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Figure 3-2: Ecological conceptual exposure model for fly ash concrete.
3.2 FGD Gypsum Wallboard This subsection describes the potential exposure pathways and receptors for mercury, the single COPC identified for FGD gypsum wallboard. While mercury may change valence states, form complexes with other ions or compounds, or undergo other reactions that reduce its mobility or bioavailability, it will not naturally degrade. Once released into the environment, mercury will persist indefinitely. Therefore, where releases are possible, so are exposures.
3.2.1 Potential Exposure Pathways for FGD Gypsum Wallboard Exposure to Air Wallboard is a porous solid; therefore, gases and vapors are able to diffuse through the interstitial pores and emanate into indoor air. Elemental mercury is the only COPC identified that readily vaporizes within the range of standard temperature and pressure conditions found in habitable buildings.
Inhalation of mercury vapor may occur in closed indoor environments as mercury vapor accumulates due to low air circulation. Therefore, this evaluation retained the inhalation of indoor air as an exposure pathway of potential concern for human receptors. Ecological receptors are not anticipated to have any appreciable direct contact with indoor air.
The evaluation also considered dermal contact with mercury vapor to be a negligible exposure pathway. Past studies have demonstrated that the amount of inorganic mercury adsorbed through the skin is small when compared to the amount adsorbed through the lungs (Hursh et al., 1989 as cited in US EPA, 1997a). Because dermal contact with mercury vapor is not a pathway that may drive exposures, this evaluation did not consider it in Step 4 (Screening Assessment).
3-5 3.2.2 Potential Receptors for FGD Gypsum Wallboard Human Receptor Due to the prevalence of FGD gypsum wallboard as a building material, human receptors may be exposed to COPCs in industrial, commercial, and residential settings. Of these receptor types, residential receptors are the most likely to be HEIs, due to the longer duration of time spent in residential buildings, as well as the generally smaller ratio of air volume to wall surface area of residential buildings compared to offices or industrial workspaces. Figure 3-3 shows the conceptual exposure model developed for human receptors. Dashed lines represent releases, exposure pathways, or receptors that may be present, but were not directly evaluated in Step 4 (Screening Assessment) because they do not drive high-end exposures.
Figure 3-3: Human conceptual exposure model for FGD gypsum wallboard.
Ecological Receptors Ecological receptors are not anticipated to have any appreciable direct contact with indoor air, and were not retained for further evaluation.
3.3 Conclusions of Step 3The evaluation did not eliminate any releases or associated COPCs in this step based on a review of potential exposure pathways and HEIs. Therefore, all of the COPCs identified in Step 2 (Comparison of Available Data) will proceed to Step 4 (Screening Assessment). Table 3-1 provides a list of the remaining COPCs following this step of the evaluation.
3-7 4 Step 4: Screening Assessment The purpose of this section is to apply the fourth step of the encapsulated beneficial use methodology to the current evaluation of fly ash concrete and FGD gypsum wallboard, based on the COPCs carried forward from Step 3. This screening used conservative (i.e., likely to overestimate exposures) environmental, fate and transport, and exposure data to estimate COPC exposures that may occur. The evaluation compared these conservative exposure concentrations to screening benchmarks drawn from established values (e.g., ecological soil screening levels) and/or health-based values calculated for this specific evaluation based on available toxicological and exposure data (i.e., based on a cancer risk of 1×10-5 or a hazard quotient of 1.0). Appendix B provides a discussion of the considerations involved in developing an appropriate set of screening benchmarks.
4.1 Fly Ash Concrete This subsection details the screening assessment conducted for fly ash concrete. For each exposure pathway carried forward from Step 3 (Exposure Review), this subsection discusses the different approaches used to evaluate each exposure scenario, as well as the results of the screenings.
4.1.1 Exposure to Concrete Dust In Step 3 (Exposure Review), the evaluation concluded that the highest exposures to concrete dust result from incidental ingestion of dust that has accumulated surface soils. In this step, the evaluation estimated an upper bound for the COPC concentrations in surface soil that may result from the use of fly ash in concrete, and compared these concentrations directly to relevant screening benchmarks. First, the 90th percentile contribution of fly ash to COPC concentrations in concrete dust was calculated
probabilistically by multiplying the following three distributions together:
- The range of cement use rates in concrete were drawn from the Portland Cement Association (PCA, No Date a,b). Typical values range from seven percent to 15 percent of the total concrete mass. For ease of calculation, the evaluation divided this continuous range of values into eight discrete data points in increments of one percent under the assumption that each of the data points had an equal probability of occurring.
- The range of cement replacement rates were based on the ASTM standard for blended cement concrete (ASTM C595), which limits the amount of portland cement replaced by fly ash to below 40 percent. Therefore, the evaluation selected a range of five percent to 40 percent of the cement used. For ease of calculation, the evaluation divided this continuous range of values into eight discrete data points in increments of five percent under the assumption that each of the data points had an equal probability of occurring.
- The COPC concentrations in fly ash were drawn from data in the CCR Constituent Database.
Appendix A provides more information on each of these data sources contained in this database.
Next, the evaluation conservatively accounted for the dilution and attenuation that can occur during the transport of concrete dust to nearby surface soil. The evaluation did not identify any literature that 4-8 specifically addressed this topic, and instead relied on the findings of US EPA (2002), as discussed in US EPA (2010a). These findings pertain to unmitigated transport of ash from uncovered CCR landfills by wind and overland runoff, and show that these overland transport routes could result in CCRs accounting for up to 10 percent of nearby surface soil. Therefore, the current evaluation divided the 90th percentile dust concentrations by a dilution and attenuation factor of 10 to account for incorporation into surface soil. This represents a conservative assumption for the current evaluation because a much greater quantity of fly ash is available for transport from an uncovered landfill, compared to an intact concrete road, at any given time. Table 4-1 presents the comparison of these soil concentrations to relevant screening benchmarks.
4.1.2 Exposure to Ground and Surface Water In Step 3 (Exposure Review), the evaluation concluded that the highest exposure to concrete leachate may result from the use of impacted ground water as a source of drinking water, ingestion of fish from impacted surface water, or direct contact with impacted surface water. In this step, the evaluation conducted ground and surface water screening in two separate stages. The intent of these stepwise screenings was to eliminate COPCs that did not warrant further consideration under a more realistic, resource intensive modeling scenario.
First, a preliminary conservative screening was conducted by comparing the maximum COPC concentrations drawn from Kosson et al. (2013) and Garrabrants et al. (2013) to relevant screening benchmarks. This approach assumed that the COPC concentrations present in both ground and surface water were the same as in undiluted leachate. Appendix C provides a detailed discussion of the data and methods used to calculate these leachate concentrations. Table 4-2 presents the results of the comparison to screening benchmarks for human health. For ease of comparison, all screening benchmarks were standardized to represent the ground or surface water concentrations below which no further evaluation is warranted. COPC concentrations higher than the benchmarks in this preliminary screening do not indicate the presence of elevated risks; only that further evaluation may be warranted.
Table 4-3 presents the results of the comparison to screening benchmarks for ecological receptors.
For ease of comparison, all screening benchmarks were standardized to represent the surface water concentrations below which no further evaluation is warranted. COPC concentrations higher than the benchmarks in this preliminary screening do not indicate the presence of elevated risks; only that further evaluation may be warranted. This table does not include antimony because the current evaluation did not identify a relevant screening benchmark for this COPC.
The results of the comparison indicate that the concentrations of antimony, boron, and selenium fall below all relevant screening benchmarks identified for ground and surface water. Therefore, the evaluation did not retain these three constituents for further consideration. The undiluted concrete leachate concentration of chromium (VI) was higher than the human health-based number (HBN) for tap water ingestion. Therefore, a second round of screening was conducted that conservatively accounted for the dilution and attenuation that occurs in the environment prior to exposure. This evaluation used the Industrial Waste Evaluation Model (IWEM) as the most appropriate ground water model. Appendix C provides a detailed discussion of the inputs and assumptions used in this model. Table 4-4 compares the modeled 90th percentile well concentration of chromium (VI) to the same HBN for tap water ingestion.
The results of the comparison indicate that chromium (VI) falls below the relevant screening benchmark for tap water ingestion. Therefore, the evaluation did not retain chromium (VI) as a COPC for further consideration. Based on this comparison, exposures to fly ash concrete leachate do not warrant further consideration for either human or ecological receptors.
- Mercury emanation rates of 4.4 and 15.6 ng/m2-hr measured by Golightly et al. (2005; 2009) were identified as the most appropriate values for this comparison because they represent concrete allowed to cure for 56 days. It has been shown that the concrete matrix becomes gradually denser with time for at least a year after mixing (Garboczi, 1995). A denser concrete matrix reduces the size of interstitial pores and, consequently, reduces the rate at which mercury vapor can escape to indoor air. Therefore, of the available data summarized in Section 2.1.3, the samples collected after 56 days are most representative of long-term exposures. The evaluation adjusted these emanation rates using the range of fly ash mercury concentrations in the CCR Constituent Database (Appendix A) under the assumption that the mercury emanation rate from concrete changes linearly as a function of the mercury concentration in concrete.
- The air exchange rate is the number of times that the total volume of air in a housing unit is exchanged with outside air during a given time period. Values were drawn from Koontz and Rector (1995), cited in the 1997 Exposure Factors Handbook (US EPA, 1997). The current evaluation incorporated the reported distribution of national air exchange rates between the 5th percentile [0.15 air changes per hour (ACH)] and the 95th percentile (1.74 ACH). The maximum air exchange rate of 23.3 ACH was omitted because it is unlikely to reflect the scenario under evaluation. No minimum air exchange rate was reported by this study.
- Product surface area is the total surface area of the CCR product exposed to indoor air. It was conservatively assumed that (at a minimum) the ceiling, floor, and four exterior walls of the residence were constructed with concrete. This evaluation assumed a square floor plan with a ceiling 2.4 m (8 ft) high. The International Building Code (IBC) was consulted to determine the total number of interior walls that may be present in a building of the size modeled (ICC, 2006b). 10 The 2006 IBC requires that the floor area of at least one room in a housing unit be 11 m2 (120 ft2) or larger, while all remaining habitable rooms must have floor areas of at least 6.6 m2 (70 ft2). Based on these parameters and the range of home unit volumes, the evaluation The International Building Codes (IBC) are building codes developed and maintained by the International Code Council.
At present, many state and local governments have adopted the 2006 IBC or a more recent iteration either statewide or by an individual county.
4-12 determined that a maximum of between three and four full-length walls may be present. Adding additional walls would result in a building that is out of code. Because both sides of the interior walls are exposed to indoor air, their surface area is twice that of the external walls. The evaluation assumed that all possible surface areas between this minimum and maximum were equally likely to occur.
- Housing unit volume is the total internal volume of a housing unit. This evaluation considered volumes between 153 m3 (5,439 ft3) and 492 m3 (5,439 ft3), which are the 10th percentile and average values, respectively, for owned and rented properties listed in Table 19-1 of the 2011 Exposure Factors Handbook (US EPA, 2011). These data were drawn from the 2011 edition because the 1997 edition only reports a median value. This evaluation used the volume of the total housing unit, rather than a single room, because the air exchange rates measured are for entire buildings, rather than individual rooms. The evaluation assumed that all possible housing volumes between these two sizes were equally likely to occur.