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A 90th percentile indoor air mercury concentration was calculated by probabilistically varying the inputs in Equation 4-1. The evaluation then compared this concentration to the relevant screening benchmark identified in Appendix B. The comparison results are presented in Table 4-5.
This comparison shows that the mercury concentration in indoor air resulting from fly ash concrete is below the relevant screening benchmark, even for the conservative scenario evaluated. Based on this comparison, exposures to mercury vapor emitted by fly ash concrete do not warrant further consideration for human receptors.
4.2 FGD Gypsum Wallboard This subsection details the screening assessment conducted for FGD gypsum wallboard. Inhalation of mercury vapor was the single exposure pathway carried forward from Step 3 (Exposure Review) for this CCR product. Thus, this subsection describes the approach used to evaluate this exposure scenario, as well as the results of the screening.
4.2.1 Exposure to Indoor Air In Step 3 (Exposure Review), the evaluation concluded that the highest exposures to mercury vapor result from inhalation of the air inside residences constructed with FGD gypsum wallboard. In this step, the evaluation first estimated an upper bound on the mercury concentrations that may occur in indoor air 4-13 as a result of the use of FGD gypsum in wallboard. Equation 4.1 was used to probabilistically calculate a 90th percentile air concentration for a conservative exposure scenario.
- Because of the relatively small number of measurements available, the evaluation selected the highest measured mercury emanation rate in the available literature of 0.34 ng/m2-hr (Shock et al., 2009). Unlike concrete, the internal structure of wallboard does not change appreciably after the product has been put into use. Therefore, it is unlikely that the mercury emanation rate of FGD gypsum wallboard will decrease over time like concrete. Because much of the available data on mercury concentrations were summary statistics, it was difficult to develop a distribution of emanation rates. Instead, to ensure that this screen effectively captured an upper bound of the potential releases of mercury, the current evaluation used the maximum reported mercury concentration in FGD gypsum of 3.1 mg/kg to adjust the emanation rate from Shock et al. (2009) under the assumption that the mercury emanation rate from the wallboard changes linearly as a function of the mercury concentration in the wallboard.
- The distribution of air exchange rates used was the same as described in Section 4.1.3.
- The distribution of product surface areas used was the same as described in Section 4.1.3, except it was assumed that wallboard would not be used in floor construction.
- The distribution of housing unit volumes used was the same as described in Section 4.1.3.
A 90th percentile indoor air mercury concentration was calculated by probabilistically varying the inputs in Equation 4-1. This evaluation then compared the calculated concentration to the relevant screening benchmark identified in Appendix B. The comparison results are presented in Table 4-6.
This comparison shows that the concentration of mercury in indoor air resulting from FGD gypsum wallboard is below the relevant screening benchmark, even for the conservative scenario evaluated.
Based on this comparison, exposures to mercury vapor emitted by FGD gypsum wallboard do not warrant further consideration for human receptors.
4.3 Conclusions of Step 4 By the end of this step, all of the COPCs identified for fly ash concrete and FGD gypsum wallboard in Step 1 (Literature Review and Data Collection) were eliminated from further consideration. Thus, the evaluation did not proceed on to Step 5 (Risk Assessment). The analytical results of the first four steps indicate that environmental releases from these CCR products are comparable to or lower than those from analogous products, or are at or below relevant screening benchmarks. To confirm these findings, a
4-15 5 Section 5: Results, Uncertainties, and Conclusions The purpose of this section is to summarize the results of the current evaluation of fly ash concrete and FGD gypsum wallboard. This evaluation was conducted according to the Methodology for Evaluating Encapsulated Beneficial Uses of Coal Combustion Residuals (US EPA, 2013a). Thus, this section summarizes the results of the evaluation, the uncertainties present in the evaluation, the potential impact of these uncertainties on the results of the evaluation, and ultimate conclusions regarding these beneficial uses.
5.1 Summary of Results The primary goal of this document is to determine whether EPA should support the continued use of coal fly ash in concrete and FGD gypsum in wallboard. In addition, this document provides a clear example of how to conduct such an analysis and demonstrate an appropriate level of documentation. For these purposes, the current evaluation considered the two largest encapsulated beneficial uses of CCRs in the United States: fly ash used as a direct substitute for portland cement in concrete and FGD gypsum used as a replacement for mined gypsum in wallboard. These CCR products may be variable in their composition. However, this evaluation only addressed those products that meet relevant physical and performance standards established for these products by voluntary consensus standard bodies, that conform to specific design criteria identified in this evaluation (e.g., ≤ a 40 percent fly ash replacement rate), and that incorporate fly ash and FGD gypsum from common pollution control devices used in the United States. This evaluation also did not address products that contain additional additives or industrial materials that may alter releases from the products.
5.1.1 Fly Ash Concrete Step 1 (Literature Review and Data Collection): From the available literature, the evaluation identified the generation of dust, emanation to air, leaching to ground and surface water and decay of naturally occurring radionuclides as potential releases that may occur from fly ash concrete during use.
During the review of collected literature, the evaluation also identified several existing evaluations of
sufficient applicability and quality to rely upon. This review found:
- None of the existing evaluations identified provided a sufficient rationale to eliminate COPCs from the evaluation of dust releases from fly ash concrete. As a result, a total of 23 COPCs were identified based on the chemical composition of fly ash and available toxicological data. These COPCs included: aluminum, antimony, arsenic, barium, beryllium, boron, cadmium, chromium, cobalt, copper, iron, lead, manganese, mercury, molybdenum, nickel, selenium, silver, strontium, thallium, uranium, vanadium, and zinc.
- Several existing evaluations were found to be relevant to releases to ground and surface water (US EPA, 2010a; 2012a). Based on these existing evaluations, the evaluation retained a total of nine COPCs for more detailed examination. These COPCs included: antimony, arsenic, boron, cadmium, chromium, lead, molybdenum, selenium, and thallium.
- None of the existing evaluations identified provided a sufficient rationale to eliminate COPCs from the evaluation of releases to air. The constituent associated with the fly ash concrete that may volatilize under standard environmental conditions is mercury. Therefore, mercury was retained as a COPC for this release.
- Several existing evaluations were found relevant to radionuclides in fly ash concrete. The cumulative body of evidence provided by these evaluations was considered sufficient to eliminate radionuclides from further consideration.
Step 2 (Comparison of Available Data): To the extent practicable, the evaluation aggregated the data identified in Step 1 to allow a comparison of the releases of COPCs from fly ash concrete and portland cement concrete. The type of comparison conducted depended on the amount of data available. If a given COPC demonstrated the potential to be released at a higher rate from fly ash concrete than from portland cement concrete, the current evaluation retained that COPC for further evaluation in subsequent
steps of the evaluation. The following comparisons were conducted:
- A statistical comparison of COPC concentrations in raw fly ash and portland cement using the ProUCL statistical software for dust releases. This evaluation used these raw materials as a surrogate for fly ash concrete and portland cement concrete, respectively. The results of the comparison showed that concentrations of manganese and silver are likely to be higher in portland cement concrete than fly ash concrete. Therefore, this evaluation did not retain these two COPCs for further consideration. The remaining 21 COPCs either demonstrated the potential to be higher in fly ash concrete, or did not have sufficient portland cement data to conduct a comparison, and were retained for further consideration.
- A direct comparison of the range of measured leachate concentrations was conducted for releases to ground and surface water. Because this leachate is the source of releases to both ground and surface water, the current evaluation used the same comparison for both media. The results of the comparison showed that all available measurements of arsenic, cadmium, lead, molybdenum, and thallium were below detection. Based on these findings, this evaluation did not retain these COPCs for further evaluation. Antimony, boron, chromium, and selenium demonstrated the potential to be released at higher rates from fly ash concrete than from portland cement concrete in one or more samples. Therefore, these four COPCs were retained for further consideration.
- A direct comparison of the range of measured mercury emanation rates was conducted for releases to air. The available data indicated the potential for mercury to be released at higher rates from fly ash concrete than from portland cement concrete. Therefore, this COPC was carried forward to the next step of the evaluation.
Step 3 (Exposure Review): This evaluation reviewed the releases carried forward from Step 2 to identify any exposures that may occur. Where the evaluation identified multiple exposure scenarios, it retained the scenarios likely to result in the highest chronic exposures for further evaluation. The
- Incidental ingestion of dust generated from concrete roadways exposed to traffic with studded tires was identified as a potentially complete exposure pathway. This evaluation identified nearby residential receptors and foraging ecological receptors as the highly exposed receptors.
- Use of ground water contaminated by concrete leachate as a source of potable water was identified as a potentially complete exposure pathway for humans. The evaluation identified nearby residential human receptors as HEIs in this scenario. Ecological receptors were assumed to have negligible contact with ground water and were not retained as potential receptors.
- Ingestion of fish caught from surface water bodies that receive runoff or ground water discharge that has been contaminated by leachate from fly ash concrete was identified as a potentially complete exposure pathway. The evaluation identified recreational fishers and their families as HEIs in this scenario. This evaluation also identified direct contact and ingestion of impacted surface water as a scenario of concern for ecological receptors. Any receptor living in or near the water body may be exposed.
- Inhalation of mercury vapor in indoor air was identified as a potentially complete exposure pathway. The evaluation identified residential receptors as HEIs in this scenario.
Step 4 (Screening Assessment): This evaluation conducted a conservative screening assessment for each exposure pathway identified in Step 3. This evaluation used conservative (i.e., likely to overestimate exposures) environmental, fate and transport, and exposure data to estimate COPC concentrations at the point of exposure. It then compared these concentrations to relevant regulatory and health-based screening benchmarks to determine if more in-depth modeling was warranted.
- The evaluation calculated the 90th percentile contribution of fly ash concrete to COPC concentrations in surface soil mixed with concrete dust and compared the calculated concentration to relevant screening benchmarks. The evaluation found concentrations of all COPCs to be below all relevant screening benchmarks for human and ecological receptors. Therefore, the evaluation did not retain this exposure pathway for further consideration.
- The evaluation compared the maximum leachate concentrations from fly ash concrete directly to the relevant screening benchmarks in the first stage of the screening for both ground and surface water. These undiluted leachate concentrations were below all relevant surface water screening benchmarks. Therefore, the evaluation did not retain this exposure pathway for further consideration. The undiluted leachate concentrations were also below the relevant ground water screening benchmarks for antimony, boron, and selenium. However, the undiluted concentration of chromium (VI) exceeded the screening benchmark for ingestion of ground water. Therefore, the evaluation conducted a second round of screening to conservatively account for dilution and attenuation in the environment prior to receptor exposures (see Appendix C). The revised chromium (VI) concentration was below all screening benchmarks. Therefore, the evaluation did not retain this exposure pathway for further consideration.
- The evaluation probabilistically calculated a 90th percentile indoor air mercury concentration based on a conservative scenario. This concentration was compared to the relevant screening benchmark
5.1.2 Flue Gas Desulfurization Gypsum Wallboard Step 1 (Literature Review and Data Collection): Drawing from the available literature, this evaluation identified generation of dust, emanation to air, leaching to ground and surface water and decay of naturally occurring radionuclides as potential releases that may occur from FGD gypsum wallboard.
During the review of collected literature, several existing evaluations were of sufficient applicability and