&EPA
United States
Environmental Protection
Agency
Office of Water
4304T
EPA-820R25001
January 2025
Responses to Select Science
Advisory Board Comments
Relevant to the Draft Sewage
Sludge Risk Assessment for PFOA
and PFOS
January 2025
U.S. Environmental Protection Agency Office of Water, Office of
Science and Technology, Health and Ecological Criteria Division
Washington, D.C.
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INTRODUCTION
In 2023, the U.S. Environmental Protection Agency (EPA) presented the Science Advisory Board (SAB)
with the Biosolids Program's proposed framework for 1) prioritizing chemicals found in sewage sludge, 2)
conducting screening-level human health and ecological risk assessments using the Biosolids Tool (BST),
and 3) conducting refined risk assessments for those chemicals that pose the greatest potential risk to
human health and the environment. In October of 2023, the SAB provided a report with feedback on all
three items in the proposed framework. The EPA's proposed framework, the charge questions to the
SAB, and the final SAB report are available on the EPA's SAB webpage.
The EPA is in the process of responding to all feedback provided by the SAB and revising the general
framework for prioritizing, screening, and conducting refined risk assessment for chemicals found in
sewage sludge. Although the focus of the SAB review was on the general framework rather than on any
particular chemical, some comments provided by the SAB are relevant to the EPA's 2025 Draft Sewage
Sludge Risk Assessment for Perfluorooctanoic Acid (PFOA) and Perfluorooctane Sulfonic Acid (PFOS)
("Draft Risk Assessment"). For thoroughness, the EPA is responding to these comments below. The EPA
expects to publish responses to the rest of the SAB's comments later in 2025.
The SAB prioritized their recommendations to indicate relative importance and were defined by the SAB
as follows:
• Tier 1: Key Revisions - Actions that are necessary to improve the critical scientific concepts,
issues, and/or narrative within the assessment/document/model/guidelines.
• Tier 2: Suggestions - Actions that are encouraged to strengthen the scientific concepts, issues,
and/or narrative within the assessment/document/model/guidelines, but other factors (e.g., EPA
need) should be considered by the EPA before undertaking these revisions.
• Tier 3: Future Considerations - Useful and informative scientific exploration that may inform
future evaluations of key science issues and/or the development of future
assessments/documents/models/guidelines. These recommendations are likely outside the
immediate scope and/or needs of the current review.
The EPA typically uses the term "biosolids" to mean sewage sludge treated to meet the requirements in
Title 40 of the Code of Federal Regulations (CFR) part 503 and intended to be applied to land as a soil
amendment. This document contains comments from SAB panel members who used the terms
interchangeably, so for the purposes of this document, "biosolids" should be interpreted as "sewage
sludge."
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SAB Comment
Tier Level
EPA Response
While the role of air-water interfacial sorption may
not impact most of the chemicals on the list to be
evaluated, PFAS transport to groundwater is
known to be greatly impacted by this process in
the vadose zone. Given the significance of PFAS in
the current regulatory framework, the SAB urges
EPA to consider how to address this transport
process.
Tier 2
The EPA considered models that include air-water interfacial sorption for
the PFOA and PFOS risk assessments for these chemicals. A detailed
discussion of the model selection process is included in the Draft Risk
Assessment under Section 2 and Appendix C. Additionally, the Draft Risk
Assessment underwent independent, external peer review by five experts,
and the EPA requested feedback specifically on model selection for PFOA
and PFOS. The external peer reviewers agreed with the EPA's model
selections (see question 4 on EPA Response to External Peer Review
Comments on the Draft Sewage Sludge Risk Assessment for PFOA and
PFOS). These air-water interfacial sorption models may also be considered
during refined risk assessment for other chemicals with similar physical and
chemical properties. Seethe Draft Sewage Sludge Risk Assessment for
PFOA and PFOS and responses to peer-review comments for additional
information.
The SAB recommends that EPA consider
occupational exposures to chemicals in biosolids
for dedicated workers who may be responsible for
their application.
Tier 2
Under CWA section 405, the EPA is directed to periodically identify
pollutants and on the basis of toxicity, persistence, concentration, mobility,
or potential for exposure, may be present in sewage sludge in
concentrations which may adversely affect human health and the
environment. The EPA considered how existing pathways of exposure for
farm families may compare to potential exposures for biosolids land
appliers (see Section 2.7 of the Draft Risk Assessment), but several
significant data limitations on worker exposures were identified. The Draft
Risk Assessment concludes that there is insufficient information available to
model occupational exposures for workers that repeatedly apply biosolids
at different farms throughout the year or to determine whether the farm
family or farm worker exposures will exceed the exposures of these
professional biosolids applicators. The assessment acknowledges that if this
type of worker is repeatedly spray applying biosolids on farm fields, this
could lead to airborne exposures over many days of the year and this type
of exposure is not represented within the modeled pathways for the farm
family. However, the EPA does not currently have survey or other data to
estimate the behavior patterns of these types of workers with missing
information including amount of biosolids mass aerosolized during
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SAB Comment
Tier Level
EPA Response
application, time spent per day applying biosolids, and number of days
worked per year. Should this information become available, the EPA would
consider adding this receptor group and the appropriate pathways of
potential exposure to the BST and future refined risk assessments.
Additionally, PFOA and PFOS are expected to occur primarily in their
ionized phase in most farm soils due to the chemicals' acid dissociation
constant (pKa) values and are not expected to volatilize; thus, inhalation is
not included as a relevant exposure pathway.
The SAB recommends EPA review the data
regarding fish consumption rates for an adult
farmer to confirm the correct values are used
corresponding with trophic level 3 and 4 fish
consumption.
Tier 1
In revisiting the fish consumption rate and trophic level assignments used
for the BST and risk assessments, the EPA has elected to use a fish
consumption rate (FCR) that reflects surveys targeted at consumers of
home-produced foods because this group is most representative of the
agricultural settings in the US where biosolids are often land applied.
Specifically, the EPA is now using fish consumption rate values from the
EPA's Exposure Factors Handbook Chapter 13, Table 13-20. The Exposure
Factors Handbook also includes information on the species offish
consumed by freshwater fishers in the US (Chapter 10, Table 10-74); this
data was used to apportion the trophic level-specific consumption patterns
specific to freshwater fishers. See the EPA's Draft Sewage Sludge Risk
Assessment for PFOA and PFOS, Section 2.9.3.8.1 for more information on
these datasets.
The SAB recommends EPA consider using the adult
farmer fish consumption exposure scenario for fish
consumption of trophic levels 3 and 4 fish at a
central tendency consumption rate (e.g., 50th
percentile consumption rate).
Tier 2
See above response. Note that in the Draft Risk Assessment for PFOA and
PFOS, for reasons explained in Section 2.9, the EPA selected the 50th
percentile values from the Exposure Factors Handbook Chapter 13, Table
13-20.
The SAB recommends that at the refined risk
assessment stage, EPA consider models that can
differentiate between the total chemical
concentration and bioavailable concentration in
biosolids (i.e., the biosolids matrix).
Tier 1
The refined risk assessment stage accounts for bioavailable fraction in soils
in that the models are parameterized using field-based biosolids-specific
studies, when available. Using field-based biosolids-specific studies for
plant uptake, groundwater leaching, and animal uptake parameters
implicitly considers only the "available" fraction of the chemical in
biosolids-amended soil and in the other exposure media like feed and
water for livestock. However, in some cases, these biosolids-specific field
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SAB Comment
Tier Level
EPA Response
data are not available. This topic of how to consider total as compared to
bioavailable concentrations of chemicals when data specific to soils are not
available was discussed extensively in the process of developing the 1993
regulations that included numeric limits for metals in land-applied
biosolids. As stated in the preamble to Part 503 "Recent data from long-
term field studies have shown that sewage sludge properties influence
pollutant bioavailability through binding of the pollutants by the sewage
sludge itself. However, the research data base on the fate of many sewage
sludge-borne pollutants is still extremely limited. As a result, uncertainties
about the health effects and threshold exposures of these pollutants has
made the risk/exposure assessment for these pollutants difficult.
Therefore, the Agency has decided to continue to use RE [relative
effectiveness of exposure] values equal to one for those pollutants with
limited data but has revised its RE values where sufficient scientifically
defensible information was available indicating the bioavailability of the
pollutant was less than 100 percent. The Agency recognizes that in some
cases this may result in numerical limits that may be more protective than
necessary; however, EPA believes that it is prudent to have a balance of
mid-range and bounding parameters in order to protect highly exposed
individuals" (Preamble to 40 C.F.R. § 503 (58 FR 9295 (Feb 19, 1993)). The
EPA plans to continue to follow this longstanding practice of assuming 100
percent bioavailability of the pollutant in biosolids in the absence of data
indicating otherwise.
The SAB recommends that EPA model land
reclamation scenarios that reflect the use of large
one-time biosolids application rates (i.e., > 100 dry
tons/acre) and its potential impact on public
health and ecological risks (Pepper et al., 2013).
Tier 1
The risk assessment framework submitted to the SAB and the Draft Risk
Assessment for PFOA and PFOS include a land reclamation scenario with a
one-time application of biosolids at a rate of 50 dry tons per acre. This
value is five times the expected agronomic rate (see Appendix E.l of the
BST User Guide Appendixes and Draft Risk Assessment, Section 2.9.3).
Though it is possible for biosolids to be land-applied at rates higher than
five times a typical agronomic rate, the EPA anticipates that these scenarios
do not frequently occur. A probabilistic risk assessment could consider a
range of application rates for a reclamation scenario (note that as
described in Section 2.9 of the Draft Risk Assessment, the EPA will not be
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SAB Comment
Tier Level
EPA Response
developing a probabilistic risk assessment for PFOA or PFOS, but
probabilistic approaches would be considered for risk assessments of other
chemicals).
The SAB recommends that EPA consider the
following to inform future evaluations/revisions of
the refined assessment: Land reclamation is
currently limited within the refined assessment to
the restoration of mining sites. There are several
other potential land reclamation scenarios where
biosolids could be utilized including being
employed to restore vegetation on wildfire-
damaged land, sand dunes, construction sites, and
over-grazed rangelands (McFarland et al., 2009).
The potential contribution of domestic septage
land application on human health and ecological
chemical exposure within the model farm scenario
should be considered. While domestic septage
applied to non-public contact sites (i.e., private
farms or ranches) does not have numerical
pollutant limits, domestic septage applied to public
contact sites (i.e., parks, cemeteries, home
gardens, etc.) must meet the same numerical
pollutant limits as land-applied sewage sludge.
Tier 3
There is considerable variability in the scale and types of land reclamation
projects that use biosolids to improve soil conditions. In the Draft Risk
Assessment, the EPA considered additional reclamation scenarios in the
refined risk assessment phase to better understand what reasonably
anticipated scenarios may present risks.
Regarding domestic septage, in support of Part 503, the EPA compared the
concentration of chemical pollutants in domestic septage to municipal
sewage sludge and found that an application rate based on nitrogen
content was protective of the risk derived chemical pollutant limits
(Technical Support Document for Land Application of Sewage Sludge
Volume II, 1992). The EPA will continue to evaluate whether domestic
septage application rates based on nitrogen are protective of human health
and the environment; however, data related to management practices and
concentrations of chemicals in domestic septage are often sparse.
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SAB Comment
Tier Level
EPA Response
Artificial drainage enhancements of agricultural
fields are not accounted for in any models despite
their prevalence, especially in the US Midwest
(USDA, 2019). Subsurface, tile drainage involves
placement of a perforated tile approximately 1-m
below the soil surface to improve field drainage,
thus reducing runoff, but allowing for direct
transport from immediately below the rooting
zone to streams. Therefore, the role of runoff in
these cases will be overpredicted, thus impacting
exposure estimates of more highly retained
compounds of interest, but possibly
underestimating the impact to streams of more
mobile/soluble chemicals. For addressing tile-drain
networks, it could be plausible to use the
Multimedia, Multi-Pathway, Multi-Receptor
Exposure and Risk Assessment (3MRA) to 1 meter
(vs 2 m) and then direct discharge to stream
coupled with the Variable Volume Water Model
versus the dilution-attenuation factor (DAF).
Not
Tiered
The EPA agrees that the use of tile drains would impact the fate and
transport of chemicals being land applied on agricultural fields. In general,
tile drainage systems would be expected to increase risks observed in
surface water-related exposure pathways (surface water as a source of
drinking water, fish consumption, aquatic life ecological receivers). In the
Draft Risk Assessment, the modeling of tile drained agricultural fields would
not change the overall risk conclusions because the Draft Risk Assessment
models many pathways with risk levels above acceptable thresholds (i.e.,
fish consumption, surface water used as a source of drinking water, milk
consumption) that would likely result in the same or higher risks if tile
drainage were modeled. If a refined risk assessment finds that tile drains
would be key risk drivers, the EPA would consider modeling this
hydrological setting.
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