contamination from CCR WMUs that provide the best available comparison
for the results of the risk assessment. This comparison is presented in
Section 5 of the revised risk assessment. EPA also provided extensive
EPACMTP validation results relative to theoretical models and field
data in appendix D of the EPACMTP technical background document (U.S.
EPA, 2003a,b).\135\
---------------------------------------------------------------------------

    \135\ U.S. EPA (Environmental Protection Agency). 2003a. EPA's
Composite Model for Leachate Migration with Transformation Products
(EPACMTP). Technical Background Document. EPA 53-R-03-002. Office of
Solid Waste, Washington, DC.
---------------------------------------------------------------------------

    COMMENT: Comments relating to the number of wells contaminated, the
realistic risk of exposure, well placement within the plume, distance
to receptor wells, identification of surface water receptors, surface
water interception modeling, the appropriateness of receiving water
reaches (e.g. the nearest surface water body), and other receptor or
well-related issues were received from public commenters.
    Surface Water Interception Modeling:
    Regarding surface water interception, many comments were supportive
of EPA's approach for simulating the interception of groundwater by
surface water bodies, which has been added to the revised risk
assessment. However, some commenters indicated that a meaningful
allocation of the groundwater plume between a surface water body and a
downgradient well receptor can only be determined reliably with
assessment of the system at a local scale.
    Commenters also raised questions regarding the specific surface
water interception methodology, including the base data and algorithms
used to calculate stream base flow, net groundwater flow, and the
contaminant mass loss to groundwater. Concern was expressed about the
large range of possible values used for Monte Carlo sampling without
calibrating models to site specific conditions and the potential to
mismatch parameters. Additionally, concerns were raised that the
assessment assumed transport directly to the nearest water body without
reflecting complexities that are often present and could lead to longer
transport pathways or to pathways to water bodies other than the
nearest.
    Commenters noted that the vicinity of many WMUs is serviced by a
municipal water supply, and; therefore, there would be no drinking
water receptors associated with these WMUs. Comments were also received
that the one mile distance considered by the transport model is not
sufficient, because actual receptor wells in many cases are further
than one mile from facilities. Comments also highlighted the
possibility that modeled receptor well concentrations may incorrectly
represent actual

exposures by sampling from a single aquifer depth. Comments on
dispersivity noted the need for greater transparency in the report.
    Placement of Receptor Wells, EPACMTP Well Inputs and Assumptions:
    Comments related to the risk assessment's use of water well
distances from MSWLFs and the Agency's belief that these distances
would be protective for CCR WMUs. Additional comments focused on the
assumption that the wells used in this assessment are contaminated
(i.e., located within the plume), even if the well location used
reflects a deeper well that may be screened in an uncontaminated
aquifer; the manner in which the assessment handles uncontaminated
wells, plume characteristics, groundwater-surface water interactions,
vertical contaminant concentration across a screened interval in an
aquifer; and the values used for plume dispersivity.
    EPA RESPONSE: The following is EPA's response broken out by
subtopic.
    Surface Water Interception Modeling:
    In cases where receptor wells are located downgradient from a
surface water body that intersects the groundwater table, some or all
of the groundwater, along with the mass of constituents contained
therein, is intercepted by the water body before it can reach the well.
This interception was not modelled in the 2010 Draft Risk Assessment.
However, a review of the input database for the 2010 Draft Risk
Assessment found that such a water body was present in approximately
two-thirds of the Monte Carlo runs. Furthermore, ignoring the loss of
constituent mass had the effect of overestimating exposures. Thus, in
the revised risk assessment an EPACMTP model post-processor was created
to account for surface water interception by removing constituent mass
flowing into the water body from the groundwater plume, and leaving
only the remaining groundwater available to migrate to a drinking water
receptor. The approach used to account for interception is discussed in
further detail in Section 4 and appendix J of the revised risk
assessment.
    While commenters were generally supportive of the proposed
approach, some indicated that a meaningful allocation of constituent
mass from groundwater into a surface water body required site-specific
data. Concerns were raised about the assumption that transport occurred
directly to the nearest water body without reflecting complexities that
are often present and could lead to longer transport pathways or to
pathways to water bodies other than the nearest. EPA acknowledges that
local conditions can make groundwater flow conditions complex, and
detailed, local-scale assessments would be required to describe these
conditions accurately. While EPA agrees that local-scale conditions
must be considered for precise estimation for specific systems, it was
impractical for EPA to characterize, simulate, and calibrate models for
the numerous locations across the nation. Discussion of the
uncertainties associated with this approach has been added to Section 5
of the revised risk assessment.
    Several questions about the surface water interception methodology
were raised by the public. The qBaseflow input parameter was derived
from the NHDplus mean recharge parameter (MEAN_RCHRG) \136\ and the
size of the water body catchment and reach (see appendix B of the
revised risk assessment). The approach assumes that all streams
intersect the shallow aquifer and that all streams either gain water
from the aquifer or do not interact with the aquifer at all (for
simplicity and conservatism). As the commenter indicates, qNetflow is a
key result calculated by subtracting the stream baseflow from the
average groundwater flow upgradient of the stream. The qNetflow value
becomes the adjusted groundwater flow beyond the stream, reflecting
groundwater losses to the stream. One commenter raised a specific
question about how the methodology handles cases where qNetflow is less
than zero, but greater than the average groundwater flow. This case
does not occur with the methodology adopted by EPA, because qNetflow is
always equal to or less than the average groundwater flow (i.e. streams
are assumed not to be losing). If qNetflow is negative (i.e., a losing
stream), all of the groundwater is assumed to migrate to any wells on
the opposite side of the stream.
---------------------------------------------------------------------------

    \136\ Available online at: water.usgs.gov/GIS/metadata/usgswrd/XML/nhd_recharge.xml.
---------------------------------------------------------------------------

    Model Validation/Calibration:
    Concern was expressed about the large range of possible values used
in the probabilistic analysis for certain parameters and the potential
for this to result in a mismatch of input parameters without proper
site-specific calibration. EPA notes that the revised risk assessment
is not intended to capture the exact risks at each disposal site.
Instead, the revised assessment combines the best resolution of site-
based, regional and national data available to provide an estimate of
potential risks that may occur from current disposal practices. While
the assigned data for any given model iteration may not reflect the
exact conditions at a real-world site, the resulting sum of all model
iterations reflect the range of potential conditions near each WMU,
weighted by prevalence, across the conterminous United States.
    Placement of Receptor Wells, EPACMTP Well Inputs and Assumptions:
    Comments regarding placement of receptor wells in the probabilistic
analysis (also known as the appropriateness of receiving water reaches)
are the result of a fundamental misinterpretation regarding the
constraints placed on groundwater receptor location to be, as described
in the 2010 Draft Risk Assessment, ``within the contaminant plume.''
This constraint is more fully explained in Section <IP_ADDRESS> of the
EPACMTP technical background document.\137\ A citation referring
readers to that document has been placed in Section 4 of the revised
risk assessment. Because the comment resulted from a misunderstanding,
EPA does not believe the sensitivity analysis suggested by the
commenter is necessary.
---------------------------------------------------------------------------

    \137\ U.S. EPA (Environmental Protection Agency). 2003a. EPA's
Composite Model for Leachate Migration with Transformation Products
(EPACMTP). Technical Background Document. EPA 53-R-03-002. Office of
Solid Waste, Washington, DC.
---------------------------------------------------------------------------

    Some commenters were concerned that many residents in the vicinity
of some WMUs may be serviced by a municipal water supply. Because these
residents would not be exposed to groundwater, the risk assessment
could overestimate exposures. EPA acknowledges that there may be a
large percentage of the population that does not rely on groundwater as
a source of potable water; however, the aim of the risk assessment is
to estimate the magnitude of potential risk to the exposed population.
Thus, this does not represent a significant source of uncertainty in
the risk assessment.
    Comments were also received that the one-mile distance considered
by the transport model is not sufficient, because actual receptor wells
in many cases are further distant than one mile from facilities. EPA
conducted a sensitivity analysis, discussed in Section 5 of the revised
risk assessment, which indicates that risks beyond the one-mile
distance are appreciably lower than risks within one mile. Given that
the highly exposed population was adequately captured by a one-mile
radius, the significant additional effort required to extend the
analysis further downgradient was unjustified.

    With respect to comments related to the placement of wells within
deeper aquifers, EPA has a policy of addressing uncertainty by erring
in favor of the protection of human health and environmental quality.
Consistent with this practice, wells screened within vulnerable,
surficial aquifers (i.e., the top 10 meters of the saturated zone)
continue to be the primary focus of the Agency's national-scale
modeling efforts. Comments also highlighted the possibility that
modeled receptor well concentrations may incorrectly represent actual
exposures by sampling from a single aquifer depth. Wells are typically
screened across an extended depth, and may capture both contaminated
and pristine groundwater. Due to the constraints of EPACMTP, EPA
maintained the current approach of modeling exposures at a single
depth. A discussion of the uncertainties associated with this approach
has been added to Section 5 of the revised risk assessment.
    In response to comments on the use of MSW landfill data to predict
the distance to private wells, EPA did not use the MSW data in the
revised risk assessment. Instead, EPA used synthetic population
representations of U.S. Census data to place each household and its
occupants at discrete points across the landscape surrounding CCR WMUs.
Synthetic populations are realistic representations of households and
individual residents and their attributes in a given census area, and
are based on methods that identify realistic locations within each
block by using LandScan 90-meter night-time population distributions to
place each household across the landscape.\138\ From these households,
a distribution of the distances to the nearest well was created. This
approach is discussed in more detail in appendix B of the revised risk
assessment. Some commenters suggested that EPA develop site-specific
estimates of actual populations around facilities rather than relying
on synthetic populations to determine potential receptor locations. The
synthetic approach provides the maximum spatial resolution possible for
publically available population data from the U.S. Census. More site-
specific estimates would be costly, but not necessarily more accurate.
---------------------------------------------------------------------------

    \138\ Bhaduri, B., E. Bright, P. Coleman, and M. Urban. 2007.
LandScan USA: A high resolution geospatial and temporal modeling
approach for population distribution and dynamics. GeoJournal
69:103-117.
---------------------------------------------------------------------------

    Some commenters were also concerned that the assessment did not
consider direct discharges from surface impoundments to surface water.
This pathway was outside the scope of the assessment, because it is
regulated by the NPDES program. However, this pathway was evaluated in
Environmental Assessment for the Proposed Effluent Limitation
Guidelines and Standards for the Steam Electric Power Generating Point
Source Category,\139\ which will be revised in support of final
effluent limitation guidelines due to be released in September of 2015.
---------------------------------------------------------------------------

    \139\ U.S. EPA. 2013. Environmental Assessment for the Proposed
Effluent Limitation Guidelines and Standards for the Steam Electric
Power Generating Point Source Category. EPA-821-R-13-003. Office of
Water. Washington, DC. 20460. April.
---------------------------------------------------------------------------

2. Comments Related to Source Modeling
    COMMENT: The majority of the public commentary in this subcategory
was dominated by the assertion that Toxicity Characteristic Leaching
Procedure (TCLP), Synthetic Precipitation Leaching Procedure (SPLP) and
other laboratory leachate test data are not applicable to CCR wastes.
Comments specifically regarding the use of Leaching Environmental
Assessment Framework (LEAF) data for modeling leaching behavior noted
that the data should be applied appropriately and pointed out the
following: (1) That the range of conditions (i.e., range of pH)
encompassed by the LEAF data is broader than those conditions found in
the field for CCR disposal; (2) high pH limits the mobility of leaching
constituents; (3) the need for validating LEAF leachate concentrations
against field data if available; and (4) the reliability of the LEAF
data is questionable as a result of inconsistencies identified in the
LeachXS LiteTM database.
    EPA RESPONSE: Only pore water and impoundment water data were used
to characterize surface impoundments. Therefore, the comments received
on the use of laboratory leachate data are not relevant for the surface
impoundment scenario. For landfills, EPA agrees that TCLP, SPLP and
other single pH test methods may not be the most appropriate leachate
extraction methods for all waste streams and all disposal scenarios.
The 2010 Draft Risk Assessment relied on a hierarchy of dissolved
concentration data to characterize leaching from landfills, ranging in
order of preference from field leachate data to TCLP. However, new data
collected using the LEAF test methods have been made available through
a series of EPA reports.140 141 142 LEAF were collected with
three LEAF methods, specifically:
---------------------------------------------------------------------------

    \140\ U.S. EPA. 2006. Characterization of Mercury-Enriched Coal
Combustion Residues from Electric Utilities Using Enhanced Sorbents
for Mercury Control. EPA-600/R-06/008. Prepared by F. Sanchez, R.
Keeney, D. Kosson, and R. DeLapp for the U.S. Environmental
Protection Agency, Air Pollution Prevention and Control Division.
February.
    \141\ U.S. EPA. 2008. Characterization of Coal Combustion
Residues from Electric Utilities Using Wet Scrubbers for Multi-
Pollutant Control. EPA/600/R-08/077. Prepared by F. Sanchez, D.
Kosson, R. Keeney, R. DeLapp, L. Turner, and P. Kariher for the U.S.
Environmental Protection Agency, Air Pollution Prevention and
Control Division. July.
    \142\ U.S. EPA. 2009. Characterization of Coal Combustion
Residues from Electric Utilities--Leaching and Characterization
Data. EPA-600/R-09/151. Office of Research and Development, National
Risk Management Research Laboratory, Research Triangle Park, NC.
December.
---------------------------------------------------------------------------

    [ssquf] SW-846 Method 1313 (and its predecessor, Method SR02);
    [ssquf] SW-846 Method 1314; and
    [ssquf] SW-846 Method 1316 (and its predecessor, Method SR03).\143\
---------------------------------------------------------------------------

    \143\ Methods SR02 and SR03 are predecessor methods to SW-846
Methods 1313 and 1316.
---------------------------------------------------------------------------

    With the availability of the LEAF data, EPA no longer relied on
other data sources to model landfills because the inability to identify
trends in leaching behavior from single pH tests made it impossible to
link these data together with the LEAF data in the probabilistic
analysis. The LEAF data provide information on the leaching behavior of
CCR for a range of pH values observed in CCR landfills, as well as the
liquid-to-solid ratio of the pore water. The data from these three
methods were used in conjunction to characterize landfill leaching.
While the natural pH range for any individual sample may be narrower
than the full range analyzed with the LEAF methods, many facilities
burn a range of coal types under varying operating conditions, and co-
dispose with other materials, so the range of pH for a specific CCR
sample may be exposed to is wider than the pH estimated based on one
sample alone.
    EPA agrees that appropriate use of the data is needed to ensure
that data represent likely conditions of leaching occurring at range of
facilities nationwide taking into account local specific environmental
conditions, the geometry of monofill, type of coal, air pollution
control, and other factors that affect leaching. Since the NODAs were
released, a report comparing leachate from field and laboratory
analyses has been completed.\144\ The report includes the use of
geochemical speciation modeling as needed to reflect site-

specific factors affecting leaching, and shows that LEAF methods
provide realistic predictions of environmental releases across the
range of pH.
---------------------------------------------------------------------------

    \144\ U.S. EPA. 2014. Leaching Test Relationships, Laboratory-
to-Field Comparisons and Recommendations for Leaching Evaluation
using the Leaching Environmental Assessment Framework (LEAF). EPA-
600/R-14/061. EPA Office of Research and Development, National Risk
Management Research Laboratory, Research Triangle Park, NC 27711.
November.
---------------------------------------------------------------------------

    All three LEAF methods are summarized in appendix C, with the
leachate data provided in Attachment C-5 of the revised risk
assessment. Additionally, the inter-laboratory validation for these
methods are described in U.S. EPA (2012a, b) 145 146 while
Kosson et al. (2002) \147\ provides the detailed test methodology for
the predecessor methods, SR02 and SR03. The noted discrepancies and
classification errors within LeachXS Lite have been corrected.
---------------------------------------------------------------------------

    \145\ U.S. EPA. 2012. Interlaboratory Validation of the Leaching
Environmental Assessment Framework (LEAF) Method 1314 and Method
1315. EPA/600/R-12/624. Prepared by A.C. Garrabrants, D.S. Kosson,
R. DeLapp, P. Kariher, P.F.A.B. Seignette, H.A. van der Sloot, L.
Stefanski, and M. Baldwin for the U.S. EPA Office of Research and
Development, Air Pollution Control Division. September.
    \146\ U.S. EPA. 2012b. Interlaboratory Validation of the
Leaching Environmental Assessment Framework (LEAF) Method 1313 and
Method 1316. EPA/600/R-12/623. Prepared by A.C. Garrabrants, D.S.
Kosson, L. Sefanski, R. DeLapp, P.F.A.B. Seignette, H.A. van der
Sloot, P. Kariher, and M. Baldwin for the U.S. EPA Office of
Research and Development, Air Pollution Control Division. September.
    \147\ Kosson, D.S., H.A. van der Sloot, F. Sanchez and A.C.
Garrabrants. 2002. An integrated framework for evaluating leaching
in waste management and utilization of secondary materials.
Environmental Engineering Science 19(3):159-204.
---------------------------------------------------------------------------

    COMMENT: Public comments focused on the general relevance of the
facility data based on age and noted that newer data should be used to
more accurately reflect the current state of CCR management. Related
comments cited that the grouping of waste and liner types by facility
is not representative of current conditions. Another commenter
suggested that the outcomes for different liner types were not
comparable and should not be used to make relative conclusions about
liner performance. It was also suggested that the assumed three-foot
clay layer underlying composite liners is too thick, and two feet would
be more representative of current practice. Commenters also described
existing management controls required in some geographical locations
that mitigate potential risks (e.g., liners, leachate collection) and
requested that EPA reflect the existence of those controls in their
analysis, as well as mismanagement scenarios when these controls are
not in place.
    EPA RESPONSE: Since the purpose of the risk assessment was to
evaluate risks for the universe of currently operating facilities and
WMUs, EPA generally agrees with the commenter that the 1995 EPRI and
2006 DOE survey data relied on in the 2010 Draft Risk Assessment may be
outdated. Thus, EPA collected data from several new sources of
information on the facilities, WMUs, and liners that are present at the
time of this analysis. Further discussion of these data sources is
available in Section 2 and appendix A of the revised risk assessment.
    Regarding the inclusion of mismanagement scenarios, EPA reviewed
the high-end pore water concentrations and determined that these data
represent actual CCR samples and therefore represent possible high-end
risks from current management practices. To better understand which
practices may lead to the highest risks, EPA conducted sensitivity
analyses that consider the influence of liner type, liner design, waste
type and other variables on model results. The results of these
analyses are presented in Section 5 of the revised risk assessment.
    Several commenters described existing management controls required
in some geographical locations that mitigate potential risks (e.g.,
liners, leachate collection) and requested that EPA reflect the
existence of those controls in the final risk analysis. The Agency's
analysis reflects the presence of different management scenarios at
WMUs to the extent the available data allowed (e.g., WMUs were assumed
to have liners if the information indicated such). A key objective of
the analysis was to compare the effectiveness of management options
(e.g., liners; surface impoundments versus landfills) at preventing
potential releases and exposures. Because the population of WMUs
considered in the analysis included a range of management controls, the
analysis does provide such comparative results between management
options. The uncertainties associated with the updated facility, WMU
and liner data are discussed in Section 5 of the revised risk
assessment.
    COMMENT: One commenter suggested that the risk assessment applied
risk results for fly ash to bottom ash, FGD sludge, and other CCR
wastes, which may result in an incorrect estimate of risks for these
other wastes. Other commenters called for EPA to evaluate each CCR
waste independently. A public commenter expressed concern about whether
the risk assessment adequately considered alternative CCR disposal
scenarios. Specifically, it was noted that CCR codisposed with coal
refuse generate more acidic conditions (i.e., lower pH) due to higher-
levels of sulfide minerals, which may significantly impact the mobility
of metals.
    EPA RESPONSE: In the revised risk assessment, EPA modeled a
combined ash waste types for the majority of surface impoundments and
all landfills. Although commenters are correct that different CCR
wastes may behave differently when monofilled, the 2009/2010 EPA survey
data indicates that the CCR are codiposed in a majority of units. Thus,
EPA believes this approach appropriately reflects current disposal
practices.
    With regard to the evaluation of CCR codisposed with coal refuse,
EPA notes that the pore water data used to characterize surface
impoundments were broken out separately for this waste type evaluation.
These data reflect samples collected in the field and are
representative of the pH at which these samples are managed. While some
ash and coal refuse samples are highly acidic, others are more neutral
or slightly basic (full pH range of 1.7 to 8.2). The development and
application of these waste types is discussed in Section 3, Section 4
and appendix H of the revised risk assessment, while the associated
uncertainties are discussed in Section 5. For landfills, waste pH,
which is the major driver of variations in Kd values used to
distinguish waste types, was known with great accuracy for CCR
nationwide because U.S. EPA (2009a) \148\ compiled a full, nationwide
distribution of CCR pH. In this distribution, disposal of ash with coal
refuse is reflected is the acidic tail of the distribution. For the
national probabilistic analysis, EPA aggregated model runs for ash and
coal refuse (surface impoundments) and acidic waste (landfills) with
other wastes so that risks reflected the prevalence of these disposal
practices. However, EPA also performed sensitivity analyses to
understand the extent that the lower pH of co-managed wastes could
affect risks, which is discussed in Section 5 of the revised risk
assessment.
---------------------------------------------------------------------------

    \148\ U.S. EPA. 2009. Characterization of Coal Combustion
Residues from Electric Utilities--Leaching and Characterization
Data. EPA-600/R-09/151. Office of Research and Development, National
Risk Management Research Laboratory, Research Triangle Park, NC.
December.
---------------------------------------------------------------------------

    COMMENT: Commenters stated that it is unclear why EPA chose to
approximate infiltration through composite liner systems based on leak
detection system flow rates from industrial landfills that use a
different construction design than projected for CCR landfills.
    EPA RESPONSE: The composite liner leakage rates used for this risk
assessment correspond to leakage rates developed for the peer-reviewed
Industrial Waste Management

Evaluation Model (IWEM).\149\ The types of synthetic liners used are
likely to be the same, regardless of the type of waste present. EPA is
unaware of any factors specific to CCR that would exacerbate leakage
rates, nor did the commenter provide any. Thus, in the absence of any
information to the contrary, EPA finds these to be the best available
data.
---------------------------------------------------------------------------

    \149\ U.S. EPA. 2002. Industrial Waste Management Evaluation
Model (IWEM) Technical Background Document. EPA530-R-02-012. Office
of Solid Waste, Washington, DC. August.
---------------------------------------------------------------------------

    Because there is currently no approach for differentiating between
flow from unimpacted water released by the consolidation of clay and
from contaminated leakage through the liner, EPA excluded data on the
subset of composite liners constructed with natural clay from the
distribution of composite liner leakage rates. EPA did consider the
potential impact of incorporating these additional data into the risk
assessment as part of sensitivity analysis, presented in Section 5 of
the revised risk assessment.
    COMMENT: Concerning the treatment of non-detect values in the risk
assessment, one commenter recognized that the use of one half the
detection limit in calculations has become an accepted protocol.
However, it was suggested that this approach may not be appropriate in
all cases, and that newer or more straightforward methods can be
applied to improve precision and minimize biasing of the dataset.
Another commenter noted that mercury was excluded from the analysis due
to the high number of non-detects.
    EPA RESPONSE: Additional constituent data measured with lower
detection limits have been made available to EPA since completion of
the 2010 Draft Risk Assessment. However, the overall CCR constituent
database still contains a large number of non-detect data for some
constituents. EPA continues to incorporate all available with the use
half the reported detection limit as the most appropriate method to
account for these non-detects. The commenter is correct that much of
the pre-2010 mercury data has high detection limits and a large
proportion of non-detects. In this one instance, EPA relied only on the
newer data made available to the Agency since the 2010 Risk Assessment,
which was collected through newer methods with significantly lower
detection limits. A more detailed rationale for this approach is
provided in Section 3 of the revised risk assessment, along with
further discussion of the uncertainty in Section 5.
    COMMENT: Comments received related to the effect of waste
compaction in landfills focused on changes to hydrologic properties of
waste materials, such as porosity and hydraulic conductivity. These
changes may result from compaction, consolidation, hydration or
geochemical changes, and have the potential to result in either an
underestimation or overestimation of risks.
    EPA RESPONSE: EPA acknowledges that the landfill source model does
not consider the compaction of CCR waste that may occur over time as a
result of anthropogenic activities, gravity or infiltrating water.
However, no data on either the rate or degree to which these processes
may occur were provided by commenters or identified elsewhere. EPA
considered the impacts of this uncertainty in Section 5 of the revised
risk assessment.
    COMMENT: Public comments focused on assumptions relating to the
variability of unlined landfill design, landfill clay liner materials,
and construction of landfill cover materials and construction. Specific
comments emphasized that the clay liner and cover thickness assumptions
(three feet) were too conservative and not conservative enough,
respectively. Commenters also questioned why composite covers and
leachate collection systems were not considered for clay-lined
landfills. Additionally, commenters stated that there was a high degree
of variability in the material and design and construction for unlined
landfills that was not accounted for in the HELP modeling. One
commenter also pointed out that the assessment may overestimate
percolation rates from landfills by underestimating the use of
engineering controls. In addition, a commenter stated that the
assessment assumes that States will require liners in all cases which
may not be the case, thereby weakening the regulation.
    EPA RESPONSE: For both unlined and clay-lined landfills, EPA used
Hydrologic Evaluation of Landfill Performance (HELP) model-derived
infiltration rates. These infiltration rates assume that the cap placed
on top of the landfill at the end of its useful life will remain intact
for the duration of the risk assessment, up to a maximum 10,000 years
of modeling. A commenter pointed out that hydraulic conductivity of a
clay liner is likely to increase by orders of magnitude due to
desiccation resulting from natural temperature cycles. Additionally,
commenters stated that there was a high degree of variability in the
material and design and construction for unlined landfills that was not
accounted for in the HELP modeling. EPA has adopted the use of the HELP
model, which was subject to both peer and administrative review, as the
source of unlined and clay-lined infiltration rates for landfill for
nearly two decades. EPA acknowledges that there are limitations in
using HELP. However, the model has been tested and verified as
discussed in the EPACMTP Parameter/Data Background Document.\150\ To
the extent that the performance of the cap will decrease over time, EPA
acknowledges that unlined and clay-lined infiltration rates calculated
by HELP may be underestimated, however the degree of that underestimate
is unknown. Discussion of this uncertainty has been added to Section 5
of the revised risk assessment.
---------------------------------------------------------------------------

    \150\ U.S. EPA. 2003. EPA's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Parameters/Data
Background Document. EPA 530-R-03-003. Office of Solid Waste,
Washington, DC. April.
---------------------------------------------------------------------------

    COMMENT: One commenter expressed concern over the fact that the
assessment modeled all disposal sites above the water table. The
commenter indicated that many surface impoundments and landfills are
deep and can come in direct contact with the water table. This will
result in an underestimation of peak concentrations, arrival times and
risks for these WMUs. Furthermore, the commenter emphasized that the
use of the unsaturated zone flow module to calculate infiltration from
the bottom of impoundments underestimates true risks in the
consolidated sediment, and noted that clogged soil layers should be
treated as saturated rather than unsaturated.
    EPA RESPONSE: EPA acknowledges that EPACMTP is not designed to
handle scenarios where the water table is above the bottom of the
landfill. However, EPACMTP can accommodate surface impoundments in
direct contact with the water table. If unit geometry and the selected
depth to the water table create a scenario where the bottom of the unit
is in contact with the water table, then the entire soil column is
considered saturated. Otherwise, even for very high infiltration rates,
regions beneath impoundments will remain partially saturated when there
is sufficient distance between the unit and the water table. EPA has
added a discussion of the uncertainties associated with WMU source
terms and EPACMTP in Section 5 of the revised risk assessment.
    EPA believes the commenter misunderstood how the sediments were
modeled for surface impoundments. The EPACMTP unsaturated zone

module assumes that the 0.2 m of consolidated sediments at the bottom
of a surface impoundment are always saturated whereas the 0.5 m of
clogged native soil are assumed to be unsaturated when the bottom of
the surface impoundment is above the water table.
    COMMENT: Public commenters recommended that EPA address the future
increase in mercury and NOX compounds levels in CCR that
will result from mercury capture from flue gas under new emission
control regulations. Commentary pointed out that the recent Vanderbilt
study should provide data that could be used to expand the risk
assessment in this area.
    EPA RESPONSE: The risk assessment was designed to evaluate the
risks associated with current management practices and, as such, draws
no conclusions about the potential for future air pollution
technologies to alter the composition or leaching behavior of CCR
wastes. However, it has been shown that newer mercury pollution control
technologies currently in place have the potential to affect leaching
behavior.151 152 153 Thus, EPA conducted a sensitivity
analysis to evaluate the risks associated with existing units that
dispose of this waste; however, the data were too few to allow EPA to
draw conclusions about the effect of pollution control technologies on
the risks. This sensitivity analysis is presented in Section 5 of the
revised risk assessment.
---------------------------------------------------------------------------

    \151\ U.S. EPA. 2006. Characterization of Mercury-Enriched Coal
Combustion Residues from Electric Utilities Using Enhanced Sorbents
for Mercury Control. EPA-600/R-06/008. Prepared by F. Sanchez, R.
Keeney, D. Kosson, and R. DeLapp for the U.S. Environmental
Protection Agency, Air Pollution Prevention and Control Division.
February.
    \152\ U.S. EPA. 2008. Characterization of Coal Combustion
Residues from Electric Utilities Using Wet Scrubbers for Multi-
Pollutant Control. EPA/600/R-08/077. Prepared by F. Sanchez, D.
Kosson, R. Keeney, R. DeLapp, L. Turner, and P. Kariher for the U.S.
Environmental Protection Agency, Air Pollution Prevention and
Control Division. July.
    \153\ U.S. EPA. 2009. Characterization of Coal Combustion
Residues from Electric Utilities--Leaching and Characterization
Data. EPA-600/R-09/151. Office of Research and Development, National
Risk Management Research Laboratory, Research Triangle Park, NC.
December.
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    COMMENT: Multiple public commenters noted that additional pore
water will improve the risk assessment, but TCLP and SPLP data are not
appropriate for use as source concentrations. Additionally, commenters
stated that EPA applies the LEAF data to pH conditions that are not
realistic to CCR disposal scenarios. Although LEAF provides a more
representative and scientifically sound approach, it must be correctly
adapted. Alternative statistical methods to represent the input data as
a range is certainly feasible and could enhance the risk assessment if
the range of data is used as an input to the risk assessment.
    Commenters agree that the LEAF data does provide useful
information, but point out that it is associated with the potential for
leaching and does not represent actual leaching of a specific CCR under
actual field conditions. Commenters argues that field leaching data
should not be mixed with laboratory data, and that EPA's field leachate
dataset (for landfills and impoundments) is not adequate for use in the
CCR risk assessment. Specific efforts recommended to properly utilize
the LEAF data include: Use of probability density functions for
leachate concentrations based on pH and/or L/S ratios in the Monte
Carlo process; selection of leachate concentrations based on pH and L/S
and tied to the geographic location of the WMU and CCR type; and
geochemical modeling to incorporate reactions once leachate impacts
groundwater.
    A few commenters pointed out that the pore water data are generally
representative, although concerns were raised about the highest arsenic
concentration (81 mg/L) in the dataset. One commenter believed that
although the addition of new data is an improvement, EPA could greatly
improve the accuracy of the model's results by removing the extreme and
unsubstantiated outlier data driving its high risk cases. Another
commenter believed the assumption that concentration of contaminants in
the sediment pores (applicable to a post closure scenario) would be
equal to the concentration assigned to in the impoundment water would
result in underestimated risks. Additionally, commenters noted that EPA
should classify the data according to CCR type and coal type.
    Overall, commenters support updates to the pore water data and the
use of statistical method to normalize the data curve. However, one
commenter noted that EPA should not use commenter-submitted CCR pore
water data unless it meets requisite applicable data quality
requirements. Another commenter stated that EPA needs to provide better
clarity on these solicited comments (on the use of older pore water
data) and provide these documents in the docket. Without these
documents, the reader does not have a complete understanding of co-
managed material containing CCR. Another comment noted that properly
collected field pore water (freely draining) samples should take
priority over any of the laboratory generated data and freely draining
pore water is more representative of leachate releases than tightly
held pore water.
    EPA RESPONSE: The use of pore water data is still considered the
most appropriate approach to estimate constituent fluxes to groundwater
for CCR surface impoundments. This is because pore water better
represents the leachate seeping from the bottom of the impoundment than
impoundment water samples. EPA did not use available LEAF data for
surface impoundments because a national distribution of pH was not
available to allow the Agency to probabilistically assign LEAF
concentrations to these units, and because there was no way to account
for partitioning of the leachate into wastewater versus porewater.
Thus, EPA has continued to rely on pore water data, supplemented with
data from the 2010 comments. EPA appreciates commenter support on the
use of pore water data and statistical methods for data analysis for
surface impoundments. EPA agrees that data available for minefill sites
may not be representative of disposal in surface impoundments. Thus,
these data were not considered in the revised risk assessment. The
specific handling of pore water concentration data with site quartiles,
rather than site averages, is discussed in Section 4 and Section 5 of
the revised risk assessment report.
    EPA agrees that TCLP and SPLP data are less appropriate for CCR
disposal scenarios and no longer uses these data in the revised risk
assessment. EPA adapted the LEAF methods and data for landfills, as
this is the best available approach and data to represent CCR landfill
leachates, and does not mix or use field data with LEAF laboratory
results for landfill leachate. The LEAF data are considered the most
robust and technically defensible data available. As noted in the 2010
Environmental Science and Technology publication,\154\ the data
represents the largest collection of comprehensive characteristic
leaching data to date.
---------------------------------------------------------------------------

    \154\ Thorneloe, S., D. Kosson, F. Sanchez, A. Garrabrants, and
G. Helms. 2010. Evaluating the Fate of Metals in Air Pollution
Control Residues from Coal-Fired Power Plants. Environ. Sci.
Technol. 44:7351-7356.
---------------------------------------------------------------------------

    A commenter noted that the LEAF data provide the potential for
leaching and not actual leaching of a specific CCR under actual field
conditions. The commenter suggests using probability distribution of
key factors affecting leaching behavior [i.e., pH and liquid/solid
ratio (L/S)] and site specific data tied to the geographic location of
the management unit and the type of CCR being managed. In the revised
risk assessment, pH is expressed as a

national distribution for selecting leachate concentrations developed
to represent CCR nationally, and L/S is considered in estimating
washout leachate concentrations based on field data observations. The
use of the pH distribution developed in U.S. EPA (2009) \155\ does
capture the range of potential variability in pH conditions at CCR
sites nationwide and is the best approach possible given the current
availability of information on site-specific coal ash chemistry.
Although leachate concentrations were selected considering pH and L/S
conditions that are nationally representative, EPA does not have the
detailed and extensive site-specific measurements that would be needed
to tie CCR and leachate concentrations to specific WMU locations.
Instead, EPA adopted a national probabilistic approach that is site-
based and representative of risks to human and ecological health across
the country. The revised risk assessment also provides details
regarding how the LEAF data are used in combination of geographical
specific data such as hydrology, precipitation, fill configuration, CCR
type, pH, L/S ratio, and other factors that take the leaching potential
as an input to fate and transport models accounting for attenuation and
dilution. Additionally, an effort was made to collect CCR samples that
characterize the range and quantity of coal usage in the U.S. along
with likely air pollution control configurations. While the data is not
statistically representative on a site-specific basis, it is adequate
to identify trends in leaching behavior that relate to differences in
materials types, APC technology, and coal rank. Geochemical speciation
modeling was not conducted because the source term as measured and
interpreted is conservative, provided that oxidizing conditions occur.
---------------------------------------------------------------------------

    \155\ U.S. EPA. 2009. Characterization of Coal Combustion
Residues from Electric Utilities--Leaching and Characterization
Data. EPA-600/R-09/151. Office of Research and Development, National
Risk Management Research Laboratory, Research Triangle Park, NC.
December.
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    Regarding the number and concentration of pore water samples, EPA
reviewed the high-end pore water concentrations and determined that
these represent actual CCR samples that therefore represent possible
high-end risks if CCR is inadequately regulated and managed. EPA
recognizes that more pore water data would potentially improve the
representativeness of the dataset, but is convinced that the current
dataset adequately captures the possible high end risks that are of
most interest in the rulemaking, including risks from the mismanagement
of CCR through codisposal with coal refuse.
    The assumption that saturated contaminant concentrations in surface
impoundment sediments are at equilibrium with the impoundment waters is
a conservative assumption that is unlikely to significantly
underestimate risks. This assumption is further discussed in Section 5
of the revised risk assessment report.
    Regarding commenter-submitted pore water data, EPA conducted a
review of the additional datasets provided by the commenters with
respect to relevance and data quality. Based on the available
information, EPA determined that the selected datasets were relevant
and acceptable in terms of data quality requirements. However, EPA does
not have sufficient data to distinguish between freely draining and
tightly bound pore water data at this time. Overall, EPA agrees that
the use of these data introduces some uncertainty into the analysis,
which is discussed in Section 5 of the revised risk assessment.
    COMMENT: One commenter questioned the assumption that there will be
no net addition of waste into a surface impoundment during and after
the operational life, noting that impoundments are frequently deepened.
Additionally, many surface impoundment wastes are left in place at the
time of closure, so that the waste behaves more as a landfill than a
surface impoundment (and increasingly, with new landfills being
constructed on top of previous surface impoundments). Another commenter
questioned why the conceptual model assumes that impoundments are
always full during their operating life, which overestimates releases
to the subsurface. Additionally, a commenter noted that the assumption
of only 0.2 m of sediment accumulation underestimates the amount of
sedimentation and subsequently overestimates the amount of percolation
to the subsurface. The commenter stated that in actual operation, ash
thickness can increase up to 30 feet or more, eventually filling the
impoundment, which results in a significant decrease in percolation
through the base. Furthermore, the commenter questioned the assumption
that post-closure percolation continues at the same rate as during
active operations.
    EPA RESPONSE: Based on the 2009/2010 EPA surveys, it was assumed
that the majority of the surface impoundments are storage impoundments,
which are continuously dredged. Because these facilities have other
units (whether onsite or offsite) established for disposition, it
likely that the majority of waste in the dredged impoundments would be
removed by the end of the unit's operating life. Regardless, an
uncertainty analysis provided in appendix K demonstrates that the risks
during the operating life of surface impoundments are greater because
the higher hydraulic head drives leachate into underlying soils with
greater force than gravity alone post-closure. Therefore, EPA did not
explicitly model the post-closure phase of surface impoundments. The
uncertainties resulting from this decision are discussed in Section 5
of the revised risk assessment.
    EPA acknowledges that EPACMTP is restricted to modeling flow as
steady state with the assumption that an impoundment always has a fixed
depth of wastewater. EPA further acknowledges that such an assumption
may overestimate infiltration. The surface impoundment conceptual model
assumes that sediments are periodically dredged and removed and that
the long-term average thickness of the sediment is approximately 0.4 m,
with half of that layer consolidated. EPA has used EPACMTP and its
predecessor model versions for a longstanding time period and it has
undergone multiple rounds of internal and external review. The reviews
associated with EPACMTP and its limitations are further discussed in
Section 5 of the revised risk assessment report.
    COMMENT: Public commenters suggested that risks from operating
landfills should be considered along with those that occur post-
closure. These commenters questioned whether greater risks may occur
during site operations when wastes are uncovered and exposed directly
to precipitation. Additional commenters noted that complete leaching of
all constituent mass at a constant concentration is overly
conservative.
    EPA RESPONSE: The landfill source model used in this risk
assessment is not able to address landfills during operation because
the non-linear sorption isotherms used require a constant, annualized
infiltration rate throughout the duration of leaching. Instead, the
revised risk assessment assumed that the full footprint of the landfill
is filled to capacity with a cap no less permeable than the soil or
liner underlying the WMU is present at the start of leaching. EPA
acknowledges that this approach introduces some uncertainty into the
analysis, the potential impacts of which are discussed in Section 5 of
the revised risk assessment.
    With respect to comments that complete leaching of all constituent
mass is overly conservative, EPA now

models landfills using leachable mass as discussed in Section 4 and
appendix C of the revised risk assessment. Alternatively, EPA presents
a sensitivity analysis of these results compared with the results
generated using total mass in Section 5.
3. Comments Related to Exposure Scenarios
    COMMENT: The commenter emphasized that the risk assessment does not
consider direct discharges to ground and surface water systems other
than groundwater infiltration (e.g., direct injection to groundwater,
point and nonpoint discharges to surface water systems). It was
recommended that EPA consider combining contributions from these
sources with CCR groundwater leaching impacts to calculate the full
load of CCR constituents to groundwater and surface water systems. The
commenter continues by suggesting that the use of liners in
impoundments does not reduce overall hazards if direct discharges are
considered in the risk assessment.
    EPA RESPONSE: RCRA waste disposal risk assessments do not address
direct discharges from impoundments to surface waters because they are
regulated as permitted point source discharges under the Clean Water
Act by EPA's Office of Water. Since this pathway is outside the scope
of the risk assessment, the revised risk assessment does not consider
these releases. However, this pathway was evaluated in the
Environmental Assessment for the Proposed Effluent Limitation
Guidelines and Standards for the Steam Electric Power Generating Point
Source Category,\156\ which will be revised in support of final
effluent limitation guidelines (ELG) due to be released in September of
2015. The revised risk assessment was updated to note this fact.
---------------------------------------------------------------------------

    \156\ U.S. EPA. 2013. Environmental Assessment for the Proposed
Effluent Limitation Guidelines and Standards for the Steam Electric
Power Generating Point Source Category. EPA-821-R-13-003. Office of
Water. Washington, DC 20460. April.
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    EPA is not aware of any CCR disposal where waste is directly
injected into groundwater aquifers, and absent any data on this
practice declines to evaluate it.
    COMMENT: Public comments were received on the methodology applied
to evaluate exposure to fugitive dust during landfill operations
(before closure). The majority of these comments focused on the
fugitive analysis as presented in Inhalation of Fugitive Dust: A
Screening Assessment of the Risks Posed by Coal Combustion Waste
Landfills,\157\ and EPA's proposed approach for refining the analysis.
Comments received on the initial fugitive dust analysis methodology and
modeling ranged from emphasizing that the approach was overly
conservative in some cases to underestimating risk in other cases.
---------------------------------------------------------------------------

    \157\ U.S. EPA. 2010. Inhalation of Fugitive Dust: A Screening
Assessment of the Risks Posed by Coal Combustion Waste Landfills.
OSWER. Washington, DC. September.
---------------------------------------------------------------------------

    Multiple comments were provided on the proposed methodology for
refining the fugitive dust analysis that was applied in the revised
risk assessment. One commenter recommended that 2010/2011 EPA survey
data should be used to refine the fugitive dust analysis for landfills.
Specifically, the current OW data indicate that active portions of the
landfills are significantly smaller than the landfills identified in
the 1995 EPRI survey. Several comments were received that pointed out
that the application of AERSCREEN and AERMOD is appropriate if
representative or realistic inputs are used including meteorological
data, material silt content, source areas for subcells of ash
management units and consideration of common operating and control
practices, which are in some cases defined by the states (e.g.,
Virginia). However, one commenter expressed concern that no previous or
current EPA regulatory model; including SCREEN3, AERSCREEN or AERMOD;
has been rigorously tested and evaluated for performance in modeling
fugitive emissions associated with CCR landfills.
    In general, the commenters supported or recommended the use of
appropriate AP-42 factors and other techniques to estimate emissions.
Others noted that consideration of deposition impacts and constituent-
specific modeling is appropriate. One commenter recommended that EPA
should conduct a full-scale assessment that considers fugitive dust as
well as emissions from landfills and emissions of diesel particulate
matter from haul trucks, on-site heavy-duty landfill equipment, and
diesel-powered pumps and generators, with potential receptors of
interest as residents and sensitive subpopulations living near the
power plant, along the transportation route and at the landfill.
Another commenter expressed concern over the lack of metal speciation
data, while another comment concerned gas emissions from the landfills
(e.g., hydrogen sulfide). One final commenter voiced concern that
insufficient information was provided on the modeling approach and the
model inputs to support evaluation and allow comments on the overall
validity or propriety of the suggested modeling.
    EPA RESPONSE: The majority of the comments received concerning
exposures during landfill operation (before closure) focused on the
assessment of fugitive dust. EPA acknowledges that the 2010 Draft Risk
Assessment did not evaluate the inhalation pathway, relying instead on
the findings of a previous evaluation, Inhalation of Fugitive Dust: A
Screening Assessment of the Risks Posed by Coal Combustion Waste
Landfills.\158\ This previous evaluation only considered releases from
windblown emissions and the potential to exceed national ambient air
quality standards (NAAQS) for particulate matter.
---------------------------------------------------------------------------

    \158\ U.S. EPA. 2010. Inhalation of Fugitive Dust: A Screening
Assessment of the Risks Posed by Coal Combustion Waste Landfills.
OSWER. Washington, DC. September.
---------------------------------------------------------------------------

    Based on the comments received, EPA updated the screening analysis
of fugitive dust. EPA agrees that there are potential risks posed by
fugitive emissions from sources beyond wind and revised the analysis to
consider emissions from a range of activities, such as vehicular
activity, unloading operations and spreading/compacting operations.
Emissions from these sources were calculated using techniques that have
undergone extensive peer-review, including AP-42: Compilation of Air
Pollutant Emission Factors.\159\ Screening level modeling was performed
with a combination of AERSCREEN and AERMOD to estimate dust dispersion
and deposition rates. Model inputs were selected to be representative
of current landfills, environmental settings (e.g., meteorological
conditions) and common dust management practices. Estimated air
concentrations were used to screen acute and chronic health risks from
inhalation, as well as the potential to exceed NAAQS standards.
Furthermore, EPA considered exposures that may result from the offsite
deposition on and accumulation in downgradient media. This was done for
all relevant metal species. In contrast, EPA did not evaluate emissions
of hydrogen sulfide to air as EPA has no data on the extent to which
this constituent is present in CCR or released into the surrounding
environment. Further discussion of this screening analysis is presented
in

Section 3 and appendix F of the revised risk assessment.
---------------------------------------------------------------------------

    \159\ U.S. EPA. 1985. Compilation of Air Pollutant Emission
Factors. Volume I: Stationary Point and Area Sources (Fourth
Edition). AP-42. U.S. Environmental Protection Agency, Office of Air
and Radiation and Office of Air Quality Planning and Standards,
Research Triangle Park, NC. September.
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    COMMENT: Comments both supported and disagreed with the
appropriateness of a screening analysis to eliminate pathways from
consideration in the full-scale probabilistic analysis. One commenter
pointed out that the EPA conducted a very conservative, but
appropriate, screen to identify constituents to include in the full-
scale probabilistic analysis. Another commenter emphasized that a full-
scale risk assessment should be conducted that assesses exposures
concurrently through all pathways (e.g., including surface pathways
with inhalation exposure) for all chemical constituents. In particular,
they emphasized that inhalation exposures to human carcinogens, such as
hexavalent chromium, as well as noncarcinogens may occur through the
aboveground pathway. Although the commenters disagreed over the use of
a screening approach, both expressed concerns over the use of risk
attenuation factors to scale screening risks to the full-scale risks
for the subset of constituents that did not pass the screen and were
not evaluated under the full scale assessment. Both commenters believe
that this approach ignores the unique fate and transport properties of
the omitted constituents and that the use of a simplistic, attenuation
factor is not an appropriate way to estimate risk.
    EPA RESPONSE: By first conducting the screening analysis presented
in Section 3 of the revised risk assessment, EPA was able to focus
available resources on the characterization of risks for exposure
routes and constituents with the greatest potential to pose risks. The
screening analysis conducted for the revised risk assessment considered
all of the potential exposure routes identified in the conceptual
models for surface impoundments and landfills, which included
aboveground exposures to ambient air, soil, sediment, produce, and
animal products. Each exposure pathway was evaluated for all
constituents (and individual species, as appropriate) for which both
concentration and toxicity data were available.
    The screening analysis was developed to be protective of highly
exposed individuals. Due to the conservative nature of the screening,
the calculated risks represent a protective, but unlikely, combination
of conditions that most likely reflect an upper bound on potential
exposures for each individual constituent. The revised screening
assessment did not rely on risk attenuation factors to screen out
constituents. All constituents that resulted in screening-level risks
above human health or ecological criteria, and for which
characterization of fate and transport could be refined, were carried
forward for further consideration in the probabilistic analysis,
described in Section 4 of the revised risk assessment. It is possible
that consideration of exposure to multiple constituents through a
single pathway or to the same constituent through multiple pathways may
have resulted in the retention of some additional constituents.
However, it is highly unlikely that these additional constituents would
remain risk drivers once more realistic dilution and attenuation in the
environment is considered.
    COMMENT: Multiple commenters noted that there may be additional
constituents present in CCR wastes beyond those quantitatively
evaluated in the risk assessment. In particular, multiple commenters
referenced organics and radionuclides. Some commenters called on EPA to
quantify the risks associated with these additional constituents.
Others claimed that these constituents are present in low levels and do
not pose risk to receptors.
    EPA RESPONSE: In the Report to Congress: Wastes from the Combustion
of Fossil Fuels: Volume 2--Methods, Findings, and Recommendations,\160\
EPA reviewed the available data on organic constituents, such as
polyaromatic hydrocarbons and dioxins. These data indicated that
concentrations of all organics are near or below analytical detection
limits both in CCR and in the leachate released from CCR. Based on the
findings of this report, the Agency concluded that organic constituents
were not risk drivers and did not require further evaluation. In the
absence of additional data that demonstrate the organic composition of
CCR wastes have markedly changed, EPA continues to rely on these
findings.
---------------------------------------------------------------------------

    \160\ U.S. EPA (Environmental Protection Agency). 1999b. Report
to Congress: Wastes from the Combustion of Fossil Fuels: Volume 2--
Methods, Findings, and Recommendations (EPA 530-R-99-010). Office of
Solid Waste and Emergency Response. Washington, DC.
---------------------------------------------------------------------------

    EPA acknowledges that, like other inorganic constituents,
naturally-occurring radionuclides may be concentrated in CCR waste
through the combustion of coal. However, due to a lack of data that
could be used to characterize leachate concentrations for individual
radionuclides, a quantitative evaluation of risk was not conducted. To
address this data gap, EPA has included radionuclides in the list of
constituents for groundwater monitoring. Furthermore, potential
transport of these constituents downgradient by windblown dust and
storm run-off are addressed through requirements for fugitive dust
controls and run-on/run-off controls.
4. Comments Related to Human Exposure and Toxicity
    COMMENT: Some commenters argued that EPA underestimated risks by
not considering combined chemical effects, additive risk and concurrent
exposures through multiple pathways. One commenter indicated that EPA
should conduct a full scale assessment that considers concurrent
exposure from ingestion of fish and groundwater. Commenters also raised
concerns that some chemical constituents share a common mechanism of
toxicity and may affect the same body organ or system, resulting in
greater risks than predicted through the consideration of each
constituent separately.
    One commenter noted that the combination of risks from different
constituents would not change the overall results of the risk
assessment. Constituents concentrations found to result in an HQ less
than 1 in the screening analysis are unlikely to make a meaningful
contribution to overall risk regardless of whether multiple compounds
share the same toxicological endpoints. Additionally, the commenter
expressed that it would be inappropriate to add the risks from
different constituents as modeled because the constituents do not all
arrive at a hypothetical receptor at the same time, due to differing
mobility in the subsurface environment.
    EPA RESPONSE: EPA acknowledges that this risk assessment considered
potential risks to human health from individual constituents and
individual pathways. EPA acknowledges that not explicitly evaluating
cumulative risk is a source of uncertainty that may result in some
underestimation of risks. It is possible that an individual could be
exposed to risks from drinking contaminated groundwater, as well as
eating contaminated fish from a local surface water body, but it is
unlikely that these two exposure pathways would occur simultaneously
with any appreciable frequency in the real world. It is even more
unlikely that a receptor would be exposed to both media at the high-end
concentrations modeled. Therefore, the magnitude of the uncertainty
introduced into the risk assessment is likely to be small. It is also
possible for an individual to be exposed to multiple constituents
through a single pathway. This is a more likely scenario because, as
demonstrated

by the available data, CCR typically leach multiple inorganic
constituents. Where exposure to multiple constituents is likely to
occur, EPA policy is to assume that the risks resulting from these
exposures are additive.\161\ The current probabilistic analysis
identified individual constituents above risk criteria. Many of the
other constituents modeled resulted in risks an order of magnitude or
more below risk criteria. Thus, the consideration of additive risk,
even with the high-end risks modeled in this risk assessment, is
unlikely alter the principal results of the probabilistic analysis.
Similarly, because the risks for individual constituents were found to
be above levels of concern, consideration of additive risk is unlikely
to meaningfully change the results of the analysis. EPA updated the
revised risk assessment to include a discussion of the associated
uncertainties in Section 5.
---------------------------------------------------------------------------

    \161\ U.S. EPA. 2000. Supplementary Guidance for Conducting
Health Risk Assessment of Chemical Mixtures. Risk Assessment Forum,
Washington, DC. August.
---------------------------------------------------------------------------

    COMMENT: Some commenters identified incorrect and inconsistent
reporting of toxicity benchmark values and recommended conducting a
thorough review of literature to ensure the use of the most current
values were used. One commenter expressed concern over the use of the
current IRIS value for arsenic carcinogenic effects and believes it
underestimates risk. Other commenters emphasized that it would be
inappropriate for EPA to consider using the draft oral cancer slope
factor (CSF) for arsenic and the oral CSF for hexavalent chromium
[chromium (VI)] published by the New Jersey Department of Environmental
Protection (NJDEP). Concerning lead, one commenter supported a peer
reviewer's recommendation to use the Integrated Exposure Uptake
Biokinetic (IEUBK) model to calculate human health risks, especially
for young children. Additionally, a commenter requested chemical-
specific information on toxicity criteria derivation, as well as
information on the relationship between environmental exposures to
specific chemicals and adverse health effects. The commenter emphasized
that this information would provide an uncertainty discussion regarding
toxicity values, facilitate communication with the public, and provide
a balanced perspective on risk.
    EPA RESPONSE: Human health benchmarks were chosen based on the
Office of Solid Waste and Emergency Response hierarchy (OSWER Directive
9285.7-53).\162\ EPA reviewed the benchmarks to confirm their accuracy
and determine whether newer values have become available from EPA or
other sources used by EPA since the CCR draft risk assessment was
conducted. The current, updated list of human health benchmarks is
provided in appendix E of the revised risk assessment, and the
references cited in that appendix provide further information on the
potential adverse effects and derivation of toxicity criteria.
---------------------------------------------------------------------------

    \162\ U.S. EPA. 2003. Human Health Toxicity Values in Superfund
Risk Assessments. Office of Solid Waste and Emergency Response
Directive 9285.7-53. December.
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    For lead, EPA used the drinking water maximum contaminant level
(MCL) to estimate risks from drinking water exposure in the draft risk
assessment. In the revised risk assessment EPA continued to rely on the
MCL, but also used IEUBK model for lead in children as described in
Section 5 of the revised risk assessment. While lead failed the
screening assessment, risks from lead exposure in the probabilistic
assessment were well below the risk criterion, and did not drive risks
in either the probabilistic or any sensitivity analyses.
    COMMENT: The commenters questioned why the cancer benchmark of 1 x
10-\5\ was selected while the typical range used by OSWER
and EPA guidance is a range from 1 x 10-\4\ to 1 x
10-\6\. The commenters suggested that an explanation is
necessary. In particular, one commenter requests clarification on the
phrase ``point of departure'' when supporting the use of the cancer
benchmark of 1 x 10-\5\. Concerning non-cancer criteria, a
commenter suggested that non-cancer risks should be report as follows:
Worst Case--Assume maximum exposure scenarios including exposure 24-
hours/day, 365 days/year for 70 years; High End--95th percentile based
on national human activity pattern distributions; Central Tendency--
50th percentile (or median) risk based on national human activity
pattern distributions. Furthermore, another commenter believed that it
is more appropriate to consider 95th percentiles, rather than 90th
percentile, of exposure and risk estimates for humans and ecological
receptors.
    EPA RESPONSE: The rationale for the selected cancer and non-cancer
risk criteria, based on Agency policy, is discussed in Section 2 of the
revised risk assessment. A citation to the where ``point of departure''
was originally defined is provided for reference. The rationale for use
of 90th percentile risk generated by a Monte Carlo simulation is
discussed in Section 4 of the revised risk assessment.
    COMMENT: Commenters questioned the evaluation of only the
reasonable maximum exposure scenario. Specifically, it was noted that
the receptor placement downgradient of an unlined management unit does
not represent the entire population exposure distribution. One
commenter suggested that EPA clearly define the exposed population of
interest.
    EPA RESPONSE: In risk assessments used to develop regulations under
RCRA, EPA has historically assessed potential risks resulting from a
reasonable maximum exposure (RME) scenario in order to ensure that the
resulting regulation is adequately protective of human health without
being excessively conservative. The types of data necessary to define
the exact population that relied on groundwater wells as a source of
drinking water or consumes fish from impacted water bodies are not
available. EPA believes that consideration of RME is a reasonable and
protective alternative, given the available data. Uncertainties
associated with the revised risk assessment are further discussed in
Section 5 of the revised risk assessment.
    COMMENT: The commenters questioned the use of data from the 1997
Exposure Factors Handbook in the development of intake rate
distributions for various exposures, because more current data are
currently available. Commenters recommended that EPA make updates to
these parameters using more current sources of information, including
the recently released 2011 Exposure Factors Handbook.\163\ In addition,
some commenters pointed out the potential for the available exposure
factor data to underestimate or overestimate exposures. One commenter
noted that the risk assessment did not fully account for the dependence
of input variables (e.g., the interdependence of body weight and water
ingestion rates for children and link between the rate of fish consumed
from a water body). Another commenter suggested that a sensitivity
analysis of human health exposure factors be conducted to add to the
sensitivity analysis conducted by EPA in 2009.
---------------------------------------------------------------------------

    \163\ U.S. EPA. 2011. Exposure Factors Handbook: 2011 Edition.
EPA/600/R-090/052F. National Center for Environmental Assessment,
Office of Research and Development, Washington, DC. September.
---------------------------------------------------------------------------

    Regarding fish consumption rates, commenters questioned the
representativeness of a fixed fish consumption rate drawn from a single

study. It was suggested for transparency that the risk assessment
provide the results of the chi-square tests to demonstrate how well the
fish consumption rate data fit a log normal distribution. Additionally,
it was suggested that fish consumption rates should be determined from
other studies and more robust data sets. One commenter suggested the
incorporation of fish consumption rates representative of subsistence
fishers, such as Native American populations that harvest and consume
fish as part of their native traditions and culture.
    Regarding drinking water ingestion rates, one commenter voiced
concern about the assumption that groundwater is the source of all
drinking water. The commenter indicated that this is an overly
conservative and atypical assumption, as a majority of individuals will
consume liquids from other sources (e.g., milk, juice, sodas, bottled
water, sports and energy drinks).
    EPA RESPONSE: This revised risk assessment relied on both the 1997
Exposure Factors Handbook (EFH) \164\ and the 2008 Child-Specific
Exposure Factors Handbook (CSEFH) \165\ for information on human
exposure factors for the U.S. population. The 2011 Exposure Factors
Handbook \166\ has been completed and updates some of the data from the
1997 EFH. During the finalization of this risk assessment, EPA released
OSWER Directive 9200.1-120.\167\ Although this document provides
default exposure factors to use for point estimates, EPA is still in
the process of updating the full distributions necessary for
probabilistic analysis. Therefore, this risk assessment does not
incorporate the data from the 2011 EFH.
---------------------------------------------------------------------------

    \164\ U.S. EPA. 1997. Exposure Factors Handbook, Volume III,
Activity Factors. EPA/600/P-95/002Fa. Office of Research and
Development, Washington, DC. August.
    \165\ U.S. EPA. 2008. Child-Specific Exposure Factors Handbook.
EPA/600/R-06-096F. National Center for Environmental Assessment,
Cincinnati, OH.
    \166\ U.S. EPA. 2011. Exposure Factors Handbook: 2011 Edition.
EPA/600/R-090/052F. National Center for Environmental Assessment,
Office of Research and Development, Washington, DC. September.
    \167\ U.S. EPA. 2014. Human Health Evaluation Manual,
Supplemental Guidance: Update of Standard Default Exposure Factors.
OSWER Directive 9200.1-120. February.
---------------------------------------------------------------------------

    Exposure data used for the fish ingestion rates are described in
appendix D of the revised risk assessment. Data on site-specific fish
consumption rates were not available for use in this analysis. Instead,
the full distribution of fish consumption rates were drawn from a study
of adult anglers from Maine that fished from streams, rivers, and
ponds. Because age-specific data for children were not available, all
child cohorts were assumed to consume fish at the same rate as the
adult cohort. Data on fish ingestion rates for Native American
subsistence fishers are currently limited and can vary widely
geographically, to the point that the 2011 EFH makes no recommendation
for representative values. EPA acknowledges that these issues introduce
uncertainty into the analysis, which are further discussed in Section 5
of the revised risk assessment.
    COMMENT: Commenters emphasized the need to update exposure factors
for childhood exposures and recommended that updates include data from
the 2011 EFH. One commenter stated that the risk assessment
appropriately considered the potential fish exposures for children.
However, they pointed out that the fish consumption rates for children
should be lower than those applied for adults. Another commenter
suggested that the risk assessment should provide a clear description
of how the exposure duration of child cohorts were used in the risk
calculations. Specifically, the commenter questioned whether exposure
durations were truncated at the end of each age cohort or aged through
the different cohorts.
    EPA RESPONSE: The revised risk assessment makes use of the 1997 EFH
\168\ and the 2008 CSEFH \169\ for information on human exposure
factors for the U.S. population. Although, as discussed in the preamble
sections above, the revised risk assessment does not incorporate data
from the recent 2011 EFH,\170\ all child data included in this document
was derived from the 2008 EFH. In addition to child ingestion of
drinking water, EPA's evaluation has been revised to also account for
infant exposures that may occur from formula mixed with contaminated
groundwater. These data are presented in appendix D of the revised risk
assessment. Consistent with the commenter's recommendation for cohort
aging, the risk assessment aged receptors through each age cohort using
age-specific data for exposure factors and physical characteristics
that were weighted proportionally by the corresponding time period and
then summed. Specific discussion of truncation values is provided in
later in this preamble.
---------------------------------------------------------------------------

    \168\ U.S. EPA. 1997. Exposure Factors Handbook, Volume III,
Activity Factors. EPA/600/P-95/002Fa. Office of Research and
Development, Washington, DC. August.
    \169\ U.S. EPA. 2008. Child-Specific Exposure Factors Handbook.
EPA/600/R-06-096F. National Center for Environmental Assessment,
Cincinnati, OH.
    \170\ U.S. EPA. 2011. Exposure Factors Handbook: 2011 Edition.
EPA/600/R-090/052F. National Center for Environmental Assessment,
Office of Research and Development, Washington, DC. September.
---------------------------------------------------------------------------

    COMMENT: Public commenters recommended updating BCF values with
more current references. One commenter questioned why bioconcentration
factors were zero for some constituents that are essential nutrients
(i.e., cobalt and copper). Another commenter voiced concern that EPA
had not fully considered the appropriateness of using BCFs to describe
metals bioaccumulation, suggesting that current science (including EPA
guidance documents) indicates that BCFs are poor predictors of tissue
metal concentrations due to wide variation in uptake patterns governed
by several chemical and biological factors. Another commenter
recommended the use of an approach that would be more robust than the
single BCF approach, establishing and applying distributions of BCFs.
This commenter also recommended that the assessment adhere to the EPA
policy of using dissolved metals in the calculating the
bioconcentration of metals in fish, or should provide the rationale for
using a different approach.
    EPA RESPONSE: EPA recognizes that the use of BCFs may not represent
the most current approaches available to estimate metal bioaccumulation
at individual sites, where fish tissue data can be collected. However,
as noted by public commenters, BCFs are useful in a screening-level
assessment and EPA believes they are also appropriate for a national-
level risk assessment, where site-specific data are not available and
collection of site-specific data is not viable.
    In some cases, insufficient data to determine a BCF value meant
that these constituents could not be quantitatively evaluated for this
pathway. Regarding the concern expressed with respect to zero BCF
values, the commenter did not provide alternative BCFs that EPA could
consider for the constituents at issue. Additionally, EPA agrees that,
given the latest scientific information, distributions of BAFs/BCFs may
be better than single BAFs/BCFs because they account for changes in
bioaccumulation/bioconcentration at different water concentrations. EPA
is working to develop BAF/BCF distributions for several CCR pollutants
of concern but does not yet have a robust enough dataset for use for
the final CCR Rule. In lieu of this, EPA is proceeding with the single
BAF/BCF approach for the current analysis. EPA does recognize this
issue as a limitation for the BCF calculations and considers it as an
uncertainty in the risk

characterization. Overall, EPA agrees that the use of this older data
introduces some uncertainty into the analysis. These uncertainties are
discussed in greater detail in Section 5 of the revised risk
assessment.
    With the exception of mercury, EPA evaluated bioconcentration based
on water column concentrations that include contributions from
dissolved and solid phases because available BCFs represent
contributions from both. Because a BAF based only on dissolved-phase
concentrations was available for mercury, EPA evaluated this
constituent using only dissolved concentrations. Applying this
conservative approach for most constituents ensured protection of human
health. Even with this conservative assumption, the 90th percentile
risks for the probabilistic analysis (Section 4) did not exceed risk
criteria for the fish ingestion pathway. Therefore, this approach is
unlikely to have affected the principal findings of the risk
assessment.
    For the revised risk assessment, EPA reviewed the available
literature and identified BCFs for additional constituents that
previously had no values. As noted in appendix G of the revised risk
assessment, the following source hierarchy was used for fish BCFs:
     Primary literature: These are generally papers focused on
a single chemical 171 172 173 174 or may contain data on
multiple chemicals.175 176
---------------------------------------------------------------------------

    \171\ Eisler, R. 1989. Molybdenum Hazards to Fish, Wildlife, and
Invertebrates: A Synoptic Review. Contaminant Hazard Reviews, Report
No. 19, Biological Report 85(1.19). Laurel, MD. August.
    \172\ Kumada, H., et al. 1973. Acute and chronic toxicity,
uptake and retention of cadmium in freshwater organisms. Bull.
Freshwater Fish. Res. Lab. 22: 157
    \173\ Lemly AD. 1985. Toxicology of selenium in a freshwater
reservoir: implications for environmental hazard evaluation and
safety. Ecotoxicology and Environmental Safety. 10(3): 314-338.
    \174\ Murphy, B.R., G.J. Atchison, and A.W. McIntosh. 1978.
Cadmium and zinc in muscle of bluegill (Lepomis macrochirus) and
largemouth bass (Micropterus salmoides) from an industrially
contaminated lake. Environmental Pollution 17:253-257.
    \175\ Barrows ME, Petrocelli SR, Macek KJ, Carroll JJ. 1980.
Bioconcentration and elimination of selected water pollutants by
bluegill sunfish (Lepomis macrochirus). In: Haque R, ed. Dynamics,
exposure and hazard assessment of toxic chemicals. Ann Arbor,
Michigan, U.S.A.: American Chemical Society. p. 379-392.
    \176\ Stephan, C.E. 1993. Derivation of Proposed Human Health
and Wildlife Bioaccumulation Factors for the Great Lakes Initiative.
Draft. Environmental Research Laboratory, Office of Research and
Development, U.S. Environmental Protection Agency, Duluth, MN.
March.
---------------------------------------------------------------------------

     U.S. EPA databases/publications: These included ECOTOX
\177\ and the Mercury Report to Congress.\178\
---------------------------------------------------------------------------

    \177\ U.S. EPA (Environmental Protection Agency). 2009b. ECOTOX
User Guide: ECOTOXicology Database System. Version 4.0. Available
online at www.epa.gov/ecotox/.
    \178\ U.S. EPA (Environmental Protection Agency). 1997d. Mercury
Study Report to Congress. Volume III--Fate and Transport of Mercury
in the Environment. EPA 452/R-97/005. Office of Air Quality Planning
and Standards and Office of Research and Development, Washington,
DC.
---------------------------------------------------------------------------

     Other government agency resources: These included ATSDR
Toxicological Profiles \179\ and the Hazardous Substances Data
Bank.\180\
---------------------------------------------------------------------------

    \179\ ATSDR (Agency for Toxic Substances and Disease Registry).
2008. Minimal Risk Levels (MRLs) for Hazardous Substances. Available
at www.atsdr.cdc.gov/mrls.html.
    \180\ U.S. NLM (National Library of Medicine). 2011. Hazardous
Substances Data Bank (HSDB). Available online at:
toxnet.nlm.nih.gov/cgi-bin/sis/htmlgen?HSDB.
---------------------------------------------------------------------------

    EPA also finds that the references provided by commenters provided
primarily phytotoxicity and accumulation data for terrestrial plants,
and were therefore not relevant to EPA's explicit solicitation on
whether the bioconcentration factors drawn from Baes et al. (1984)
should be considered in the final risk assessment.\181\
---------------------------------------------------------------------------

    \181\ Baes, C.F., III, R.D. Sharp, A.L. Sjoreen, and R.W. Shor.
1984. A Review and Analysis of Parameters for Assessing Transport of
Environmentally Released Radionuclides Through Agriculture. ORNL-
5786. Oak Ridge National Laboratory, Oak Ridge, TN. September.
---------------------------------------------------------------------------

5. Comments Related to Ecological Exposure and Toxicity
    COMMENT: Public commenters emphasized the potential importance of
cumulative ecological risk, whereby an ecological receptor may be
exposed to multiple constituents and/or pathways concurrently. For
example, amphibians may be subject to both dermal and ingestion
exposure. Public commenters noted that ecological risks were
underestimated because the following scenarios were not considered for
ecological receptors: Aboveground pathways, contaminant transport to
nearby uncontaminated environments, and the inclusion of field data in
the analysis.
    EPA RESPONSE: EPA acknowledges that cumulative effects can be
important for ecological receptors. However, just as EPA did not
consider cumulative human health risks from exposures to groundwater
(discussed in the previous sections of this preamble), they were not
modeled for ecological receptors. In the national, probabilistic
analysis (Section 4 of the revised risk assessment), risks for all
constituents fell below the ecological criteria. Even the sum of
modeled risks for all constituents fell below the ecological criteria.
In sensitivity analyses (Section 5 of the revised risk assessment),
which considered different subsets of national disposal practices that
may drive risks, boron and cadmium were the two constituents found to
result in risks above ecological criteria. To the extent that
cumulative exposures were not evaluated, EPA acknowledges that
ecological risk could be underestimated to some degree. However, these
uncertainties are unlikely to affect the principal findings of the risk
assessment. In addition, EPA also notes that all surface water risks
are orders of magnitude lower than the risks resulting from direct
discharges modeled in U.S. EPA (2013).\182\
---------------------------------------------------------------------------

    \182\ U.S. EPA. 2013. Environmental Assessment for the Proposed
Effluent Limitation Guidelines and Standards for the Steam Electric
Power Generating Point Source Category. EPA-821-R-13-003. Office of
Water. Washington, DC 20460. April.
---------------------------------------------------------------------------

    In contrast to the surface water and sediment exposures, ecological
risks for individual constituents were appreciably above risk criteria
for direct exposure to impoundment wastewater. As a result, it is clear
that CCR disposal in surface impoundments have the potential to pose
risk to ecological receptors, even without consideration of cumulative
exposures.
    COMMENT: Public commenters stated that the risk assessment does not
consider sensitive habitats or species. Commenters requested additional
consideration of threatened and endangered species and the inclusion of
ecological field data.
    EPA RESPONSE: EPA did not evaluate these sensitive habitats and
sensitive/endangered ecological receptors because these are inherently
site-specific issues for which data on potential impacts are often not
available and can be difficult to quantify, even on a site-specific
basis. EPA acknowledges that the inability to quantitatively evaluate
the potential for these adverse effects represents a source of
uncertainty. Discussion of these uncertainties is presented in Section
5 of the revised risk assessment.
    COMMENT: Public commenters were concerned that a more conservative
approach was needed to derive the ecological benchmarks. Multiple
commenters also stated that the use of risk attenuation factors to
scale the screening risks to full-scale risks was inappropriate.
Several commenters noted that the ecological boron benchmark used for
surface water exposures contained incorrect units based on an incorrect
transcription in the peer-reviewed article. Another commenter noted
that the ecological

cadmium benchmark used for direct contact with surface water was
incorrect.
    EPA RESPONSE: Ecological benchmarks were obtained for CCR
constituents when available and compared with the modeled media
concentrations (e.g., surface water, sediment) to estimate the HQs used
to characterize ecological risk. These benchmarks represent the best
available estimates of receptor responses based ``no effects'' (NOAEL)
or ``lowest effects'' (LOAEL) study data. In some scenarios, these
benchmarks may represent species not actually present in the field. In
others, these benchmarks may not capture the most sensitive possible
receptor at every site or for each constituent. While some benchmarks
have factors of safety included to account for these or other
uncertainties, there remains the potential for these ecological
benchmarks to underestimate risks for the specific species and
communities that live in surface waters impacted by CCR WMUs. The
magnitude of this uncertainty is unknown. Consideration of any
additional sensitive species not captured by the current benchmarks may
result in some additional constituents above risk criteria in the
probabilistic analysis. EPA notes that ecological risks to some of
these additional sensitive receptors may be reflected in damage cases.
However, this site-specific uncertainty is unlikely to affect the
national conclusions of the risk assessment.
    Regarding incorrect benchmark values, an updated boron benchmark
was used in the revised risk assessment. The units in the fish study
from which the previous SCV was derived \183\ had been erroneously
transcribed in Suter and Tsao (1996) \184\ as [micro]g/L instead of mg/
L. The updated SCV was recalculated using the corrected units. The
revised value has been corroborated with the authors. Additionally, a
continuous criteria concentration (CCC) was used for the cadmium
surface water benchmark in the revised risk assessment, replacing the
previous value. The updated values are presented in appendix E of the
revised risk assessment report.
---------------------------------------------------------------------------

    \183\ Hamilton, S.J. 1995. Hazard assessment of inorganics to
three endangered fish in the Green River, Utah. Ecotoxicol Environ
Saf 30:134-142.
    \184\ Suter, G.W., and C.L. Tsao. 1996. Toxicological Benchmarks
for Screening Potential Contaminants of Concern for Effects on
Aquatic Biota: 1996 Revision. U.S. Department of Energy, Oak Ridge
National Laboratory, Oak Ridge, TN. June.
---------------------------------------------------------------------------

6. Comments Related to the Monte Carlo Analysis Approach
    COMMENT: While some public commenters stated that the human health
probability distributions appeared appropriate, others expressed
concern regarding a conservative bias in input parameter probability
distributions used and the resulting potential for overestimation of
risks. These commenters noted that the ideal approach would be to
estimate the actual risk and associated uncertainty rather than
weighting the results conservatively.
    EPA RESPONSE: The revised risk assessment conducted a full-scale,
probabilistic Monte Carlo analysis to quantify human and ecological
risks. EPA agrees it would be ideal to produce best estimates of actual
risk. All input data distributions (e.g., aquifer data, soil type, WMU
data, climate data, distance to groundwater wells, distance to surface
water bodies, constituent concentrations, water flow data, human
exposure factors) were developed in line with this objective. However,
these distributions were developed from available data and are subject
to the limitations of these data. In cases where data were not
sufficient to fully characterize the input distribution, conservative
values and assumptions were used to fill data gaps to remain protective
of human health and the environment. Further discussion of these
uncertainties has been added to Section 5 of the revised risk
assessment.
    COMMENT: Public commenters pointed out that the risk assessment
does not formally differentiate variability from uncertainty or show
confidence limits for risk results, which makes it challenging to
identify opportunities to reduce uncertainty. One commenter requested
that EPA discuss the implications of the relatively wide risk
distributions, including the reasons why some risk distributions are
larger than others based on the Monte Carlo results.
    EPA RESPONSE: EPA acknowledges it would be ideal to separate
variability from uncertainty when possible in a probabilistic risk
assessment. EPA was able to reduce a substantial number of the
uncertainties in the revised risk assessment through the acquisition of
additional data on facilities, environmental parameters, and
constituent concentrations. Variability and uncertainty are still
comingled in a large number of cases due to remaining data gaps;
however, EPA conducted multiple sensitivity analyses to determine the
potential for different inputs to affect risk results. Additional
discussion of the differences between parameter variability, data
uncertainty, and model error, as well as discussions of the sensitivity
and uncertainty analyses, is presented in Section 5 of the revised risk
assessment.
    EPA disagrees that there are wide risk distributions. While the
commenter correctly points to other risk assessments that had closer
central tendency and high-end results, those were either site-specific
assessments or involved no fate or transport modeling. National-scale
risk assessments will necessarily have wider variability in their
results compared to risk assessments that are specific to a single
site. Thus, the ``wider'' risk distributions simply reflect the fact
that different sites with different CCR can have very disparate impacts
on human health and the environment.
7. Miscellaneous Comments
    COMMENT: Some commenters stated that the documentation is
incomplete and that an independent reviewer could not reproduce the
analysis. Another commenter performed an independent review and cancer
risk estimate and noted that the EPA used a reasonable approach for
calculating cancer cases in the risk assessment.
    EPA RESPONSE: EPA acknowledges that the documentation of the inputs
and intermediate outputs could have been more transparent for the 2010
Draft Risk Assessment. In the revised risk assessment, many of the
inputs EPA used are directly discernible from the appendices. A summary
of the data available in each appendix is presented in Section 1 of the
revised risk assessment. EPA also acknowledges that the additions and
discussions of inputs in the document were not sufficient for complete
duplication of the results. Thus, the input and output files for the
draft risk assessment were made available in the docket of the proposed
rule via an FTP site,\185\ and final input and output files are being
placed in the docket for the final rule.
---------------------------------------------------------------------------

    \185\ Available online at: ftp://ftp.epa.gov/coal-combustion-residues.
---------------------------------------------------------------------------

    COMMENT: Commenters requested improvement on the graphical
presentation of risk results. Additionally, commenters requested
further explanation of the minimum and maximum truncating values, as
truncated values may reduce risk estimates.
    EPA RESPONSE: While EPA did not provide a graphical presentation of
the risk results, this information is more clearly discernible from the
full input and output files. For discussion of the

full inputs and outputs files, see the responses in the preamble
section above. With regard to truncation, EPA no longer manually
truncates input distributions for the human exposure factors. Instead,
exposure factor distributions in the revised risk assessment were
generated with the @Risk software (Palisade Co., Newfield, NY),\186\ as
described in appendix D. EPA has also added further discussion of the
cohorts to revised risk assessment, with tables comparing each cohort's
risk presented in Section 5 of the revised risk assessment.
---------------------------------------------------------------------------

    \186\ Available online at: www.palisade.com/risk/.
---------------------------------------------------------------------------

    COMMENT: Commenters requested more complete documentation of the
sensitivity analysis. Other comments included a request to add human
health exposure factor variables to the sensitivity analysis, and to
conduct additional sensitivity analyses on different topics (e.g., well
distance distribution).
    EPA RESPONSE: EPA acknowledges the omission of the original
sensitivity analysis from the docket. EPA updated the sensitivity
analysis \187\ so that it clearly describes the methodology that
underlies the results summarized in Section 5 of the revised risk
assessment. This sensitivity analysis was placed in the docket for the
proposed rule.
---------------------------------------------------------------------------

    \187\ U.S. EPA. 2009. Sensitivity Analysis for the Coal
Combustion Waste Risk Assessment. Draft Technical Report. Prepared
by RTI International for U.S. EPA, Office of Solid Waste,
Washington, DC.
---------------------------------------------------------------------------

    Human health exposure factor variables were not evaluated in the
sensitivity analysis. Human exposure factor variables have well-
established, peer-reviewed, national distributions that are regularly
used in probabilistic risk analyses conducted by EPA based on Agency
policy. Therefore, the contribution of variability in the exposure
factors to the variability in risk was not particularly useful for
understanding the aspects of CCR disposal practices that may drive
risk. Additional sensitivity analyses such as leachate duration versus
leachable content and liner performance by thickness were conducted in
the revised risk assessment and are summarized in Section 5.

B. Summary of Risk Assessment and Results

1. Problem Formulation
    EPA first developed conceptual models to illustrate a general
layout of surface impoundments and landfills, the chemical constituents
that may be released from these WMUs, the routes through which these
constituents may migrate through environmental media, and the types of
exposures that may result. These conceptual models were used as the
basis for all subsequent data collection efforts. EPA first collected
data on the coal-fired power plants and CCR WMUs located across the
United States. EPA then collected regional and national data to
characterize the environment and receptor population surrounding each
WMU. The data assembled represent the most current and comprehensive
information available to the Agency at the time this risk assessment
was conducted. Using the data collected, EPA first conducted a
simplified hazard identification to determine which constituents
warranted further evaluation. At this stage, EPA considered the
presence of a constituent in CCR waste, combined with the availability
of at least one toxicity benchmark, sufficient evidence of hazard
potential. Table 1 presents a summary of the different chemical
constituents retained as constituents of potential concern (COPCs) for
further analysis.

    Table 1--List of Chemical Constituents Evaluated in the CCR Risk
                               Assessment
------------------------------------------------------------------------

-------------------------------------------------------------------------
Aluminum
Ammonia
Antimony
Arsenic
Barium
Beryllium
Boron
Cadmium
Calcium
Chloride
Chromium
Cobalt
Copper
Fluoride
Iron
Lanthanum
Lead
Lithium
Magnesium
Manganese
Mercury
Molybdenum
Nickel
Nitrate/Nitrite
Selenium
Silicon
Silver
Sodium
Strontium
Sulfate
Sulfide
Thallium
Uranium
Vanadium
Zinc
------------------------------------------------------------------------

    All risks identified in subsequent analyses were compared against
risk criteria of cancer risk greater than 1 x 10-\5\ or a
noncancer hazard quotient (HQ) greater than 1. EPA typically relies on
a risk range to determine the point at which regulation is appropriate.
EPA uses as an initial cancer risk ``level of concern'' a calculated
risk level of 1 x 10-\5\ (one in one hundred thousand) or an
HQ above 1.0 for any noncarcinogens. For example, waste streams for
which the calculated high-end individual cancer-risk level is 1 x
10-\5\ or higher generally are considered candidates for
regulation. Waste streams whose risks are calculated to be 1 x
10-\4\ or higher generally will be considered to pose a
substantial present or potential hazard to human health and the
environment and generally will be regulated. Waste streams for which
these risks are calculated to be 1 x 10-\6\ or lower, and
lower than 1.0 HQs or EQs for any noncarcinogens, generally will be
considered not to pose a substantial present or potential hazard to
human health and the environment and generally will not regulated. See
59 FR 66075-66077, December 22, 1994.
2. Screening Analysis
    EPA conducted separate screening analyses for each exposure pathway
to identify which COPCs are most likely to pose risk to receptors. The
results of this screening generally do not provide a precise
characterization of individual risks that may occur, but rather
identify those COPCs that are most likely to exceed risk criteria. In
cases where well established, post-construction management practices
(``controls'') have been shown to minimize releases from WMUs, EPA
considered exposures for both an uncontrolled and controlled management
scenario.
    This screening analysis identified potential risks to human and
ecological receptors resulting from the releases of particulate matter
and the chemical constituents contained therein through wind and run-
off. Under an uncontrolled management scenario, risks to human
receptors resulted from the inhalation of windblown particulates in
ambient air and the ingestion of soil and animal products (i.e., meat
and dairy), while risks to ecological receptors resulted from exposures
to soil and sediment. Under a controlled management scenario, which
consisted of fugitive dust controls and run-on/run-off controls, all
risks associated with these exposure pathways decreased to below the
criteria. Due to the conservative nature of the screening, there is a
great deal of uncertainty surrounding the specific risks calculated for
these exposure pathways. These risks represent a protective, but
unlikely, combination of conditions that reflect at least an upper

bound on potential exposures. Thus, the cumulative effect of these
uncertainties results in an overestimation of nationwide risks to most
or all receptors. Therefore, EPA makes no direct findings concerning
the magnitude of the risks that may occur under either an uncontrolled
or controlled management scenario, but concludes with a high degree of
confidence that the reductions achievable with standard management
practices are sufficient to be protective even under this conservative
screening assessment. Based on these lines of evidence, EPA concluded
that no further characterization was warranted for these exposure
pathways.
    These screening analyses identified potential risks to human and
ecological receptors from leaching of chemical constituents from CCR
waste into surrounding environmental media. Risks to human health
resulted from ingestion of groundwater and fish, while risks to
ecological receptors resulted from exposure to surface water. There was
no simple method to estimate the effect controls may have for these
pathways. However, considerable dilution and attenuation may occur
before COPCs reach downgradient private wells and surface water bodies.
Therefore, EPA retained all of the COPCs found to be above risk
criteria in groundwater and surface water for further characterization.
In addition, EPA used the uncontrolled screening results for the above
ground sediment pathway as a conservative proxy for the groundwater to
surface water sediment pathway. As a result, sediment exposures of four
COPCs were retained for further characterization. Table 2 presents a
summary of the chemical constituents retained as COPCs for each
pathway.

                   Table 2--List of Chemical Constituents Retained for Probabilistic Analysis
----------------------------------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------------------------
                         Human health                     Ecological
----------------------------------------------------------------------------------------------------------------
Ingestion of groundwater              Ingestion of fish         Surface water exposure   Sediment exposure
----------------------------------------------------------------------------------------------------------------
Antimony                              Arsenic                   Aluminum                 Antimony
Arsenic                               Cadmium                   Arsenic                  Arsenic
Boron                                 Mercury                   Barium                   Silver
Cadmium                               Selenium                  Beryllium                Vanadium
Cobalt                                Thallium                  Boron                    .......................
Fluoride                              ........................  Cadmium                  .......................
Lead                                  ........................  Chloride                 .......................
Lithium                               ........................  Chromium                 .......................
Molybdenum                            ........................  Cobalt                   .......................
Thallium                              ........................  Copper                   .......................
                                      ........................  Iron                     .......................
                                      ........................  Lead                     .......................
                                      ........................  Molybdenum               .......................
                                      ........................  Nickel                   .......................
                                      ........................  Selenium                 .......................
                                      ........................  Silver                   .......................
                                      ........................  Vanadium                 .......................
                                      ........................  Zinc                     .......................
----------------------------------------------------------------------------------------------------------------

    These screening analyses also identified potential risks to
ecological receptors from direct exposure to impoundment wastewater.
Unlike the other exposure pathways, no dilution or attenuation will
occur within impoundment wastewater prior to ecological exposures.
Thus, the direct exposures considered in the screening analysis provide
a reasonable estimate of the relative magnitude of risks. Based on the
screening analyses, EPA concluded that HQs for ecological receptors
exceeded 1 for the following constituents (listed from highest to
lowest potential): Arsenic (100), barium (50), aluminum (30), boron
(30), selenium (20), cadmium (10), vanadium (10), beryllium (2),
chloride (2) and chromium (2). Because the screening analysis provides
sufficient characterization of these exposures, this pathway was not
carried forward for further analysis.
3. Probabilistic Analysis
    EPA conducted a national-scale, probabilistic analysis to better
characterize the potential risks to human and ecological receptors
associated with leachate released from surface impoundments and
landfills. The specific exposure routes evaluated for these releases
were human ingestion of groundwater used as a source of drinking water
and fish caught from freshwater lakes or streams, as well as ecological
contact with and ingestion of surface water and sediment. A combination
of models was used to predict COPC fate and transport through the
environment, receptor exposures, and the resulting risks. Site-specific
data were used, supplemented by regional and national data sets, to
capture the national variability of disposal practices, environmental
conditions and receptor behavior. EPA modeled risks for both highly
exposed individuals (90th percentile risks) and more moderately exposed
individuals (50th percentile risks). In instances where the speciation
of a COPC has been shown to greatly affect fate and transport, EPA
modeled multiple species to provide a bounding on potential exposures.
    Table 3 shows the 90th percentile human health risks to the most
sensitive age cohorts for constituents that exceeded the risk criteria.
Risks are presented for arsenic modeled entirely as two different
species (III and V) to provide a bounding on potential risks. Values
that exceed the selected risk criteria are shown in bold. No 90th
percentile risks above ecological criteria were identified for either
surface impoundment or landfills. No 50th percentile risks above human
health or ecological criteria were identified for either surface
impoundment or landfills.

     Table 3--90th Percentile Nationwide Probabilistic Risk Results
------------------------------------------------------------------------
                                               Ingestion of groundwater
                                            ----------------------------
                    COPC                         Surface
                                              impoundments    Landfills
------------------------------------------------------------------------
                              Cancer Risks
------------------------------------------------------------------------
Arsenic III................................        2 x 10	4     5 x 10	6
Arsenic V..................................        1 x 10	5     7 x 10	8
------------------------------------------------------------------------
                             Noncancer Risks
------------------------------------------------------------------------
Arsenic III................................               5          0.1
Arsenic V..................................             0.4        <0.01
Lithium....................................               2          (a)
Molybdenum.................................               2        <0.01
------------------------------------------------------------------------
\a\ Leachate data were not available to model this COPC for landfills.

     Surface Impoundments:
    Ingestion of groundwater was the only exposure pathway that
resulted in risks above 1 x 10-5. 90th percentile cancer
risks above 1 x 10-5 were identified for arsenic III (2 x
10-4). The 90th percentile noncancer risks above an HQ of 1
were identified for arsenic III (5), lithium (2), and molybdenum (2).
     Landfills:
    All 90th percentile cancer and non-cancer risks were below human
health criteria.
    High-end risks identified for surface impoundments are consistently
higher than those for landfills. These results are attributed to the
higher infiltration rates through surface impoundments, which are
driven by the hydraulic head of the ponded water. Median risks for both
surface impoundments and landfills were substantially lower than both
the high-end risks in this risk assessment and the median risks modeled
in the 2010 Risk Assessment. This decrease is attributed primarily to
the interception of groundwater by surface water bodies, which is
accounted for in the revised risk assessment to provide a more accurate
mass balance of constituent mass during transport. It is common for
coal-fired utilities to be located near water bodies, which are used as
a source of cooling water and conveyance of waste. As a result, in the
majority of model iterations, the interception of groundwater by
surface water bodies resulted in negligible downstream well
concentrations.
    Based on the results of the probabilistic analysis, EPA concludes
that leaching from CCR waste management units has the potential to pose
risk to receptors. Arsenic, lithium, and molybdenum are the chemical
constituents found to pose the greatest risks from surface
impoundments, while arsenic posed the greatest risks from landfills.
Available toxicological profiles indicate that risks from arsenic
ingestion are linked to an increased likelihood of cancer in the skin,
liver, bladder and lungs, as well as nausea, vomiting, abnormal heart
rhythm, and damage to blood vessels; \188\ risks from lithium ingestion
are linked to neurological and psychiatric effects, decreased thyroid
function, renal effects, cardiovascular effects, skin eruptions, and
gastrointestinal effects; \189\ and risks from molybdenum ingestion are
linked to higher levels of uric acid in the blood, gout-like symptoms,
and anemia.\190\
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    \188\ Profile for arsenic available online at: www.epa.gov/iris/subst/0278.htm and www.atsdr.cdc.gov/toxprofiles/tp2.pdf.
    \189\ Profile for lithium available online at: hhpprtv.ornl.gov/issue_papers/Lithium.pdf.
    \190\ Profile for molybdenum available online at: www.epa.gov/iris/subst/0425.htm.
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4. Sensitivity and Uncertainty Analysis
    The modeled probabilistic risks capture the range of current,
nationwide CCR disposal practices. However, because of the broad scope
of the analysis, there are a number of sources of variability and
uncertainty present. Therefore, to confirm the results of the
probabilistic analysis and to better understand whether any particular
subset of disposal practices drives the risks identified, EPA conducted
additional sensitivity and uncertainty analyses.
    EPA reviewed the models used, as well as the data and assumptions
input into these models, to better understand the sources of
variability and uncertainty inherent in the probabilistic analysis. The
Agency then qualitatively and, to the extent possible, quantitatively
analyzed these sources to understand the potential effects each may
have on the modeled risk results. During this review, specific
attention was focused on the parameters shown to have the greatest
influence on model results. As a further method of validation, EPA
compared the results of the sensitivity and uncertainty analyses with
proven and potential damage cases. Together these analyses and
comparisons show that there is a high degree of confidence in the
principal findings of the probabilistic analysis. However, the review
of sensitive parameters revealed some specific disposal practices that
may result in greater risks than identified in the probabilistic
modeling.
    Through these additional sensitivity and uncertainty analyses,
which explored different subsets of national disposal practices, EPA
identified the potential for higher risks than those identified in the
broader, national analysis. In particular, consideration of different
waste pH values showed higher risks for arsenic at more acidic and
basic pH values, as well as additional risks for boron, cobalt,
fluoride and mercury at these more extreme pH values. Consideration of
specific liner types showed that groundwater risks are driven by
disposal in unlined units and, in particular, unlined surface
impoundments. For these units, EPA identified higher risks for arsenic,
lithium, and molybdenum, as well as additional risks for thallium.
Clay-lined units were found to pose lower risks than unlined units.
Composite-lined units were found to be the most protective disposal
practice, resulting in risks far below all criteria identified in this
risk assessment.

C. Conclusions

    Based on the analyses presented in this document, EPA concludes
that current management practice of placing CCR waste in surface
impoundments and landfills poses risks to human health and the
environment within the range that OSWER typically regulates. On a
national scale, surface impoundments presented higher risks than
landfills. Risks to ecological receptors were identified from exposures
to aluminum, arsenic, barium, beryllium, boron, cadmium, chloride,
chromium, selenium and vanadium through direct exposure to impoundment
wastewater. Risks to residential receptors were identified primarily
from exposures to arsenic, lithium, and molybdenum in groundwater used
as a source of drinking water, but additional risks from boron,
cadmium, cobalt, fluoride, mercury and thallium were identified for
specific subsets of national disposal practices.
    Sensitivity analyses on liner type indicate that disposal of CCR
wastes in unlined surface impoundments and landfills presents the
greatest risks to human health and the environment. As modeled, the
national risks from clay-lined units are lower than those for unlined
units, but such units can exceed risk criteria at individual sites.
Composite liners were the only liner type modeled that effectively
reduced risks from all pathways and constituents far below human health
and ecological criteria in every sensitivity analysis conducted.
Sensitivity analyses on waste type indicate that the acidic conditions
that result from codisposal of CCR waste with coal refuse and the basic
conditions that result from disposal of FGD waste result in higher

risks from arsenic and other constituents than CCR waste disposed
alone.
    The risk results are consistent with the groundwater damage cases
compiled by EPA. These damage cases were primarily associated with
unlined units and were most frequently associated with releases of
arsenic. Recent surveys of the industry indicate the majority of newly
constructed units are lined, and that that the practice of codisposal
with coal refuse has declined. However, this risk assessment presents a
static snapshot of current disposal practices. While newer units may be
managed in a more protective manner, older units, which still comprise
the majority of current units, continue to operate in a manner that
poses risks to human health and the environment that OSWER typically
regulates.

XI. Summary of Damage Cases

    EPA has a long history of considering damage cases in its
regulatory decisions under RCRA. As discussed earlier in this preamble,
the statute specifically directs EPA to consider ``documented cases in
which danger to human health and the environment from surface run-off
or leachate has been proved,'' in reaching its Regulatory Determination
for these wastes, demonstrating that such information is to carry great
weight in decisions under this section. 42 U.S.C. 6982(n)(4). Damage
cases, even if only potential damage cases, are also relevant under the
third Bevill factor: ``potential danger, if any, to human health and
the environment from the disposal and reuse of such materials.'' 42
U.S.C. 6982(n)(4). In addition, damage cases are among the criteria EPA
must consider under its regulations for determining whether to list a
waste as a ``hazardous waste.'' See 40 CFR 261.11(a)(3)(ix). Damage
cases generally provide extremely potent evidence in hazardous waste
listings. In this regard, EPA notes that the number of damage cases
collected for this rulemaking (157) is by far the largest number of
documented cases in the history of the RCRA program.
    EPA considers that both proven and potential damage cases provide
information directly relevant to this rulemaking. First, damage cases
provide evidence of both the extent and nature of the potential risks
to human health and the environment. The primary difference between a
proven and a potential damage case is whether the contamination has
migrated off-site of the facility. But the mere fact that groundwater
contamination has not yet migrated off-site does not change the fact
that a potentially harmful constituent has leached from the unit into
groundwater. Whether the constituent ultimately causes further damage
by migrating into drinking water wells does not diminish the
significance of the environmental damage caused to the groundwater
under the site, even where it is only a future source of drinking
water. As EPA explained in the preamble to the original 1979 open
dumping criteria, which are currently applicable to these facilities,
EPA is concerned with groundwater contamination even if the aquifer is
not currently used as a source of drinking water. Sources of drinking
water are finite, and future users' interests must also be protected.
(See 44 FR 53445-53448.) (``The Act and its legislative history clearly
reflect Congressional intent that protection of groundwater is to be a
prime concern of the criterion. . . . EPA believes that solid waste
activities should not be allowed to contaminate underground drinking
water sources to exceed established drinking water standards. Future
users of the aquifer will not be protected unless such an approach is
taken.'')
    In the June 21, 2010 proposed rule, EPA presented for public
comment an assessment of CCR damage cases, and requested comments and
other information related to damage cases EPA had previously received
from industry, environmental groups, and citizen groups. EPA later
requested public comment on additional damage case information in a
Notice of Data Availability (NODA) published in the Federal Register on
October 12, 2011 (76 FR 63252). As discussed in Section IV of this
preamble, the Agency is deferring making a Bevill determination;
however, EPA is still presenting its findings with regard to damage
cases (including information submitted during the comment periods for
the June 2010 proposal and the October 2011 NODA) because as described
above, this information supports actions taken in the present final
rule.\191\
---------------------------------------------------------------------------

    \191\ Damage Case Compendium (Technical Support Document on
Damage Cases), U.S. EPA, December 2014.
---------------------------------------------------------------------------

A. Damage Cases Presented in June 21, 2010 Proposed Rule

    In the June 2010 proposed rule, the Agency summarized its database
on damage cases that had expanded since the May 2000 Regulatory
Determination.\192\ This summary included two cases of CCR slurry spill