F
Wednesday,
June 12, 2002
m
Part VI
Environmental
Protection Agency
Standards for the Use or Disposal of
Sewage Sludge; Notice
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Federal Register/Vol. 67, No. 113/Wednesday, June 12, 2002/Notices
ENVIRONMENTAL PROTECTION
AGENCY
[FRL -7228-9]
STANDARDS FOR THE USE OR
DISPOSAL OF SEWAGE SLUDGE
AGENCY: Environmental Protection
Agency.
ACTION: Notice of data availability.
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^llOl),
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 OW-
Docket@epamail.epa.gov. 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. plunkett.arleen@epa.gov
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
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40555
http :II 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/m3—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
Ql*—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
1 Section 405(d)(2)(A) of the Clean Water Act
(CWA), 33 U.S.C. § 1345(d)(2)(A) required EPA to
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.
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
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.
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
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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.
Gearhartv. 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 MED-
IO 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
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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",
"V2 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
Method
Mean
Std. Dev.
Max.
99th %
98th %
95th %
90th %
50th %
Total Dioxin and Furan TEQs (nanograms/kilogram dry matter basis)
0
16 DL
DL
21.70
21.70
21.80
47.5
47.5
47.5
682.00
682.00
682.00
100.00
100.00
100.00
54.40
54.40
54.40
33.30
33.30
33.30
31.40
31.60
31.70
15.50
15.50
15.50
Total Coplanar PCB TEQs (nanograms/kilogram dry matter basis)
0
16 DL
DL
5.22
9.87
14.50
10.3
14.0
22.4
58.30
58.30
103.00
50.60
55.10
97.2
44.80
54.50
91.60
13.10
49.40
78.00
9.66
19.20
35.00
2.05
6.04
8.11
Total Dioxin and Dioxin-Like TEQs (nanograms/kilogram dry matter basis)
0
16 DL
DL
26.90
31.60
36.30
49.6
50.0
52.7
718.00
718.00
718.00
114.00
115.00
138.00
76.60
80.10
96.00
59.30
73.50
113.00
42.80
55.10
69.10
19.70
23.40
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.
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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
DL for Non-
detects
DL for Nondetects
Estimate
<1 MGD
>1 MGD
<1 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 nondetects
DL for 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.
/. 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
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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
WH09s 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 :II www. amsa-clean water, 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
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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 VILA, 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.
D. How Did the Framework Change?
In the exposure analysis, the risk
assessment evaluated a revised scenario
for exposure to sewage sludge: exposure
2 Information was not available to allow the
weighting of these 41 climate regions based on the
number of farm families in each region.
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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
Receptor
Inhala-
tion of
ambi-
ent air
Inges-
tion of
soil
Inges-
tion of
above-
and be-
low-
ground
produce
Inges-
tion of
beef
and
dairy
prod-
ucts
Inges-
tion of
poultry
and
egg
prod-
ucts
Inges-
tion of
fish
Inges-
tion of
breast
milk
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 unfilled 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 (unfilled). 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
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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
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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
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
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).
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
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.
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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.
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.
/. 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.
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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)
Percentile
Adult*
Child **
Risk
Daily Expo-
sure pg
TEQ/kg-d
Risk
Daily Expo-
sure, pg
TEQ/kg-d
50th
1 x10~6
4 x 10~6
1 x10-5
2 x 10-5
4x10-5
7.3
7.3
7.3
7.3
7.3
1 x 10-6
3x10-6
7 x 10-s
1 x 10-5
2 x 10-5
7.3
7.3
7.3
7.3
7.3
75th
90th
95th
99th
* 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 ac-
Pathway and Sensitivity variables
counted
for by
variable
Beef:
Exposure Duration
60
Consumption Rate
26
Sewage sludge Application
1
Rate.
Average year that the farm
1
family moves in.
Dairy products:
Exposure Duration
54
Consumption Rate
32
Average year that the farm
1
family moves in.
Sewage sludge Application
1
rate.
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
(Ql* 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
unfilled pasture. An important
assumption in terms of characterizing
the risk is that the dioxin in each
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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 Ql*. 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
• 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.
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).
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
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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
WHOgs 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 l0~3/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
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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 l0~3/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 l0~3/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 Ex-
posed Farm Adult and Child for
All Exposure Pathways—(Q*=1
x 10-3 pg JEQ/kg=d)
Percentile
Adult*
Child **
50th
6x10-6
6x10-6
75th
2 x10-5
2 x10-5
90th
6 x10-5
4x10-5
95th
1 xIO-4
6 x10-5
99th
2x10-4
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 l0~3/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.)
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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 (HQ>1). 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
Parameter
Phase 1—High end exposures
Phase 2—deterministic exposures
Cogeners addressed
All
All.
35 representative mammals and birds.
Representative diets.
Fixed values
Receptors
Dietary composition
Biouptake factors
Four highly exposed mammals and birds
Diets reflecting maximum exposure
Fixed values
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40571
Table 8.—Values and Assumptions for the Screening Ecological Risk Analysis—Continued
Parameter
Percent of diet taken from contaminated area ..
Ecological benchmarks
Media concentrations used to estimate expo-
sure.
Phase 1—High end exposures
100% ...
NOAELs
50th and 90th percentiles and maximum for
sewage sludge.
Phase 2—deterministic exposures
100%.
Maximum allowable toxicant level, calculated
as the geometric means of the NOAELs
and LOAELs.
90th percentile for modeled concentrations in
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
Species
HQ: Terrestrial habitats
HQ: Waterbody margin habitats
American kestrel ....
American robin
American woodcock
Bald eagle
Beaver
Belted kingfisher ....
Black bear
Canada goose
Cooper's hawk
Coyote
Deer mouse
3.5E-02
1.2E-02
1.8E-01
not assigned
not assigned
not assigned
8.1E-02
3.5E-03
2.9E-02
2.2E-01
3.0E-01
not assigned,
not assigned,
not assigned.
2.8E-03.
2.5E-02.
9.0E-03.
not assigned,
not assigned,
not assigned,
not assigned,
not assigned.
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Table 9.—Phase 2 Results for Screening Ecological Risk Analysis—Continued
Species
Eastern cottontail rabbit
Great blue heron
Green heron
Herring gull
Least weasel
Lesser scaup
Little brown bat
Long-tailed weasel
Mallard
Meadow vole
Mink
Muskrat
Northern bobwhite
Osprey
Prairie vole
Raccoon
Red fox
Red-tailed hawk
River otter
Short-tailed shrew
Short-tailed weasel
Tree swallow
Western meadowlark
White-tailed deer
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
HQ: Terrestrial habitats
4.4E-02
not assigned
not assigned
not assigned
1.6E-01
not assigned
6.2E-02
2.2E-01
not assigned
1.7E-02
not assigned
not assigned
1.3E-02
not assigned
2.3E-02
4.4E-02
1.7E-01
1.9E-02
not assigned
3.6E-01
1.8E-01
2.8E-02
1.7E-02
6.1E-02
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> 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
HQ: Waterbody margin habitats
not assigned.
3.5E-03.
6.3E-03.
8.8E-03.
not assigned.
2.1E-02.
not assigned,
not assigned.
1.0E-02.
not assigned.
2.3E-02.
8.1E-02.
not assigned.
3.6E-03.
not assigned.
1.3E-01.
not assigned,
not assigned.
2.6E-02.
not assigned,
not assigned,
not assigned,
not assigned,
not assigned.
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
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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 ofl.7xl0~5
(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 pptTEQ 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
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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,
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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.).
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40576
Federal Register/Vol. 67, No. 113/Wednesday, June 12, 2002/Notices
XIV. List of References
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Biosolids: Statistical Analyses
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Sewage Sludge Using an Increment
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National Center for Environmental
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April, 2002.
USDA, 1997. Census of Agriculture.
Washington, DC.
USEPA, 1985. Health Assessment
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Washington, DC September, 1985.
USEPA, 1989. Interim Procedures for
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Forum. Washington, DC March 1989.
USEPA, 1990. National Sewage Sludge
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Federal Register 55 (218): 47210-
47283.
USEPA, 1992. Guidelines for Exposure
Assessment, EPA/600Z-92/001,
National Center for Environmental
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USEPA, 1994a. Health Assessment for
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EPA/600/BP-92/OOla-c, ( Vol. I: 420
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USEPA, 1994b. EPA Method 1613:
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Dilution High-resolution Gas
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October 1994.
USEPA, 1997. Exposure Factors
Handbook. National Center for
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Washington, DC EPA/600/P-95/
002F(a-c). Vol. I: 208 pp. Vol. II: 336
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NTIS (Vol. I PB98—124225, Vol. II
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USEPA, 1998a. Methodology for
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with Multiple Pathways of Exposure
to Combustion Emissions. EPA/600/
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USEPA, 1998b. Guidelines for
Ecological Risk Assessment (Final).
EPA/630/R-95/002F. Risk Assessment
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USEPA, 1999a. EPA Method 1668:
Polychlorinated Biphenyls by Isotope
Dilution High-resolution Gas
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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
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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.
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Bb, Be, Bd, Be, Bg). Available online
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USEPA, 2000b. Risk Characterization
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Science Policy Council, Washington,
DC.
USEPA, 2001a. Sampling Procedures for
the 2001 National Sewage Sludge
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USEPA, 2001b. Analytical Data for
Dioxins in Sewage Sludge Submitted
by Three Wastewater Treatment
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USEPA, 2001c. The Role of Screening-
Level Risk Assessments and Refining
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Update, Publication 9345.0-14. EPA/
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Washington, DC.
USEPA, 2002a. Statistical Support
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Polychlorinated Biphenyls in Sewage
Sludge-Technical Background
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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
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in Biosolids. Office of Science and
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Van den Berg M, et al. 1998. Toxic
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Wildlife. Environ. Health Perspect.
106(12): 775-792.
Dated: June 5, 2002.
G. Tracy Mehan III,
Assistan t Administrator for Water.
[FR Doc. 02-14761 Filed 6-11-02; 8:45 am]
BILLING CODE 6560-50-P
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