There were several comments regarding the cancer slope factor for arsenic and arsenic bioavailability provided to EPA during the public review period. We believe that the comments do not reflect the wealth of information the Agency is currently considering regarding the health effects of arsenic. First. the cancer slope factor for arsenic is likely to be revised downward. EPA should be aware of the considerable effort currently underway in the Office of Water to comply with the I996 Safe Drinking Water Act provision that requires EPA to establish a MCL for arsenic by January 1, 2000. This effort has included the establishment of a Subcommittee on Arsenic of the Committee of Toxicology of the National Research Council ( NRC) and an Expert Panel on Arsenic Carcinogenicity under the Integrated Risk Information System ( NRC, 1999, EPA, 1999). The findings of these efforts reflect the state of the science by the leading experts on the health effects of arsenic. In addition, there is an extensive research program that has been established under the 1996 Strategic Plan for the Office of Research and Development to involve Agency wide research activities to contribute to the development of an arsenic drinking water regulation. It is beyond the scope of these comments to provide a summary of these substantive activities currently underway at the Agency. However, it is imperative that EPA actively engage the Office of Water in the FFC waste regulation to respond to public comments that suggests that the slope factor overestimates cancer risk, because this viewpoint is not supported by the current analysis underway at your Agency. EPA should consider the information in the risk characterization of FFC waste. ( ALA00292)

The Subcommittee on Arsenic of the Committee of Toxicology of the National Research Council published a final report in March 1999, which concluded that recent studies suggest that drinking water with high levels of arsenic can lead to bladder, and lung cancer, which can be more fatal than skin cancer. The Subcommittee examined seven International epidemiological studies, including research from Taiwan, Argentina, and Chile. All these studies show that in addition to causing skin, bladder, and lung cancer  exposure to arsenic in drinking water can cause skin lesions, anemia, nerve damage and circulatory problems. Finally, and most importantly with regard to the groundwater risk assessment of FFC waste, the report concluded that " the epidemiological findings, experimental data on the mode of action of arsenic, and available information on the variations in human susceptibility, it is the Subcommittee's consensus that the current EPA MCL in drinking water of 50 ug/ L does not achieve EPA's goal for public health protection and, therefore, requires downward revision as promptly as possible." Based on the incidence of bladder cancer from Taiwan studies, that are qualitatively confirmed by data from Chile and Argentina the NRC determined that the difference in the potency at the modeled point of departure ( POD) is around 10 fold less than the current cancer slope factor. There are Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 28 XIV  28 uncertainties in this extrapolation. Extrapolations below 100 ppb become nonlinear and should be caveated as probable overestimates and both the POD and the environmental exposure ( real or projected) should be examined as the NRC did. Based on this analysis by the NRC, the suggestion that arsenic is less toxic than and the state of the science regarding the health effects of arsenic should be incorporated into the Report to Congress. ( ALA00292) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 29 XIV  29 XIV. RISK METHODOLOGY IN GENERAL G. Site Averaging Industry commenters suggested that site averaging overestimated risk. One of the commenters stated that using a numerical average to represent a typical value for concentrations that are likely characterized by a lognormal distribution would be an overestimate. A geometric mean would have been a more appropriate representation of a typical value for a given site. Another of the commenters stated that EPA should have used the all the data as independent observations and fitted the best probability density function, rather than use the 95th percentile siteaveraged concentrations. According to the commenter, under this approach, the 95th percentile porewater concentration for arsenic would have been less than 0.5 mg/ L, rather than 9.64 mg/ L. The commenter also suggested that EPA overlooked the variations within each waste management unit and inappropriately assumed the release of uniform concentrations across the unit.

Public interest group commenters, on the other hand, argued that site averaging underestimated risk by cutting off extreme observations. These extremes reflect " hot spots" of non uniform mixing that should be modeled. One of these commenters further stated that variation within facilities is at least as important as variation between facilities. Another of these commenters stated that EPA should have used the 95th percentile upper confidence limit of the arithmetic mean of a lognormal distribution. This commenter also specifically addressed the site averaging approach for OCW ( oil combustion wastes), which changed between the draft risk assessment and the sensitivity analysis, and requested an explanation of why the revised approach would not underestimate high end concentrations.

Response: EPA believes its approach to site averaging was both reasonable and appropriate as a method of producing a typical, yet conservative input value for use in the risk assessment. The approach allowed characterization of the variations between sites without overweighting sites with more characterization data and without overweighting unusually high or low values that would not be typical of entire management units. EPA believes use of this approach produced an appropriate estimate of upper bound and probabilistic risk for purposes of this study without unnecessarily over or under estimating risk. EPA checked this approach in each case with the available site data to be sure mischaracterization would be minimized, if not avoided entirely. EPA used a log normal distribution as suggested by one commenter.

EPA disagrees with the comment that it assumed uniform release of contaminants from a particular comanaged waste management unit. Contaminant supply is finite, and release will vary over time depending on opening and closing of landfill cells, closing of an impoundment and many environmental factors. Variations of measured contaminant within each management unit are reflected in the data. To simplify its analysis, EPA calculated a single contaminant concentration for each site, as noted above, to represent the overall effects of this variability, but this calculation does not imply that contaminant release is uniform. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 30 XIV  30 EPA disagrees that its site averaging approach underestimates risks. The management units in question are large in size and have compositional variations. The management units may contain " hot spots" but may also contain wastes with relatively low contaminant release. While EPA intended its analysis to be conservative, an analysis which focuses on the extremes posed within individual sites would be unrealistic. While EPA has found instances of localized high contaminant concentrations, it has not found instances where an entire management unit has these characteristics and believes such an instance is unrealistic. Therefore EPA disagrees with the commenters that these localized effects were not adequately addressed in the risk assessment. EPA contends that its use of site averaging, with the subsequent selection of the 95th percentile concentration, represents a defensible high end initial concentration. EPA utilized the data collected and provided by industry, in reaching the risk findings noted for coal combustion wastes.

The above issue concerns the best way to characterize a waste management unit, and EPA acknowledges that opinions may vary. At one site, measured concentrations varied from below .01 mg/ l to over 80 mg/ l. This reflected a very thorough data collection effort; yet no argument can be made that either extreme is representative of the volume of waste in that unit.

EPA also maintains that the site averaging methodology adopted in the sensitivity analysis is appropriate for OCWs. EPA has found that oil combustion facilities operating surface impoundments commonly manage their fly ash along with low volume wastes in a single impoundment or series of impoundments. The dredged solids are subsequently landfilled offsite. The bottom ash is typically managed separately from the fly ash but also landfilled offsite. Therefore, all of these wastes are eventually landfilled, and it would be plausible for a generating facility to use a single landfill location for the disposal of its different wastes. In conclusion, because facilities may combine different types of oil combustion wastes in a single management unit, EPA believes that its calculations are appropriate and do not underestimate potential risks for the landfill scenario. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 31 XIV  31 XIV. RISK METHODOLOGY IN GENERAL G. Site Averaging Verbatim Commenter Statements The TCLP input data used for EPA's groundwater modeling represent " facility averaged" values, and thus do not represent actual high end values. EPA rationalized the use of average values because " the Agency was more interested in variation between facilities than variation within facilities." This rationale is both misguided and fails to support the intended objective anyway. The Report to Congress and the associated regulatory determination are supposed to ascertain whether FFCW wastes warrant regulation. Thus for this purpose, variation at individual facilities is at least as important to capture as the variation between facilities. Moreover, high and low values at individual facilities are actually better indications of the full extent of variations on a national basis when aggregated. Perhaps more importantly, the measured high end values at individual facilities represent real world " hot spots" reflective of non uniform mixing of various wastes within very large units. Indeed, it is not unreasonable to expect that contaminants migrating from these units may preferentially reflect areas where one or more low volume wastes have been placed, subsequently enhancing the migratory potential of other wastes coming into contact with them. Therefore, unless EPA can demonstrate uniform mixing and/ or only " average" waste contamination migration at these facilities, the " potential" risks from the co management units in the Report to Congress are equally or more accurately reflected by releases from " hot spot" areas. ( EDF00021)

EPA also cannot vouch for the representativeness of the TCLP data on a facility specific basis, therefore the Agency hasn't a clue what it is combining to achieve a so called " average" result. The term itself is misleading in this regard. Averaging non representative data for a facility does not magically yield a representative facility average. For all these reasons, EPA's objective of capturing variations between facilities using facility averaged data is extremely problematic based upon the data now available, and in fact national variation would be better captured by incorporating true high end values measured at particular facilities. As a consequence of relying upon facility averaged data instead of actual high end measured values, the " screening analyses" relied upon by EPA to exclude most hazardous constituents that " might" be a threat were far from the conservative analyses EPA describes in the Report to Congress. For these assessments, EPA claimed to use the 95th percentile waste concentrations, but by using the 95th percentile of the facility average data, the Agency failed to utilize actual high end values measured consistent with risk assessments performed in support of listing determinations and other RCRA proceedings. The same is true of EPA's subsequent monte carlo analyses, because none of the 2,000 model runs reflect actually measured high end values. ( EDF00021)

Some of the documents I have examined indicate that the sample giving the 9.64 ppm arsenic value from the deterministic case came from a site in Missouri where pore water sample were taken up and down the length of the core from above the water table and that the concentrations were Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 32 XIV  32 attenuating rapidly downward through the sample before the water table was reached. If this is true then the use of this sample in the deterministic case was a technical catastrophe. Not only was the wrong kind of sample used and averaged as if it represented an intercept with the water table down the entire length, but the sample showed that attenuation far beyond the value used had occurred befor contact with the water table ... Using a simple averaging process to reduce data from many levels in several cores to a single value for a site was a statistical blunder. The fact that the procedure was allowed to mask data in direct conflict with the way the model was run is exceptionally hurtful to good science has resulted in claiming a water table contact concentration that was nearly two orders of magnitude off from what the data actually showed. ( NMA00024A)

EPA first averaged pore water concentrations numerically up and down the entire length of the cores sampled. The problem with taking numerical averages of samples from a lognormal distribution ( trace element distributions are almost alway lognormal and coversations with the EPA's contractor on the modeling indicated that they had confirmed lognormality of the data   another fact not apparently in the report) is that values from the upper X% of tail are much further from the value of the bulk of the samples than the corresponding X% value from the lower tail. Although mathematicians give a little more hairy an explanation of what is happening the result is that the numerical average value will ( except by pure luck) always drastically over estimate the typical value for the population. Taking numerical averages also distorts the shape of the distribution of averages relative to the original population. Those familiar with statistics usually know that the most basic theorem is the " Central Limit Theorem" which says that the distribution of average values will tend to be a normal distribution regardless of what the original underlying distribution was ( ie., EPA's action of creating secondary populations by taking numerical averages was already reducing skewness and distorting the data toward a normal distribution). EPA then took all the already once averaged pore water concentrations from samples over entire sites and took another numerical average pore water concentration for the site ... Remember, however, that the typical values of this population have already been shifted upwards through the use of numerical averages instead of geometric means and that the skewness of the distribution has been reduced shifting more weight into the upper tails. ( NMA00024A)

The high end deterministic assessment was designed to provide an estimate of exposure ( and risk) that is greater than or equal to the risk expected at any site. To establish this, EPA determined the 50th and 95th percentile concentrations by averaging all the samples from a given site to arrive at a single leachate concentration. These average values were then arrayed to develop the 50th and 95th percentile for each constituent in each waste type. In other words, the 50th and 95th percentile concentrations were generated from facility wide averages and do not necessarily reflect high end estimates. ( ALA00036)

EPA changed the concentration data for individual wastes for the oil ash scenarios. The facility average 95th percentile concentration that was calculated for each waste type was revised to include all wastes. Since there are significant differences in the concentration of chemicals in the various waste types, EPA needs to provide an explanation. as to why this approach would not Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 33 XIV  33 underestimate the high end concentrations by combining waste that have relatively high levels with wastes that have relatively low levels of contamination. ( ALA00036)

For inputs to the deterministic model for each waste management scenario and each constituent of concern, EPA calculated the average value of all porewater data for each waste management scenario and each constituent of concern for each given site. EPA then chose the conservative 95th percentile concentration as the " high end" value for model input. Where there were data available from fewer than 20 sites, EPA used the highest available value as the input concentration. This approach accentuated and magnified anomalous data points. ( USWAG00037)

As discussed above, a more representative selection of high end values would have provided a more accurate assessment. EPRI provided over 100 porewater concentration data points taken from over 15 landfills and impoundments. Rather than the 95th percentile site averaged concentration, EPA should have used these data as independent observations and treated them as random values for statistical purposes. This approach would have provided the best definition of porewater concentration probability distribution. The Agency should then have fitted the best probability density function to the data to obtain an estimate of the central tendency as well as the high end value. An examination of the arsenic data, for example, indicates that the 95th percentile concentration should have been less than 0.5 mg/ L, compared to the value 9.64mglL used in the risk assessment. Thus, EPA used an input concentration value almost 20 times too high. ( USWAG00037)

The waste characterization data were averaged for each facility and then the averages were averaged  which completely masks any high values, and is inconsistent with a conservative approach. ( 49CAO00058)

The exposure and risk assessments seemingly do not represent a " high end" analysis, but rather represent averaged data. ( 49CA00058).

Demonstrating that the upper bound concentrations of FFC waste constituents have been evaluated in the Report to Congress is central to EPA's contention that the risks have been overstated and, therefore, should be dismissed. EPA calculated facility wide averages for 14 sites. These averages were then arrayed and the 95th percentile was determined. Because facility wide averages were used, the 95 percentile values represents the 95th percentile of the average FFC waste concentrations and not of the 95th percentile of the distribution. This concept is illustrated in Figure 1. According to EPA policy these statistical values are considered central tendency and not high end values. Figure 1 also illustrates that as the sample size increases the UCL of the mean moves closer to the true mean, while the 95th percentile of the distribution remains at the upper end of the distribution ( EPA, 1992). ( ALA00292)

The waste characterization data presented in the Report to Congress shows that FFC waste is highly variable. SAIC observed that the concentration of some metals in waste leachates was Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 34 XIV  34 found to vary by up to three or four orders of magnitude ( SAIC, 1998, page 2 l). To address the uncertainties associated with estimating the true average concentration that results from variability of environmental contaminants, the Agency has established the use of the 95th percent upper confidence limit ( UCL) of the arithmetic mean in exposure assessments ( USEPA, 1992). It does not appear that the facility wide averages calculated from the 14 site investigations is the most appropriate statistical approach to address the limited sampling data and variability in the FFC waste constituents. However, we are unable to calculate the 95 percent UCL of the average for FFC wastes and compare it to the values used in the Report to Congress because, as previously mentioned, the analytical data are not provided in the Report to Congress or technical background documents. The June 25th SAIC memo to CATF does indicate that the FFC waste characterization data are log normally distributed. Therefore, the following equation ( Figure 2) should be used to calculate the upper confidence limit of the arithmetic mean for a lognormal distribution. ( ALA00292) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 25 FF2P S0361, Attachment 1 ( Deterministic Results).

35 XIV  35 XIV. RISK METHODOLOGY IN GENERAL H. Inclusion of Montour An industry commenter indicated that the inclusion of the Montour site skewed the risk results because this site is not representative of actual management practices at the majority of sites. A public interest group commenter was concerned that the Monte Carlo analysis did not incorporate the concentrations from this site and another site with a high end arsenic concentration.

Response: EPA believes the inclusion of the Montour site was reasonable and appropriate. The highest arsenic pore water concentrations were found at this site, and were subsequently used as input concentrations in the risk assessment. EPA agrees with the commenter that this particular site has unusual management practices and may not represent a " typical" facility. One objective of EPA's risk assessment was to identify high end risks. Therefore, the assessment used high end initial concentrations as source concentrations. High end initial concentrations may be found at a minority of sites, such as Montour. These data were provided, voluntarily, by industry, presumably to be used and not dismissed as typical of an " outlier." There is no assurance that such data might in fact represent an " outlier".

EPA simply used the data it had, and described the data used in each case. The data point used was not the highest reported at Montour by a factor of approximately ten; the data point used was the average of the data set provided by industry.

At the same time, EPA appreciates the commenter's concern that inclusion of this particular site may have given excessive weight to the worst case observations of arsenic concentration in the relatively small sample size. As part of its revised analyses in October 1998, EPA performed a high end deterministic analysis removing the Montour site from the distribution of arsenic data. The next highest site averaged arsenic concentration was approximately half of the value observed at Montour ( 5.4 mg/ L at CASJ site vs 9.6 mg/ L at the Montour site). Resulting risk decreased by the same amount ( i. e., approximately half). 25 Therefore, this change in input concentration did not result in a statistically significant reduction in risk... e. g. one half of an estimated 4x10 4 is 2x10 4.

In response to the public interest group commenter, the Monte Carlo analysis does incorporate the full distribution of calculated input concentrations from all sites, including Montour and the CASJ site ( i. e., the site with the second highest site average arsenic concentration). Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 36 XIV  36 XIV. RISK METHODOLOGY IN GENERAL H. Inclusion of Montour Verbatim Commenter Statements In the case of arsenic, it is clear that a statistical outlier skewed the entire modeling effort and rendered the results unfit for a generic nationwide determination to regulate. Arsenic concentrations in surface impoundment pore water samples ranged from 0.0075 to 9.64 with a mean value of 1.57 mg/ L. EPA used the high end value 9.64 mg/ L to characterize all electric utility industry co management surface impoundments. This high end value was the maximum observed value at the Montour (" MO") site. After a simple review of the full set of available data, it becomes evident that the arsenic concentrations at the MO site are not representative of the arsenic concentrations in waste management units in the industry. For that matter, this value is not representative of arsenic concentrations at this site, as discussed in detail in the next bullet point. Nonetheless, EPA used the MO concentration data to model arsenic risk that is orders of magnitude higher than would be the case if a representative sample were used. ( USWAG00037)

EPA's arsenic source term concentration is based upon anomalous management practices at one site. EPA's source term selection process also overlooked the variations occuring within each waste management unit, and the Agency inappropriately assumed the release of uniform concentrations across the unit. The high arsenic concentrations at the MO site were observed only in porewater in the upper portion of the mill rejects area, which contained only mill rejects over an ash base. This dedicated mill rejects area represented less than 7 percent of the total area of the 144 acre impoundment. The assumption that arsenic is being released to groundwater from the entire facility at the porewater concentration in the mill rejects area grossly overestimates the arsenic release to groundwater. Segregated management of mill rejects as was practiced at the MO site is not typical of most utility impoundments. ( USWAG00037)

It does also appear that the runs did not include the higher end concentrations reported in two site investigations. ( ALA00292) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 26 FF2P S0363.

37 XIV  37 XIV. RISK METHODOLOGY IN GENERAL I. Adequacy of Coordination Public interest group commenters expressed concern that the ground water and nongroundwater risk assessments were not adequately coordinated. One of the commenters was specifically concerned that the assessments were not conducted in such a way as to allow aggregation of risks. The commenter was also concerned that the assessments used different receptor characteristics.

Response: EPA believes that the ground water and non groundwater risk assessments were adequately coordinated. Both assessments used data from the same waste characterization sources and used identical values for waste management unit characteristics. Both assessments were double high end assessments, used similar screening procedures, presented results in similar format, and were subject to the same degree of peer review. The Agency acknowledges that it did not attempt to aggregate risks from the ground water and non groundwater pathways, but does not believe such aggregation is appropriate, given that very different populations would be exposed via these pathways. Exposures to above ground risks from agricultural use are to current populations. Ground water contamination may take generations or hundreds of years to reach human populations.

The commenter's concern regarding the use of different receptor characteristics centered on the premise that using different receptor characteristics " makes it impossible to aggregate the ground water and non groundwater risks into a total human health risk." As mentioned in the preceding paragraph, EPA asserts that it is inappropriate to aggregate risks that are occurring at different time periods. Review of Table 5 23 in Technical Background Document for the Supplemental Report to Congress on Remaining Fossil Fuel Combustion Wastes: Ground Water Pathway Human Health Risk Assessment26 shows that the time to reach risk generally requires time periods that are in the thousands of years. However, the time to reach maximum risk for the nongroundwater pathways is achieved during the operating life of the waste management unit which, depending on the waste management scenario, is assumed to be 30 to 40 years. The one possible exception to this involves vanadium in oil combustion wastes managed in an onsite monofill where the time to peak ground water concentration was 80 years. For this scenario the non groundwater analysis assumed a monofill life of 30 years and therefore maximum risks would have been achieved within that 30 year period. While the times to maximum risk were much closer for this scenario, EPA still considers the aggregation of ground water and non groundwater risks inappropriate, primarily because, as noted in other responses, they occur at times that may be as much as hundreds of years apart. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 38 XIV  38 XIV. RISK METHODOLOGY IN GENERAL I. Adequacy of Coordination Verbatim Commenter Statements There is inconsistency between the dose response assessment used in the groundwater and nongroundwater risk assessment. ( ALA00036)

Despite the Agency's assurance that every effort was made to coordinate the groundwater pathway analysis and the aboveground exposure assessment, a close reading reveals that the modeling assumptions for each are such that the exposures are evaluated separately. ( ALA00036)

While the Agency claims that every effort was made to coordinate the groundwater pathway analysis and the aboveground exposure assessment, it appears that these evaluations were done completely separately. ( 49CAO00058)

EPA has not included such aggregate human health risk figures in its Report to Congress. It seems that this was not done. in part. because the analyses were not conducted in such a way as to facilitate or allow for it. ( ALA00292)

However, as stated in the EPA's own draft Groundwater Pathway Human Health Risk Assessment, the groundwater and non groundwater analyses are inconsistent with respect to the characteristics of the receptors. This is the inconsistency that makes it impossible to aggregate the groundwater and non groundwater risks into a total human health risk estimate. ( ALA00292) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 27 FF2P S0326.

39 XIV  39 XIV. RISK METHODOLOGY IN GENERAL J. Waste Management Unit Size A public interest group commenter stated that no data were available on the area of oil combustion waste surface impoundments and argued that EPA should collect this data. The commenter also argued that EPA consistently underestimated waste management unit sizes.

Response: Data were available on the area of oil combustion waste surface impoundments in the EPRI report on oil combustion waste management. 27 EPA used these data to describe oil combustion impoundment size in the Report to Congress and to calculate input values for the risk assessment. EPA also disagrees that waste management unit sizes were consistently underestimated. For its assessments, EPA used data from surveys sponsored by industry associations to estimate the size and characteristics of the waste management units. These data represented the best available for characterizing these particular waste management units. EPA used the distribution of these values as inputs in its Monte Carlo analyses, including very large units. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 40 XIV  40 XIV. RISK METHODOLOGY IN GENERAL J. Waste Management Unit Size Verbatim Commenter Statements In the assumptions for developing hypothetical waste management units, EPA desired to use 50th and 95th percentile values for estimating the unit size. However, comparison the area selected in the assessment with data from an industry survey indicates that EPA consistently underestimated the size of the units in the assessment compared to actual field data. ( ALA00036)

lt is difficult to assess the parameters chosen for WMU area as only summary data are provided in the docket, from the EPRI survey of waste management practices. Even the technical background document fails to provide any detailed information. Based on the summary statistics provided, the WMU area chosen for the CCW and FBC landfills seem reasonable. In contrast, no data are available regarding the area of oil waste impoundments. As a result. only one oil waste unit was modeled, with dimensions calculated from an estimate of oil waste generation. The Agency should request impoundment area information from the oil waste facilities that participated in the EPRI survey. This information should be readily available and is critical to evaluating wind erosion from surface impoundments. ( ALA00292) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 1 XV  1 XV. GROUNDWATER RISK MODELING Many commenters provided input on the design of EPACMTP, the groundwater model used in the analysis. One of the commenters stated that even the selection and use of EPACMTP was inappropriate. These and other commenters questioned one or more of the specific assumptions or input values used in its application. Specific concerns are addressed below in this section following this general response.

Response: EPA is carefully reviewing all of the comments on the model. The process of thoroughly investigating all of the comments will take substantially more time to complete than is available within the court deadline for issuing this regulatory determination. Accordingly, we are not relying on the results of our ground water pathway risk analysis in support of today's regulatory determination on fossil fuel combustion wastes. As explained elsewhere, in making today's regulatory determination we have relied on other information related to the potential danger that may result from the management of fossil fuel combustion wastes.

Meanwhile, we will continue with our analysis of comments on the groundwater model and risk analysis. At this time, we have not determined which, if any, changes are appropriate to make to the model. If our investigations reveal that a re analysis of groundwater risks is appropriate, we will re evaluate today's decisions as warranted.

In addition to our ongoing review of comments on the groundwater model, one element of the model   the metals partitioning component called " MINTEQ"   has been proposed for review by EPA's Science Advisory Board ( SAB). When such additional review is completed, we will take this into account in any overall decision to re evaluate today's regulatory determination.

Aside from model design issues, EPA believes the assumptions and input values it used in this application of EPACMTP were appropriate, given the specific characteristics of FFC wastes and the environmental settings of FFC facilities. The concerns raised by the commenters with regard to individual FFC waste specific EPACMTP inputs and assumptions are addressed in the responses below. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 2 XV  2 XV. GROUNDWATER RISK MODELING Verbatim Commenter Statements The workhorse model that the EPA has adopted for deterministic risk assessment does not appear to be robust enough to adequately address the many and varied conditions that are encountered with the management practices of FBC ashes. It was developed as a " generalized" model to fit national needs. ( ARIPPA00019)

The extreme conservatism has resulted in the application of unreliable, mutually exclusive data that, on occasion, results in scenario parameters that break basic scientific principals. Selecting the input parameters more carefully in order to more realistically represent even extreme conditions for managed FBC ashes should dramatically change the outcome of the Report and position of the EPA on FBC ash. ( ARIPPA00019)

The human health risk assessment used a hypothetical groundwater exposure that is overly conservative. The EPA should have used more realistic exposure point concentrations within a realistic time frame. In PG& E Gen's view, the selection of the EPACMTP fate and transport model, input parameters to that model ... are overly conservative, as discussed below. ( PG& E00023)

The selection and use of EPACMTP to model the fate and transport of metals in the environment is inappropriate. The model oversimplifies what happens to metals in the environment and fails to consider likely documented reactions ( see comments below). EPA, in fact, recommends the use of other models ( such as MINTEQA2, GEOCHEM) for predicting metals fate and transport ( McLean and Bledsoe, 1992). ( PG& E00023)

Dr. Paul of Southern Illinois University also noted some fundamental flaws in the EPA computer model used to construct a risk assessment on the groundwater pathway. Dr. Paul highlighted some of those concerns in his May 2 1 testimony, and his final written comments provide considerably greater detail. ( NMA00024)

It is my belief that the real world could not validate even the isolated harms identified in the computer models because the models were fed extreme inputs and because the model does not treat important natural attenuation processes. ( NMA00024A)

To summarize, the pertinent part of EPA's groundwater modeling methodology was not reviewed by SAB, and the modeling methodology suffers from substantial flaws as identified in the enclosed report ( which in some cases were previously acknowledged by EPA in other contexts but never properly revised for this effort). These flaws produce substantial understatements of potential groundwater risks, and consequently explain ( at least in large part) why EPA's metals modeling is inconsistent with EPA damage case information in a variety of RCRA contexts. ( EDF00021) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 3 XV  3 The inadequacies of the risk assessment suggest that the depiction of risks is incorrect in the risk characterization portion of the analysis. ( ALA00036)

By using extremely conservative assumptions in the model that fail to mimic real world conditions, the model produces exaggerated risk conclusions divorced from reality and ceases to be a tool for sound regulatory decision making. ( USWAG00037)

As discussed in detail below, the chosen model, Composite Model for Leachate Migration with Transformation Product (" CMTP"), has been applied to conditions beyond its capabilities, and extremely conservative assumptions have compounded the error. ( USWAG00037)

Not only did EPA neglect the wealth of real world data in favor of a model that is ill equipped to simulate the conditions occurring in FFC waste landfills and surface impoundments, but the Agency modeled conditions that are too far removed from reality to form a defensible foundation for regulatory action. ( USWAG00037)

However, with the decision to use a predictive model as a policymaking tool comes the obligation to base the selection of input parameters on sound science. The assumptions that form the basis of the Bevill modeling effort run afoul of that duty and undermine their national applicability. As one of the two peer reviewers of the non groundwater risk analysis stated, " The scenarios addressed by the document cannot be applied on a nationwide basis. " ( USWAG00037)

USWAG comments provide detailed, technical information demonstrating the risks are overstated by several orders of magnitude. Reiteration of the technical arguments is unnecessary. However, APS strongly agrees with USWAG's position that ... the modeling incorporated numerous overly conservative and technically inappropriate assumptions including the questionable application of pore water data ... Cumulatively, the numerous technical issues associated with the model invalidate the agency's concern with respect to the groundwater risk associated with arsenic. ( APSC00043)

Because of their limitations, environmental models ( including risk assessment models) quite often substantially overstate real world exposure and risks. They are even more conservative when their screening levels are set at unrealistically low thresholds and their internal (" default") assumptions maximize projected impacts, defining " significant" impacts very stringently. ( CIBO00052)

The groundwater modeling on which the analysis is based, according to peer review comments, has deficiencies in at least 13 steps, including but not limited to fracture flow, the understanding of the complex hydrogeologic conditions found in mine land areas, and a tendency to overestimate leachate production rates and leachate concentrations. ( CIBO00052)

The risk assessment that models the fate and transport of contaminants in ground water dramatically and falsely understates the real risks that are occurring from disposal of fossil fuel Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 4 XV  4 wastes. The methodology used in the risk assessment that is based on EPACMTP modeling, is fundamentally flawed. There are errors of logic, of implementation, of programming, and simple quality control, These errors almost universally understate risks. Even under the conditions that the Agency purportedly models, the modeling undercalculates by orders of magnitudes the risks from FFCW, ( See Use of EPACMTP to Estimate Groundwater Pathway Risks From Land Disposal of Selected Inorganic Compounds by Norris and Hubbard). ( HEC00056)

At this time, USWAG finds it necessary to provide additional comment on EPA's risk modeling to specifically address comments filed by the Environmental Defense Fund ( EDF). Those comments allege that EPA's modeling under predicts concentrations of contaminants. However, those views are not based upon a plausible scientific foundation and do not stand up to simple field validation. EDF's comments are based upon a report by Noms & Hubbard designed to support the argument that EPA's model significantly understates the risk of coal combustion waste management. ( USWAG00275)

In earlier comments, CIBO pointed out with specificity the many flaws in the models so heavily relied on by EPA. ( CIBO00280)

There are significant deficiencies in each of the steps in EPA's assessment of the human health impacts of current FFC waste disposal practices. The cumulative effect of these deficiencies indicates that EPA has underestimated the human health impacts associated with exposure to FFC wastes, particularly to individuals with above average exposures. ( ALA00292)

The modeling that forms the basis of the recommendations fails to consider the strong bias impressed by natural and manmade geologic processes. ( RICE00041)

The risk assessments are not adequate. There are several ways in which the risk assessment and exposure analyses contained in the Report are inadequate and inconsistent with Agency policy. ( 49CAO00058)

It appears that EPA's concern is based on the results of a model that indicates that the concentration of arsenic in groundwater at a receptor well could reach the health based level of arsenic after 3,000 years. EPA's model, among other items, does not appear to account for the potential for these wastes to exhibit self cementing properties leading to very low hydraulic conductivity ... Other issues raised by EPA's model were discussed in ARIPPA's 6/ l2/ 99 comments, which are incorporated herein by reference. ( ARIPPA00273)

PG& E Gen's initial comments included risk assessment criticism of EPA's model and assumptions that were used to generate the remote risk to human health for arsenic from agricultural use of FBC coal ash. PG& E Gen's review identified numerous problems that cause the model to overestimate the potential risk presented by this beneficial use of coal ash, such as the choice of an inappropriate fate and transport model, unrealistic geochemical process Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 5 XV  5 assumptions. ( PG& E00274)

The bulk of the [ USWAG] comments discuss ... the Norris and Hubbard report on EPACMTP modeling. These are the topics to which I will respond. ( GHIL0012)

It is observed that the EPACMTP modeling results under predict concentrations of metals relative to the observations in the real world data. ( GHIL0012) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 6 XV  6 XV. GROUNDWATER RISK MODELING A. Model Design In General EPA's groundwater model, EPACMTP, was subjected to careful review by stakeholders on both sides of today's rulemaking. The following captures the essence of this review.

Industry commenters stated that prior SAB review of the model concluded that the model needs validation and has deficiencies in manifesting an understanding of the complex hydrogeologic conditions found in waste placement areas with the resultant tendency to overestimate leachate production rates and leachate concentrations. Other specific design concerns raised by industry commenters included the following: the model uses insufficient thermodynamic data for metal species and interactions; the model does not account for the effect of redox potential on metal speciation and mobility; the model has the potential to produce initial concentrations for metals that exceed known chemical solubilities; and the model does not addresses the situation in which a background concentration of the metal of concern exists in either the vadose or saturated zone. One of these commenters suggested that the Department of Energy ( DOE) has developed technically sophisticated models to assess certain complex geologies and that EPA should coordinate with DOE to improve its model design.

Public interest group commenters also raised specific questions about the design of the model. These included the following major concerns: The model incorrectly assumes an unsaturated soil zone that will absorb contaminant metals; the model assumes there is no change to ground water except for the addition of one RCRA metal as a contaminant ( thus ignoring metals loading from multiple contaminants that compete for a finite number of adsorption sites); the model assumes ground water will remain buffered with respect to pH and not change as a result of waste leachate; and the model produces a virtually inexhaustible and unrealistic pollution sink in the unsaturated zone. One of these commenters provided a detailed technical report documenting model deficiencies that may have led to an underestimate of risk.

Response: As a direct result of comments received, EPA/ OSW and EPA/ ORD are engaged in an item by item review of EPACMTP and its metals partitioning component, MINTEQA2. As appropriate, we will review other modeling approaches. While it is not known at this time whether risk estimates will change, either up or down, EPA expects when the review is completed to have greater assurance of the model's reliability. The review may result in a reevaluation of the potential groundwater risks posed by management of fossil fuel combustion wastes and EPA action to revise today's determination, if appropriate. Specific comments are addressed below, but in many cases must await the model/ MINTEQ review already noted. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 7 XV  7 XV. GROUNDWATER RISK MODELING A. Model Design in General Verbatim Commenter Statements However, when attempts to apply it to site specific or regional applications, many of which are very complex due to pre existing deep mines, the over simplifications that are required to be made in order to make it function result in " high end" scenarios that are not only unrealistic but which violate basic scientific principals. ( ARIPPA00019)

If we are to assess the " quality" of the EPACMTP risk assessment code, we can only judge it on the basis of the public documents provided by the EPA. The latest retrievable citation to this code is EPA SAB EEC 95 010, which constitutes a peer review of the code initiated by the EPA. In this review, the committee is critical of the coding and has recommended 13 steps to insure its validity. Since this recommendation is the last public reference to the code, we can not make any assumptions on the status of the implementation of the peer committee's recommendations and can only go by the peer review committees published statements. ( ARIPPA00019)

Recent publications by Nishimura and Robins [ 1998] and Bothe and Brown [ 1998,1999] both contribute new thermodynamic data relating to the solubilities of calcium arsenate compounds. Indeed these new data support the identification of new compounds in the phase stability ranges that encompasses the chemical and environmental conditions under consideration in this review. The Ground Water Pathway Human Health Risk Assessment suggests that only 16 metal species were considered in the modeling activity. This restricted use of thermodynamic data is clearly inadequate especially in light of the importance of ion pairing in this predictive calculation and the impact of ionic strength. Additionally, Eh can and does play an important role for poly valent metals where solubilities can vary significantly for different valence states e. g. the EPA's reevaluation of chromium. ( ARIPPA00019)

Insufficient thermodynamic data for metal species and their interactions were considered in the model. ( ARIPPA00019)

The assessment that is required of EPA is not at all different from the activities that are being undertaken for the assessment of a Federal nuclearwaste repository. The performance assessment groups must deal with release and the transport and fate of radionuclides in the environment. This community of researchers has developed models that address fluid flow in fractured media, in saturated and unsaturated zone and structurally complex geology. Both the Performance Assessment Group at Sandia National Laboratory and Argonne National Laboratory  West have the capabilities to conduct such risk assessments and have performed them on comparably complex designs. Golder Associates of Seattle, WA have developed what appears to be the code that will establish the benchmark for all future assessment calculations. Lawrence Livermore Laboratory Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 8 XV  8 has for decades maintained the state of the art geochemical codes for thermodynamic calculation of reaction pathways. Perhaps inter agency cooperation between the DOE and EPA is appropriate at this point. ( ARIPPA00019)

The mobility of metals is also largely dependent upon metal speciation. The redox potential or oxidation/ reduction potential ( ORP) of the surrounding aquifer materials determines the speciation of a metal ( Deutsch, 1997). The EPACMTP model does not account for the effects of ORP on metal specification, and hence, mobility. Natural changes in the speciation can significantly further decrease the dissolved metal concentration downgradient from source areas. ( PG& E00023)

While EPA claims EPACMTP was reviewed by EPA's Science Advisory Board ( SAB), this assertion is only partially true. In fact, MINTEQA2 was expressly not evaluated, and SAB indicated " the accuracy of the model estimates must be verified and the documentation of this use needs to be clarified". Therefore, SAB has not reviewed one of the most crucial components of EPA's groundwater modeling underlying the Report to Congress. ( EDF00021)

As the CKD experience demonstrates, the releases from these large volume waste disposal units can overwhelm any neutralization capacity the uncontaminated groundwater may offer, and thus background pH offers only temporary protection in such circumstances. In ascertaining " potential" risks, it is appropriate for EPA to assume the groundwater underneath and in close proximity to the management units will reflect the pH of the waste when large quantities of waste is disposed in or near or the water table. ( EDF00021)

In effect, the errors in EPA's modeling for the Report to Congress produce a virtually inexhaustible and unrealistic pollution sink in the unsaturated zone, effectively capturing most of the pollutants so that they never reach the groundwater. ( EDF00021)

EPA's CMTP groundwater fate and transport model is ill suited for modeling some of the complex physical chemical phenomena present in the utility waste management units under study. In particular, the model's defects preclude accurate modeling of arsenic fate and transport. ( USWAG00037)

The groundwater modeling on which the analysis is based, according to peer review comments, has deficiencies in at least 13 steps, including but not limited to fracture flow, the understanding of the complex hydrogeologic conditions found in mine land areas, and a tendency to overestimate leachate production rates and leachate concentrations. For example, the peer review panel recognized in 1995 that the model had the potential to produce initial concentrations for metals that exceed the known chemical solubilities. The Science Advisory Board ( EPA SAB EEC 92 003) first disclosed this defect in 1992. Again in its draft of the Ground Water Impact Assessment Document, EPA freely admits that it " most likely overestimated leachate generation and leachate concentrations." Furthermore, no mechanism appears to be available to address the situation in which a background concentration of the metal of concern exists in either the vadose or saturated Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 9 XV  9 zone, thus further exacerbating the overestimate of leachate generation and its impact. ( CIBO00052)

To cite but one flaw, one cannot determine whether the EPA CMTP risk assessment code utilized by EPA included the utilization of the 13 steps recommended by the peer review committee. This flaw is fully explained in materials prepared by Professor Barry E. Sheetz ( Pennsylvania State University), which have been submitied to EPA. ( CIBO00280)

We recommend that EPA refer to the Agency's Soil Screening Guidance that addresses the risks associated with ground water contamination from contaminants leaching from soil. This guidance provides an extensive review of the EPACMTP model and other unsaturated zone models that was conducted by EPA's ORD laboratories in Oklahoma and Georgia ( EPA, 1998, pages 68 80). ( ALA00292) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 10 XV  10 XV. GROUNDWATER RISK MODELING B. Precipitation/ co precipitation Several industry commenters indicated that EPACMTP is not capable of modeling the significant precipitation and co precipitation phenomena that they believe takes place in waste management units. As a result of not considering these natural attenuation phenomena, industry argues contaminant transport was overestimated and risks, therefore, were overestimated.

Response: The extent of precipitation/ co precipitation occurring will be governed by chemistry of the vadose zone and the saturated zone, and EPA acknowledges that the methodology for calculating this may be improved as part of the examination of broader geochemical interactions as planned for the overall model review. Precipitation and co precipitation are dependent phenomena. This issue is discussed further under Topix XV. L, below. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 11 XV  11 XV. GROUNDWATER RISK MODELING B. Precipitation/ co precipitation Verbatim Commenter Statements The concentration of dissolved metals can be attenuated in the subsurface environment by three main processes: 1) dispersion, 2) adsorption, and 3) precipitation ( Deutsch, 1997) as well as others ( McLean and Bledsoe, 1992). The EPACMTP model assumes that metals undergo attenuation by dispersion and adsorption o& and does not account for the effects of precipitation. This is a shortcoming because the precipitation of metal minerals in the subsurface has been well documented to occur naturally ( Deutsch, 1997; Lindsay, 1979; McBride, 1980; McLean and Bledsoe, 1992; Rai, et al., 1987; and Sposito, 1989) and can significantly decrease the metal concentrations in groundwater downgradient from source areas. ( PG& E00023)

EPA modeling did not consider all natural attenuation factors ... The model appears to neglect coprecipitation with iron or adsorptive processes on soils or even the coal combustion product itself. In my work I have found these processes to be extremely significant. ( NM00024A)

While CMTP can model adsorption and desorption effects, it cannot account for precipitation and co precipitation, which are significant processes that are undeniably occurring in the waste management units. In the co management unit and mine placement contexts, these chemical processes are controlling." Without consideration of attenuation by precipitation and coprecipitation EPA used the input porewater concentrations obtained from within the waste management unit or mine in effect to represent the leachate emerging from the unit. As a result, the modeled dilution and attenuation factors (" DAFs") are much lower than the real world data demonstrate. ( USWAG00037)

CMTP is not capable of modeling the geochemistry that occurs in the waste management units under study. Precipitation and co precipitation of arsenic in co management units is a significant and well understood process. The oxidation of pyrites in the waste management unit yields a low pH and high concentrations of iron. When the resulting acidic leachate flows through the ash, neutralization and co precipitation occur very rapidly and within a short distance. This geochemical process is very clearly demonstrated in the vertical concentration distribution in porewater samples. The full data set indicates that concentrations attenuated rapidly downward through the sample before the water table was reached. The high end value of 9.64 mg/ L chosen by EPA is two orders of magnitude higher than the value measured at contact with the water table. Figure 4 5 of the MO report shows that the arsenic concentration dropped to approximately 0.025 mg/ L vertically through the sample core. Since the model does not perform an attenuation calculation, EPA has effectively multiplied the arsenic concentration entering the water table by two orders of magnitude. Dr. Paul labeled EPA's failure to account for this natural attenuation a " technical catastrophe." ( USWAG00037) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 12 XV  12 In light of the field data and knowledge of the prevalence of precipitation and co precipitation in these units, EPA used unrealistically low, fixed values for distribution coefficient ( Kd) for several metals. The low Kd resulted in an unnecessarily long modeled transport distance and high modeled concentration of constituents in groundwater. The retardation factor ( Rd) value ( calculated based on an assumed Kd value) used by EPA is representative of reversible arsenic adsorption reactions but not irreversible precipitation and co precipitation reactions. The EPRI field data indicate that precipation and co precipitation are the controlling reactions. ( USWAG00037)

APS strongly agrees with USWAG's position that ... precipitation and co precipitation are significant processes occurring in waste management units, and the use of the model Composite Model for Leachate Migration with Transformation Products ( CMTP), as applied, is inappropriate since CMTP can not account for these processes. ( APSC00043)

The risk assessment that models the fate and transport of contaminants in ground water dramatically and falsely understates the real risks that are occurring from disposal of fossil fuel wastes. The methodology used in the risk assessment that is based on EPACMTP modeling, is fundamentally flawed. There are errors of logic, of implementation, of programming, and simple quality control. These errors almost universally understate risks. Even under the conditions that the Agency purportedly models, the modeling undercalculates by orders of magnitudes the risks from FFCW. ( See Use of EPACMTP to Estimate Groundwater Pathway Risks From Land Disposal of Selected Inorganic Compounds by Norris and Hubbard). ( HEC00056)

The agency modeling assumes an unsaturated soil zone beneath the site that will absorb contaminant metals before they reach groundwater, and a saturated aquifer of clean water that will serve to dilute contaminants. These assumptions ignore the fact that FFCW, and CCW in particular, are sometimes currently disposed and are proposed to be disposed in a setting below the water table and where the waste mass itself replaces the entire thickness of the aquifer( s). The agency assumes there is no change to groundwater except the addition of one RCRA metal as a contaminant. This assumption ignores the fact that metals loading from multiple contaminants compete for a finite number of absorption sites. ( HEC00056)

The Agency modeling assumes a pristine soil and a dilute groundwater, with no natural, predisposal loading of absorption sites. This assumption ignores natural soil conditions and the fact that millions of tons of FFCW disposed in lagoons and monofills below the water table will not leave the natural groundwater chemistry intact. Rather, the leachate will displace natural groundwater with vastly higher concentrations of contaminants other than RCRA metals. Studies performed for EPA's Determination on cement kiln dust ( CKD) demonstrated the enhanced migration and increased risk afforded by leachates with high dissolved solid loads, but these lessons are ignored in this Determination. ( HEC00056) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 13 XV  13 The Agency modeling assumes the natural groundwater will remain buffered with respect to pH. This assumption ignores the fact that natural groundwater used in the exercise has very little buffering capacity and the mountainous masses of wastes not only generate dangerous pHs but also incredible concentrations of alkalinity or acidity. This choice to ignore this fact is, again, in spite of what was learned with the earlier CKD determination. And, while FFCWs can range from alkaline to acidic, the ability of soil to retard contamination migration is highest in the neutral range modeled by the agency and lowest at both the low and high pH ranges demonstrated by the FFCW. ( HEC00056) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 28 FF2P S0363, Section 8.

29 FF2P S0361, Attachment 6 ( Sensitivity Analysis): Mass Balance Check.

14 XV  14 XV. GROUNDWATER RISK MODELING C and D. Constant Source and Ultimate Leachability Several industry commenters stated that the model inappropriately used a constant source concentration, while actual source concentrations should decline over time. Commenters also expressed concern that the model appeared to assume that 100 percent of the mass of trace constituents is available for leaching, and suggested that only 10 percent of the constituents in CCW are leachable.

Response: EPA agrees that the ` constant source' assumption may be conservative, depending on the total contaminant available, and configuration and life of the waste unit.. The commenters correctly state that EPACMTP uses a constant source concentration for the landfill scenario. The concentration of the contaminant as well as the infiltration rate ( leachate generation rate) are both held constant for the duration of the study period. EPA acknowledges that these assumptions would overstate risk if the actual leachate concentration from the management unit decreases over time. 28 The Agency did not assume 100 percent availability for every management scenario. For surface impoundments, the model assumes a constant leaching rate for the life of the impoundment, while for landfills, the model assumes a constant leaching rate for the study period or until 100 percent of the contaminant is released from the management unit, whichever comes first. EPA has conducted follow up analyses to verify that these scenarios did not overstate the quantity of constituents available for leaching. For all constituents showing a risk greater than 10 6 or HQ= 1 in the June 1998 report for any scenario, a mass balance was conducted to determine if the total quantity of contaminant that leaches out was less than the maximum quantity that reasonably be placed in the unit.

Results of these analyses are presented in the October 1998 Sensitivity analysis. 29 In this analysis, the total quantity of contaminant released from the landfill scenario as assumed by the model was found to be no more that 13 percent for the principal contaminants of interest, which is consistent with the leachable proportion of 10 percent suggested by the commenter. These percentages are based on the initial leachate concentrations presented in the June report; lower concentrations were used in the October revised analyses. The percentages for the October analyses, if calculated, would be even lower than those presented here.

For the surface impoundment scenario, the results show that 33 percent of arsenic would be released under the model assumptions in the impoundment scenario. EPA conducted a sensitivity analysis for the impoundment infiltration rate which can be used for assessing the commenter's Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 30 FF2P S0361, Attachment 6 ( Sensitivity Analysis): Hydraulic Conductivity of Surface Impoundment Liners.

15 XV  15 suggestion that 10 percent leachability would be more reasonable. The results of this sensitivity analysis are presented in the October 1998 report. 30 The results suggest that the peak concentration, and therefore the risk, would decrease by approximately 3.3 times using this assumption ( i. e., the sensitivity analysis suggests a linear relationship between peak receptor concentration and infiltration rate). This would decrease the high end receptor risk from 5 x 10 4 to about 2 x 10 4. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 16 XV  16 XV. GROUNDWATER RISK MODELING C and D. Constant Source and Ultimate Leachability Verbatim Commenter Statements The model defines the concentration of metals present in the ash and then assumes a linear release with time. This erroneous assumption can result in a total inventory of metals released that exceeds the original inventory of metals specified in the model. The linear release of the metals over an arbitrary time interval has no scientific basis. ( ARIPPA00019)

Because of the inconsistency in the availability of data, EPA chose to make sweeping assumptions in order to implement the modeling. The impact of this action is to effectively provide an infinite source of metals with infinite availability to groundwater, which can also result in a total inventory of metals released that exceeds the original inventory of metals specified in the model. The linear release assumption presupposes a zeroth order dissolution mechanism for the constituents of the FBC ash when in reality the mechanistic controls are diffusion related. A review of the data presented to the EPA by CIBO and the accompanying ARIPPA data clearly demonstrate the 3 to 4 order of magnitude difference in the bulk content of arsenic in the FBC ash and the arsenic that is released upon leaching that can be expected. The presence of a metal in the solid waste does not necessarily mean that it is labile when placed in contact with water. Data in the scientific literature published since this review would support the observations made by CIBO and ARIPPA. ( ARIPPA00019)

An incorrect dissolution mechanism for the FBC ash is used. ( ARIPPA00019)

The metals in the ash are not released linearly over time as the model assumes. ( ARIPPA00019)

The models used a constant source concentration while real coal combustion products exhibit order of magnitude plus declining concentrations ... The computer models used by EPA in their risk assessment did not reach the well concentrations used for evaluation for 500 or 3000 years or more at constant leachate source concentration. Coal combustion products could not be expected to maintain the source concentrations at these original levels for even the right order of magnitude in the number of years to support this outcome. The model is thus spitting out data that simply cannot happen outside of a programmed microchip. ( NMA00024A)

The computer model assumed that all of the trace metals in a mostly vitrified material were leachable. Under ordinary water leaching conditions, a glass phase can be one of the most " leach proof" structures into which trace elements can be placed ... Although the EPA drew heavily on EPRI reports to obtain the concentration data for their database, it appears the parts of those reports stating that only around 10% of the elements in a typical coal combustion product were leachable got lost somewhere before the data reached the input lines of the model. ( NMA00024A) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 17 XV  17 One related problem with the analysis is that trace elements concentrations in the solids was set using a ratio of leachate to solid concentration at either the median or 95th percentile. To say that trace element concentration in a solid is a linear function of initial concentration requires a leap of faith I could not make, especially when I had a set of data on directly measured concentrations in the solids. ( NMA00024A)

EPA inappropriately used a constant source concentration. In the real world, coal combustion waste constituent source concentrations are expected to decline by an order of magnitude or greater over a short period of time. However, EPA's modeling exercise assume a constant highend source term. The decaying source term concentration is readily apparent by reference to the coal combustion process. Combustion of coal results in vitrification of much of the ash, producing a leach resistant glass phase. Mineral phases settle and condense in layers around the glass nuclei, resulting in a layered structure. The outer layers have a very porous structure that provides a high surface area and high leachability. However, the inner layers are more resistant to leaching. As a result, open column leach testing of coal ash generally exhibits rapid arsenic leaching from the outer layers, followed by a decline to non detect levels after several months. ( USWAG00037)

EPA Incorrectly assumed total availability of arsenic in coal combustion wastes for leaching. EPRI reports demonstrate that only approximately 10% of the constituents in a typical coal combustion waste are leachable. Nonetheless, EPA's modeling assumed that 100% of the mass of trace constituents is available for leaching. This single erroneous assumption resulted in excessive modeled risk by an order of magnitude. ( USWAG00037)

Professor Sheetz further expressed concern about the method in the model for defining the concentration of metals in the ash and assuming a linear release time: " This erroneous assumption can result in a total inventory of metals released that exceeds the original inventory of metals specified in the model. The linear release of the metals over an arbitrary time interval haa no scientific basis." As part of this discussion, Professor Sheetz pointed out that a review of data presented to EPA by CIBO and the accompanying ARIPPA data clearly demonstrate the 3  to 4 order of magnitude difference in the bulk contentof arsenic in the FBC ash and the arsenic that is released upon leaching that can be expected. ( CIBO00280) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 18 XV  18 XV. GROUNDWATER RISK MODELING E. Duration of Modeling An industry commenter suggested that modeling to a time horizon of 10,000 years was too long a period ( given, for example, uncertainty about climatic conditions), and suggested a period of 100 years would be more appropriate. In contrast, a public interest group commenter expressed concern that the modeling accounted for release during an impoundment's active lifetime only, without accounting for a post closure period that may have been preceded by impoundment drainage and reclamation.

Response: EPA disagrees that a 100 year time frame should be used in evaluating receptor risks from landfill disposal of fossil fuel combustion wastes. EPA believes such a time frame to be far too brief. EPA has used the 10,000 year time period assumption in other solid waste risk assessments for landfill disposal. EPA reported both the peak receptor well concentration and the time to reach this concentration in the Report to Congress, in order to show the time period for which risks may be anticipated.

It is true that EPACMTP assumes that releases from a surface impoundment end at the end of the impoundment's useful life. In other words, the model assumes that impoundments are dredged and clean closed. In some cases, however, fossil fuel combustion wastes managed in a surface impoundment may be left in place, rather than removed, at closure. In these cases, releases might continue after closure, but would not be considered by the modeled impoundment scenario. EPA believes that the release mechanisms during the post closure period of a surface impoundment would be similar to those from a landfill. For each waste where a surface impoundment scenario was modeled, EPA also modeled a landfill scenario. While there are some differences in the input parameters used between the two scenarios, EPA believes that the landfill scenarios provide a reasonable bound on the risks that would be expected from surface impoundments in the post closure period.

EPA believes that factors that would tend to under or overestimate transport are in general balanced, within the state of the art of nationwide ground water risk assessment. ( See response to Topic XV. L, below) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 19 XV  19 XV. GROUNDWATER RISK MODELING E. Duration of Modeling Verbatim Commenter Statements As described in Table A 4 ( page A 13) of the Technical Background Document for the Supplemental Report to Congress on Remaining Fossil Fuel Combustion Wastes: Ground Water Pathway Human Health Risk Assessment ( revised Draft Final, June 1998), the source for recharge ( RCHRG) and infiltration ( SINFIL) rate input parameters is historical ( throughout the last 60 years) climatological data generated from the HELP model. The assumption that these rates remain constant over the length of the study period ( 10,000 years) is highly unlikely. For example, 10,000 years ago the earth was near the end of the last Ice Age and large portions of the globe were under sheets of ice at least one mile thick ( Press and Siever, 1974; Montgomery, C. W., 1990). The uncertainty in the variation of climatic conditions over a 10,000 year period leads to a large uncertainty in the predicted concentrations of metals in the receptor well. These time durations are unrealistic and add yet another layer of conservative assumptions than is typical in models. A more practical time duration used in fate and transport modeling is 100 years. ( PG& E00023)

For onsite surface impoundments for coal and oil fired managed waste, EPA assumed that the leachate is released during the active lifetime of 40 years for the waste management unit only. At the end of the operational life, it was assumed that leaching would cease and all waste would be removed from the impoundment. The model did not account for a post closure period that may have been preceded by impoundment drainage and reclamation. For the onsite landfill for coaland oil fired managed waste, no leaching would take place during the operational life of the unit ( 40 years). At the close of the landfill, the unit would be capped and leaching would begin at a constant infiltration rate and an initial leachate concentration. ( ALA00036)

Waste management unit scenarios assumed no leaching from active unlined impoundments over a 40 year period. For landfills it assumes no leaching during the operational life and that leaching would begin at a constant infiltration rate and leachate rate at the end of the active iife of the unit. ( ALA00292) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 31 FF2P S0363, Sections 7 and 8.

20 XV  20 XV. GROUNDWATER RISK MODELING F. Hydraulic Conductivity Industry commenters disagreed with the hydraulic conductivity input values used in modeling. Specifically, the comment noted that modeling did not address the physical and chemical properties of FBC wastes that may result in much lower conductivity and hencelower infiltration rates for these wastes. The commenters stated that this omission resulted in hydraulic conductivity being overstated by 2 to 3 orders of magnitude.

Response: The characteristics of FBC wastes may be different than those used in its modeling, as suggested by the commenter. 31 Such effects might result in modeled receptor well risks lower than those calculated by EPACMTP. However, the potential variance in wastes, for which no data were available, made estimating such differences problematical. This was acknowledged in the RTC in which it was stated that FBC risk as calculated by the model may be overstated for this reason. ( As noted elsewhere in this set of responses to comments, EPA believes the likelihood of either over or underestimating modeled risk to be approximately equal, all factors considered, pending review of the model.) While EPA agrees with the commenter's suggestion that the hydraulic conductivity is probably lower for FBC wastes, sufficient data were not available to determine a more appropriate value for this parameter. EPA reported the results of sensitivity testing of this variable in its October 1998 sensitivity analysis, and agrees, if groundwater modeling is re visited, to re examine this issue.. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 21 XV  21 XV. GROUNDWATER RISK MODELING F. Hydraulic Conductivity Verbatim Commenter Statements EPA's model does not take into consideration any of the characteristics of the FBC ash that beneficially contribute to retention of metals. ( ARIPPA00019)

The modeling activities of EPA also did not take into consideration the physical characteristics of the engineered ash minefills. A properly placed FBC minefill develops physical characteristics that begin to approach the properties of portland cement concrete; modest strength [ 1200psi to 4500psi], a monolithic structure [ i. e. minimization of surface area for contacting waters] and low hydraulic conductivity [ 10 5 to 10 7cm/ secl. All of these characteristics contribute to a significant reduction of labile metals to contacting groundwater. In contrast, the EPA modelers assumed a lateral hydraulic conductivity for the placed ash of 300m/ y [ 1 x 10 3cm/ sec] which is fully 2 to 3 orders of magnitude larger than what might be anticipated in engineered minefills composed of FBC ash. ( ARIPPA00019)

The high compaction and cementitious behavior of the ash and its resultant low hydraulic conductivity are not properly considered. ( ARIPPA00019)

The hydraulic conductivity of the placed ash was overstated by 2 to 3 orders of magnitude. ( ARIPPA00019)

The model failed to account for the beneficial ongoing reaction of the lime ( CaO) and the Anhydrite ( CaS04) in the presence of water. The lime ( CaO) will react with water to form Ca( OH) 2 that further produces cemetitous reactions. In addition, the Anhydrite reacts with water to form Gypsum ( CaS04 + 2/ H20). The presence of the lime ( CaO) and Anhydrite ( CaS04) and their on going reactions with water along with the interactions with aluminosilicates from calcined clays provide the ash with some of its beneficial physical properties, such as compressive ash strengths measured of 1000 to 4000 pounds PSI; expansive nature due to " hydration reactions;" self cementing properties; high densities and very low permeabilities; and highly alkaline quality. ( CIBO00052)

It appears that EPA's concern is based on the results of a model that indicates that the concentration of arsenic in groundwater at a receptor well could reach the health based level of arsenic after 3,000 years. EPA's model, among other items, does not appear to account for the potential for these wastes to exhibit self cementing properties leading to very low hydraulic conductivity. This apparent oversight is far from trivial, and renders the results of EPA's model and risk assessment invalid. The cementitious nature of the ash from a waste coal plant is such that, if a truck back hauling ash to a mine site has a flat tire and is delayed by more than a couple of hours, the ash has to be jack hammered out of the truck. In short, CFB ash has Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 22 XV  22 physical characteristics that begin to approach the properties of Portland cement concrete, including a hydraulic conductivity of l0( 5) to l0( 7) cm/ sec. Other issues raised by EPA's model were discussed in ARIPPA's 6/ l 2/ 99 comments, which are incorporated herein by reference. ( ARIPPA00273) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 32 FF2P S0363, Table A 1.

33 Gelhar, L. W.; C. Welty; K. R. Rehfldet. A Critical Review of Data on Field scale Dispersion in Aquifers. Water Resources Research, 28( 7). 1992.

34 See discussion in EPA's Composite Model for Leachate Migration with Transformation Products, EPACMTP User's Guide, 1995, Section 6.5.9. This discussion references L. W. Gelhar, C. Welty, K. R. Rehfeldt, A Critical Review of Data on Field Scale Dispersion in Aquifers, Water Resour. Res., 28( 7), 1992.

35 FF2P S0363, Appendix K ( Table K 3).

36 FF2P S0363, Section 5.

23 XV  23 XV. GROUNDWATER RISK MODELING G. Dispersivity USWAG suggested that EPA selected artificially low values for transverse and longitudinal dispersivity.

Response: In its evaluation of comanaged coal combustion wastes, EPA used a longitudinal dispersivity value of 4.64 m and a transverse dispersivity value which is 1/ 8 of this value32. The dispersivity values used in the model were the median values based on a scale dependent dispersivity distribution given by Gelhar. 33 The value of 4.64 m was selected because this represented the 50th percentile from this data. 34 EPA also conducted a sensitivity analysis to examine the effects of changing dispersivity values on results. It conducted this analysis using a longitudinal dispersivity value of 68 m/ y, which is consistent with the value recommended by the commenter. For the comanaged coal combustion wastes in both a landfill and a surface impoundment, the effect of this parameter on receptor well concentration was found to be much less than the effect of other parameters. 35 EPA notes that in its Monte Carlo analysis the full distribution of dispersivity values ( from the Gelhar source) were used. These results were presented together with the deterministic highend results. 36 EPA notes that, ceteris paribus, an increase in longitudinal dispersivity may increase longitudinal contaminant transport. Of course, this may be offset by reduced concentration if in fact a higher value is selected for transverse dispersivity as well. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 24 XV  24 XV. GROUNDWATER RISK MODELING G. Dispersivity Verbatim Commenter Statements EPA selected artificially low dispersivity values. For example, EPA set the value for longitudinal dispersivity at 4.84 m, whereas a value in excess of 15 m would be reasonable. Transverse dispersivity values are likewise unrealistic. The combination of increased dispersivity and higher values for Kd ( to reflect the chemical precipitation process discussed above) would yield higher DAFs that better reflect real world conditions and that will be corroborated by real world data. ( USWAG00037) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 37 FF2P S0361.

38 FF2P S0363, Section 5.

25 XV  25 XV. GROUNDWATER RISK MODELING H. Well Locations not Realistic Industry commenters stated that the assumption of a well location of 150 meters away and on the centerline of the plume was unrealistic. One of the commenters suggested that real well locations are on the order of kilometers away. A public interest group commenter believed that this fixed well location was not justified with any information about actual well locations and stated that EPA should conduct a review of actual well locations. This commenter further stated that the receptor well location was changed to 430 meters for EPA's October sensitivity analysis and stated that the available data demonstrate receptors living much closer than this distance.

Response: As one of its high end assumptions, EPA selected a well location which was 150 meters from the source, on the plume centerline. The results of its sensitivity analyses suggest that well location is very important in determining receptor exposure, and therefore was set equal to this high end value to maintain a conservative analysis. The public interest group commenter is correct that some of the scenarios modeled in the October sensitivity analysis37 used a well location of 430 meters. These scenarios were the deterministic central tendency scenarios used in the sensitivity analysis to provide estimates of central tendency risk. These scenarios, therefore, appropriately used a central tendency estimate for well location. The high end scenarios modeled in the October sensitivity analysis and presented in the Report to Congress, however, all used a high end receptor well location of 150 meters.

EPA found that its data characterizing receptor well distances from fossil fuel combustion management units is incomplete. In the absence of such information, EPA is using the well distances from the Subtitle D Survey Database, used for the 1995 HWIR analyses. EPA is also employing this data because wells may be placed closer to the waste management unit in the future, over the study period. For these reasons EPA relied on distances obtained from the OSW database as representative of potential well locations at the high end.

EPA notes that in its Monte Carlo analysis, the full distribution of available receptor well distances was used including wells at distances up to one mile and off of the plume centerline. These results were presented together with the deterministic high end results. 38 Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 26 XV  26 XV. GROUNDWATER RISK MODELING H. Well Locations Not Realistic Verbatim Commenter Statements As the second of the two most important parameters that control the outcome of the deterministic risk assessment calculation, the model assumes, as a " high end" scenario, that an adult individual places a well directly into the water table 150 yards along the down gradient flow path from the fly ash site. Break through calculation show that the transit times to this well are measured mostly in the thousands of years, with the exception of arsenic, which was 500 years. The decision to select this distance is therefore extremely critical to the outcome of the model. Place it a little farther away and the transit time increases. In the CIBO survey, proximity to the ash site is generally measured on the order of kilometers, or approximately an order of magnitude farther away from the fly ash source than was used in the model. ( ARIPPA00019)

The likelihood of a drinking water well being placed 150 meters on plume centerline are infmitesimaly small and, therefore, the predicted contaminant concentrations and the time for them to reach the receptor well are greatly overstated. ( PG& E00023)

EPA well locations are probably not realistic ... EPA set the location of the critical well just 150 meters away from the edge of the coal combustion product fill. Although EPA modeling of mine sites was limited and more of an interpolation off of the impoundment results the notion that the users well is 150 meters away is highly questionable given typical State regulatory programs for mine sites in the Midwest. ( NMA00024A)

In the groundwater risk assessment EPA used a fixed well location at the 50th percentile   or 430 meters   without justification. There are no data to indicate the extent to which this is a realistic value or whether there are receptor wells located closer to waste management units. EPA does not provide data that are available on groundwater contamination around these waste management units. In addition, this major change occurred after the draft risk assessment was subject to peer review ( SAIC memo, 1998). This change resulted in reducing the risk estimates for groundwater ingestion posed by FFC waste. ( ALA00036)

EPA assumed that the adult resident ( and child) drank tap water derived from ground water that had been contaminated by the migration of metals to a receptor well 430 meters from the WMU ( about one quarter of a mile). EPA's sensitivity analysis detemlined that the waste concentration and the receptor well location are the driving parameters in the risk assessment. In the October 1998 risk assessment supplement the contractor reported that the receptor well location was extended from 150 meters to 430 meters. The contractor states that the 430 m value corresponds to the 50th percentile used in the HWIR analysis ( SAIC, 1998). Since receptor well location is critical to estimating risks associated with FFC w'astes, it seems appropriate for EPA to justify Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 27 XV  27 this significant change in the risk assessment to a less conservative distance at the 50th percentile. This is particularly important since drinking water wells have been reported to be 150 feet from waste management units surveyed in the site investigations. ( ALA00292)

The Agency has not, to date, analyzed the proximity of private drinking water wells to FFC waste management and disposal facilities. It is clearly a responsibility of the Agency to conduct such a review. It is likely that either the Agency or industry has already obtained this information for at least some facilities. In a memorandum to the docket. SAIC describes one site as having 17 wells within 1 mile, with the closest within 150 feet of the facility. Another memorandum states that 415 people using well water within 1 mile of that facility, yet EPRI had characterized only 5 wells. SAIC recommended that EPRI better characterize the wells near this site. A third example is yet another SAIC memorandum to EPA that describes a site as having numerous wells within 1 mile. with groundwater betng affecting to 600 feet from the facility. With this information in hand. why did the EPA choose the receptor well for the groundwater analysts to be 1.477 feet ( 450 meters) from the facility? Secondly, why has the Agency not conducted a review of private drinking wells near these facilities when it has had some knowledge of the location of these wells for almost two years ( and more than 3 years in the case of one facility)? ( ALA00292) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 39 FF2P S0361, Attachment 2 ( Monte Carlo Results).

28 XV  28 XV. GROUNDWATER RISK MODELING I. Characterization of Source Term ( TCLP versus Porewater)

An industry commenter stated that EPA may have used defective concentration distributions ( i. e., uniform or normal distributions) in their Monte Carlo simulations resulting in too many high end concentration runs. Several industry commenters also questioned whether the specific analyses ( e. g., porewater data) used were appropriately representative of leachate from waste management units. One of the commenters suggested that vacuum drawn pore water samples will almost always overpredict the concentration that will be exchanging out of a source layer of ash and into a productive aquifer system. One of the commenters was concerned that the Agency interchangeably used data from porewaters, TCLP results, SPLP results, and data for FBC waste and mixed waste. The commenter also stated that non standard leachate procedures ( i. e., 2: 1 water extracts) were mixed with the data. This commenter also was concerned that the source term did not consider the alkalinity of FBC waste.

Public interest group commenters also questioned the adequacy of porewater data to represent surface impoundment leachate and TCLP data to represent landfills. One of the commenters stated that, based on comparison with pore water data, TCLP data consistently fail to predict the actual concentration of contaminants available for migration into ground water. The commenter also expressed concern that EPA could not vouch for the representativeness of the TCLP data on a facility specific basis. Another commenter questioned the change from porewater data to represent landfills in the draft risk assessment to TCLP data for landfills in the sensitivity analysis. This commenter also was concerned that the assessment did not consider waste effluents such as sluice wastes and low volume wastes. The commenter further expressed concern that no distinction was drawn between filtered and unfiltered samples.

Response: In the assessments performed for the October 1998 analyses, EPA did in fact use lognormal distributions of the site averaged waste leachate concentrations in all of the Monte Carlo assessments presented. 39 Thus, in its Monte Carlo assessments EPA used the type of distribution recommended by the commenter.

While responding to specific comments below, EPA notes that it used all the data made available and in accordance with its best judgement knowing the advantages and disadvantages of data collected by varying means.

EPA believes that, given the constraints imposed by available data, it appropriately used specific analyses to represent the appropriate wastes and management scenarios. For example, for comanaged wastes, EPA believes its use of porewater and TCLP analyses was appropriate to represent surface impoundments and landfills, respectively. Each of these tests were performed to Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 40 FF2P S0363, Section 7.1.

41 FF2P S0361.

42 FF2P S0363, Section 3.3.2. The data were recalculated in the October report ( FF2P S0361, Attachment 1) with no difference in the types of data included ( i. e., TCLP and EP data were used in both instances).

29 XV  29 simulate different leaching environments. Specifically, the porewater data were collected principally from surface impoundments, and, therefore, these data were used in the surface impoundment scenarios as the best available representation of leachate from these units.

In its June 1998 report, EPA used the porewater data for its landfill assessment but acknowledged that the resulting analysis was expected to be very conservative. 40 One reason for this is that the porewater data driving the analysis were collected from impoundments with standing water and high infiltration, a situation different than a landfill with relatively little free liquid. The TCLP is intended to simulate leaching in a particular landfill environment. While neither the impoundment porewater data nor the TCLP data may perfectly reflect the conditions in a comanaged landfill, EPA expects that the TCLP data better reflect these conditions. The Agency, therefore, revised its analysis for landfills to use TCLP data and presented these revised results in the October analyses. 41 EPA acknowledges that the TCLP data collected from each facility may not be representative of the conditions at the site but disagrees that the collected data are necessarily over or under predicting the site conditions. These data were collected from a portion of the samples collected at the site; typically between 2 and 4 were analyzed for TCLP. These samples were collected from different cores throughout the site. EPA did not identify any particular bias in the selection of samples analyzed for TCLP.

EPA also disagrees with the comments that leaching test results were improperly interchanged. The specific test results used for the comanaged waste scenarios and EPA's logic in selecting them are described above. In the case of the modeling of FBC wastes, EPA used data specific to FBC wastes using only the TCLP and EP leaching procedures. 42 Similarly, in modeling OCWs, EPA used TCLP and EP data specific to OCWs. While EPA acknowledges that the TCLP and EP tests represent two different leaching procedures, they are similar in that both use organic acids as extraction media. EPA did not have available any data representing FBC waste or OCW pore fluids; it also did not combine the TCLP/ EP data with SPLP data.

EPA disagrees that low volume and sluice wastes should have been considered in its assessment. EPA's study for this determination was of large volume wastes comanaged with lowvolume wastes, and, therefore, EPA used data that represent these comanagement scenarios. Lowvolume wastes and sluice waters managed alone are outside the scope of this determination. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 30 XV  30 With regard to its evaluation of filtered and unfiltered data, EPA attempted to consistently use the same type of data in its analyses ( i. e., filtered). However, at some sites with comanagement data, only unfiltered data were available ( or the type of analyses was not specified). In these cases, EPA elected to use the data available because the remaining alternative would be to disregard the data or evaluate it separately. To maximize the use of all its data and develop relevant statistical parameters, EPA elected to combine these types of analyses.

In sum, EPA made every effort to use the only data it had in a balanced, representative, and consistent manner, while minimizing known data deficiencies. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 31 XV  31 XV. GROUNDWATER RISK MODELING I. Characterization of Source Term ( TCLP versus Porewater) Verbatim Commenter Statements Data from different leachants with different metal chelating properties are mixed inappropriately. ( ARIPPA00019)

The reference data used for the leachate concentrations are also questionable. It is reported that EPA's data can be pore fluids, TCLP leachate data or SPLP leachate data, or the numbers may be derived from FBC wastes or mixed wastes. In one instance, the attempts to recover samples from cores drilled into a fly ash pile did not produce any liquids, a typical condition for engineered minefill fly ash placements. However, in order to report some numbers for the site, the ash was mixed 2: l with water and leached to produce a liquid sample. [ The Ground Water Pathway Human Health Risk Assessment, p7 3, 1998]. These data are derived from a non standard leach procedure, which is uncontrolled and hence the data should not be utilized. In the modeling activities, these various leachates have been used interchangeable when in fact the nature of the different leachants is radically different and results in different amounts of metals in solution. Organic acids will tend to chealate certain metals and hold them in solution giving artificially inflated solubilities. ( ARIPPA00019)

To summarize our concerns with the Source term of the EPA's model, we believe that the model is flawed because... The alkalinity of the FBC ash was not considered. ( ARIPPA00019)

Non standard leachate procedures were used to collect a portion of the data and the data from these tests was mixed with standard test data. ( ARIPPA00019)

The data used for input concentrations may have been misinterpreted allowing extreme values to be used inappropriately in the model. This is a critical issue because, according to EPA's Report to Congress, the source concentration is the most sensitive parameter in the whole model for controlling outcome. As one who has personally run laboratory analysis on coal combustion products for years I have found that solid to liquid ratio has a profound influence on leachate concentration for most trace elements ... I have grave concerns with the way pore water samples were used in the impoundment case. EPA averaged the concentration down the length of the entire sample ( and indeed the sample values over the entire site) ... Much of the data in the data base was apparently vacuum drawn pore water samples, but these values were averaged and used as if they were samples drawn up and down the length of an as used aquifer unit. Remember that vacuum drawn pour water samples do not represent flowing and exchanging water moving through an aquifer unit ... vacuum drawn pour water samples will almost always over predict badly the concentration that will be exchanging out of a source layer of ash and into a productive aquifer system ... The problem is that the sample was averaged and treated like a sample across a Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 32 XV  32 productive aquifer when it was not. The results of the deterministic analysis are completely invalid as a result. ( NMA00024A)

EPA used defective concentration distributions in their Monte Carlo simulations resulting in too many high end concentration runs. Obviously this problem will cause the Monte Carlo runs to overpredict risk. From what is written in the risk assessment report of Oct. 1998 and the Report to Congress the problem would be a catastrophe, because these reports talk about taking a midpoint central tendency value plus or minus 50% with parameters having a similar role to concentration. This would cause may readers, including myself, to assume that a uniform distribution or maybe on a good day a normal distribution had been used. I understand from conversations with EPA personnel and their contractor following the hearing on May 21, 1999 that a lognormal distribution of source concentrations was used. If this action was taken and simply omitted from the report, the agency avoided compounding an already critical problem. High concentration values are far more scarce in lognormal distributions than in uniform or normal distributions and running the Monte Carlo simulation with the number of high concentration cases way off from reality would be a fatal flaw. ( NMA00024A)

A good indication of the weakness of the TCLP for the wastes in question can be found in Table 3 9 of the Report to Congress. In this table, EPA provides TCLP data ( covering wastes in both landfills and surface impoundments) and pore water sample data ( covering surface impoundments) for co managed coal combustion wastes. For every toxic contaminant but mercury, both the mean and upper range of the pore water data exceed the TCLP data, typically by an order of magnitude or more. In the case of arsenic wastes, a particularly important contaminant in coal combustion the difference between the mean pore water and TCLP data is almost two orders of magnitude. Insofar as the pore water data represent " actual leachate" results for surface impoundments, it is clear that the TCLP data consistently fail to predict the actual concentration of contaminants available for migration into groundwater. ( EDF00021)

EPA also cannot vouch for the representativeness of the TCLP data on a facility specific basis, therefore the Agency hasn't a clue what it is combining to achieve a so called " average" result. The term itself is misleading in this regard. Averaging non representative data for a facility does not magically yield a representative facility average. ( EDF00021)

In the groundwater risk assessment, concentrations may not be reflective of actual conditions due to the tests used. Specifically, the tests used to estimate the extent to which the metals leach from the waste maybe inadequate. EPA has not demonstrated that the leachate tests ( Toxicity Characteristic Leaching Procedure, TCLP) for landfills and leachate represented by pore water samples for impoundments is adequate for risk assessment. ( ALA00036)

EPA replaced the pore water samples used in the April 1998 groundwater risk assessment with TCLP data. The use of TCLP data resulted in a significant decrease in the concentration of Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 33 XV  33 toxicologically significant metals and a decrease in the risk estimates to levels above one in ten thousand for cancer risks and below a HQ of 1 for non cancer risks ( see table below). EPA does not explain this significant change in the groundwater risk assessment, which occurred after the peer review in the Report. ( ALA00036)

Data from pond waters from comanaged coal waste also did not include waste effluents   such as sluice wastes and low volume wastes. These were not considered because it was believed that these wastes represent only localized effects. It should be noted, however, that these aqueous wastes have the potential to be transported off site. In addition, although most of the samples were filtered, some were not and no distinction was between these two sampling methods in the data compilation. ( ALA00036)

We requested clarification of the waste characterization data in the Report to Congress during the public comment period. EPA responded through their contractor, SAIC, on June 25, 1990. SAIC indicated that the porewater data used in the June 1998 draft version of the ground water risk assessment was changed to the Toxicity Characteristic Leaching Proccdure ( TCLP) data in the October 1998 final risk assessment. This highly unusual exchange of analytical results after the risk assessment was completed and peer reviewed resulted in substantial understatement of the risks associated with leaching of FFC waste constttuents from a hypothetical landfill to a drinking water source. Although EPA justtlies this change because only 3 landfills of a known population of 500 were included in the site in\, estigations, the use of TCLP data appears equally problematic ... In addition, this major change is not adequately explained in the technical background document and simply not mentioned in the Report to Congress. Since this change occurred after the risk assessment was peer reviewed EPA should, at a minimum, explain their rationale and discuss the uncertainties associated with the use of TCLP data in the ground water risk assessment. ( ALA00292) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 43 Exposure Factors Handbook, Draft. Volume I  General Factors. EPA, Office of Research and Development. April, 1996.

44 Johnson, T. and Capel, J. A Monte Carlo Approach to Simulating Residential Occupancy Periods and its Application to the General U. S. Population. EPA, Office of Air Quality and Standards. 1992.

34 XV  34 XV. GROUNDWATER RISK MODELING J. Fixed Exposure Assumptions Some public interest group commenters expressed concern that the exposure parameters are based on central tendency values for such factors as ingestion rates, and residence time and, therefore, do not account for above average exposures. One of the commenters suggested EPA use 90th percentile values for exposure factors. The commenter also specifically stated that the exposure duration for elderly residents may be far longer than 9 years.

Response: The commenter correctly notes that exposure duration is dependent on age. In EPA's Exposure Factors Handbook, 43 EPA compiles various studies regarding population mobility and concluded that a 9 year residence time is representative of the 50th percentile from the studies. One of these studies44 presented residence time by age, showing that the elderly population ( age 63 to 84) had a median residence time of 19 years or more, while a 33 year old had a median residence time of 9 years ( i. e., the residence time used in the risk assessment). All such data are taken directly form the Exposure Factors Handbook.

EPA's Office of Solid Waste includes above average exposures as a part of its risk analyses. EPA did so in this case. As part of its deterministic high end analysis, EPA conducted a sensitivity analysis seeking those parameters most affecting exposure to ground water. EPA found that setting two parameters to their high end values ( leachate concentration and well location) resulted in the case of highest double high end risk. EPA did not set additional parameters, such as length of exposure, to their high end values ( e. g., 90th percentile exposure duration or a duration more appropriate to elderly subpopulations) because the end result would have been an overly conservative ( i. e., multiple high end, instead of double high end) analysis. Again, this is a matter of EPA Office of Solid Waste policy and is not unique to this study. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 35 XV  35 XV. GROUNDWATER RISK MODELING J. Fixed Exposure Assumptions Verbatim Commenter Statements Because the concentrations of chemicals in FFC waste are suspect, the exposure estimates are also suspect. As a result, the analysis could underestimate risks, especially to populations with above average exposures. ( ALA00036)

The exposure parameters are based on central tendency values for such factors as ingestion rates, and residence time and, therefore, do not account for above average exposures. In the groundwater risk assessment, EPA assumed that drinking water exposure would occur for only nine years ( residence time) and at an ingestion rate of 1.4 liters per day. These are average exposure factors and do not protect people who, for examples, live in their homes throughout their childhood and high school years and drink more than one and one half liters of water per day. ( ALA00036)

EPA is required to assess the risks associated with highly exposed subgroups. " This descriptor is useful when there is ( or is expected to be) a subgroup experiencing significantly different exposures or doses from that of the large population. These subgroups may be identified by age, sex, lifestyle, economic factors. or other demographic variables." ( EPA. 1995) ... the reasonable maximum exposure concentration is determined by setting the exposure frequency and duration at the 90th percentile concentration ... Based on a comparison of the values used by EPA in the ground water risk assessment and the values representing the 90th percentile for the U. S. population, it is apparent that EPA has not evaluated the high end reasonable exposures. ( ALA00292)

These facility specific maps illustrate that there are real people living near power plant FFC waste facilities, not just " hypothetical receptors". Included in these populations are children, who are a more sensitive subpopulation and the elderly whose duration of exposure may be far longer than the 9 years estimated for the " high end" groundwater exposure. ( ALA00292)

Some of the drinking water risk assessment assumptions seem to be inadequate for example, the assumption that an adult resides in a home and is exposed to contaminated groundwater for only 9 years and drinks only 1.4 liters of water per day. What about the adult who lives in the same home for 18 years and consumes twice that amount of water a completely reasonable assumption? ( 49CAO00058) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 45 FF2P S0363, Section 7.

36 XV  36 XV. GROUNDWATER RISK MODELING K. Other Input Assumptions Fixed Industry and academic commenters stated that modeling did not account for all the variable geologic conditions or site specific situations possible. One of these commenters specifically noted that the model is not capable of evaluating landfills in downgradient areas, or landfilling below the water table ( a common practice on the Texas Gulf Coast)

A public interest commenter argued that EPA has not taken into account variable conditions that exist throughout the U. S. regarding proximity to receptor well, unit liner characteristics and infiltration rate, meteorological and hydrologeological settings, unit size and waste characteristics. Specifically, the commenter was concerned about not accounting for karst terrain or sensitive environments such as wetlands. This commenter also was concerned that EPA did not vary waste characteristics to reflect variability in feed and operating conditions over time.

Response: EPA notes that downgradient and below watertable fill may be minefilling; the commenter was not clear on this. EPA does agree that current modeling does not address this problem well. Section VII of these comments and responses discusses minefilling and EPA's planned action. We will address site specific below watertable landfilling issues in the same manner.

EPA agrees that the EPACMTP modeling does not address several of the site or regionally speciifc variable geologic conditions noted by the commenters. As is well known, this is not a comment unique to today's rulemaking. EPACMTP is targeted on assessing risk on a nationwide basis and EPA has previously acknowledged areas in which the modeling results are not applicable. 45 In any event, EPA determined not to rely on modeling in making its regulatory determination for minefilling. For other waste management scenarios, the Agency believes that its selection of typical geologic conditions was appropriate for a national level risk assessment, as noted next.

EPA disagrees with the comments that other variable conditions such as receptor well location and infiltration rate were not considered. The variability of such factors were explicitly considered in its Monte Carlo and deterministic sensitivity analyses, where values used as input parameters were allowed to vary over a plausible range. In deterministic analysis, EPA evaluated all factors adjudged to affect results most significantly.

EPA also disagrees that waste management unit sizes were consistently underestimated and that waste characteristics did not account for variable conditions. For its assessments, EPA used Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 37 XV  37 data from surveys sponsored by industry associations to estimate the size and characteristics of the waste management units. These data represented the best available for characterizing particular waste management units. EPA used the distribution of these values as inputs in its Monte Carlo analyses, including very large units. In sensitivity analysis for its deterministic analyses, EPA identified central tendency and high end values for waste management unit size. EPA found that these parameters were not as significant as others, such as initial waste concentration.

EPA also disagrees that waste characteristics did not account for variable operating conditions. EPA used the complete distributions of site averaged waste characteristics as inputs in its Monte Carlo analyses, thus reflecting the full range of variation represented in the available data. In its deterministic analyses, EPA used a high end value for initial leachate concentration. The use of this value would account for higher levels of contaminant release that would occur in a small section of the population. These higher levels of release represent the high end scenario over the range of operating conditions for which data were available. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 38 XV  38 XV. GROUNDWATER RISK MODELING K. Other Input Assumptions Fixed Verbatim Commenter Statements An assumption was made that the sampling data reflect the overall population of landfill wastes and that the leachate composition is not variable. Although EPA notes that the assumptions were not tested and would be affected by variability in feed and operating conditions over time, there is no further consideration of these effects. ( ALA00036)

Each management scenario was described and modeled in terms of the proximity to the nearest groundwater receptor well, unit liner characteristics and infiltration rate, the meteorological an hydrogeological settings affecting the unit, unit size and waste characteristics ( discussed above). In all cases EPA has not taken into account variable conditions that exist throughout the U. S. regarding these parameters. ( ALA00036)

There are concerns that the use of model parameters does not account for the variability of conditions across the U. S. For example, the different geologic characteristics of the areas where waste landfills or impoundments are located are not taken into account. Sensitive environments, such as facilities located in karst terrain, could allow contaminants to quickly and easily migrate to groundwater. Under certain circumstances, this migration may occur largely unattenuated and in an undiluted fashion. In addition, waste sites near wetlands areas may have a greater likelihood of contaminating groundwater due to the role of wetlands as groundwater recharge areas ( EPA, 1995). These factors are critical to EPA's assessment because EPA dismisses significant public health risks because modeling predictions indicate that the contamination will not occur for several hundred years. Suppose the model under predicted migration? ( ALA00036)

EPA used central tendency values for all the driving parameters in the risk assessments   starting waste concentration, size of unit, exposure duration, and distance to receptor. ( ALA00036)

The comment at this point is that it does not appear that the modeling, in either the Deterministic or Monte Carlo case, deals with the realities of the geologic situation. I have had a little experience with review of these techniques as a Citizen member of the EPA focus group on Industrial Nonhazardous Waste Guidance, and it certainly appears that there is opportunity to add geologic reality to at least some of the modeling. The response to this comment may be that this is built into the models. I would not agree. ( RICE00041)

The study did not evaluate several situations that are typical in the Texas Gulf Coast lignite area. One was minefilling  because it is difficult to evaluate because of the interference by the backfilling operation. The same could be said for landfills in areas subsequently backfilled on the downdip or downgradient side. Another admitted shortcoming was the present inability to model the results of landfilling below the water table  which SAI [ Revised Sensitivity Study] regretted. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 39 XV  39 But again, this landfilling below the water table operation is common in the Gulf Coast, for both municipal and coal mine landfills. Most of the lignite mines dewater the associated sands by wellpointing long before mining begins. That is, groundwater has a significant involvement in the Gulf Coast regiorrwhere the study failed to deal with at least two very real situations. ( RICE00041)

The comment is that the tentative conclusions fail to consider basic real situations in the Gulf Coast region that involves a significant number of coal waste disposal situations. ( RICE00041) Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 40 XV  40 XV. GROUNDWATER RISK MODELING L. Specific Technical Comments on Model Design As noted under Topic XV. A, a public interest group commenter provided a detailed technical report addressing issues related to the ground water model ( EPACMTP). These comments were directed primarily at MINTEQA2, the metals partitioning input component of CMTP, but many comments affect the overall model ( EPACMTP) itself. An industry commenter stated that it shares some of EDF's general concerns with EPA's utilization of CMTP beyond its capabilities, and supports a thorough review of MINTEQA2. The industry commenter argued, however, that the specific modifications suggested by EDF commenters are unrealistic.

This industry commenter also cautioned that the specific risk results presented in the EDF report were based on unrealistic input concentrations, and are, therefore, designed to produce extreme results to draw attention to deficiencies in EPA's approach. The contractor that prepared the report for EDF clarified that the report was intended to be a general review and critique of the model the Agency is using in a broad spectrum of regulatory determinations.

EDF's comments were summarized in two tables for delivery to EPA. For ease of presentation and general review, these are complied into the several points summarized below where both industry response and the status of EPA's review is noted.

General Response: EPA is carefully reviewing all of these very specific comments on the model. The process of thoroughly investigating all of the comments will take substantially more time to complete than is available within the court deadline for issuing this regulatory determination. At this time, we are uncertain of the overall outcome of our analysis of the issues raised in the comments. Accordingly, we are not relying on the results of our ground water pathway risk analysis in support of today's regulatory determination on fossil fuel combustion wastes. In making today's regulatory determination, we have relied on other information related to the potential danger that may result from the management of fossil fuel combustion wastes.

Because all of these comments are the subject of the thorough, long term analysis cited several times throughout this section, it is premature to attempt responses to each individual comment at this time. Therefore, we have not attempted to respond to these points for today's rulemaking.

At this time, we have not determined what, if any, changes are appropriate to make to the model. However, the conclusions of our analyses may may involve changing or re structuring various aspects of the model, if appropriate. It may also include additional analyses to determine whether any changes to the model or modeling methodology would materially affect the groundwater risk analysis results that were reported in the RTC. If our investigations reveal that a re analysis of groundwater risks is appropriate, we will conduct the analysis and re evaluate today's decisions as warranted by the reanalysis. Comment Summary and Response Document Report to Congress: Wastes from the Combustion of Fossil Fuels April 2000 41 XV  41 In addition to our ongoing review of comments on the overall groundwater model, one element of the model   the metals partitioning component called " MINTEQ"   has been proposed for review by EPA's Science Advisory Board ( SAB). We will take the findings of this review into account in any overall decision to re evaluate today's regulatory determination.

In response to the industry commenter, EPA is aware that the results presented in EDF's analysis are the results of a theoretical exercise designed to call attention to issues with the model and do not duplicate or replace the analysis conducted for the FFC risk assessment. Specific concerns with the suggested modifications raised by the industry commenter are included in the numbered items below. Specific FFC chemical contaminations were not used in this critique, despite the industry commenter stating to the contrary.

Below are listed the specific comments as submitted by EDF, the initial comment in response by industry commenters and the status of EPA's review. As noted, EPA's analysis of these comments is underway and it would be very premature to offer judgments as to how these might impact on risk modeling. EPA believes certain comments to have merit, but the collective effect remains to be studied. As noted elsewhere in this response , this will take far more time than was available under this court schedule, and, if warranted, groundwater isk modeling will be revisited.

Comments in EDF table entitled " Summary of MINTEQA2 Critique and Improvements to Isotherm Generation Method"

 Flawed Chemistry of the Basic Ground Water  EDF raised concerns with regard to ion charge imbalance in ground water, to improper geochemical relationships in the STORET data base, to the presence of exclusively oxidized elements ( carbon, sulfur, iron, nitrogen), to mineral phase disequilibrium, to low input concentrations for calcium, magnesium, phosphorous and sulfur, and relative to ignoring of colloidal phase iron and aluminum. USWAG disagreed with EDF's concerns about colloidal transport and EDF's interpretation of the STORET data. EPAs study of these interrelated issues is underway. ( These are items 1 7 in the EDF critique.)

 Leachate Chemistry  EDF raised concerns that leachate chemistry was not considered in isotherm calculations, that other contaminants may be present in leachate, and that leachate pH may overwhelm an aquifer. USWAG disagreed with the potential for leachate pH to overwhelm aquifer pH. EPA is just beginning to study these. ( These are items 8 10 of the EDF critique..

EPA is just beginning to address the following comments.