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Overview of Methodology Steps The beneficial use evaluation of fly ash concrete and FGD gypsum wallboard follows the steps laid out in the Methodology for Evaluating Encapsulated Beneficial Uses of Coal Combustion Residuals (US EPA, 2013a). This methodology has undergone an independent external letter peer review. A summary of the comments received from peer reviewers is available in the document Peer Review Summary Report: Independent External Peer Review of the Preliminary Draft Report Methodology for Evaluating Encapsulated Beneficial Uses of Coal Combustion Residuals (US EPA, 2012a). Responses to these comments are available in the document Responses to External Peer Review Comments: Methodology for Evaluating Encapsulated Beneficial Uses of Coal Combustion Residuals (US EPA, 2013b).
This methodology is flexible and allows evaluation of the range of possible encapsulated beneficial uses for any CCR. The evaluation process is divided into five individual steps. As developed, the party conducting the evaluation can choose to begin at the first step and follow the methodology in the order presented or, based on the type and amount of data available, can choose to begin the evaluation at any other step of the methodology. If, at any point in the evaluation, all releases of COPCs are found to be comparable to or lower than those from an analogous non-CCR product, or to be at or below relevant regulatory and health-based benchmarks, then no further evaluation of the CCR product is necessary.
This specific evaluation began with the first step and followed subsequent steps in the order presented in the methodology.
Step 1 (Literature Review and Data Collection): This step involves the collection and review of available literature on a specific CCR and associated beneficial use. The purpose of this step is to establish whether existing evaluations are sufficient to demonstrate that releases from the CCR products under evaluation are comparable to or lower than those from analogous products, or are at or below relevant regulatory and health-based benchmarks, and to collect data on COPCs that may be present in and released from the CCR products, but were not sufficiently addressed by existing evaluations.
Step 2 (Comparison of Available Data): This step involves using the data collected in Step 1 to conduct a comparison of the COPC releases from the CCR products with those from the analogous products that they replace. The purpose of this step is to determine whether COPC releases from the CCR products are comparable to or lower than those from an analogous product.
Step 3 (Exposure Review): This step involves the review of those COPCs that were not comparable to or lower than those from an analogous product and were carried forward from Step 2. The purpose of this step is to identify potential exposure pathways, determine whether these exposure pathways are complete, and to develop a conceptual exposure model to organize and communicate this information.
Step 4 (Screening Analysis): This step involves a screening analysis of the COPC exposures carried forward from Steps 2 and 3. This screening uses a combination of conservative (i.e., likely to overestimate exposures) environmental, fate and transport, and exposure data to estimate the magnitude of COPC concentrations at the point of exposure. The analysis then compares these conservative COPC concentrations to relevant regulatory and health-based screening benchmarks. The purpose of this step is to eliminate any COPC exposures that do not warrant further consideration with more realistic, resource intensive modeling.
Step 5 (Risk Analysis): This final step involves a revised analysis of COPC exposures that were found to be above screening benchmarks in Step 4. This analysis is intended to be more realistic than the screening analysis, and is conducted using environmental, fate and transport, and exposure data that are more representative of real world conditions. The evaluation uses these revised COPC concentrations to estimate corresponding risks. The purpose of this step is to reduce conservatisms remaining in the evaluation to a level at which a final conclusion can be made.
1 Step 1: Literature Review and Data Collection This section applies the first step of the methodology to the evaluation of fly ash concrete and FGD gypsum wallboard. This step involves collecting and reviewing the available literature relevant to a specific CCR and its beneficial use. The purpose of this step is twofold. The first purpose is to determine whether any existing evaluations have already demonstrated that releases of COPCs from fly ash concrete or FGD gypsum wallboard are comparable to or lower than those from analogous products, or are at or below relevant regulatory and health-based benchmarks. The second purpose is to collect data on the COPCs present in releases from fly ash concrete and FGD gypsum wallboard that have not been sufficiently addressed by the existing evaluations. The following subsections provide a discussion of the existing evaluations used to identify the releases and associated COPCs that required further evaluation, followed by a brief summary of the data sources relied upon in the current evaluation.
1.1 Fly Ash Concrete 1.1.1 Existing Evaluations for Fly Ash Concrete The current beneficial use evaluation reviewed all existing evaluations identified from the available literature according to the recommendations of Summary of General Assessment Factors for Evaluating the Quality of Scientific and Technical Information (US EPA, 2003a). 3 The focus of this review was to determine whether these existing evaluations could form the basis for defensible conclusions regarding fly ash concrete. The review determined whether the existing evaluations pertained to fly ash concrete, clearly and sufficiently explained the data and assumptions relied upon, accounted for major sources of uncertainty and variability, and had undergone an independent review in some form. The remainder of this subsection summarizes the existing evaluations used to identify releases and associated COPCs for further consideration. Under the title of each evaluation, a brief summary of relevant findings is provided. Where multiple existing evaluations were pertinent to a given topic, all the summaries are combined under a list of the evaluation titles.
US EPA (1998): Supplemental Report to Congress on Remaining Wastes from Fossil Fuel Combustion Technical Background Document: Beneficial Use of Fossil Fuel Combustion Wastes This report identified the following types of releases to the surrounding environment that may occur from CCR products: 1) generation of dust, 2) emanation to air, 3) leaching to ground and surface water, and 4) decay of naturally occurring radionuclides. Because this report addresses the beneficial use of CCRs, it is directly applicable to the current evaluation of fly ash concrete. Therefore, each identified type of release was retained for further consideration.
EPA developed this document in response to guidelines issued by OMB (US OMB, 2002) under section 515 of the Treasury and General Government Appropriations Act for Fiscal Year 2001 (Public Law 106-554; H.R. 5658).
1-1 US EPA (1999): Report to Congress: Wastes from the Combustion of Fossil Fuels: Volume 2 Methods, Findings, and Recommendations This report reviewed all of the data available to the Agency at the time of publication on releases from CCRs generated in the United States. The data indicated that all concentrations of organic constituents, such as polyaromatic hydrocarbons and dioxins, were near or below analytical detection limits both in CCRs and in leachate released from CCRs. Based on these data, the report concluded that organic constituents are not COPCs associated with CCRs. Consideration of updated toxicity values does not alter the conclusions of this report. Furthermore, no additional data have been identified since the completion of this report that would indicate the potential for higher organic levels. Although this report addressed CCRs, the conclusions are also applicable to fly ash concrete. The beneficial use of fly ash in concrete will dilute concentrations of any organic constituents present in the fly ash through mixing with other concrete components. In addition, these organic constituents are not volatile under standard environmental conditions and are often highly hydrophobic, making a release of these complex organic compounds from fly ash concrete at rates higher than those from pure fly ash unlikely. Therefore, the current evaluation eliminated organic constituents from further consideration.
US EPA (2002): Constituent Screening for Coal Combustion Wastes US EPA (2010a): Human and Ecological Risk Assessment of Coal Combustion Wastes US EPA (2002), as discussed in US EPA (2010a), evaluated transport of CCRs disposed in uncovered landfills by wind and precipitation. This screening assessment found that all potential exposures were below levels of concern. Although this 2002 evaluation addressed CCRs that were disposed, the findings are also applicable to fly ash concrete. The beneficial use of fly ash in concrete will dilute constituent concentrations present in the fly ash through mixing with other raw materials, and will also reduce the rate of release because of the high strength of the intact concrete matrix. Therefore, the results of this 2002 evaluation provide a conservative estimate of dust release from concrete. However, more than a decade has passed since the 2002 screening assessment was conducted. Since that time, the Agency has obtained additional data through public comments and published literature that characterize constituent concentrations in fly ash, including data that reflect the effects of some new pollution control technologies and plant configurations installed in response to updated Clean Air Act requirements. In addition, revised toxicity values have resulted in updated human health and ecological benchmarks. As a result, the current evaluation retained generation of dust for further consideration, and identified all constituents for which sufficient data were available as COPCs for this release. These include aluminum, antimony, arsenic, barium, beryllium, boron, cadmium, chromium, cobalt, copper, iron, lead, manganese, mercury, molybdenum, nickel, selenium, silver, strontium, thallium, uranium, vanadium, and zinc. While other inorganic constituents may also be present in CCRs, they were not retained for further evaluation. Some constituents were not retained because of the absence of human health and ecological toxicity values (i.e., calcium, chloride, magnesium, phosphate, potassium, sodium, silicon, sulfate, sulfide). Other constituents were not retained because the newly available data, which were either for CCRs other than fly ash or for fly ash mixed with other CCRs, were not representative of the CCRs assessed in 1-2 the current evaluation, and do not add to the information used to draw conclusions in U.S EPA (2010a) (i.e., cyanide, fluoride, nitrate/nitrite).
US EPA (2010a): Human and Ecological Risk Assessment of Coal Combustion Wastes US EPA (2012b): The Impact of Coal Combustion Fly Ash Used as a Supplemental Cementitious Material on the Leaching of Constituents from Cements and Concretes Kosson et al. (2013): pH-dependent Leaching of Constituents of Potential Concern from Concrete Materials Containing Coal Combustion Fly Ash US EPA (2010a) conducted a national evaluation of potential human health and ecological risks resulting from releases to ground and surface water from CCRs disposed in landfills and surface impoundments. The risk assessment used leachate data from a range of different analytical methods to consider a wide range of management scenarios and environmental conditions. Leaching from disposed fly ash does not directly reflect leaching from fly ash concrete placed on the land.
However, the results of US EPA (2010a) are still applicable to this evaluation of fly ash concrete based on the findings of Kosson et al. (2013), which evaluated the leaching behavior of fly ash concretes and demonstrated that these CCR products consistently leach at lower levels than fly ash alone when subjected to similar environmental conditions. Thus, the current evaluation used the findings of US EPA (2010a) to identify a conservative set of COPCs for leaching from fly ash concrete placed on the land.
The modeled results for unlined landfills provide a conservative, yet appropriate, surrogate for fly ash concrete. Although US EPA (2010a) found leaching from surface impoundments to be higher than from landfills, the effects of large hydraulic heads that drive leaching from surface impoundments would be greatly diminished by the low permeability of an intact concrete matrix.
Therefore, the current evaluation did not consider leaching results for surface impoundments. The results for unlined landfills show that antimony, arsenic, boron, cadmium, lead, molybdenum, selenium, and thallium may be released at levels posing unacceptable risk to downgradient receptors.
Therefore, the evaluation retained these eight constituents as COPCs for further consideration. US EPA (2012b) also conducted a conservative screening analysis by comparing the undiluted leachate from fly ash mortars and concretes to screening benchmarks. However, because US EPA (2010a) conducted more robust, full-scale modeling that took into account dilution and attenuation in the environment, it provided a more realistic estimate of exposures. Therefore, with one exception, the current evaluation did not rely on the results of US EPA (2012b) to identify COPCs. Chromium was retained as a COPC based on the screening results of US EPA (2012b). US EPA (2010a) did not evaluate cancer risks from chromium because the revised oral cancer benchmark was not available at that time.