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- Kairies et al. (2006): Mercury in Gypsum Produced from Flue Gas Desulfurization. This literature source contains data on mercury concentrations in FGD gypsum and FGD gypsum wallboard.
- Shock et al. (2009): Evaluation of Potential for Mercury Volatilization from Natural and FGD Gypsum Products Using Flux-Chamber Tests. This literature source contains data on mercury concentrations in and mercury emanation rates from FGD gypsum and mined gypsum wallboards.
- US EPA (2009a): Characterization of Coal Combustion Residues from Electric Utilities – Leaching and Characterization Data. This literature source contains data on mercury concentrations in FGD gypsum.
- United States Department of Energy (DOE) (2008): Fate of Mercury in Synthetic Gypsum Used for Wallboard Production. This literature source contains data on mercury concentrations in FGD gypsum, mined gypsum, and FGD gypsum wallboard.
- Yost et al. (2010): Lack of Complete Exposure Pathways for Metals in Natural and FGD Gypsum.
This literature source contains data on mercury concentrations in FGD gypsum and mined gypsum.
1.2.3 Summary of Releases Identified for FGD Gypsum Wallboard Based on the review of the available literature, the current evaluation initially identified four potential releases from FGD gypsum wallboard that may occur during use: 1) generation of dust, 2) emanation to air, 3) leaching to ground and surface water, and 4) decay of naturally occurring radionuclides. A review of existing evaluations found them to be of sufficient quality and applicability to eliminate all releases from further consideration except for emanation to air. The one COPC identified for this release was mercury.
1.3 Conclusions of Step 1 Based on a review of the available literature, the current evaluation identified COPCs that may be released from fly ash concrete and FGD gypsum wallboard, but have not been sufficiently addressed by existing evaluations. For fly ash concrete, potential releases retained for further consideration were those to dust, ground and surface water, and air. COPCs for dust include aluminum, antimony, arsenic, 1-13 barium, beryllium, boron, cadmium, chromium, cobalt, copper, iron, lead, mercury, manganese, molybdenum, nickel, selenium, silver, strontium, thallium, uranium, vanadium, and zinc. COPCs for ground and surface water include antimony, arsenic, boron, cadmium, chromium, lead, molybdenum, selenium, and thallium. The one COPC identified for emanation to air was mercury. For FGD gypsum wallboard, the single potential release retained was to air. The one COPC identified for this release was mercury. Table 1-2 provides a summary of the releases and associated COPCs.
1-14 2 Step 2: Comparison of Available Data This section applies the second step of the methodology to the current beneficial use evaluation of fly ash concrete and FGD gypsum wallboard, focused on the COPCs identified in Step 1 (Literature Review and Data Collection). This evaluation aggregated and used data from the literature to compare the range of potential COPC concentrations and determine whether releases from fly ash concrete and FGD gypsum wallboard are comparable to or lower than those from analogous products. This section details the comparisons conducted, the assumptions built into these comparisons, and the results.
Appendix A presents the raw data used in these comparisons.
2.1 Releases from Fly Ash Concrete and Portland Cement Concrete This subsection presents the comparisons of potential COPC releases from fly ash concrete and the analogous product, portland cement concrete, during use. The type and amount of data available determined the types of comparisons conducted. These comparisons considered all available lines of evidence to determine whether releases of COPCs from fly ash concrete are comparable to or lower than those from portland cement concrete. The evaluation retained COPCs with the potential to be released from fly ash concrete at rates that are higher than those from portland cement concrete, or for which portland cement concrete data are not available, for further consideration in subsequent steps of the evaluation.
2.1.1 Generation of Dust Dust is generated during use when disturbances to the concrete matrix results in the transport of particulate matter away from the encapsulated matrix. The COPC concentrations in these releases will be similar to those in the source concrete because they are both composed of the same materials. The current evaluation did not identify sufficient data on the range of COPC concentrations in finished fly ash concrete and portland cement concrete to compare these products directly. Instead, the evaluation used COPC concentrations in raw fly ash and portland cement as a surrogate in this comparison. A surrogate is defined in Methodology for Evaluating Encapsulated Beneficial Uses of Coal Combustion Residuals as “data on one variable that can be used to reliably approximate the behavior of another variable and, as a result, can substitute for that variable in the comparison” (US EPA, 2013a). Because the substitution of portland cement with fly ash is the primary difference in the compositions of these two types of concrete, any difference in COPC concentrations in these raw materials will drive differences in the resulting concrete dust. The evaluation drew fly ash data from the CCR Constituent Database and portland cement data from three sources (PCA, 1992; Pflughoeft-Hassett et al., 1993; and Eckert and Guo, 1998). This evaluation used a statistical comparison to determine whether differences in the two data sets were significant.
This evaluation used ProUCL Version 4.1.01 to compare the distribution of different COPC concentrations found in fly ash and portland cement (US EPA, 2010b,c). This statistical software allows for appropriate consideration of datasets with non-detect values. To help select the most appropriate
Based on the statistical evaluation, the median concentrations of aluminum, antimony, arsenic, barium, beryllium, boron, cadmium, chromium, cobalt, copper, lead, mercury, molybdenum, nickel, selenium, thallium, vanadium, and zinc were found to be higher in fly ash than in portland cement.
Therefore, the evaluation retained each of these COPCs for further consideration. Iron, strontium, and uranium were also retained because no portland cement data were available for comparison.
Concentrations of manganese and silver were found to be less than or equal to those in portland cement.
As a result, the evaluation did not carry either of these COPCs forward for further consideration.
2.1.2 Leaching to Ground and Surface Water Leaching occurs during use when COPCs diffuse out of the concrete matrix and into surrounding liquids. The current evaluation applied the data from Garrabrants et al. (2013) as the most relevant to this release pathway. Garrabrants et al. (2013) used the EPA Leaching Evaluation Assessment Framework (LEAF) Method 1315 to estimate the cumulative release of COPCs from monolithic concrete blocks that had been allowed to cure for three months prior to sampling. Because these data provide information on releases of COPCs as a function of time, they allow a more precise evaluation of concrete leaching behavior. The available data consist of three sets of concrete samples. The first and second sets were concrete with a 20 percent and 45 percent fly ash replacement, respectively. The third set was micro-concrete with a 45 percent fly ash replacement. 9 For each of these sample sets, the study also collected a single control sample of portland cement concrete. The current evaluation includes a direct comparison of these data because the type and amount of available data do not support a robust statistical comparison.
Micro-concretes have a similar composition as standard concrete, but lack large aggregate. The micro-concrete mixture is intended to mimic the rheological properties of standard concrete. These samples were included in Kosson et al. (2013) and Garrabrants et al. (2013) to evaluate their use as a surrogate for standard concrete in leaching studies.
2-3 The following figures compare the data drawn from Garrabrants et al. (2013) on the cumulative constituent mass released per unit of surface area (mg/m2) from fly ash concrete and portland cement concrete. Garrabrants et al. (2013) examined the cumulative release of each COPC over time using EPA LEAF Method 1315. Each value listed in the following graphs represents the COPC concentration measured at a discrete time step added to the concentration measured at the previous time step. In each graph, the leachate data are plotted along with the associated method detection limit (MDL) and minimum level of quantitation (ML). The MDL is the minimum concentration that can reliably be differentiated from background noise, while the ML is the minimum concentration that can be quantified with accuracy. Following the recommendations in Risk Assessment Guidance for Superfund (RAGS) Part A (US EPA, 1989) and EPA Region 3 Guidance on Handling Chemical Concentration Data near the Detection Limit in Risk Assessments (US EPA, 1991), the current evaluation added samples that were not detected above the MDL to the subsequent time step at half the MDL. In addition, this evaluation considered samples detected above the MDL, but below the ML, to be estimated values and added those samples to the subsequent time step at the reported value. While the graphs below show an increasing MDL and ML, these values are constant for each individual time step. The summation of each discrete sample results in cumulative MDL and ML curves.
Figure 2-1 provides a comparison of antimony leaching from fly ash concrete and portland cement concrete. The amount of antimony released from two fly ash concrete samples was consistently higher than all portland cement concrete samples. The higher leaching profiles of these samples indicate the potential for fly ash concrete to leach antimony at higher rates than those of portland cement concrete.
Figure 2-2: Comparison of cumulative leaching of arsenic from concrete.
Figure 2-3 presents a comparison of boron leaching from fly ash concrete and portland cement concrete. Boron was detected in one sample of fly ash concrete, but was not detected in any samples of portland cement concrete. The higher leaching profile of this one fly ash concrete sample indicates a potential for fly ash concrete to leach boron at higher rates than portland cement concrete.
Figure 2-4: Comparison of cumulative leaching of cadmium from concrete.
Figure 2-5 presents a comparison of cumulative leaching of chromium from fly ash concrete and portland cement concrete. Several samples of fly ash concrete exhibited higher leaching rates during the first several time steps. As time progressed, releases from some portland cement concrete approach, but never exceed, those from fly ash concrete. The higher leaching profiles of these samples indicate the potential for fly ash concrete to leach chromium at higher rates than from portland cement concrete.
Figure 2-6: Comparison of cumulative leaching of lead from concrete.
Figure 2-7 presents a comparison of molybdenum leaching from fly ash concrete and portland cement concrete. All samples were below the MDL for every measured time step. Therefore, the current evaluation could not identify any differences between fly ash concrete and portland cement concrete with respect to molybdenum leaching.
Figure 2-7: Comparison of cumulative leaching of molybdenum from concrete.
Figure 2-8 presents a comparison of selenium leaching from fly ash concrete and portland cement concrete. All samples were below the MDL for every measured time step. Therefore, the current evaluation could not identify any differences between fly ash concrete and portland cement concrete with respect to selenium leaching.
Figure 2-8: Comparison of cumulative leaching of selenium from concrete.
Figure 2-9: Comparison of cumulative leaching of thallium from concrete.
In summary, the comparison of cumulative leaching data presented in Garrabrants et al. (2013) indicates that antimony, boron, and chromium have the potential to leach at higher rates from fly ash concrete than from portland cement concrete. Because these three constituents demonstrated the potential to leach at higher rates from fly ash concrete, the current evaluation retained them as COPCs for further consideration. Concentrations of arsenic, cadmium, lead, molybdenum, selenium, and thallium were below the MDL in all the measured concrete samples that were allowed to cure for three months. With the exception of selenium, these six constituents exhibited similar leaching behavior in an additional, unpublished set of samples collected by the authors of Kosson et al. (2013) and Garrabrants et al. (2013). These additional samples were identical in composition to those reported in Kosson et al.
(2013) and Garrabrants et al. (2013), but were allowed to cure for only 28 days. Concentrations of arsenic, cadmium, lead, molybdenum, and thallium were all below the MDL in samples tested using Method 1315, but selenium was detected slightly above the MDL in two samples of fly ash concrete (Kosson and Garrabrants, 2012). Because selenium demonstrated the potential to leach at higher rates from fly ash concrete, the current evaluation retained it as a COPC for further consideration.
As previously noted, arsenic, cadmium, lead, molybdenum, and thallium were below the MDL in all measured concrete samples. At the most extreme, these samples represent micro-concretes that were cured for only 28 days and placed in contact with water for 14 consecutive days. Kosson et al. (2013) and Garrabrants (2013) found that while micro-concretes provide a good approximation of concrete leaching behavior, these samples generally leach at higher concentrations and rates than standard concrete. As demonstrated by these two studies, concrete leaching decreases as the concrete cures.
Therefore, long-term concrete leaching behavior will be lower than measured after only 28 days.