-5 7.3 2 x 10-5 7.3
----------------------------------------------------------------------------------------------------------------
* Initial exposure begins when the individual is an adult.
** Initial exposure begins when the individual is a child.
K. How Did EPA Analyze the Relative Importance of Inputs to the Risk
Model?
In addition to the revised risk assessment, EPA conducted a
sensitivity analysis to identify the effects of variability and
uncertainty in the risk model on the risk estimates. These steps are
performed on the inputs and outputs of the Monte Carlo analysis. In the
Monte Carlo analysis, probability distributions were assumed for each
of the variable input parameters, and a distribution of 3,000 media
concentrations and risk results were generated as outputs in the
analysis. In the sensitivity analysis, statistical methods were applied
to this sample of inputs and outputs to evaluate the influence of the
individual inputs on the model outputs. Several different indices of
sensitivity were derived from the simulated sample to quantify the
influence of the inputs and identify the most influential parameters.
Finally, a regression analysis was applied to a linear equation to
estimate the relative change in the output of a Monte Carlo simulation
relative to the changes in the input parameters.
Table 6 presents the results of the sensitivity analysis for the
beef and dairy products exposure pathways. The consumption of beef and
dairy products by the farm family represent over 90 percent of dioxin
exposure and subsequent cancer risk associated with land application of
sewage sludge. For the beef products pathway, exposure duration and
beef consumption rate combine to account for 86 percent of the
variation in the estimation of dioxin exposure. The two variables which
account for the next highest contributions to variation in the
estimation of exposure (i.e., sewage sludge application rate and
average year that the farm family moves in) combined for 2 percent of
the variation. Similarly, for dairy products, exposure duration and
dairy products consumption rate also represent 86 percent of the
variation in the estimation of exposure, with the next two highest
variables again representing a combined 2 percent of the variation. A
detailed discussion of the entire sensitivity analysis can be found in
the land application risk assessment Technical Background Document
(USEPA, 2002b).
Table 6.--Results of Sensitivity Analysis
------------------------------------------------------------------------
Percent of risk accounted
Pathway and Sensitivity variables for by variable
------------------------------------------------------------------------
Beef:
Exposure Duration................... 60
Consumption Rate.................... 26
Sewage sludge Application Rate...... 1
Average year that the farm family 1
moves in.
Dairy products:
Exposure Duration................... 54
Consumption Rate.................... 32
Average year that the farm family 1
moves in.
Sewage sludge Application rate...... 1
------------------------------------------------------------------------
L. How Does EPA Characterize the Risk?
As previously noted, EPA developed a revised risk assessment using
a probabilistic approach as a basis for the Agency final action on
development of a numerical standard for dioxins in sewage sludge
applied to agricultural land. In order to protect the general public
from adverse health impacts from dioxins in land-applied sewage sludge
with an adequate margin of safety, the risk assessment calculates the
risk to the most highly exposed population (i.e., a farm family
consuming 50 percent of their diet from products grown on sewage sludge
amended soil) . The following discussion characterizes the key elements
of EPA's risk assessment and compares them according to the principles
in EPA's guidance for exposure assessment and for risk characterization
(USEPA, 1992 and USEPA, 2000b).
Approximately 95 percent of the U.S. population's exposure to
dioxins results from the consumption of animal products in the diet
where dioxin is concentrated in the fatty portion of the meats and
dairy products (USEPA, 2000a. See Part I, Vol. 3, Chap. 3.). EPA chose
the farm family as the highly exposed population to be modeled, using a
key assumption that their diets have significant percentages of meat
and dairy products from their own farms where sewage sludge is land
applied as a fertilizer or soil amendment. Members of such a farm
family are at greater risk from exposure to dioxins associated with
land application as compared with the overall U.S. population because
their diets would be based on products from their farm. As previously
noted, a decision that is protective of this highly exposed modeled
population is thus protective of the general population from the same
pathways of dioxin exposure with a greater margin of safety since the
diet of the general population contains only a small fraction of meat
and dairy products grown on farms with land-applied sewage sludge.
The following discussion characterizes the three principal
components of the risk assessment: the exposure scenario; key
assumptions and data used in the exposure assessment modeling; and the
cancer slope factor (Q1* or potency factor). Each of these components
is characterized as either ``high end'' or ``central tendency.''
As previously noted, sewage sludge is assumed to be applied at
agronomic rates to tilled crop land used for the production of
vegetables, fruits, and root crops, and to pasture land which is not
tilled. Fifty percent of the farm family's agricultural land is assumed
to be tilled crop land and the other fifty percent untilled pasture. An
important assumption in terms of characterizing the risk is that the
dioxin in each
[[Page 40567]]
application of sewage sludge to pasture is assumed to permanently
remain in the top two centimeters of the land surface and is not
diluted over time. This is a key assumption since volatilization from
soil to the leaf surfaces of crops consumed by animals and humans is
the principal mechanism by which dioxins are transported from sewage
sludge applied to the land. This assumption predicts a maximum amount
of transport of dioxins for subsequent consumption by pastured animals.
In addition, this pasturing scenario is not varied; EPA assumes that
the farmer does not rotate the pasture to grow row crops where tilling
of sewage sludge in the soil would mitigate dioxin volatilization
transport. Thus, this assumption is likely to contribute to an
overestimation of risk.
Another important assumption contributing to the risk estimate is
that the family is simultaneously exposed to a combination of
agricultural products produced on the farm. For the purpose of the
exposure assessment and risk assessment, all pathways of exposure to
dioxins are summed.
As previously noted, the cancer slope factor used in the revised
risk assessment is 1.56 x 10-4/pg TEQ/kg-d. This value is
characterized as the upper bound (i.e., at the 95th percentile
confidence level) on the slope of the dose-response curve in the low-
dose region and is generally assumed to be linear. Use of upper bound
slope factors also results in calculation of high-end risks of cancer
for individuals in the target population of highly exposed farm
families (i.e., 95% likelihood that risk to such highly exposed
individuals is lower) (USEPA, 2000a. See Part III, Chap. 6).
As described above in the description of the risk assessment, most
of the parameters used in the Monte Carlo simulations were
distributions of a range of observed values for each parameter. Where a
range of data was not available, ``fixed'' data points or assumptions
were used. The sources of information for the fixed point inputs
necessary to conduct the risk assessment include the EPA Exposure
Factors Handbook (USEPA, 1997), peer reviewed scientific literature,
and other assumptions specifically related to land application of
sewage sludge based on actual practice.
The following is a listing of some of the key fixed parameters used
in the Monte Carlo simulations and their characterizations. Some of the
fixed assumptions characterized as ``high end'' have the greatest
impact on the risk estimate based on the results of the sensitivity
analysis discussed above (see Section VI.K.). These assumptions include
the farm family simultaneously exposed to multiple pathways including a
certain percentage of their own products; dioxin remaining in the top
two centimeters on pasture lands; and the upper bound Q1*. The
following ``fixed'' parameters are important to note, but have a lesser
impact on the risk estimate.
Other ``High End'' Assumptions
Exposure Frequency--350 days per year.
Fraction of diet for home-caught fish--100%.
Fraction of soil ingested that is contaminated--100%.
Fraction of ingested dioxin absorbed by the mother--100%.
Use of potential dose rather than applied or internal
dose.
Mean or Central Tendency Values from EPA Exposure Factors Handbook
Fraction of food preparation loss for exposed fruit,
exposed vegetables, and root vegetables.
Percent cooking and percent post-cooking loss for beef and
poultry.
Fraction of home-caught fish that are at trophic levels 3
and 4 (high dioxin bio-accumulating fish).
Soil ingestion rates for children and adults.
Values from Scientific Literature \5\
---------------------------------------------------------------------------
\5\ USEPA, 1998a. Methodology for Assessing Health Risks
Associated with Multiple Pathways of Exposure To Combustor
Emissions. These values were gathered from various sources and are
either mean values or representative ranges (not high end).
---------------------------------------------------------------------------
Biological half life of dioxin in lactating women.
Concentration of dioxin in aqueous phase of maternal milk.
Fraction of fat in maternal breast milk. (mean value)
Fraction of ingested dioxin absorbed by the infant.
Fraction of mother's weight that is fat. (mean value)
Proportion of dioxin stored in maternal fat.
The probabilistic methodology facilitates risk estimates for
individuals in any percentile of the assessed population. The revised
land application risk assessment reports high-end estimates of risks
for individuals at the 50th, 75th, 90th, 95th and 99th percentiles of
exposure within the population defined for this analysis as ``highly
exposed.'' USEPA, 2002b. It may also be acceptable to characterize the
risk assessment as the ``high end of the high end'' within this modeled
population of highly exposed farm families.
The incremental cancer risk for land application of sewage sludge
was estimated considering all exposure pathways for three scenarios:
baseline (all samples from the EPA 2001 dioxin update survey); 300 ppt
TEQ cutoff (samples greater than 300 ppt TEQ excluded); and 100 ppt TEQ
cutoff (samples greater than 100 ppt TEQ excluded). The estimated
lifetime risks for adults using this cancer slope factor range from 4 x
10-5 at the 99th percentile to 1 x 10-6 at the
50th percentile for multi-pathway exposure to dioxins through land-
applied sewage sludge (see Table 5). (As indicated in Table 5, the
estimated risks for children are less than or equal to the estimated
risks for adults.) No quantifiable decrease in risk is calculated if
sewage sludge with greater than 300 ppt TEQ dioxins or greater than 100
ppt TEQ dioxins were restricted from being land applied. The reason
that the estimated risk does not decrease when sewage sludge limits of
300 ppt TEQ dioxins or 100 ppt TEQ dioxins are assumed is that, based
on the representative sampling, there is so little sewage sludge that
contains dioxin at or above these concentrations.
Continual application of sewage sludge with significantly higher
concentrations of dioxins than currently measured would be necessary to
predict quantifiable increases in risk. However, comparison of data
from the 1988 NSSS (USEPA, 1990) and the EPA 2001 dioxin update survey
(USEPA, 2002a) indicate that ``spikes'' (i.e., higher concentrations)
of dioxins in sewage sludge appear to be transient. Specifically, all
ten sewage sludge samples with the highest concentrations of dioxins
and furans measured in the 1988 Survey (concentrations ranging from 97
ppt TEQ to 827 ppt TEQ) had greatly reduced concentrations of dioxins
and furans in the 2001 dioxin update survey (concentrations ranging
from 2 ppt TEQ to 53 ppt TEQ) (USEPA, 1990 and USEPA, 2002a).
Conversely, the four sewage sludge samples with the highest
concentrations of dioxins and furans measured in the 2001 dioxin update
survey (concentrations ranging from 93 ppt TEQ to 682 ppt TEQ) had
markedly lower concentrations of dioxins and furans in the 1988 Survey
(concentrations ranging from 2 ppt TEQ to 41 ppt TEQ) (USEPA, 2002a and
USEPA, 1990).
[Note: These comparisons are based on dioxin and furan
concentrations since only dioxins and furans were measured in the
1988 Survey.] Thus, it is highly unlikely that a single family would
be exposed to one of these sewage sludges with a high
[[Page 40568]]
concentration of dioxin long enough to produce a quantifiable
increase in risk.
Finally, the Agency calculated the maximum number of cancer cases
in the highly exposed population that could be predicted from exposure
to dioxins in land applied sewage sludge (USEPA, 2002c). To make this
calculation the Agency used data from the EPA Exposure Factors Handbook
(USEPA, 1997) that indicates that 2 percent of the United States
population are in farm families whose diets consist of 50 percent of
products produced on their own farm (5.6 million people). The Agency
then estimated the maximum percentage of farmland to which sewage
sludge could be applied annually is 0.2 percent. This estimate was
derived by dividing the amount of farmland which could receive sewage
sludge if all 8 million metric tons of sewage sludge produced annually
in the United States (USEPA, 1999c) were land-applied at an agronomic
rate of 10 metric tons/hectare (800,000 hectares) by the total amount
of farmland in the United States (377 million hectares; USDA, 1997). On
this basis EPA estimates that the highly exposed farm family population
is no greater than 11,000 (i.e., 0.2% of the 5.6 million people whose
diets consist of 50% percent of products produced on their own farm).
The number of lifetime cancer cases is estimated by multiplying the
risk by the number of individuals in the modeled population. The
estimated lifetime cancer cases for the modeled population is 0.224 if
the 95th percentile adult risk from land application of sewage sludge
(2 x 10-5, see Table 5) is used for this calculation, and
0.112 using the 90th percentile adult risk (1 x 10-5, see
Table 5). The number of annual cases is estimated by dividing the
lifetime cancer cases by 70 years of exposure. The estimated annual
cancer cases is 0.006 if the 99th percentile adult risk is assumed,
0.003 if the 95th percentile adult risk is assumed, and 0.002 if the
90th percentile adult risk is assumed.
EPA requests comments on the Agency's characterization of the key
elements of the revised land application risk assessment. EPA will
consider these comments to characterize the overall estimate of risk to
the modeled population.
VII. What Are the Implications of EPA's Dioxin Reassessment Process for
This Rulemaking?
Since 1991 EPA has been conducting a scientific reassessment of the
health risks of exposure to dioxin and dioxin-like compounds. EPA began
this task in light of significant advances in the Agency's scientific
understanding of mechanisms of dioxin toxicity, significant new studies
of dioxin's carcinogenic potential in humans, and increased evidence of
other adverse health effects. These efforts have included the
involvement of outside scientists as principal authors of several
chapters, frequent public meetings to report progress and take public
comment, and publication of early drafts for public comment and peer
review. The review process for the Dioxin Reassessment has also
involved extensive use of outside scientists from other federal
agencies and the general scientific community.
As previously stated, aspects of the Agency's Draft Dioxin
Reassessment that are considered state of the science or the best
available information about dioxin have been incorporated into the
revised exposure analysis and risk assessment for dioxins in land-
applied sewage sludge. (See Section VI.D. of this Notice). However, the
Agency has not finalized its policy and/or factual conclusions with
respect to other aspects of the Draft Dioxin Reassessment, and any
decisions on these policy and factual conclusions made in part as a
result of the Dioxin Reassessment could affect the sewage sludge land
application exposure analysis and risk assessment, and therefore could
affect the Agency's decisions with respect to this rulemaking.
Therefore, EPA is seeking comment on the implications of this
information in the event that, prior to taking final action on the
Round Two rule, EPA finalizes a cancer slope factor, an approach to
assessing risk of non-cancer health effects from dioxins, or other
aspects of the current Draft Dioxin Reassessment. If EPA issues a final
Dioxin Reassessment that is substantially similar to the current draft
as discussed in this Notice, EPA does not expect to provide further
notice and opportunity for public comment with respect to the effect of
the Dioxin Reassessment on this rulemaking. The following is a brief
summary of the EPA Dioxin Reassessment process, and a discussion of how
the Agency may integrate key decisions on dioxins policy resulting from
the Dioxin Reassessment into the Round Two rulemaking.
EPA first released the external review drafts of the Dioxin
Reassessment health effects and exposure documents in September 1994
(USEPA 1994a). The Agency took public comment on the drafts, followed
by the Agency's Science Advisory Board (SAB) review of the Draft Dioxin
Reassessment in May 1995. The documents were revised based on these
reviews and were again released for external peer review. EPA made
additional revisions to the documents based on the external peer review
and submitted them once again to the SAB. After a public meeting on May
15, 2001, the SAB's Executive Committee endorsed a review report of the
Draft Dioxin Reassessment contingent upon changes to address some of
the differing scientific opinions raised in the review report.
Based on the overall endorsement of the content of the Draft Dioxin
Reassessment by the SAB, EPA used many aspects of the Reassessment in
the revised Part 503 exposure analysis and risk assessment. These
include the TEQ approach based on the toxicity of 2,3,7,8-TCDD, the use
of the current WHO98 TEQs, and the numerous physical,
chemical, occurrence, and exposure factors used in the Dioxin
Reassessment to evaluate and characterize human health risks from
dioxins.
Two of the key areas which the SAB identified as having differing
scientific opinions are the cancer slope factor for 2,3,7,8-TCDD and
the use of a margin of exposure (MOE) approach to evaluate the
likelihood that non-cancer effects may occur in the human population at
environmental exposure levels. The Draft 2000 Dioxin Reassessment notes
that, while major uncertainties remain, efforts to bring more data into
the evaluation of cancer potency have resulted in an estimate of 1 x
10-3/pg TEQ/kg-d. According to the Draft 2000 Dioxin
Reassessment, this cancer slope factor represents a plausible upper
bound on risk based on evaluation of human and animal data. These
values are approximately six times higher than previous estimates
(USEPA, 1985 and USEPA, 1994a) which were based on fewer data. However,
the EPA SAB panel was not able to reach consensus on a single value for
a dioxin potency factor. The SAB panel cited differences of opinion on
the adequacy of data and modeling approaches and assumptions as their
reasons for not reaching consensus on a dioxin cancer slope factor.
The revised Round Two land application risk assessment uses the
cancer slope factor currently used by EPA in risk assessments (USEPA,
1994a). If EPA adopts a different cancer slope factor for assessing the
risk of cancer from dioxin prior to taking final action on the proposed
Round Two rule, EPA will evaluate the risk of cancer from land-applied
sewage sludge using any such revised cancer slope factor. Similarly, to
the extent EPA adopts a policy regarding risks of non-cancer health
effects from dioxin prior to the
[[Page 40569]]
final decision on the proposed Round Two rule, the Agency will evaluate
non-cancer effects associated with dioxins in land-applied sewage
sludge using any such policy.
In order to give the public an opportunity to understand and
comment on how the particular approaches contained in the Draft Dioxin
Reassessment could potentially affect the proposed Round Two
rulemaking, EPA is presenting a discussion of the potential impacts of
the revised cancer slope factor and approaches for estimating non-
cancer effects contained in the Draft Dioxin Reassessment on EPA's
revised land application risk assessment. This includes a discussion of
background exposures and risks based on information in the Draft Dioxin
Reassessment, such as existing body burden, although EPA has not made a
final decision regarding these findings or adopted any policy with
respect to regulating dioxins in light of background exposures and
existing body burden.
A. How Would the Dioxin Cancer Risk from Land Application Compare to
Background Dioxin Cancer Risk?
Dioxin and dioxin-like compounds always exist in nature as complex
mixtures. These compounds can be quantified in environmental media and
their potential effects assessed as a mixture. As previously noted, the
contribution of the other ``dioxin-like'' compounds is quantified by
treating each as having a defined ``toxicity equivalence'' to dioxin
(toxicity equivalent factor, TEF). The TEQ concentration is calculated
by multiplying the concentration of each congener in the sewage sludge
by its corresponding TEF, and then summing the resulting products from
this calculation for all 29 congeners.
The significance of the incremental exposure and risk due to a
specific source such as land application of sewage sludge is best
understood by discussing it in the context of general population
background exposure to dioxins. The fact that background exposures and
body burden of dioxins are currently high for the general population
means that any incremental exposure from a particular source needs to
be considered in context of its contribution to overall risk. The
following is a comparison of the dioxin cancer risk the EPA calculated
from the Agency's revised risk assessment to the background dioxin
cancer risk estimated from the Agency's 2000 Draft Dioxin Reassessment.
This comparison considers both the current cancer slope factor the
Agency has been using since 1985 and the revised cancer slope factor
contained in EPA's 2000 Draft Dioxin Reassessment.
The revised risk assessment for land application of sewage sludge
uses the current cancer slope factor of 1.56 x 10-4/pg TEQ/
kg-d. The estimated upper bound lifetime risks for highly exposed farm
family adults using this cancer slope factor range from 4 x
10-5 at the 99th percentile to 1 x 10-6 at the
50th percentile for multi-pathway exposure to dioxins through land-
applied sewage sludge (see Table 5). As indicated in Table 5, the
estimated risks for children are less than or equal to the estimated
risks for adults. These risks correspond to an estimated daily
exposures (adult) ranging from 0.3 pg TEQ/kg-d at the 99th percentile
to 0.006 pg TEQ/kg-d at the 50th percentile. Use of the 1 x
10-3/pg TEQ/kg-d cancer slope factor being considered in the
2000 Draft Dioxin Reassessment would result in estimated high-end
multi-pathway lifetime risks for highly exposed farm family adults
ranging from 2.4 x 10-4 at the 99th percentile to 6 x
10-6 at the 50th percentile (see Table 7, below). These
estimated risks using a 1 x 10-3/pg TEQ/kg-d cancer slope
factor are based on the same daily exposures indicated in Table 5.
Again, the estimated risks for children would be less than or equal to
the estimated risks for adults (see table 7).
Table 7.--Risks for Highly Exposed Farm Adult and Child for All Exposure
Pathways--(Q*=1 x 10-3 pg TEQ/kg=d)
------------------------------------------------------------------------
Percentile Adult * Child **
------------------------------------------------------------------------
50th.......................................... 6 x 10-6 6 x 10-6
75th.......................................... 2 x 10-5 2 x 10-5
90th.......................................... 6 x 10-5 4 x 10-5
95th.......................................... 1 x 10-4 6 x 10-5
99th.......................................... 2 x 10-4 1 x 10-4
------------------------------------------------------------------------
* Initial exposure begins when the individual is an adult.
** Initial exposure begins when the individual is a child.
For this comparison EPA considered ``background risk'' to be the
upper bound risk for the general population. Using the current cancer
slope factor of 1.56 x 10-4/pg TEQ/kg-d and current body
burden and exposure levels, the background risk for the general
population is estimated to be approximately 1 x 10-4. By
comparison, EPA's 2000 Draft Dioxin Reassessment estimates that the
upper bound risk for the general population exceeds 1 x 10-3
using a revised cancer slope factor of 1 x 10-3/pg TEQ/kg-d.
Note that actual risks for individuals are a function primarily of
dietary habits and could be higher or lower. Thus, high-end incremental
risk estimates for highly exposed farm families from land application
of sewage sludge are approximately an order of magnitude (i.e., ten
times) lower than background risks for the general population.
These risk calculations are a function of dioxin TEQ dietary
intake. Adult daily intakes of dioxins, furans and coplanar PCBs are
estimated to average 65 picograms toxic equivalents per day (pg TEQ/
day) from all sources for the general population. By comparison, land
application of sewage sludge results in an estimated incremental intake
for a highly exposed adult farmer of 0.45 pg TEQ/day at the 50th
percentile of exposure; 1.7 pg TEQ/day at the 75th percentile; 4.5 pg
TEQ/day at the 90th percentile; 9.1 pg TEQ/day at the 95th percentile;
and 19.6 pg TEQ/day at the 99th percentile. These estimates of total
intake of dioxin for highly exposed adult farmers are calculated by
multiplying the estimated daily exposures from land application of
sewage sludge (in pg TEQ/kg-d; see Table 5) by an assumed adult body
weight of 70 kg.
B. How Would the Non-Cancer Dioxin Risk from Land Application Compare
to Background Non-Cancer Dioxin Risk?
EPA traditionally uses a ``reference dose'' (RfD) for evaluating
the potential for non-cancer effects for an incremental exposure that
results from a specific source of contamination. The RfD is an estimate
of a daily oral exposure to the human population that is likely to be
without an appreciable risk of deleterious non-cancer effects during a
lifetime. RfDs for a particular contaminant are a useful health
benchmark when background exposures are low or nonexistent. Background
exposures for dioxin-like compounds have been quantified by EPA as
being in the range of 1 pg TEQ/kg body weight-day for adults. On a body
burden basis, the background exposure for adults in the United States
has been quantified at 5 ng TEQ/kg whole weight basis (USEPA, 2000a.
See Part I, Vol. 3, Chap. 4.). The Draft Dioxin Reassessment concluded
that traditional approaches for setting an RfD would result in an RfD
for dioxin TEQs that is likely to be substantially below current
background intakes. For this reason, EPA believes that establishment of
an RfD that is below typical background exposures is uninformative in
judging the significance of incremental exposures. Consequently, EPA
has not developed an RfD in the Draft Dioxin Reassessment (USEPA,
2000a. See Part III, Chap. 6.)
[[Page 40570]]
Instead, the Draft Dioxin Reassessment promotes the concept of
evaluating an incremental percentage increase over background approach
for assessing potential non-cancer risk. There are two approaches to
evaluating the incremental percent increase. One is based on dose or
intake, and the second is based on body burden. The Draft Dioxin
Reassessment states that body burden, rather than daily dose, is a more
appropriate metric for quantifying risks of cancer as well as non-
cancer health effects. For long-term exposures to a steady dose (i.e.,
15-20 years or more), dose and body burden are correlated since the
body burden will tend to approach a steady state with long term steady
exposures. However, a short term change in dose will not result in the
same relative change in body burden. For example, a short term elevated
exposure to dioxin, say an exposure ten times higher on average for one
year, will not result in a proportional increase in body burden, a ten-
fold increase in body burden in this example. However, over long
periods of time, 20 years or more for example, a ten-fold increase in
an average dose will result in a ten-fold increase in body burden.
High-end incremental dioxin body burdens to the modeled highly
exposed farm population associated with land application of sewage
sludge are estimated to be 0.019 ng TEQ/kg body weight at the 50th
percentile of exposure, 0.072 ng TEQ/kg body weight at the 75th
percentile of exposure, 0.19 ng TEQ/kg body weight at the 90th
percentile of exposure, 0.39 ng TEQ/kg body weight at the 95th
percentile of exposure, and 0.84 ng TEQ/kg body weight at the 99th
percentile of exposure (Lorber 2002). These body burden estimates are
based on the estimated daily exposure from land application of sewage
sludge for highly exposed adult farmers (see Table 5) and an assumed
exposure time of at least 20 years. As described in the Draft Dioxin
Reassessment, the general population body burden spans a range of
younger to older adults. Evidence clearly indicates that older
individuals have body burdens that are higher than younger individuals,
mainly because of much higher exposures in past decades. The average
body burden of younger adults is more likely to be approximately 3 ng
TEQ/kg body weight, while the body burden of older adults would be
higher than the overall population average of 5 ng TEQ/kg body weight.
Women of childbearing age, a population of concern, would more likely
have body burdens in the range of 3 ng TEQ/kg body weight. (USEPA,
2000a. See Part I, Vol. 3, Chap. 6.). Using this background body burden
and the high-end incremental exposures associated with land application
of sewage sludge, the percentage increases in body burdens of dioxins
for highly exposed adult farmers from land application of sewage sludge
are estimated to be 0.6 percent at the 50th percentile of this modeled
population, 2 percent at the 75th percentile, 6 percent at the 90th
percentile,13 percent at the 95th percentile and 28 percent at the 99th
percentile.
VIII. What Is EPA's Assessment of Effects on Ecological Species?
A. What Approach Did EPA Use for the Screening Ecological Risk Analysis
of Dioxins in Land-Applied Sewage Sludge?
In response to public and peer review comments EPA performed a
screening ecological risk analysis (SERA) since the December 1999 Round
Two proposal. The SERA uses a two-phased approach that includes (1) an
initial bounding estimate to assess the upper bound potential for
ecological effects at a high-end of exposure and (2) a deterministic
assessment focused on representative ecological receptors.
The risk measurement chosen for this SERA is the hazard quotient
(HQ), the ratio of the exposure (in dose or concentration) to an
ecological benchmark. Media concentrations (e.g., sediment, soil) from
the human health risk assessment modeling simulations were used to
predict exposure doses, and HQs were calculated on a dioxin TEQ basis.
Calculation of HQs has a binary outcome: either the chemical
concentration (or dose) is below the protective ecological benchmark
(HQ<1), or it is equal to or greater than the benchmark (HQr1). Given
the assumptions and data inputs for each stage, the HQ results are
presumed to progress from highly uncertain and highly conservative in
the first phase to somewhat less conservative and more certain in the
second phase.
Screening-level ecological risk assessments are designed to
provide, for those chemicals and receptors that pass the screen, a high
level of confidence that there is a low probability of adverse effects
to ecological receptors (U.S. EPA, 2001c). The SERA was not designed or
intended to provide definitive estimates of risk; rather, the SERA
provides insight into the potential for ecological risk. The SERA was
designed to be consistent with EPA's Guidelines for Ecological Risk
Assessment (USEPA, 1998b).
B. How Did EPA Conduct the Screening Ecological Risk Analysis?
The screening ecological risk analysis addresses the 29 dioxin
congeners modeled in ``Exposure Analysis for Dioxins, Dibenzofurans,
and Coplanar Polychlorinated Biphenyls in Sewage Sludge'' (USEPA,
2002b) and was based on media concentrations generated in that
assessment.
The analysis phase of the SERA began with a highly conservative
approach to determine whether any of the habitats, receptor categories,
and exposure routes might be of concern. The second phase consisted of
more refined deterministic analyses based on somewhat more
representative exposure scenarios. Both phases predicted exposure doses
and compared those estimates to ecological benchmarks (i.e., the HQ).
HQs greater than 1 in the first phase analysis indicated that a more
refined analysis was needed to determine whether ecological effects are
expected.
The exposure estimates were derived from modeled media
concentrations generated in the human health risk assessment (USEPA,
2002b). For the SERA, annual soil, sediment, and surface water
concentrations were used as the basis for estimating exposure in all
phases of the analysis. Thus, the SERA inherently assumes a one-year
exposure duration for ecological receptors. The model calculates
average annual exposures. We use these values as high end
representations of exposures over the lifetimes of the evaluated
receptors.
Table 8 compares the values and assumptions used in each phase of
the analysis.
Table 8.--Values and Assumptions for the Screening Ecological Risk
Analysis
------------------------------------------------------------------------
Phase 2--
Parameter Phase 1--High end deterministic
exposures exposures
------------------------------------------------------------------------
Cogeners addressed.............. All............... All.
Receptors....................... Four highly 35 representative
exposed mammals mammals and
and birds. birds.
Dietary composition............. Diets reflecting Representative
maximum exposure. diets.
Biouptake factors............... Fixed values...... Fixed values
[[Page 40571]]
Percent of diet taken from 100%.............. 100%.
contaminated area.
Ecological benchmarks........... NOAELs............ Maximum allowable
toxicant level,
calculated as the
geometric means
of the NOAELs and
LOAELs.
Media concentrations used to 50th and 90th 90th percentile
estimate exposure. percentiles and for modeled
maximum for concentrations in
sewage sludge. environmental
media.
------------------------------------------------------------------------
The exposure scenarios considered in the SERA include the
agricultural application of sewage sludge in crop fields and pastures,
silvicultural application, and application to reclaimed lands. However,
only the agricultural application in crop fields and pastures was
assessed quantitatively; the other scenarios were addressed
qualitatively through comparison with agricultural application. For
agricultural application, the SERA addressed two types of habitats. The
first habitat consisted of receptors feeding and foraging in the
agricultural fields where sewage sludge is applied (i.e., terrestrial
habitat). These receptors are terrestrial vertebrates that eat the
crops and pasture vegetation (e.g., the white-tailed deer), or that eat
small birds and mammals that live and feed in the fields (e.g., the red
fox). In addition, the agricultural field includes soil invertebrates
that are exposed through direct contact with the land-applied sewage
sludge.
The second type of habitat consisted of receptors exposed through
living in or feeding from nearby surface water bodies that receive
dioxin loads through runoff (i.e., waterbody margin habitat). Aquatic
species, such as fish and aquatic invertebrates, were assumed to be
exposed through direct contact with dioxins in water and sediment and
through ingesting sediment and aquatic prey items. Terrestrial species,
such as the raccoon or the osprey, were assumed to be exposed when they
eat aquatic prey, such as fish, mussels, and snails from contaminated
water bodies.
Exposure in both of these habitat types was based on the common
characteristics of terrestrial and waterbody margin habitats,
respectively. Exposure in waterbody margin habitats is influenced by
variables such as water body size, position in the landscape, water
flow rate, bed sediment composition, periodicity of flood events, and
the presence of aquatic vegetation. Exposure in terrestrial systems is
dependent upon many important factors such as regional location,
vegetative cover type, wildlife community structure, and adequacy of
food sources. While the generalized representative habitats are a
simplification of exposure scenarios, they capture the basic elements
characteristic of most terrestrial and waterbody margin habitats. The
use of generalized terrestrial and waterbody margin habitats provided a
screening-level context for this analysis.
Given the generalized habitat types for the SERA, the exposed
ecological species were selected based on the following criteria: (1)
Represent all trophic levels and relevant feeding guilds (e.g.,
herbivores, carnivores), (2) represent receptors with the potential to
be highly exposed to dioxins in land-applied sewage sludge, and (3)
include receptors with as wide a geographic distribution as possible,
avoiding local receptors or those with narrow ecological niches because
sewage sludge is land applied throughout the United States. Since
adequate data were identified only for mammals and birds, assessment
endpoints (i.e., HQs) were quantitatively screened only for these
wildlife species populations.
The most significant pathway for vertebrate exposures to dioxins
(e.g., mammals, birds, amphibians) is ingestion, and exposure/risk are
expressed in terms of ingestion dose. Ingestion risk estimates for
terrestrial vertebrates reflect risk to an individual in a species
population, and risk to a population of that species is inferred
through the selection of endpoints relevant to population viability.
C. What Are the Results of the Screening Ecological Risk Analysis?
Each phase of the SERA was designed to provide insight into the
potential for adverse ecological effects. Phase 1 was a high-end
bounding analysis, and Phase 2 was a deterministic analysis based on
somewhat more representative exposure parameters and somewhat less
protective benchmarks. In the Phase 1 analysis, HQ values greater than
1 were calculated, indicating that a more refined analysis was needed.
For the Phase 2 analysis, no HQ values exceeded the target HQ of 1;
values range from a minimum of 0.0035 (Canada goose) to a maximum value
of 0.36 (short-tailed shrew). The median HQ for the receptors assigned
to waterbody margin habitats was 0.015, and the median HQ for receptors
assigned to terrestrial habitats was 0.044, indicating that the
potential for effects on terrestrial receptors may be somewhat higher
than for receptors in waterbody margin habitats. The results of the
Phase 2 analysis are summarized below in Table 9.
Table 9.--Phase 2 Results for Screening Ecological Risk Analysis
------------------------------------------------------------------------
HQ: Terrestrial HQ: Waterbody
Species habitats margin habitats
------------------------------------------------------------------------
American kestrel................ 3.5E-02........... not assigned.
American robin.................. 1.2E-02........... not assigned.
American woodcock............... 1.8E-01........... not assigned.
Bald eagle...................... not assigned...... 2.8E-03.
Beaver.......................... not assigned...... 2.5E-02.
Belted kingfisher............... not assigned...... 9.0E-03.
Black bear...................... 8.1E-02........... not assigned.
Canada goose.................... 3.5E-03........... not assigned.
Cooper's hawk................... 2.9E-02........... not assigned.
Coyote.......................... 2.2E-01........... not assigned.
Deer mouse...................... 3.0E-01........... not assigned.
[[Page 40572]]
Eastern cottontail rabbit....... 4.4E-02........... not assigned.
Great blue heron................ not assigned...... 3.5E-03.
Green heron..................... not assigned...... 6.3E-03.
Herring gull.................... not assigned...... 8.8E-03.
Least weasel.................... 1.6E-01........... not assigned.
Lesser scaup.................... not assigned...... 2.1E-02.
Little brown bat................ 6.2E-02........... not assigned.
Long-tailed weasel.............. 2.2E-01........... not assigned.
Mallard......................... not assigned...... 1.0E-02.
Meadow vole..................... 1.7E-02........... not assigned.
Mink............................ not assigned...... 2.3E-02.
Muskrat......................... not assigned...... 8.1E-02.
Northern bobwhite............... 1.3E-02........... not assigned.
Osprey.......................... not assigned...... 3.6E-03.
Prairie vole.................... 2.3E-02........... not assigned.
Raccoon......................... 4.4E-02........... 1.3E-01.
Red fox......................... 1.7E-01........... not assigned.
Red-tailed hawk................. 1.9E-02........... not assigned.
River otter..................... not assigned...... 2.6E-02.
Short-tailed shrew.............. 3.6E-01........... not assigned.
Short-tailed weasel............. 1.8E-01........... not assigned.
Tree swallow.................... 2.8E-02........... not assigned.
Western meadowlark.............. 1.7E-02........... not assigned.
White-tailed deer............... 6.1E-02........... not assigned.
------------------------------------------------------------------------
As noted above sewage sludge application to reclaimed lands and
silvicultural application of sewage sludge were addressed qualitatively
through comparison with agricultural application. In general,
reclamation and silviculture applications of sewage sludge are not well
characterized. Reclamation applications can consist of spreading sewage
sludge on reformed land surfaces as an amendment to support re-
vegetation or as fill material deposited in excavations. In the former
case, some tilling may occur with landscaping operations; for the
latter case, tilling is unlikely. In either case, the dioxins would be
expected to bind to soil particles and to display fate and transport
behavior similar to that in agricultural fields. While the application
rates and frequency are not necessarily comparable, ecological
exposures are likely to occur in a manner similar to that for
agricultural fields. Terrestrial vertebrates feeding at reclaimed sites
would generally be similar to those in an agricultural setting.
Receptors and pathways of exposure through aquatic systems would also
be expected to be similar to those modeled in the SERA.
For silvicultural application of sewage sludge, EPA assumed that
sewage sludge is land-applied once per site. Tilling is less likely to
occur except in reforestation projects where site preparation for new
plantings could include tilling of sewage sludge into the soil. Many of
the avian and mammalian species considered in the agricultural analysis
for the field habitat are also expected to feed and forage in forests
and, therefore, the screening results for field habitats are considered
relevant to the forest habitats. Although there are forest species that
are not represented in the agricultural scenario, the major trophic
elements are substantially represented. For these reasons, EPA believes
that the results of the SERA also provide a useful indicator for the
potential for adverse ecological effects at reclamation and
silvicultural sites.
Finally, EPA notes the following considerations that should be
recognized due to the screening nature of this analysis:
Because the screening methodology is based on the
exceedance of a target HQ of 1, the outcome of the screen is binary: HQ
< 1 or HQ r 1. Although large exceedances suggest a greater potential
for ecological damage, an HQ of 50 is not necessarily five times worse
than an HQ of 10.
The potential for adverse ecological effects (as indicated
by an HQ exceedance) should not be confused with the ecological
significance of those effects. Regardless of the magnitude of an HQ
exceedance, screening results can only suggest ecological damage; they
do not demonstrate actual ecological effects, nor do they indicate
whether those effects will have significant implications for ecosystems
and their components.
Ecological receptors for the screening methodology were
chosen to represent relatively common species populations. Threatened
and endangered species and/or habitats were not included in the
analysis because a different type of spatial resolution would have been
required (i.e., co-occurrence of threatened and endangered species/
habitats with sewage sludge application sites). Consequently, the
screening results do not indicate whether endangered species/habitats
are at risk.
EPA requests comments on the methodology and data used for the
screening ecological risk assessment. The Agency also requests comments
on the results derived from the screening ecological risk analysis
summarized above.
IX. How Might the New Data and Revised Risk Assessment Affect EPA's
Proposed Dioxin Concentration Limit for Land-Applied Sewage Sludge and
the Proposed Monitoring Requirements?
A. Possible Implications for Proposed Concentration Limit for Land-
Applied Sewage Sludge
As indicated above, the revised risk assessment (probabilistic) for
land application of sewage sludge estimates that the high-end
individual excess lifetime risk to the highly exposed modeled
population using the current cancer slope factor could range from 2 x
10-5 to 1 x 10-6 (``two in one-hundred thousand''
to ``one in one million'') for exposure by multiple pathways. Use of
the cancer slope factor being considered in the 2000 Draft Dioxin
Reassessment would result in
[[Page 40573]]
estimated high-end multi-pathway lifetime cancer risks ranging from 1.2
x 10-4 to 6 x 10-6 for this same highly exposed
modeled population. By comparison, the risk assessment for the December
1999 proposal (which used a deterministic methodology and a number of
different assumptions; see Section VI.D. of this Notice), estimated a
high-end cancer risk of 1.7 x 10-5 (USEPA, 1999b). As noted
in the December 1999 proposal, the Agency considers risks in the range
of 1 x 10-6 to 1 x 10-4 (``one in one million''
to ``one in ten thousand'') to be acceptable levels of risk. The
revised high-end risk estimates continue to fall within this range of
acceptable risks. The revised risk assessment also shows no measurable
change in risk from requiring all sewage sludge to meet a 300 ppt TEQ
limit.
B. Effect on Proposed Monitoring Requirements
In the December 1999 proposal, the Agency proposed two alternative
monitoring schedules based on the level of dioxins in sewage sludge to
be land applied. Specifically, treatment works and other sewage sludge
preparers that measure the level of dioxin in their sewage sludge to be
between 300 ppt TEQ and 30 ppt TEQ would be required to monitor
annually. Treatment works and sewage sludge preparers that measure
dioxin levels of 30 ppt TEQ or less for two consecutive years would be
required to monitor every five years thereafter. The proposed
monitoring schedule was based on the Agency's assumption that the level
of dioxins in sewage sludge, both nationally and from specific sources,
is relatively constant over time and may be decreasing. The Agency
noted that since the concentration of 30 ppt TEQ which would allow less
frequent monitoring is a full order of magnitude less than the proposed
numeric standard of 300 ppt TEQ (i.e., one-tenth), the chances that
such a sewage sludge would exceed the limit are small. Furthermore, the
Agency noted that any health risks associated with dioxin exposure from
land application of sewage sludge at these levels would require long-
term exposure (i.e., significantly greater than five years) to
potentially present unreasonable health risks.
As noted in Section V.H. of this Notice, the EPA 2001 dioxin update
survey indicates that dioxin levels in sewage sludge appear to have
decreased from 1988 to 2001. The new data also indicate that for most
POTWs, dioxin levels appear to not fluctuate greatly over time.
However, the sewage sludge samples which had the highest levels of
dioxins in either the 1988 NSSS or 2001 EPA update survey appeared to
evidence greater fluctuations in dioxin concentrations than the other
sewage sludges. As also previously noted, the data for facilities where
monthly data were available indicate that dioxin concentrations tend to
corroborate these observations from the EPA 2001 dioxin update survey.
The data for the facilities where monthly data were available indicate
that the dioxin concentrations are relatively consistent over time on a
month-to-month basis, but the variability appeared the greatest for the
facility with the highest dioxin concentrations measured in its sewage
sludge (see Section V.K.).
The Agency continues to believe that if it sets a dioxin limit of
300 ppt TEQ, this two-tier monitoring schedule in line with the
December 1999 proposal may be appropriate. For facilities where longer
term monitoring data was available, the maximum monthly concentration
of dioxin was within a factor of two to four times the average
concentration for that facility. By comparison, the proposed monitoring
schedule would allow reduced monitoring frequency only when two
consecutive measurements were a factor of ten less than the specified
limit. Furthermore, no POTWs in the EPA 2001 dioxin update survey had
consistently high levels of dioxins in their sewage sludge; and the
revised risk assessment predicts that even long term exposure to
dioxins in land-applied sewage sludge would result in negligible
increases in risk.
Based on the data from the EPA 2001 dioxin update survey,
approximately 31 percent of POTWs produce sewage sludge with dioxin
levels between 30 ppt TEQ and 300 ppt TEQ (USEPA, 2002a). These POTWs
would be required to monitor annually for dioxin under the proposed
monitoring schedule if their sewage sludge is land applied. (By
comparison, approximately 61 percent of POTWs previously were estimated
to produce sewage sludge with dioxin levels between 30 ppt TEQ and 300
ppt TEQ based on the data available to EPA at the time of the December
1999 proposal (USEPA, 1999d).)
The costs associated with monitoring for dioxin annually at
facilities with sewage sludge concentrations between 30 ppt TEQ and 300
ppt TEQ previously was estimated to be $1,224,000 based on the sewage
sludge dioxin data available to EPA at the time of the December 1999
proposal (USEPA, 1999d). EPA now estimates the costs associated with
monitoring for dioxin annually at facilities with sewage sludge dioxin
concentrations between 30 ppt TEQ and 300 ppt TEQ would be
approximately $656,000 (USEPA, 2002d).
Based on the new data, EPA is considering whether alternatives to
the proposed monitoring scheme would be more appropriate. Because the
data continue to show periodic ``spikes,'' and the data indicates that
these higher levels of dioxin may not remain for long periods of time,
a different monitoring schedule may be indicated. Similarly, the data
indicates that sewage sludge with lower levels of dioxins may not
fluctuate as greatly, which may indicate a different threshold or
monitoring frequency than those proposed. For example, monitoring every
two years rather than annually; or at some other interval may be more
appropriate.
The percentage of land-applied sewage sludge which would have to be
monitored annually would be reduced if the threshold for annual dioxin
monitoring was set at a higher concentration than 30 ppt TEQ. Likewise,
the percentage of land-applied sewage sludge which would have to be
monitored annually would be greater if the threshold for annual dioxin
monitoring was set at a lower concentration than 30 ppt TEQ. As an
example, 13 percent of POTWs produce sewage sludge between 50 ppt TEQ
and 300 ppt TEQ based on data from the EPA 2001 dioxin update survey
(USEPA, 2002a). This compares to 31 percent of POTWs with sewage sludge
dioxin concentrations between 30 ppt TEQ and 300 ppt TEQ, as noted
above.
The Agency requests comments on the proposed monitoring schedule
and the threshold concentration of dioxin that would allow for more or
less frequent monitoring. Specifically, EPA requests comments on
whether other schedules which would require more or less frequent
monitoring would be more appropriate. EPA also requests comment on
whether a monitoring requirement in lieu of a numeric limit should be
considered.
X. How Might the New Data and Revised Risk Assessment Affect EPA's
Proposal for Small Entities?
EPA proposed to exclude from the proposed land application
requirements relating to dioxins, sewage treatment works with a
wastewater flow of one MGD or less and sewage sludge-only entities
which prepare 290 dry metric tons or less of sewage sludge annually for
land application. (EPA estimates that a one MGD treatment works
produces approximately 290 dry metric tons of sewage sludge annually.)
Sewage sludge from these small preparers would be excluded from the
limitation on dioxins
[[Page 40574]]
in sewage sludge. Such preparers could continue to land apply their
sewage sludge with no further restriction due to the sewage sludge's
dioxin content.
The December 1999 proposal indicated that EPA believes that this
exclusion is appropriate for several reasons. First, less than eight
percent of the total sewage sludge that is land applied is produced by
sewage treatment works with flow rates of one MGD or less (USEPA,
1990). Second, the probability that this small amount of sewage sludge
(i.e., 42 dry metric tons per facility annually, which is the average
amount of sewage sludge produced by POTWs less than one MGD) could
unreasonably increase health risks for any individual is extremely
small. EPA specifically requested comment on the Agency's proposal to
exclude small preparers from any requirements relating to dioxins in
sewage sludge to be land applied.
The new data that EPA collected on the levels of dioxins found in
sewage sludge (USEPA, 2002a) and the revised land application risk
assessment (USEPA, 2002b), provide additional information which the
Agency believes supports the proposal to exclude sewage treatment works
with a wastewater flow of one MGD or less and sewage sludge-only
entities which prepare 290 dry metric tons or less of sewage sludge
annually for land application.
The levels of dioxins in sewage sludge from treatment works with a
wastewater flow of one MGD or less was measurably less than the levels
of dioxins in sewage sludge from facilities with a wastewater flow
greater than one MGD (USEPA, 2002a). The highest observed level of
dioxins from treatment works with a wastewater flow of one MGD or less
was 78.6 ppt TEQ. This compares to the highest observed value of 718
ppt TEQ for dioxins for facilities with a wastewater flow greater than
one MGD. The average (mean) and 95th percentile values dioxins for
treatment works with a wastewater flow of one MGD or less also were
measurably less compared to treatment works with flows greater than one
MGD: 26.5 ppt TEQ and 67.1 ppt TEQ, respectively for treatment works
with a wastewater flow of one MGD or less compared to 44.1 ppt TEQ and
94.8 ppt TEQ, respectively for treatment works with a wastewater flow
greater than one MGD.
The revised risk assessment methodology does not allow EPA to make
a separate risk estimate for treatment works with wastewater flows of
one MGD or less because, other than the dioxin levels in sewage sludge
discussed above, there are no relevant factors considered in the risk
assessment which vary specifically based on the capacity of the
treatment works . However, the Agency believes the revised risk
assessment provides further indication that the minimal amounts of
sewage sludge from treatment works with wastewater flows of one MGD or
less would be very unlikely to produce an unreasonable increase in
health risks for any individual.
The revised risk assessment estimates that the high-end incremental
adult lifetime risk for highly exposed farm families associated with
dioxins in land-applied sewage sludge ranges from 4 x 10-5
at the 99th percentile to 1 x 10-6 at the 50th percentile,
which equates to less than 0.006 cancer cases annually. The key
variable in this risk estimate that can be related to treatment
facility size is the distribution of farm sizes to which the sewage
sludge is land-applied. The revised risk assessment used a distribution
of median farm sizes for 41 meteorologic regions ranging from 24.2
acres to 1241.7 acres (USDA, 1997). For this distribution, the average
farm size is 487 acres and the median farm sizes is 120 acres. By
comparison, the average amount of sewage sludge produced by a treatment
works with a wastewater flow of one MGD or less (i.e., 42 dry metric
tons annually) would be applied to approximately 10 acres of farmland
when applied at agronomic rates (i.e., 4 metric tons per acre
annually). Thus, the acreage impacted by treatment works with a
wastewater flows of one MGD is significantly less than that which would
result in an estimated risk of 1 x 10-6. On this basis, EPA
believes that the amount of sewage sludge produced by treatment works
with a wastewater flow of one MGD or less is not sufficient to result
in an unreasonable risk to potentially exposed populations. Again, EPA
specifically invites comment on the Agency's proposal to exclude small
entities from any limit for dioxins in sewage sludge to be land
applied.
XI. How Does the New Data and Revised Risk Assessment Affect EPA's Cost
Estimates?
As noted in the December 1999 proposal, the increased costs which
would be imposed by the proposed regulation are the costs for initially
monitoring for dioxins by all land-applying treatment works greater
than one MGD, annual monitoring at those facilities with dioxin levels
between 30 ppt TEQ and 300 ppt TEQ, and switching to co-disposal with
municipal solid waste for current land appliers whose sewage sludge
contains over 300 ppt TEQ of dioxins. The Agency assumed that the cost
of measuring dioxins in sewage sludge is $2,000 per sample and the cost
to switch to co-disposal with municipal solid waste was $189 per dry
metric ton in 1998 dollars. For the proposal, EPA estimated that the
annualized cost of this regulation nationwide would be approximately
$18 million. Of this amount, 13 percent was for monitoring, and the
balance is for switching use or disposal practices (USEPA, 1999d).
EPA has updated these cost estimates (USEPA 2002d). The Agency
assumes that the cost to switch to co-disposal with municipal solid
waste has risen to $197 per dry metric ton in year 2000 and that the
cost of measuring dioxins in sewage sludge remains at $2,000 per
sample. On this basis, EPA now estimates that the annualized cost of
this regulation Nationwide would be approximately $4.5 million if the
dioxin limit for land application of sewage sludge is 300 ppt TEQ. The
decrease in the estimated cost results from the smaller percentage of
sewage sludge that would exceed a 300 ppt TEQ dioxin limit based on the
data from the EPA 2001 dioxin update survey (i.e., 1% vs. 5%). The
estimated benefits of a 300 ppt limit would be very low, since such a
limit would not likely produce a detectable change in lifetime cancer
risk, even to highly exposed farm families and using conservative
assumptions, and no species in the SERA has a HQ above 1, even in the
baseline with no limits.
XII. Identification and Control of Dioxin Sources that Contribute
to Elevated Dioxin Levels in Sewage Sludge.
Both the EPA 2001 dioxin update survey and the 2001 AMSA Survey
found a small percentage of sewage sludge samples with dioxin
concentrations which were significantly higher than most of the other
the sewage sludge samples in the survey. The EPA 2001 dioxin update
survey found only 1 percent of the samples with a dioxin concentration
greater than 100 ppt TEQ (compared to an average (mean) of 31.6 ppt
TEQ). The AMSA 2001 survey found less than 5 percent of the samples
analyzed in their survey with a dioxin concentration greater than 100
ppt TEQ (compared to an average (mean) of 48.6 ppt TEQ.)
Even though relatively few sewage sludge samples have elevated
concentrations of dioxins, those that do can have levels which are much
higher than the values typically observed. The highest dioxin
concentration measured in the 2001 EPA and AMSA surveys were 718 ppt
TEQ and 3,590 ppt TEQ,
[[Page 40575]]
respectively. In addition, as discussed previously in this Section of
today's notice, higher levels of dioxins in sewage sludge appear to be
transient and may not be consistently identified. While the revised
risk assessment shows no measurable change in the risk from eliminating
these spikes to individuals exposed through land application of sewage
sludge, the Agency believes it may be beneficial to develop a procedure
to identify the sources contributing to higher levels of dioxins in
sewage sludges. Relatively high levels of dioxin in sewage sludge may
be an indication of sources in the treatment works' service area with
even higher levels of dioxins.
The Agency is requesting comments on a methodology to assist
communities in identifying sources of elevated dioxins in their sewage
sludge. This methodology relies on two complementary elements to
identify sources of dioxin: (1) Identification of sources known to be
generators or sinks for dioxin (e.g., specific chemical manufacturing
operations, combustion sources or contaminated landfills); and (2)
comparison of the mix of the 29 dioxin congeners measured in a
particular sewage sludge sample to the ``fingerprint'' of 29 dioxin
congeners for known sources of dioxins. The methodology would be used
by communities to reduce levels of dioxins in their sewage sludge by
eliminating these sources of dioxins from the collection system or
remediating contaminated sites.
The first element of this methodology is identification of local
industrial, commercial and other sources with inputs to municipal
sanitary sewers which have a potential to contain significant levels of
dioxins. The primary database used to make these identifications would
be the Agency's updated 2001-2002 Toxics Release Inventory. The Toxics
Release Inventory is a valuable source of nationwide information
regarding toxic chemicals that are being used, manufactured, treated,
transported or released into the environment. Toxics Release Inventory
data includes the local discharges of chemicals to sanitary sewers by
industrial and commercial establishments. Other potential local sources
of dioxins in sewage sludge include leachate from landfills,
contaminated manufacturing and disposal sites, and scrubber water from
combustion operations.
Identification of possible sources of dioxins in sewage sludge also
will be aided by reviewing data available from local pretreatment
programs and the results of detailed studies conducted in any
communities which have attempted to identify sources of dioxins in
their sewage sludge. Industry listings for local pretreatment programs
will be reviewed to determine which are likely sources of elevated
dioxins in sewage sludge. With respect to community-specific studies,
EPA has received information which indicates that elevated
concentrations of dioxins in the sewage sludge may be due to non-point
source contamination. Non-point source contamination comes from
erodible soils that contain elevated levels of dioxins and periodically
enter either sanitary sewers as a result of infiltration during
precipitation, or combined sewers through normal stormwater flows.
The second element of a methodology to identify sources which
contribute to elevated dioxins in sewage sludge is to compare the mix
of dioxin congeners in a particular sewage sludge to the mix of dioxin
congeners in known sources of dioxins. Mixtures of the 29 congeners of
dioxins have distinct patterns (profiles or ``fingerprints'') of
relative proportions for each of the congener classes (i.e., dioxins,
dibenzofurans and coplanar PCBs) depending on the source of dioxins.
For example, dioxins produced by combustion have a different
``fingerprint'' than dioxins produced by chemical processes such as
pulp and paper mill bleaching with chlorine or pentachlorophenol
manufacturing. By examining these congener ``fingerprints'', it is
possible to identify likely manufacturing, chemical or combustion
processes that produced that particular profile. Dioxin congener
profiles from the sewage sludge samples with elevated dioxin
concentrations from the 2001 EPA and AMSA surveys will be compared
against known dioxin profiles of samples from various manufacturing,
chemical and combustion and chemical processes. These comparisons can
be used in the source identification portion of the methodology
described above.
EPA is inviting comments on this overall methodology to identify
and reduce or eliminate sources of dioxins entering wastewater
treatment plants that contribute to elevated levels of dioxins in
sewage sludge. In particular, comments are invited on the two phase
approach to identify these sources described above. Note that EPA is
not proposing use of this methodology in a regulatory context, but
rather developing it as a tool for use by POTWs and/or communities on a
voluntary basis.
XIII. Request for Public Comments
While EPA is requesting comments on all of the information
discussed in this Notice, the Agency hopes that public comments will
also focus specifically on the following aspects of this Notice:
(1) The significance of the differences in dioxin concentrations in
sewage sludge measured at facilities with wastewater flows greater than
one MGD compared to dioxin concentrations in sewage sludge at
facilities with wastewater flows less than one MGD (V.G.).
(2) The significance of the differences in dioxin concentrations in
sewage sludge measured in the EPA 2001 dioxin update survey compared to
dioxin concentrations in sewage sludge measured in the 1988 NSSS
(V.H.).
(3) Choice of the highly exposed farm family as the modeled
population for the revised risk assessment and the assumptions related
to this choice of modeled population. (VI.D.).
(4) All of the assumptions related to exposure, fate and transport
used in the revised risk assessment , including the specific
assumptions related to the farming and grazing practices used by the
modeled farm family (VI.D.),
(5) The treatment of non-detects in the revised risk assessment and
the effect on estimating risk (VI.E.).
(6) The assumptions and values used to estimate how much dioxins
are being transported to individuals in the modeled farm family (e.g.,
the sources [store-bought versus farm-produced], types and dioxin
contamination levels of poultry feeds.) (VI.F.)
(7) The methodology and data used for the screening ecological risk
assessment (VIII.A. and VIII.B); and the results derived from the
screening ecological risk analysis (VIII.C.).
(8) The significance of the finding that setting a 300 ppt TEQ
limit would make no detectable difference in the risk of cancer to the
highly exposed farm family.
(9) Taking no action with respect to regulating dioxins for land
application (IX.).
(10) The proposed monitoring schedule and the threshold
concentration of dioxin that would allow for less frequent monitoring,
and specifically, on whether other schedules which would require more
or less frequent monitoring would be more appropriate (IX.).
(11) Excluding small entities from the limit for dioxins in sewage
sludge to be land applied (X.).
(12) A methodology to assist communities in voluntarily identifying
and reducing or eliminating sources of dioxins entering wastewater
treatment plants that contribute to elevated levels of dioxins in
sewage sludge (XII.).
[[Page 40576]]
XIV. List of References
AMSA 2001. The AMSA 2000/2001 Survey of Dioxin-like Compounds in
Biosolids: Statistical Analyses
Green, et al. 1995. Comments on Estimating Exposure to Dioxin-Like
Compounds: Review Draft, Jan. 12, 1995. 204 pp. Addendum. May 11, 1995.
23 pp.
Lorber, M.N., 2002. Evaluating Non-Cancer Risk from Land Application of
Sewage Sludge Using an Increment Over Background Approach. Memorandum
from Matthew Lorber, National Center for Environmental Assessment,
Office of Research and Development, USEPA, Washington, DC to Alan B.
Hais, Health and Ecological Criteria Division, Office of Science and
Technology, Office of Water, USEPA, Washington, DC. April, 2002.
USDA, 1997. Census of Agriculture. Washington, DC.
USEPA, 1985. Health Assessment Document for Polychlorinated Dibenzo-p-
Dioxins. EPA/600/8-84/014F. Final Report. Office of Health and
Environmental Assessment. Washington, DC September, 1985.
USEPA, 1989. Interim Procedures for Estimating Risks Associated with
Exposure to Mixtures of Chlorinated Dibenzo-p-dioxins and -
dibenzofurans (CDDs and CDFs) and 1989 Update. EPA/625/3-89/016. Risk
Assessment Forum. Washington, DC March 1989.
USEPA, 1990. National Sewage Sludge Survey; Availability of Information
and Data, and Anticipated Impacts on Proposed Regulations; Proposed
Rule. Federal Register 55 (218): 47210-47283.
USEPA, 1992. Guidelines for Exposure Assessment, EPA/600Z-92/001,
National Center for Environmental Assessment, Washington, DC.
USEPA, 1994a. Health Assessment for 2,3,7,8-TCDD and Related Compounds.
External Review Draft. EPA/600/BP-92/001a-c, ( Vol. I: 420 pp., Vol.
II: 685 pp., Vol. III: 125 pp.) and Estimating Exposure to Dioxin-Like
Compounds. Volume I. Executive Summary. 128 pp. Volume II. Properties,
Sources, Occurrence, and Background Exposures 424 pp. + 260 pp. Volume
III. Site-Specific Assessment Procedures. 452 pp. External Review
Draft. EPA/600/6-88/005Ca-c. National Center for Environmental
Assessment. Washington, DC.
USEPA, 1994b. EPA Method 1613: Dioxins and Furans by Isotope Dilution
High-resolution Gas Chromatography/ Mass Spectrometry, Revision B (EPA
821-B-94-005, October 1994.
USEPA, 1997. Exposure Factors Handbook. National Center for
Environmental Assessment. Washington, DC EPA/600/P-95/002F(a-c). Vol.
I: 208 pp. Vol. II: 336 pp. Vol. III: 340 pp. Also available at NTIS
(Vol. I PB98-124225, Vol. II PB98-124233, Vol. III PB98-124241, The Set
PB98-124217). See also http://www.epa.gov/ncea/exposfac.htm
USEPA, 1998a. Methodology for Assessing Health Risks Associated with
Multiple Pathways of Exposure to Combustion Emissions. EPA/600/P-98/
137. Washington, DC.
USEPA, 1998b. Guidelines for Ecological Risk Assessment (Final). EPA/
630/R-95/002F. Risk Assessment Forum. Washington, DC.
USEPA, 1999a. EPA Method 1668: Polychlorinated Biphenyls by Isotope
Dilution High-resolution Gas Chromatography/Mass Spectrometry, Revision
A , EPA-821-R-00-002, December 1999).
USEPA, 1999b. Risk Analysis for the Round Two Biosolids Pollutants.
Office of Science and Technology. Washington, DC.
USEPA, 1999c. Biosolids Generation, Use, and Disposal in the United
States. EPA 530-R-99-009. Office of Solid Waste and Emergency Response.
Washington, DC.
USEPA, 1999d. Costs Associated with Regulating Dioxins, Furans, and
PCBs in Biosolids. Office of Science and Technology. Washington, DC.
USEPA, 2000a. Exposure and Human Health Reassessment of 2,3,7,8-
Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. Parts I-III.
Draft. Prepared by the National Center for Environmental Assessment,
Office of Research and Development. Washington, DC (EPA/600/P-00/001
Bb, Bc, Bd, Be, Bg). Available online at http://www.epa.gov/ncea.
USEPA, 2000b. Risk Characterization Handbook, EPA 100-B-00-002, Science
Policy Council, Washington, DC.
USEPA, 2001a. Sampling Procedures for the 2001 National Sewage Sludge
Survey, Office of Science and Technology, Washington, DC.
USEPA, 2001b. Analytical Data for Dioxins in Sewage Sludge Submitted by
Three Wastewater Treatment Plants, Office of Science and Technology,
Washington, DC.
USEPA, 2001c. The Role of Screening-Level Risk Assessments and Refining
Contaminants of Concern in Baseline Ecological Assessments. EPA ECO
Update, Publication 9345.0-14. EPA/540/F-01/014. Office of Solid Waste
and Emergency Response, U.S. EPA, Washington, DC.
USEPA, 2002a. Statistical Support Document for the Development of Round
2 Biosolids Use or Disposal Regulations , Office of Science and
Technology, Washington, DC.
USEPA, 2002b. Exposure Analysis for Dioxins, Dibenzofurans, and
Coplanar Polychlorinated Biphenyls in Sewage Sludge-Technical
Background Document, Office of Science and Technology, Washington, DC.
USEPA, 2002c. Estimate of Population Exposed to Dioxins from the Land
Application of Sewage Sludge and Corresponding Number of Annual Cancer
Cases from this Exposure, Office of Science and Technology, Washington,
DC.
USEPA, 2002d. Costs Associated with Regulating Dioxins, Furans, and
PCBs in Biosolids. Office of Science and Technology. Washington, DC.
Van den Berg M, et al. 1998. Toxic Equivalency Factors (TEFs) for PCBs,
PCDDs, and PCDFs for Humans and Wildlife. Environ. Health Perspect.
106(12): 775-792.
Dated: June 5, 2002.
G. Tracy Mehan III,
Assistant Administrator for Water.
[FR Doc. 02-14761 Filed 6-11-02; 8:45 am]
BILLING CODE 6560-50-P