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Standards for the Use or Disposal of Sewage Sludge
[Federal Register: June 12, 2002 (Volume 67, Number 113)]
[Notices]
[Page 40553-40576]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr12jn02-163]
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ENVIRONMENTAL PROTECTION AGENCY
[FRL -7228-9]
Standards for the Use or Disposal of Sewage Sludge
AGENCY: Environmental Protection Agency.
ACTION: Notice of data availability.
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SUMMARY: The Environmental Protection Agency (EPA) proposed to amend
the Standards for the Use or Disposal of Sewage Sludge to limit dioxin
and dioxin-like compounds ("dioxins") in sewage sludge that
is applied to the land on December 23, 1999. Since that time, EPA
collected new data on the levels of dioxins in sewage sludge. EPA also
has extensively revised the risk assessment which estimates the risks
from dioxin and dioxin-like compounds associated with land application
of sewage sludge. This document summarizes the new sewage sludge data
and risk assessment. In addition, EPA is inviting comment on the effect
of applying approaches in EPA's current Draft Dioxin Reassessment
concerning non-cancer health effects of exposure to dioxins as they
relate to land application of sewage sludge. EPA also conducted a
screening analysis of the effects of dioxins in land-applied sewage
sludge on ecological species, which is addressed in this notice. EPA is
requesting comments on the new data and risk analysis, as well as
dioxin exposure information, and any impact that this may have on the
proposed rule with respect to land application of sewage sludge.
EPA is under a court-ordered deadline to take final action on the
proposed land application rule. The deadline was recently extended to
October 17, 2003 with respect to land application; EPA met the previous
court-ordered deadline of December 15, 2001 for taking final action on
the Round Two proposal concerning surface disposal and incineration in
a sewage sludge incinerator. EPA gave final notice of its determination
that numeric standards or management practices are not warranted for
dioxin and dioxin-like compounds in sewage sludge that is disposed of
in a surface disposal site or incinerated in a sewage sludge
incinerator (66 FR 66228, Dec. 21, 2001).
DATES: Your comments on this document must be submitted to EPA in
writing and must be received or postmarked on or before midnight
September 10, 2002.
ADDRESSES: Written comments and enclosures should be mailed or hand-
delivered to: W-99-18 NODA Comment Clerk, Water Docket
(MC-4101), USEPA, 1200 Pennsylvania Ave., NW., Washington, DC
20460. Hand deliveries should be delivered to: EPA's Water Docket (MC
4101) at 401 M St., SW., Room EB57, Washington, DC 20460. Comments may
also be submitted electronically to 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
[[Page 40555]]
http://www.epa.gov/owm/bio.htm or request the document (EPA publication
number EPA/832/R-93/003) from: Municipal Technology Branch,
Office of Wastewater Management (4204M), Office of Water, U.S.
Environmental Protection Agency, 1200 Pennsylvania Avenue, NW.,
Washington, DC 20460-0001.
II. Abbreviations Used
AMSA Association of Metropolitan Sewerage Agencies
CFR Code of Federal Regulations
DL detection limit
ED01 dose corresponding to a one percent increase in an adverse
effect relative to the control response
EPA Environmental Protection Agency
HQ hazard quotient
kg/m\3\ kilograms per cubic meter
LADD lifetime average daily dose
Ln natural logarithm
LOEL lowest-observed-effect level
Max. maximum
MGD million gallons per day
mg/kg/day milligrams per kilogram per day
MOE margin of exposure
ng/kg nanograms per kilogram
NOEL no-observed-effect level
NSSS National Sewage Sludge Survey
PCBs polychlorinated biphenyls
PCDFs polychlorinated dibenzofurans
PCDDs polychlorinated dibenzo-p-dioxins
pg/kg/day picograms per kilogram per day
pg TEQ/day picograms toxic equivalents per day
pg TEQ/kg-d picograms toxic equivalents per kilogram body weight
per day
POTWs Publicly Owned Treatment Works
ppt parts per trillion
Q1* cancer slope factor
RfD reference dose
SAB Science Advisory Board
SERA screening ecological risk analysis
Std. Dev. standard deviation
TCDD tetrachlorodibenzo-p-dioxin
TEF toxicity equivalent factor
TEQ toxic equivalent
WHO World Health Organization
III. How Does This Document Relate to the Proposed Rule?
A. What EPA Proposed
In December 1999, EPA proposed to amend management standards for
sewage sludge by adding a numeric concentration limit for dioxins in
sewage sludge that is applied to the land (64 Fed. Reg. 72045, Dec. 23,
1999) ("Round Two proposal").\1\ The proposed numeric limit
would prohibit land application of sewage sludge that contains greater
than 300 parts per trillion (ppt) toxic equivalents (TEQ) of dioxins.
EPA based this proposed numeric limit on the results of a risk
assessment for dioxins in sewage sludge that is applied to the land.
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\1\ Section 405(d)(2)(A) of the Clean Water Act (CWA), 33
U.S.C. Sec. 1345(d)(2)(A) required EPA to establish numeric
limits and management practices for toxic pollutants in sewage
sludge identified on the basis of available information. In 1993,
EPA promulgated the "Round One" rule for such toxic
pollutants in sewage sludge that is applied to the land, disposed of
in surface disposal units, and incinerated in sewage sludge
incinerators. 58 Fed. Reg. 9248 (Feb. 19, 1993). Under section
405(d)(2)(B), EPA was directed to propose and promulgate regulations
for other toxic pollutants not regulated in Round One, i.e.,
"Round Two." The Round Two proposal identified dioxins,
and included proposed standards for land-applied sewage sludge, but
did not propose further regulation of sewage sludge disposed of by
surface disposal or incineration.
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EPA proposed a standard for dioxins in sewage sludge that is
applied to the land in order to protect public health and the
environment from unreasonable risks of exposure to dioxins. The purpose
of this standard would be to prohibit land application of sewage sludge
containing concentrations of dioxins above the limit, and thereby
protect the health of highly exposed individuals as well as the health
of the general population.
EPA also proposed to exclude from the proposed numeric limit and
monitoring requirements treatment works with a flow rate equal to or
less than one million gallons per day (MGD) and certain sewage sludge-
only entities that receive sewage sludge for further processing prior
to land application. This exclusion was based on the relatively small
amount of sewage sludge that is prepared by these facilities and
entities and, therefore, the low probability that land application of
these materials could significantly increase risk from dioxins to human
health or the environment.
Finally, EPA proposed technical amendments to the frequency of
monitoring requirements for pollutants other than dioxin. These
amendments were intended to clarify but, with one exception, not alter
the monitoring schedule in the existing sewage sludge rule. The one
exception would require preparers of material derived from sewage
sludge to determine the appropriate monitoring schedule based on
quantity of material derived rather than quantity of sewage sludge
received for processing.
B. Developments Since Proposal
The Agency's risk assessment for land application of sewage sludge
used for the proposal estimated that sewage sludge with concentrations
of dioxins above the proposed limit may present an unreasonable cancer
risk to specific highly exposed individuals. Subsequently, for reasons
discussed below, the Agency extensively revised the land application
risk assessment. EPA also gathered new data on dioxins in sewage sludge
that was used in the revised risk assessment. This information,
however, does not change the overall technical approach for the
proposal.
The new data and the methodology of the revised risk assessment are
summarized in this notice. In addition, the results of the revised risk
assessment are described in today's notice. Also discussed in today's
notice are the possible implications of the new data and revised risk
assessment on the proposed limit, the monitoring requirements, the
small entity exclusion, and the projected cost of the proposed
regulation.
Another development since the proposal in December 1999 concerns
EPA's Dioxin Reassessment, which began in 1991. In September 2000, EPA
provided Draft Dioxin Reassessment documents to the Science Advisory
Board (SAB) for their review, and in May 2001, the SAB issued its
report. The current Draft Dioxin Reassessment (USEPA, 2000a),
"Exposure and Human Health Reassessment of 2,3,7,8-
Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds,"
consists of three parts. Part I. Estimating Exposure to Dioxin-Like
Compounds focuses on sources, levels of dioxin-like compounds in
environmental media, and human exposures. Part II. Health Assessment
for 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds
includes information on critical human health end points, mechanisms of
toxicity, pharmacokinetics, dose-response, and toxic equivalent factors
(TEFs). Part III. Integrated Summary and Risk Characterization for
2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds
describes key findings pertinent to understanding the potential hazards
and risks of dioxins, including a discussion of important assumptions
and uncertainties.
The Draft Dioxin Reassessment documents do not represent Agency
policy or factual conclusions, and EPA has not yet issued final
findings or conclusions as a result of the Dioxin Reassessment process.
However, much of the information incorporated into the Draft Dioxin
Reassessment documents reflects the state of knowledge with respect to
dioxin, and scientific updates resulting from or reflected in these
[[Page 40556]]
documents are relevant to the assessment of risk from dioxins in sewage
sludge that is applied to the land. For example, the revised sewage
sludge land application risk assessment incorporates the latest science
and state of knowledge concerning characteristics of dioxin and
exposure pathways which are described in the Draft Dioxin Reassessment.
The Draft Dioxin Reassessment also presents conclusions and
findings which are still under review and which EPA has not applied to
the analysis of dioxins in sewage sludge. These aspects of the Draft
Dioxin Reassessment include, for example, a revised cancer slope factor
for calculating cancer risk from exposure to dioxins, and discussions
of various approaches to evaluating risks of non-cancer health effects
from exposure to dioxins. Although not incorporated into the revised
risk assessment, today's Notice also discusses potential implications
that these aspects of the Draft Dioxin Reassessment could have for this
rulemaking, when and if the Dioxin Reassessment is issued by EPA in
final form, and if the final version takes the same approaches and
reaches the same conclusions as the current draft.
Finally, EPA was under a consent decree deadline of December 15,
2001 to take final action on the proposed rule. Gearhart v. Whitman,
Civil No. 89-6266-HO (D. Ore.). In accordance with the
consent decree, EPA took final action on the proposal not to establish
numeric limits or management practices for dioxins in sewage sludge
that is disposed of in surface disposal units or incinerated in sewage
sludge incinerators. 66 Fed. Reg. 66228 (Dec. 21, 2001). The consent
decree deadline was extended to October 17, 2003, for EPA to take final
action on the land application portion of the proposed Round Two rule.
C. Proposed Definition of Dioxins
The proposed rule included a definition of "dioxins" to
specify the seven 2,3,7,8,-substituted congeners of polychlorinated
dibenzo-p-dioxins (PCDDs), the ten 2,3,7,8-substituted congeners of
polychlorinated dibenzofurans (PCDFs), and the twelve coplanar
polychlorinated biphenyl (PCB) congeners to which the numeric standard
applies. The vast majority of information on the toxicity of dioxins
relates to the congener 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).
Animals exposed to 2,3,7,8-TCDD exhibit a variety of biological
responses and adverse effects. These include both carcinogenic and non-
carcinogenic effects. These effects are primarily classified as chronic
effects and consequently they are generally associated with long term
exposure over years and decades. Relatively speaking, these exposures
and effects are observable at very low levels in the laboratory and in
the environment when compared with other environmental toxicants
(USEPA, 1994a).
Studies to elucidate the mechanism of toxicity for 2,3,7,8-TCDD in
mammalian and other species have indicated that the overall shape and
chlorine substitution of this congener are keys to its biological
potency. The fact that all of the lateral positions (the 2,3,7,8
positions) on the multi-ring system are substituted with chlorine and
that the overall molecule assumes a flat or planar configuration
apparently are essential factors that make this congener biologically
active. Other congeners with a similar structure and chlorine
substitution pattern are assumed to exhibit similar biological
properties. These include the other six 2,3,7,8-chlorinated substituted
dibenzo-p-dioxin congeners, the ten 2,3,7,8-chlorinated substituted
dibenzofuran congeners and the 12 coplanar PCB congeners. Coplanar PCB
congeners are those congeners with no more than one ortho position and
both para positions substituted with chlorine in the biphenyl ring
system. Additionally, the coplanar PCB molecule assumes a relatively
planar (i.e., flat) configuration.
The proposed TEQ numeric limit would apply to these 29 congeners in
ppt TEQ or nanograms TEQ per kilogram of dry sewage sludge. The TEQ
concentration is calculated by multiplying the concentration of each
congener in the sewage sludge by its corresponding "toxicity
equivalent factor," or TEF, and then summing the resulting
products from this calculation for all 29 congeners. The TEFs (relative
potencies) are based on expert judgment about toxicity and other
biological effects for the individual compounds. The TEQs of these
compounds are summed because they are believed to act by the same
mechanism of toxicity. The December 1999 proposal specified that the
International TEF scheme described in USEPA, 1989, would be used for
the 17 2,3,7,8-substituted PCDDs and PCDFs, and the World Health
Organization's TEF scheme (Van den Berg M, et al., 1998) would be used
for the 12 coplanar PCBs, because the sewage sludge data EPA had at
that time used these TEF schemes. The World Health Organization (WHO)
has subsequently recommended and developed a single TEF scheme which
includes all relevant information on dioxins, furans and dioxin-like
(coplanar) PCBs. As part of this process, various terminologies or
definitions applicable to TEFs were reviewed and standardized.
The 2001 sewage sludge data and the revised risk assessment use the
WHO's 1998 TEF scheme (Van den Berg M, et al., 1998) for all 29 dioxin,
furan and coplanar PCB congeners. EPA intends to use the 1998 WHO TEF
scheme (or later, if the WHO adopts a revised scheme) for any final
Part 503 TEQ numeric limit.
A 1997 WHO meeting of experts concluded that an additive TEF model
remained the most feasible risk assessment method for complex mixtures
of dioxin-like compounds. The WHO panel indicated that although
uncertainties in the TEF methodology have been identified, one must
examine this method in the broader context of the need to evaluate the
public health impact of complex mixtures of persistent bioaccumulative
chemicals. On this basis, EPA has used the 1998 WHO TEF methodology for
the Agency's Draft Dioxin Reassessment, noting that it decreases the
overall uncertainties in the risk assessment process.
A Panel of EPA's Science Advisory Board has reviewed the Agency's
use of the 1998 WHO TEF scheme. The consensus of the Panel was that
this is a reasonable and widely accepted way of dealing with the joint
effects of dioxin-like compounds on human health. The majority of the
Panel noted that the TEF approach is well accepted internationally.
IV. Why Did EPA Collect New Data and Revise the Land Application Risk
Assessment?
The proposal to amend the Standards for the Use or Disposal of
Sewage Sludge to limit dioxins in sewage sludge that is applied to the
land was followed by a 90 day public comment period. During this time
the risk assessment which supported the proposed rulemaking also was
peer reviewed in accordance with EPA peer review procedures. Both the
public comments and the peer review comments raised significant issues
concerning the methodology and assumptions used for the land
application risk assessment. The public and peer review comments also
emphasized the need to collect new data on dioxins in sewage sludge.
This data is used in the risk assessment, economic analysis, and other
aspects of the rulemaking.
The data on dioxins in sewage sludge used for the proposal came
from two separate sources. The data on dioxin and furan congeners was
from the 1988 EPA National Sewage Sludge Survey
[[Page 40557]]
(USEPA, 1990). Since the National Sewage Sludge Survey (NSSS) did not
include specific information on coplanar PCBs, EPA used a separate
database to estimate the amount of coplanar PCBs found in sewage sludge
(Green, et al., 1995). In addition to developing a single database
which includes information on all 29 dioxin-like congeners, EPA
developed new data on dioxins in sewage sludge to test the Agency's
assumption that dioxin levels in sewage sludge have changed over time,
and to more accurately determine dioxin levels in sewage sludge using
analytical methods with lower limits of detection. The Agency is also
using this more recent data to more reliably estimate the risk,
impacts, and costs associated with dioxins in land applied sewage
sludge. A discussion of the sewage sludge sampling and data analysis is
presented in Section V. of this Notice.
The principal comment concerning the risk assessment methodology
was that the Agency should use a probabilistic approach instead of the
deterministic approach that was used for the proposal. A probabilistic
approach uses values for certain input variables over the range of
available data, instead of the deterministic approach of determining,
or setting, certain input variables at particular values. Conducting a
risk analysis with a probabilistic approach can yield better
information about sources of variability and uncertainty in the final
risk estimates, compared to conducting a risk analysis with a
deterministic approach.
Other comments on the risk assessment recommended that the Agency
use an exposure analysis more consistent with that used in the Agency's
current Draft Dioxin Reassessment (USEPA, 2000a); that the Agency use
data from the current EPA Exposure Factors Handbook (USEPA, 1997); and
that the risk assessment include a sensitivity analysis of the critical
input variables.
The revised risk assessment is described in Section VI. of this
Notice. The revised risk assessment was submitted for peer review. The
consensus view of the peer reviewers agreed with the revised risk
assessment methodology and assumptions on input parameters. The revised
risk assessment, described below and available in the docket,
incorporates revisions made in response to the peer review.
V. What Information Concerning Dioxins in Sewage Sludge Does the New
Data Provide?
A. What Data Were Collected in the EPA 2001 Dioxin Update of the
National Sewage Sludge Survey?
The EPA 2001 dioxin update of the NSSS provides data that support
the calculation of unbiased national estimates (i.e., based on a random
selection of publicly owned treatment works) for dioxin and dioxin-like
compounds in sewage sludge (USEPA, 2002a). The publicly owned treatment
works (POTWs) sampled in the EPA 2001 dioxin update survey were
randomly selected from all POTWs in four size categories: <1 MGD, 1
MGD-10 MGD, 10 MGD-100 MGD and >100 MGD. This survey
updates the 1988 NSSS. The updated survey includes coplanar PCBs, which
had not been included in the 1988 NSSS because approved analytical
methods for these analytes were not available at that time. The updated
survey also uses the current TEFs, which have been revised since the
1988 NSSS. For the EPA 2001 dioxin update survey, EPA collected sewage
sludge samples from 94 POTWs selected from the 174 POTWs which had been
surveyed in the 1988 NSSS. The sample of 174 POTWs included in the 1988
NSSS were selected from the national population (as of 1988) of
approximately 10,000 POTWs with secondary treatment. EPA used a survey
design which accounted for the different numbers of POTWs in different
size categories for both the 1988 NSSS and the EPA 2001 dioxin update
survey. EPA conducted the sampling at the 94 POTWs in the first
calendar quarter of 2001 and completed the laboratory analysis, data
review, and database development by mid-2001.
B. What Techniques Were Used To Collect Samples?
Sewage sludge samples were collected, documented, preserved, and
shipped to the laboratory where the analyses for dioxins were conducted
using the protocol entitled "Sampling Procedures for the 2001
National Sewage Sludge Survey" (USEPA, 2001a). This document
specifies the sampling procedures used for the sewage sludge samples
obtained from the 94 POTWs that participated in the EPA 2001 dioxin
update survey. The procedures were used on a number of different types
of sewage sludge samples including liquids, samples with low solids
content, dewatered sewage sludges from filter presses and centrifuges,
composted products, and pellets. The sampling protocol specifies sample
preservation methods, collection devices and apparatus, containers,
types of labels, and label information. In accordance with the sampling
protocol used for the EPA 2001 dioxin update survey, duplicate samples
were collected for 15 percent of the samples collected for subsequent
analysis to determine the precision of the analyses. At each treatment
works sampled, a second sample aliquot was collected and archived for
potential future analyses. Chain of custody forms were completed for
the samples collected at each sampling site to ensure the integrity of
the results of the survey.
C. What Analytical Methods Were Used?
EPA used analytical methods that are considered state of the art
for the sewage sludge matrix. Dioxin and dibenzofuran congener
concentrations were determined by EPA Method 1613B (USEPA, 1994b) using
high resolution gas chromatography-mass spectrometry as the end point
system of measurement. The coplanar PCB analyte concentrations were
determined by EPA Method 1668A (USEPA, 1999a) which employs the same
type of measuring instrumentation. Method 1613B is an official EPA
analytical methodology codified at 40 CFR Part 136. EPA anticipates
that Method 1668A will be codified in Part 136 within the next two
years.
D. How Were the Concentrations of Dioxin Measured?
The sewage sludge samples were analyzed for 29 dioxin congeners
consisting of the 7 dioxin congeners, 10 dibenzofuran congeners, and 12
coplaner PCB congeners that EPA proposed for the definition of
"dioxins" (see Section III.B. above). For the EPA 2001
dioxin update survey, whole (wet) weight sample sizes were individually
determined for each sewage sludge sample by considering the percent
solids in each sample. Smaller whole weight sample sizes were used for
the analyses when the percent solids content of the sewage sludge
sample was greater, and vice versa. This approach led to lower and more
consistent detection limits for concentrations of target analytes for
all of the sewage sludge samples in the EPA 2001 dioxin update survey.
This procedure was a significant improvement compared to the method
used for handling the sewage sludge samples in the 1988 NSSS. For the
1988 NSSS, equal whole weight sample sizes were used regardless of the
percent solids content of the samples. This led to higher and less
consistent detection limits for the sewage sludge samples in
[[Page 40558]]
the 1988 NSSS. In addition, other improvements in the analytical
methodology and the analytical instrumentation also contributed to
lower and more consistent detection limits than those obtained in the
1988 NSSS.
E. How Were the Concentrations Reported?
All of the individual 29 congener concentrations were converted to
TEQ concentrations by multiplying the congener concentrations by the
1998 WHO TEFs. For comparison purposes, TEQs for total dioxin and
dioxin-like compounds in the 1988 NSSS samples and the EPA 2001 dioxin
update survey samples are reported in Table 1, Table 2 and Table 3 in
nanograms per kilogram (ng/kg) dry weight basis.
F. How Were the Non-Detect Measurements Handled in Developing National
Summary Statistics?
Where congeners were not detected in sample measurements, three
different substitution methods were used in calculating national
estimates of dioxin concentrations in sewage sludge: (1) Zero was
substituted for a non-detect; (2) one-half the detection limit for the
congener was substituted for a non-detect; (3) the detection limit for
the congener was substituted for a non-detect. As a result of the small
detection limits achieved in the EPA 2001 dioxin update survey, there
were only small differences in the national summary statistics among
the three substitution methods for the EPA update survey.
G. What Were the Results of the EPA 2001 Dioxin Update of the National
Sewage Sludge Survey?
Table 1 presents the mean, standard deviation, maximum and 99th,
98th, 95th, 90th and 50th percentiles dioxin TEQ values for the sewage
sludges from the 94 POTWs in the EPA 2001 dioxin update survey. Table 1
reports summary results separately for dioxins and furans, coplanar
PCBs, and total dioxin-like compounds (i.e., 29 dioxin, furan and
coplanar PCB congeners) using the three alternative substitution values
for non-detects (i.e., zero, one-half the detection limit, and equal to
the detection limit). In Table 1, the results obtained using zero, one-
half the detection limit and the detection limit are shown in the rows
denoted by "0", "\1/2\ DL" and
"DL", respectively. The complete statistical analysis of
the data from the EPA 2001 dioxin update survey is presented in
Statistical Support Document for the Development of Round Two Sewage
Sludge Use or Disposal Regulations (USEPA, 2002a).
Table 1. EPA 2001 Dioxin Update Survey National Toxic Equivalent Estimates (nanograms/kilogram dry matter basis) Total Toxic
Equivalents for POTWs
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Method Mean Std. Dev. Max. 99th % 98th % 95th % 90th % 50th %
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Total Dioxin and Furan TEQs (nanograms/kilogram dry matter basis)
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0..................................................... 21.70 47.5 682.00 100.00 54.40 33.30 31.40 15.50
\1/2\ DL.................................................... 21.70 47.5 682.00 100.00 54.40 33.30 31.60 15.50
DL.......................................................... 21.80 47.5 682.00 100.00 54.40 33.30 31.70 15.50
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Total Coplanar PCB TEQs (nanograms/kilogram dry matter basis)
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0..................................................... 5.22 10.3 58.30 50.60 44.80 13.10 9.66 2.05
\1/2\ DL.................................................... 9.87 14.0 58.30 55.10 54.50 49.40 19.20 6.04
DL.......................................................... 14.50 22.4 103.00 97.2 91.60 78.00 35.00 8.11
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Total Dioxin and Dioxin-Like TEQs (nanograms/kilogram dry matter basis)
--------------------------------------------------------------------------------------------------------------------------------------------------------
0..................................................... 26.90 49.6 718.00 114.00 76.60 59.30 42.80 19.70
\1/2\ DL.................................................... 31.60 50.0 718.00 115.00 80.10 73.50 55.10 23.40
DL.......................................................... 36.30 52.7 718.00 138.00 96.00 113.00 69.10 24.00
--------------------------------------------------------------------------------------------------------------------------------------------------------
Under the proposed rule, treatment works with a flow rate equal to
or less than one MGD and certain sewage sludge-only entities that
receive sewage sludge for further processing prior to land application
would be excluded from the proposed numeric limit and monitoring
requirements. The EPA 2001 dioxin update survey provides additional
data with respect to dioxin concentrations from POTWs that would be
excluded under the proposal. Table 2 below shows the results for dioxin
concentrations in sewage sludge for POTWs with flows of less than and
greater than one MGD. Results shown in Table 2 indicate very small
differences in the median dioxin concentrations between small and large
POTWs. At the upper percentiles, the differences between the small and
large POTW values are substantial. However, the significance of these
differences is difficult to assess due to the relatively small sample
sizes, the sensitivity of the results to the treatment of non-detect
measurements and the low precision typically associated with estimates
of upper percentiles based on small sample sizes. An additional
discussion of the proposed exclusion for small entities is presented in
Section X. of this Notice. EPA requests comments on the significance of
the differences in dioxin concentrations in sewage sludge measured at
facilities with wastewater flows greater than one MGD compared to
dioxin concentrations in sewage sludge at facilities with wastewater
flows less than one MGD.
[[Page 40559]]
Table 2. EPA 2001 Dioxin Update Survey Total Dioxin and Furan and Dioxin-Like PCB National TEQ
(nanograms/kilogram dry weight basis) Estimates POTWs by Flow Groups
----------------------------------------------------------------------------------------------------------------
Method Zero for \1/2\ DL for DL for Nondetects
----------------------------------------------------- Nondetects Nondetects -------------------
----------------------------------------
Estimate ≤1 >1 ≤1 > 1 ≤1 > 1
MGD MGD MGD MGD MGD MGD
----------------------------------------------------------------------------------------------------------------
Mean................................................ 22.10 38.50 26.50 44.10 30.80 49.60
Std. dev............................................ 16.8 86.7 18.3 86.8 24.6 88.2
dev.
Maximum............................................. 78.60 718.00 78.6 718.00 118.00 718.00
99th %.............................................. 71.80 401.00 76.40 403.00 109.00 406.00
98th %.............................................. 65.10 265.00 74.20 269.00 101.00 276.00
95th %.............................................. 46.00 62.60 67.10 94.80 77.00 134.00
90th %.............................................. 37.20 54.00 46.10 64.20 46.60 86.90
50th %.............................................. 19.90 18.90 22.90 22.60 23.80 25.80
----------------------------------------------------------------------------------------------------------------
H. How Do the Results of the EPA 1988 National Sewage Sludge Survey
Compare with the EPA 2001 Dioxin Update Survey?
A comparison of results for dioxin and furan congeners obtained in
the 1988 and 2001 surveys is presented in Table 3.
Table 3. National Estimates (nanograms/kilogram dry matter basis) for Dioxin and Furan Congeners in the
EPA 2001 Dioxin Update Survey and NSSS 1988
----------------------------------------------------------------------------------------------------------------
Method Zero for nondetects \1/2\ DL for DL for nondetects
--------------------------------------------------------------------- nondetects ---------------------
----------------------
Estimate 2001 1988 2001 1988 2001 1988
----------------------------------------------------------------------------------------------------------------
Mean.......................................... 21.70 46.50 21.70 67.30 21.80 88.20
Std. dev...................................... 47.5 153.0 47.5 153.0 47.5 157.00
Maximum....................................... 682.00 1870.00 682.00 1870.00 682.00 1870.00
99th %........................................ 100.00 450.00 100.00 453.00 100.00 466.00
98th %........................................ 54.40 402.00 54.40 404.00 54.40 455.00
95th %........................................ 33.30 301.00 33.30 303.00 33.30 340.00
90th %........................................ 31.40 56.70 31.60 152.00 31.70 226.00
50th %........................................ 15.50 5.68 15.50 34.20 15.50 52.40
----------------------------------------------------------------------------------------------------------------
The values obtained in the EPA 2001 dioxin update survey for the
upper percentiles are lower than those obtained in the 1988 NSSS. On
this basis, the concentrations of dioxins in sewage sludge appear to
have declined since 1988. However, the significance of these
differences between the two surveys is not certain due to changes in
the sampling procedures and analytic methods . These comparisons do not
include coplanar PCB congeners because the 1988 NSSS did not collect
coplanar PCB congener data. For the purposes of the December 1999
proposed rule, data on coplanar PCB levels in sewage sludge from a 1995
Association of Metropolitan Sewerage Agencies Survey (Green, et al.,
1995) were combined with the 1988 NSSS dioxin and furan results to
provide an estimate of total dioxin levels in sewage sludge. EPA
requests comments on the significance of the differences in dioxin
concentrations in sewage sludge measured in the EPA 2001 dioxin update
survey compared to dioxin concentrations in sewage sludge measured in
the 1988 NSSS.
VIII.Why Is Temporal Variability of Dioxin in Sewage Sludge Important?
The variability of dioxins in sewage sludge over time is important
for a number of reasons. First, understanding the temporal variability
of dioxin concentrations in sewage sludge is important for establishing
numerical limits for dioxins in sewage sludge which protect public
health and the environment with an adequate margin of safety.
Specifically, this information helps in assessing the likelihood that
individuals will be exposed to higher levels of dioxins from land
application of sewage sludge over time. A more complete discussion of
this issue is presented in the risk characterization in Section VI.L.
of this Notice. Second, information on the variability of dioxin
concentration in sewage sludge is important for determining the
appropriate frequency of monitoring for concentrations of dioxins in
sewage sludge that will ensure that any numerical limit that is
established will not be exceeded.
J. What Does the Variability of the Dioxin Levels Show?
It is not possible to draw general inferences with regard to the
variability or differences in dioxin levels observed in the two
surveys. This is due to a number of factors that include the large time
interval between the surveys (i.e., 13 years), changes that may have
occurred at the POTWs, and changes and improvements in analytical
methods. It is possible, however, to make a number of observations with
regard to changes in dioxin levels based on the data. Of the 94 POTWs
participating in both the 1988 NSSS and the EPA 2001 dioxin update
survey, a total of 14 POTWs have sewage sludge dioxin concentrations
(dioxins and furans only) equal to or greater than 93 ppt TEQ from at
least one of the surveys. These same 14 POTWs exhibited the greatest
differences in the dioxins and furans concentrations when comparing the
results of the 1988 and 2001 EPA surveys. The other 80 POTWs
[[Page 40560]]
participating in both surveys have substantially smaller differences,
as well as lower dioxin levels measured in both surveys. Of the 14
POTWs with the greatest differences between the two surveys, four had
large increases in sewage sludge dioxin concentrations and ten had
large decreases in sewage sludge dioxin concentrations from 1988 to
2001.
Based on these data, no POTWs had consistently high levels of
dioxins in sewage sludge. It appears that sewage sludge samples with
higher concentrations of dioxins may experience a greater variability
in dioxin concentrations over time and that higher dioxin levels may
not remain high for a significant period of time. Likewise, POTWs with
moderate or low levels of dioxins in their sewage sludge may experience
much less variability in dioxin concentrations over time. It is
possible that in the group of POTWs where higher concentrations of
dioxins were measured in their sewage sludge, there are unidentified
sources with relatively high levels of dioxins entering the sewers
intermittently. The second group of POTWs where lower concentrations of
dioxins were measured in both surveys appear to be experiencing typical
environmental background variation of dioxin levels. The possible
sources of dioxins which contribute to higher levels of dioxins in
sewage sludge are discussed in greater detail later in this Section and
Section XII of the Notice. EPA's assessment of the variability in
higher levels of dioxins in sewage sludge is discussed further as part
of the risk characterization in Section VI.L. of this Notice.
K. What Does Month-to-Month Variability in the Concentration of Dioxins
Show?
EPA also examined both long and short term variability in sewage
sludge dioxin concentrations in three wastewater treatment plants that
have routinely monitored for dioxins in their sewage sludge over
relatively long periods of time and voluntarily submitted their data to
EPA (USEPA, 2001b). EPA did this to better understand the extent of
variability using data collected on a relatively frequent basis.
Of the three POTWs which provided their data to EPA, one of the
POTWs provided data on two different sewage sludge products that they
produce. These data were standardized using the WHO98
standard for TEQs to provide consistency.
The December 1999 proposal specified annual monitoring for land
applied sewage sludges with dioxin concentrations between 30 ppt TEQ
and the proposed limit of 300 ppt TEQ. Sewage sludges with two
consecutive annual dioxin measurements less than 30 ppt TEQ would be
required to monitor once every five years. These less frequent
monitoring requirements were based on EPA's assumption that dioxin
concentrations in sewage sludge remained relatively constant over time.
The data for the facilities where monthly data were available
indicate that the dioxin concentrations are relatively consistent over
time on a month-to-month basis. The maximum monthly concentration was
within a factor of two to four times the average (mean) concentration
for the same facility. Similar to the comparison data from the 1988
NSSS and the 2001 update, the variability appeared the greatest for the
facility with the highest dioxin concentrations measured in its sewage
sludge. A complete analysis of the month-to-month data is presented in
the Statistical Support Document for the Development of Round Two
Sewage Sludge Use or Disposal Regulations (USEPA, 2002a).
The month-to-month variability in the dioxins concentration
observed in the sewage sludge for which the Agency had data, as well as
the longer term variability observed in the small percentage of sewage
sludge with higher concentrations of dioxins (discussed above), has led
us to re-evaluate the proposed monitoring frequency. A more complete
discussion of monitoring frequency is presented in Section IX. of this
Notice.
L. What Other Data Did EPA Evaluate?
The Association of Metropolitan Sewerage Agencies (AMSA)
voluntarily collected sewage sludge samples from 171 POTWs and analyzed
these samples for dioxins using the same methods used for the 2001 EPA
dioxin update survey. AMSA submitted the results of their survey to EPA
in a report entitled "AMSA 2000/2001 Survey of Dioxin-Like
Compounds in Biosolids: Statistical Analyses (Final Report)"
(AMSA, 2001). The AMSA survey began in October 2000 and was completed
in July 2001. The AMSA survey was designed to measure levels for the
same 29 dioxin and dioxin-like congeners measured in the EPA 2001
dioxin update survey. AMSA also compared the results of their 2001
survey with the results of their 1994/1995 survey of dioxins in sewage
sludge. Participation in AMSA's survey was on a voluntary basis.
Most participants in the AMSA survey were larger POTWs which make
up the bulk of the AMSA membership. Some non-AMSA members also
participated in the AMSA survey, including some smaller POTWs. Overall,
111 separate wastewater treatment agencies participated in the 2001
AMSA survey, providing 200 samples from 171 POTWs, located in 31
states. The sewage sludge dioxin concentrations measured in the AMSA
survey generally ranged from 7.1 ppt TEQ to 256 ppt TEQ, with one
sample measured at 3,590 ppt TEQ. The mean (average) concentration and
the median dioxin concentrations in sewage sludge from the AMSA survey
were 48.5 ppt TEQ and 21.7 ppt TEQ, respectively.
EPA has found the data from the AMSA survey to be useful in
describing dioxins in sewage sludge from larger POTWs. The results of
the AMSA survey tend to corroborate the results obtained from the EPA
2001 dioxin update survey. However, the AMSA results were not used by
EPA to establish national estimates of dioxin concentrations in sewage
sludges or for purposes of estimating risks from dioxins in land-
applied sewage sludge. EPA did not use these results because the POTWs
participating in the AMSA survey volunteered for this survey and were,
therefore, not randomly selected, as were the POTWs in the EPA 2001
dioxin update survey. The final report from the AMSA survey and
associated appendices are in the docket and can also be found on AMSA's
web site at: http://www.amsa-cleanwater.org/advocacy/dioxin/dioxin.cfm. 
VI. What Are the Principal Features and Assumptions of the Revised Land
Application Human Health Risk Assessment?
The revised risk assessment is entitled "Exposure Analysis
for Dioxins, Dibenzofurans, and CoPlanar Polychlorinated Biphenyls in
Sewage Sludge Technical Background Document" (USEPA,
2002b). The risk assessment methodology, assumptions, results and
characterization are summarized below.
The revised risk assessment contains the following standard
elements used in EPA human health risk assessments: hazard
identification, dose-response assessment, exposure assessment, and risk
characterization. The revised risk assessment includes a probabilistic
methodology to determine the adult and child exposure to the 29 dioxin
and dioxin-like congeners. For the proposed rule, the risk assessment
depended on a deterministic analysis based on single value inputs and
outputs. A probabilistic analysis was well-suited for this risk
assessment because sewage
[[Page 40561]]
sludge is generated nationwide and, therefore, may be used on
agricultural fields anywhere in the United States. The probabilistic
analysis not only captures the variability in sewage sludge application
practices, it also captures the differences in the environmental
settings (e.g., soils, meteorology and agricultural practices) in which
sewage sludge may be land-applied.
In addition to a new methodology of analysis, the revised risk
assessment uses new inputs which include a redefined "highly
exposed individual," new pathways and mechanisms of exposure
consistent with EPA's Draft Dioxin Reassessment (USEPA, 2000a. See Part
I, Vol. 3, Chap. 2.), a number of new exposure factors adopted from the
latest EPA Exposure Factors Handbook (USEPA, 1997), and a sensitivity
analysis to determine the relative importance of the input variables.
In this Section, EPA describes the features of the revised risk
assessment with emphasis on the new inputs used in the probabilistic
analysis.
A. What Did the Hazard Identification Analysis Conclude?
The risk assessment that EPA used for the December 1999 proposal
identified cancer as the human health endpoint, i.e., as the
"hazard" (64 FR 72051). The revised risk assessment does
not change this hazard identification and continues to assess the risk
of cancer as the human health endpoint.
B. What Did the Dose-Response Assessment Conclude?
EPA's dose-response assessment evaluated the risk of the dioxin,
dibenzofuran, and PCB congeners using cancer slope factors that are
based on the toxicity of the most highly characterized of the dioxin
congeners, 2,3,7,8-TCDD (USEPA, 2000a. See Part II, Chap. 7, Part A.).
The cancer slope factor for TCDD used by EPA in recent assessments,
including the revised sewage sludge land application risk assessment,
is 1.56 × 10−4/picograms toxic equivalents/
kilogram body weight/day (pg TEQ/kg-d) (USEPA, 1994a). The cancer slope
factor (also referred to as Q* or "cancer potency") is a
numeric value which relates the incremental probability of developing a
cancer from exposure to a particular substance. This cancer slope
factor value is expressed as a lifetime excess cancer risk per unit
exposure, and is usually quantified in terms of (milligrams of
substance per kilogram of body weight per day)−1. The
greater the numeric value of the cancer slope is, the greater the
carcinogenic potency of the substance. The same slope factor is used to
estimate cancer risks for both children and adults. For this analysis,
only the cancer endpoint was evaluated and a linear dose response
relationship was used in the analysis.
An extensive discussion of the dose response mechanism for TCDD is
provided in the Draft Dioxin Reassessment document (USEPA, 2000a. See
Part II, Chap. 8.). The Draft Dioxin Reassessment also includes a
revised cancer slope factor. Because the Draft Dioxin Reassessment is
preliminary and does not state EPA policy conclusions or factual
findings, the draft cancer slope factor was not used in the revised
risk assessment. However, for purposes of discussion and public
comment, this Notice includes a discussion of how the EPA Draft Dioxin
Reassessment could apply to the analysis of impacts from dioxins in
land-applied sewage sludge, including use of the revised cancer slope
factor, in Section VII.A. of this Notice. EPA is seeking comment on the
implications of this information in the event that, prior to taking
final action on the Round Two rule, EPA finalizes a cancer slope factor
or other policies or approaches currently reflected in the current
Draft Dioxin Reassessment and discussed in this Notice.
C. How Was the Exposure Analysis and Risk Assessment Conducted?
The primary methodology for the exposure analysis was to estimate
exposure to dioxins in land-applied sewage sludge using a probabilistic
approach. A probabilistic exposure analysis produces a distribution of
exposures which is then used to estimate the range of risks for the
highly exposed population being modeled. The distribution of exposure
is determined by varying parameter values where data is available over
multiple iterations of the exposure model. Values were varied for such
parameters as dioxin concentrations in sewage sludge, number of years
on the farm, and number of applications. While ranges of data were
available for the majority of input parameters, "single
point" values were used for some key input parameters for the
exposure analysis, including values for parameters used to define the
highly exposed population, soil ingestion rates, and number of days per
year of exposure. These assumptions are discussed in greater detail
elsewhere in this Notice.
A receptor is the entity exposed to a physical, chemical or
biological source which can cause an adverse effect. In this case the
receptors are infants, children, and adults in highly exposed farm
families living on farms where sewage sludge is applied. "Highly
exposed" farm families are defined as farm families whose diets
consist of 50 percent of products produced on their own farm. EPA
estimates that the maximum number of individuals in this highly exposed
population would be less than 11,000 even if all of the Nation's sewage
sludge were applied to family farms (see Section VI.L.). Since the
general population consumes only a small fraction of their diets from
products grown on farms with land-applied sewage sludge, EPA assumed
that a regulatory decision that is protective of this highly exposed
family is also protective of the general population.
The probabilistic analysis was performed using a Monte Carlo
simulation. In a Monte Carlo simulation, the model is run for a number
of iterations, each producing a single result (e.g., a single estimate
of cancer risk). For this assessment, 3,000 iterations were run in the
Monte Carlo simulation; therefore, the output of the probabilistic
analysis was a distribution of 3,000 values. This distribution
represents the distribution of possible outcomes, which reflects the
underlying variability in the data used in the analysis. These results
were then used to identify risk to the highly exposed population at
various percentile levels (e.g., 90th percentile risk value). As noted
above, the corresponding percentile risk values to the general
population would be significantly lower.
Some model input parameters used in the Monte Carlo simulation,
such as the concentrations of dioxin congeners in sewage sludge
samples, were drawn from statistical distributions. For others,
variability was associated with variable locations; thus, location
variability was explicitly considered in the setup of the data used for
the probabilistic analysis. For location-dependent parameters,
locations were first selected at random with equal probability of
occurrence \2\ based on the 41 climate regions. These regions
defined a set of related environmental conditions (e.g., soil type,
hydrogeologic environment) that characterized the environmental
setting. All location-specific parameters (e.g., rainfall) thus
remained correlated, while non-location-specific parameters were varied
both within and among locations.
---------------------------------------------------------------------------
\2\ Information was not available to allow the weighting
of these 41 climate regions based on the number of farm families in
each region.
---------------------------------------------------------------------------
D. How Did the Framework Change?
In the exposure analysis, the risk assessment evaluated a revised
scenario for exposure to sewage sludge: exposure
[[Page 40562]]
of a farm family that consumes 50% of its diet from home-produced crops
and animal products grown on their own sewage sludge-amended land. For
the December 1999 proposal, a rural family consuming a smaller
proportion of home-grown products derived from sewage sludge-amended
soil was modeled in the original risk assessment. EPA selected the new
scenario specifically to address groups of individuals who may have
high levels of exposure to dioxins in sewage sludge. EPA assumed that
the farm family lives immediately adjacent to the sewage sludge-amended
field and is exposed to a combination of agricultural products produced
on the farm, including beef and dairy products. The farm family also is
assumed to raise free-range chickens near their house (in the buffer
area). On the opposite side of the house from the field and pasture is
a fishable stream where a recreational fisher is assumed to catch fish
for personal consumption. There are four types of people who were
assumed to be representative of the individuals who would be exposed to
dioxin from sewage sludge: an infant of a farmer, a child of a farmer,
an adult farmer, and an adult recreational fisher. The exposure to the
adult fisher was combined with that of the adult farmer, when the total
exposure to the adult was calculated. Therefore, the fisher and farm
adult can be considered as the same adult. Table 4 summarizes the
exposure pathways for each type of individual.
Table 4. Receptors and Exposure Pathways
----------------------------------------------------------------------------------------------------------------
Ingestion Ingestion
Inhalation of above- Ingestion of Ingestion
Receptor of ambient Ingestion and of beef poultry Ingestion of breast
air of soil belowground and dairy and egg of fish milk
produce products products
----------------------------------------------------------------------------------------------------------------
Adult........................... X X X X X X
Child........................... X X X X X
Infant.......................... X
----------------------------------------------------------------------------------------------------------------
The new scenario includes new exposure pathways and exposure
mechanisms, incorporating updated scientific analysis for dioxin, which
is also reflected in EPA's Draft Dioxin Reassessment (USEPA, 2000a. See
Part I, Vol. 3, Chap. 2.). For the proposed rule, the risk assessment
evaluated pastured animals eating sewage sludge containing dioxins
after sewage sludge land application. The revised risk assessment
assumes tilled soil only for production of vegetables, fruits, and root
crops and untilled soil for pasturage to which sewage sludge is
applied. Half the acreage on the modeled farm is assumed to be used for
crop production (tilled) and half permanently used for pasturage
(untilled). Rather than assuming that cattle are exposed to dioxins
only by eating sewage sludge-containing soil, the Agency now assumes
that cattle are exposed to dioxins in sewage sludge by three
mechanisms: ingesting dioxins from the leaf surfaces of plants
containing dioxins which have volatilized from the top two centimeters
of the soil to which sewage sludge has been applied; ingesting dioxins
from sewage sludge particles which remain on the leaf surfaces of
plants after land application; and direct ingestion of sewage sludge-
containing soil by the grazing cattle. Of these three mechanisms of
dioxin transfer to cattle from the sewage sludge, the predominant
mechanism is ingestion of dioxins from leaf surfaces containing dioxins
which have volatilized from the sewage sludge-soil mixture. The dioxins
from land-applied sewage sludge that does not erode away from the land
application site are assumed to reside permanently in the top two
centimeters of the soil. Another new assumption reflecting the latest
science on dioxin and consistent with EPA's Draft Dioxin Reassessment
documents is that chickens will be ingesting dioxins from the buffer
area which receives dioxins from the pasture and crop fields through
erosion. EPA requests comments on the Agency's use of the farm family
scenario described for the revised risk assessment. EPA also requests
comments on the specific assumptions outlined above.
E. What Are the Factors in Estimating How Much Dioxin is Released to
the Environment?
Various inputs for sewage sludge characteristics were used in the
exposure analysis to determine how much dioxin is available for
volatilization, erosion or leaching. These included: concentrations of
each of the 29 congeners in sewage sludge (empirical distribution of
concentrations for each dioxin congener varied by sample), bulk density
of sewage sludge (single value), porosity of sewage sludge (single
value), percent moisture of sewage sludge when applied to agricultural
fields (single value), and fraction of organic carbon of sewage sludge
(single value). The use of the congener concentrations was different in
the revised exposure analysis. Rather than using point estimates for
the 29 congeners for the probabilistic analysis, all of the congener
concentrations measured in the 94 samples from the EPA 2001 dioxin
update survey were used. Specifically, for each iteration of the Monte
Carlo analysis, one of the 94 sewage sludge samples from the EPA 2001
dioxin update survey was randomly selected and the concentrations of
all congeners from that sample were considered in that iteration of the
analysis. For each iteration, the concentration of dioxins in the
sludge was assumed to remain constant for the entire period of
application since family farms would likely receive sewage sludge from
a single POTW.
When the chemical content of a substance is analyzed, the
assumption used to address non-detected chemicals can have a
significant impact on the reported results if the detection limits are
relatively large. Non-detects can be reported as zero, one-half the
detection limit, or the detection limit. Because of the excellent
sensitivity and limits of detection achieved by the analytical
procedures used in the EPA 2001 dioxin update survey, the reported
values for dioxin congeners in the samples of sewage sludge are
relatively unchanged whether non-detects are treated as zero, one-half
of the detection limit, or at full detection limit. For this risk
assessment, EPA assumed that non-detects are equal to one-half of the
detection limit. This assumption is prevalently used by EPA for risk
assessments based on data sets for non-detects, including the Draft
[[Page 40563]]
Dioxin Reassessment for calculating TEQ concentrations for dioxins in
environmental media (i.e., air, soil, water) and in exposure media
(i.e., food). Furthermore, it appears that there would be no
quantifiable difference in the estimated risk regardless of the
assumption made for non-detects for the reasons discussed above. EPA
requests comment on the treatment of non-detects in the revised risk
assessment and the effect on estimating risk.
Another sewage sludge characteristic, bulk density of sewage sludge
as it is applied to the agricultural field, was used to estimate the
loading of constituents to the soil in the model. Sewage sludge is
assumed not only to add constituents to the soil, but also to add
volume when mixed with the existing soil. Thus, bulk density is a
required parameter for the modeling scenario used in the exposure
analysis. Bulk density of the land-applied sewage sludge may be a
direct measurement or may be estimated using the dry bulk density, the
percent moisture, and the porosity of the sewage sludge.
F. What Are the Factors in Estimating How Much Dioxin Is Being
Transported in the Environment to the Individual in the Farm Family?
A conceptual site model was used to represent exposures to the
highly exposed modeled population from land application of sewage
sludge. To capture some of the variability in environmental settings
across the United States, the conceptual site model was placed in
different regions throughout the continental United States.
The risk assessment was intended to be representative of a national
distribution of environmental conditions. The 48 contiguous states
(excluding Hawaii, Alaska, and the off-shore possessions) were divided
into 41 meteorologic regions. These regions were selected to represent
the national variation of location-specific variables. Each area is
assumed to represent a single climate region (i.e., conditions within
that area can be modeled using the meteorologic data from a single
meteorologic observation station). Meteorologic and climate data were
used in air modeling, partitioning in the source model, and surface and
subsurface fate and transport modeling.
In addition, farm areas were assumed to be linked to geographic
area. Large farms are more common in the Midwest and western parts of
the United States, and smaller farms are more common in the eastern and
southern parts of the United States. Thus, a regional estimate for a
median farm size was developed and was used in this risk assessment.
The U.S. agricultural census contains estimates for the distribution of
farms within each county. These data were used to develop a median farm
size for each county. These county-wide median farm sizes were
classified according to the 41 geographic areas and the median of the
median farm sizes was estimated for each of the 41 regions. The median
area was then used in the air modeling and the erosion to surface water
modeling. This methodology was used to account for the regional
variation in agricultural practices throughout the nation, but it did
not consider variation in size within a single region.
A series of models was used to estimate concentrations of the
congeners in the environment with which a farm family may come into
contact. The revised risk assessment assumes that there are six direct
and indirect exposure pathways that the models describe:
· Inhalation of ambient air;
· Incidental ingestion of soil in the buffer area;
· Ingestion of above- and below-ground produce grown on
the crop land;
· Ingestion of beef and dairy products from the pasture;
· Ingestion of home-produced poultry and eggs from the
buffer area; and
· Ingestion of fish from the nearby water body.
As indicated above, a regional approach was used to define the area
surrounding the agricultural application site. A source partition model
was then used to estimate environmental releases of each constituent.
These estimated environmental releases in turn provided input to the
fate and transport models to estimate media concentrations in air,
soil, and surface water. A food chain model was used to estimate
constituent concentrations in produce, beef, dairy products, poultry,
eggs, and fish.
The source partition model determines the initial release of
congeners into the environment. Sewage sludge application to pastures
or crop land is assumed to be different and these differences affect
the behavior of constituents in the environment. The model uses
information described above on sewage sludge characteristics (e.g.,
moisture content and congener concentrations), and environmental
setting (e.g., precipitation, temperature, and soil characteristics) to
estimate environmental releases.
Fate and transport modeling procedures describe the mechanism by
which the congeners move from the source through the environment. As
described above, a source partition model was used to determine the
amount and nature of congener released from the agricultural field. A
multimedia approach was used to characterize the movement of the
dioxins through the environment. This approach considered atmospheric
concentrations, atmospheric deposition, soil concentrations, and
sediment concentrations in potentially impacted water bodies.
Air modeling procedures estimated air concentrations and deposition
of vapors and particles on the agricultural farm, onto the buffer area,
directly into the surrounding water bodies, and onto the regional
watershed. Air dispersion and deposition of vapors and particles were
modeled using the Industrial Source Complex Short Term Model. Soil
erosion comes from the crop fields and pastures, the buffer area
containing the house and chicken yard, and the remaining portion of the
watershed. Erosion was modeled using the Universal Soil Loss Equation.
All impacts in the same period of time were summed to estimate the
concentration in the stream sediment and water column.
The exposure pathways included inhalation of dioxins in ambient air
during tilling of agricultural fields, incidental ingestion of soil,
ingestion of aboveground and belowground produce (i.e., root crops),
ingestion of beef and dairy products, ingestion of eggs and poultry
products, and ingestion of fish. EPA's preliminary analysis indicated
that exposure to dioxins from the consumption of ground water was
insignificant due to the extremely low solubility of dioxins in water
and negligible leaching of dioxins to ground water (USEPA, 1999b).
With concentrations of the congeners determined for water and air,
the concentrations being delivered to humans from aboveground produce,
belowground produce, poultry, eggs, beef, dairy products, and fish were
then calculated. This was accomplished using food chain models. The
food crops (vegetables, fruits, and root vegetables) were assumed to be
grown on the sewage sludge-amended fields, and cattle (beef and dairy)
were assumed to be raised on pastures receiving sewage sludge. These
processes were modeled using a multi-pathway exposure model and the
fate and transport parameters and modeling procedures reflecting the
latest scientific knowledge on the fate and transport of dioxin. The
exposure pathways considered the transport of constituents from the
soil to plants (vegetables, fruits, roots, and pasture grass) and
ingestion of these materials by humans and animals. The transport to
plants
[[Page 40564]]
may occur through the root system, but most occurs through air-to-plant
transfer mechanisms. The contaminated plants are in turn consumed by
cattle and humans.
The latest scientific knowledge with respect to the methodology of
estimating concentration of congeners in beef and/or dairy products is
also described in the Draft Dioxin Reassessment document. This
methodology has been developed based on the transfer of congeners from
the total diet of the cattle into the fat. The method described in the
Draft Dioxin Reassessment emphasizes the importance of the differences
in diet between beef and dairy cattle in explaining different food
concentrations. While the same equation was used for all cattle,
whether they are beef cattle or dairy cattle, the differences were in
the dietary fraction assumptions. These assumptions were based on how
much of the time the cattle are pastured and how much of the time they
are confined with supplemental feed. Forage was assumed to be raised on
the sewage sludge-amended pasture where the sewage sludge was assumed
to remain on the top two centimeters of the soil and to volatilize onto
the forage. The soil was assumed to be the soil in the sewage sludge-
amended pasture. The supplemental feed for the cattle was assumed to be
grown on sewage sludge-amended crop land where the sewage sludge was
tilled into the soil. Half of the supplemental feed was assumed to be
vegetation and half was assumed to be grains. Supplemental feed was
assumed to contain a lower dioxin concentration than forage because it
was assumed to contain less volatilized dioxins (due to tilling), and
the grain portion was assumed to be free of contamination due to
stripping of the outer leaves where dioxins accumulate.
To determine the dioxin concentrations in poultry and eggs, the
risk assessment starts with the assumption that sewage sludge is not to
be applied directly to the chicken yard. The chickens are assumed to be
free range within a confined area of the buffer near the farm
residence. The chicken diet is assumed to consist of 90 percent store
bought chicken feed (uncontaminated by dioxins in sewage sludge applied
on the farm land) and 10 percent buffer soil.
As already indicated, the receptors included in the modeling are
adults and children living and working on farms where fruits,
vegetables, root crops, and farm animals are raised, and half of these
food items consumed by the adults and children living on the farm are
produced on the farm. The farm family also is assumed to be exposed to
inhalation risks from windblown and tilling emissions from the
agricultural field. Soil ingestion risks are also assessed for both
adults and children. Children are assumed to ingest soil from the
buffer area, and the adult farmer is assumed to ingest soil from the
tilled field. In addition, risks to recreational fishers who catch and
consume fish from the stream adjacent to the agricultural field is
considered and summed with the other exposure pathways on the
assumption that farmers are also recreational fishers.
EPA requests comment on the assumptions and values used in this
Section to estimate how much dioxins are being transported to
individuals in the modeled farm family (e.g., the sources (store-bought
versus farm-produced) and dioxin contamination levels of poultry
feeds).
G. What Additional Factors Are Applied to Dioxin Concentrations To
Determine How Much of the Congeners are Being Ingested or Inhaled by a
Farm Family Member?
To determine how much of the congeners adults and children are
inhaling and ingesting, exposure factors were applied to the
concentrations of the contaminants from air, produce, cattle, dairy,
poultry, eggs, and fish. The exposure factors used in this analysis
were taken from the Exposure Factors Handbook (USEPA, 1997). The
Exposure Factors Handbook summarizes data on human behaviors and
characteristics related to human exposure from relevant key studies and
provides recommendations and associated confidence estimates on the
values of exposure factors.\3\
---------------------------------------------------------------------------
\3\ EPA carefully reviewed and evaluated the quality of
the data before their inclusion in the Exposure Factors Handbook.
EPA's evaluation criteria included peer review, reproducibility,
pertinence to the United States, currency, adequacy of the data
collection period, validity of the approach, representativeness of
the population being modeled (in this case, farm families),
characterization of the variability, lack of bias in study design,
and measurement error (USEPA, 1997).
---------------------------------------------------------------------------
The proportion of home produced food commodities eaten by highly
exposed farm families was assumed to be 50% of their diet for all
iterations. This assumption defined the modeled population. Specific
distributions of other exposure factors for the general population of
farm residents were compiled from the Exposure Factors Handbook. These
include ingestion rates for adults and children for aboveground
vegetables, root vegetables, fruits, beef, dairy products, poultry, and
eggs. Distributions have been developed for adults and for three age
groups of children for these dietary categories.
Exposure factors are related to the pathways in that they describe
the rates at which dioxin doses are ingested or inhaled from the
various sources noted above (e.g., air, soil, beef, and diary, by the
highly exposed farm family adults and children). The exposure factors
used in this risk assessment are represented by a distribution or a
fixed value in the Monte Carlo probabilistic analysis.
For the probabilistic exposure analysis, probability distribution
functions were developed from the values in the Exposure Factors
Handbook. The intake factors, for which either single values or
distributions were used from the Exposure Factors Handbook, are: soil
ingestion (one value for children aged 1 to 6 and another value for all
other receptors); and fruits and vegetables ingestion, beef and dairy
ingestion, fish ingestion, and inhalation rates (all of which are
distributions of values.)
H. How Did EPA Calculate the Range of Exposure Levels?
For cancer effects, where the biological response is described in
terms of lifetime probabilities, dose is presented as a "lifetime
average daily dose" (LADD). Because exposure duration varies from
person to person (i.e., may not occur over the entire lifetime),
calculation of exposure produces a distribution of exposure levels (or
doses). In addition to exposure duration, the LADD takes a number of
variable factors into account, including when exposure begins, how
often and in what amounts sewage sludge is applied to the land, and the
length of time over which land application occurs. For this risk
assessment, the LADD takes into account: (1) A distribution of randomly
selected times when land application begins, i.e., either when the
highly exposed farm family begins applying sewage sludge to their land
or moves onto a farm where sewage is being or has been applied; (2) a
distribution of exposure durations ranging from one year to 70
years; \4\ (3) a distribution of sewage sludge application
duration, ranging from a minimum of one year up to a maximum of 40
years (i.e., a minimum of one application to a
[[Page 40565]]
maximum of 20 applications based on a fixed application frequency of
once every two years), and (4) a distribution of sewage sludge
application rates (i.e., amount of sludge applied to the land) ranging
from 5-10 metric tons per hectare per application. The LADD also
includes doses from each exposure route (i.e., inhalation and
ingestion) and body weight. A distribution of body weights for the
adult and child were taken from the Exposure Factors Handbook.
---------------------------------------------------------------------------
\4\ Exposure durations representing the residence time in
the same house were also determined using the Exposure Factors
Handbook. The lifetime of the individual was assumed to be a fixed
value of 70 years. A fixed value for exposure frequency was assumed
to be 350 days per year, accounting for two weeks away from the farm
for vacation (USEPA, 2002b). These single values were selected to be
protective and yet representative of realistic scenarios.
---------------------------------------------------------------------------
The purpose of the exposure assessment is to estimate the dose to
an exposed individual by combining media intake estimates with media
concentrations. Estimates of exposure are based on the potential dose
(e.g., the dose ingested or inhaled) rather than the applied dose
(e.g., the dose delivered to the gastrointestinal tract) or the
internal dose (e.g., the dose delivered to the target organ). Doses
from individual pathways (e.g., soil, exposed vegetables) were
calculated by multiplying the contaminant concentration in the food
product or other exposure media (e.g., air or soil) by the respective
intake rate on a per kilogram body weight basis. Doses received from
the various ingestion pathways (e.g., soil and food) were then summed
over the period of time in which exposure occurs, resulting in an
average daily dose received from ingestion exposure.
I. How Was Childhood and Infant Exposure Evaluated in the Exposure
Analysis?
Children are an important sub-population to consider in a risk
assessment because they may be more highly exposed than adults;
compared to adults, children may eat more food and drink more fluids
per unit of body weight. This higher intake-rate-to-body-weight ratio
can result in a higher average daily dose of dioxins than adults
experience. The risk assessment performed for sewage sludge application
to agricultural land includes an analysis of exposures to 3,000
individuals whose exposures begin in childhood. To account for intake
rates varying over different childhood age groups, parameters
characterizing exposures beginning in childhood were developed.
The first step in developing the time-weighted parameters is to
define the start age for the child and the length of exposure for that
individual. These two values then determine how long the individual is
in each age group. Four age groups were defined as follows: age group 1
(1-5 years of age); age group 2 (6-11 years of age); age
group 3 (12-19 years of age); and age group 4 (over 20 years of
age). After the individual is defined, age appropriate consumption
rates are chosen for each age group which are selected from the age
specific consumption rate distribution for each item considered in the
analysis. For example if the exposure begins at age 3 and continues for
20 years, a consumption rate for each age group was selected and
weighted to represent the number of years spent in each age group to
get an average intake rate for the entire exposure duration of 20 years
(i.e., age group 1= 3 years of exposure; age group 2 = 6 years; age
group 3 = 7 years; and age group 4 = 4 years, for a total of 20 years
exposure.) This time weighted intake rate is then used with the average
concentration of dioxins for the food item over the entire exposure
duration, to yield an average daily dose.
Infants are also an important sub-population to consider in this
risk assessment because they may be exposed to dioxin-like compounds
via the ingestion of breast milk. While risks to children and adults
were integrated to incorporate individuals for whom exposure first
occurs during childhood but continues into adulthood, the lifetime
risks to infants were calculated separately from the risks to older
children (i.e., ages 1 year or older) and adults. For infants, exposure
during the first year of life was averaged over an expected lifetime of
seventy years to derive a LADD that was then used to calculate risk.
The "lifetime" risk to infants thus should be thought of as
the contribution to lifetime risk that occurs during the first year of
life through ingestion of breast milk for individuals born into a farm
family exposed to dioxins from land-applied sewage sludge.
J. How Was the Cancer Risk Estimate Calculated?
Cancer risk is calculated using lifetime excess cancer risk
estimates to represent the excess probability of developing cancer over
a lifetime as a result of exposure to the constituent of interest.
Lifetime excess cancer risk estimates are the product of the lifetime
average daily dose for each of the four types of individuals exposed to
dioxin and for each exposure pathway, and the corresponding cancer
slope factor.
The exposure assessment estimates delivered doses for each of the
29 congeners to a farm family individual. Each of these congener doses
were then converted to TEQ doses by multiplying each congener dose by
its TEF. These TEQ doses for each of the 29 congeners were then summed
to yield an overall TEQ dose to the individual for that exposure
pathway (e.g., inhalation or ingestion). Finally this TEQ dose was
multiplied by the cancer slope factor to estimate the excess cancer
risk to the individual for that pathway of exposure.
Using all samples from the EPA 2001 dioxin update survey, the
estimated risks and corresponding daily exposure to dioxins for the
highly exposed farm adult and child are given below in Table 5 for
various percentiles of exposure within this population.
"Adult" means individuals whose exposure begins when they
are adults, and "child" means individuals whose exposure
begins when they are children. In most cases exposure which begins
during childhood also ends during childhood. However, in some
instances, exposures which begin when individuals are children
continued into their adult years.
Additional risk calculations were performed to estimate the impact
on the risk if sewage sludge with 300 ppt TEQ dioxin and 100 ppt TEQ
dioxin were restricted from being land applied. Eliminating sewage
sludge samples with higher concentrations of dioxins did not change the
estimated risk. The distribution of risk estimates for scenarios
excluding samples with dioxin concentrations greater than 300 ppt TEQ
and 100 ppt TEQ are the same as the distribution below shown in Table
5, which includes data from all sewage sludge samples.
[[Page 40566]]
Table 5. Risks and Daily Exposure for Highly Exposed Farm Adult and Child for All Exposure
Pathways (Q*=1.56 x 10&minus4/pg TEQ/kg-d)
----------------------------------------------------------------------------------------------------------------
Adult * Child **
-------------------------------------------------------
Percentile Daily Daily
Risk Exposure pg Risk Exposure, pg
TEQ/kg-d TEQ/kg-d
----------------------------------------------------------------------------------------------------------------
50th.................................................... 1 x 7.3 1 x 7.3
10&minus98 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
× 10−3/pg TEQ/kg-d. According to the Draft 2000
Dioxin Reassessment, this cancer slope factor represents a plausible
upper bound on risk based on evaluation of human and animal data. These
values are approximately six times higher than previous estimates
(USEPA, 1985 and USEPA, 1994a) which were based on fewer data. However,
the EPA SAB panel was not able to reach consensus on a single value for
a dioxin potency factor. The SAB panel cited differences of opinion on
the adequacy of data and modeling approaches and assumptions as their
reasons for not reaching consensus on a dioxin cancer slope factor.
The revised Round Two land application risk assessment uses the
cancer slope factor currently used by EPA in risk assessments (USEPA,
1994a). If EPA adopts a different cancer slope factor for assessing the
risk of cancer from dioxin prior to taking final action on the proposed
Round Two rule, EPA will evaluate the risk of cancer from land-applied
sewage sludge using any such revised cancer slope factor. Similarly, to
the extent EPA adopts a policy regarding risks of non-cancer health
effects from dioxin prior to the
[[Page 40569]]
final decision on the proposed Round Two rule, the Agency will evaluate
non-cancer effects associated with dioxins in land-applied sewage
sludge using any such policy.
In order to give the public an opportunity to understand and
comment on how the particular approaches contained in the Draft Dioxin
Reassessment could potentially affect the proposed Round Two
rulemaking, EPA is presenting a discussion of the potential impacts of
the revised cancer slope factor and approaches for estimating non-
cancer effects contained in the Draft Dioxin Reassessment on EPA's
revised land application risk assessment. This includes a discussion of
background exposures and risks based on information in the Draft Dioxin
Reassessment, such as existing body burden, although EPA has not made a
final decision regarding these findings or adopted any policy with
respect to regulating dioxins in light of background exposures and
existing body burden.
A. How Would the Dioxin Cancer Risk from Land Application Compare to
Background Dioxin Cancer Risk?
Dioxin and dioxin-like compounds always exist in nature as complex
mixtures. These compounds can be quantified in environmental media and
their potential effects assessed as a mixture. As previously noted, the
contribution of the other "dioxin-like" compounds is
quantified by treating each as having a defined "toxicity
equivalence" to dioxin (toxicity equivalent factor, TEF). The TEQ
concentration is calculated by multiplying the concentration of each
congener in the sewage sludge by its corresponding TEF, and then
summing the resulting products from this calculation for all 29
congeners.
The significance of the incremental exposure and risk due to a
specific source such as land application of sewage sludge is best
understood by discussing it in the context of general population
background exposure to dioxins. The fact that background exposures and
body burden of dioxins are currently high for the general population
means that any incremental exposure from a particular source needs to
be considered in context of its contribution to overall risk. The
following is a comparison of the dioxin cancer risk the EPA calculated
from the Agency's revised risk assessment to the background dioxin
cancer risk estimated from the Agency's 2000 Draft Dioxin Reassessment.
This comparison considers both the current cancer slope factor the
Agency has been using since 1985 and the revised cancer slope factor
contained in EPA's 2000 Draft Dioxin Reassessment.
The revised risk assessment for land application of sewage sludge
uses the current cancer slope factor of 1.56 ×
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 × 10−5 at the 99th
percentile to 1 × 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 ×
10−3/pg TEQ/kg-d cancer slope factor being
considered in the 2000 Draft Dioxin Reassessment would result in
estimated high-end multi-pathway lifetime risks for highly exposed farm
family adults ranging from 2.4 × 10−4 at the
99th percentile to 6 × 10−6 at the 50th
percentile (see Table 7, below). These estimated risks using a 1
× 10−3/pg TEQ/kg-d cancer slope factor are
based on the same daily exposures indicated in Table 5. Again, the
estimated risks for children would be less than or equal to the
estimated risks for adults (see table 7).
Table 7. Risks for Highly Exposed Farm Adult and Child for All
Exposure Pathways (Q*=1 × 10&minus3 pg TEQ/kg=d)
------------------------------------------------------------------------
Percentile Adult * Child **
------------------------------------------------------------------------
50th..................................... 6 × 6 ×
10&minushttp://www.epa.gov/ncea/exposfac.htm
USEPA, 1998a. Methodology for Assessing Health Risks Associated with
Multiple Pathways of Exposure to Combustion Emissions. EPA/600/
P-98/137. Washington, DC.
USEPA, 1998b. Guidelines for Ecological Risk Assessment (Final). EPA/
630/R-95/002F. Risk Assessment Forum. Washington, DC.
USEPA, 1999a. EPA Method 1668: Polychlorinated Biphenyls by Isotope
Dilution High-resolution Gas Chromatography/Mass Spectrometry, Revision
A , EPA-821-R-00-002, December 1999).
USEPA, 1999b. Risk Analysis for the Round Two Biosolids Pollutants.
Office of Science and Technology. Washington, DC.
USEPA, 1999c. Biosolids Generation, Use, and Disposal in the United
States. EPA 530-R-99-009. Office of Solid Waste and
Emergency Response. Washington, DC.
USEPA, 1999d. Costs Associated with Regulating Dioxins, Furans, and
PCBs in Biosolids. Office of Science and Technology. Washington, DC.
USEPA, 2000a. Exposure and Human Health Reassessment of 2,3,7,8-
Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. Parts
I-III. Draft. Prepared by the National Center for Environmental
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600/P-00/001 Bb, Bc, Bd, Be, Bg). Available online at http://
www.epa.gov/ncea.
USEPA, 2000b. Risk Characterization Handbook, EPA
100-B-00-002, Science Policy Council, Washington, DC.
USEPA, 2001a. Sampling Procedures for the 2001 National Sewage Sludge
Survey, Office of Science and Technology, Washington, DC.
USEPA, 2001b. Analytical Data for Dioxins in Sewage Sludge Submitted by
Three Wastewater Treatment Plants, Office of Science and Technology,
Washington, DC.
USEPA, 2001c. The Role of Screening-Level Risk Assessments and Refining
Contaminants of Concern in Baseline Ecological Assessments. EPA ECO
Update, Publication 9345.0-14. EPA/540/F-01/014. Office of
Solid Waste and Emergency Response, U.S. EPA, Washington, DC.
USEPA, 2002a. Statistical Support Document for the Development of Round
2 Biosolids Use or Disposal Regulations , Office of Science and
Technology, Washington, DC.
USEPA, 2002b. Exposure Analysis for Dioxins, Dibenzofurans, and
Coplanar Polychlorinated Biphenyls in Sewage Sludge-Technical
Background Document, Office of Science and Technology, Washington, DC.
USEPA, 2002c. Estimate of Population Exposed to Dioxins from the Land
Application of Sewage Sludge and Corresponding Number of Annual Cancer
Cases from this Exposure, Office of Science and Technology, Washington,
DC.
USEPA, 2002d. Costs Associated with Regulating Dioxins, Furans, and
PCBs in Biosolids. Office of Science and Technology. Washington, DC.
Van den Berg M, et al. 1998. Toxic Equivalency Factors (TEFs) for PCBs,
PCDDs, and PCDFs for Humans and Wildlife. Environ. Health Perspect.
106(12): 775-792.
Dated: June 5, 2002.
G. Tracy Mehan III,
Assistant Administrator for Water.
[FR Doc. 02-14761 Filed 6-11-02; 8:45 am]
BILLING CODE 6560-50-P
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