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Furthermore, a 14-day contact time is much longer than the amount of time most water will remain in contact with a concrete surface, and far longer than required by other available leaching tests (e.g., TCLP and SPLP), which call for contact times of less than a day. Thus, there is no evidence that releases of these five COPCs from fly ash concrete are substantially different than those from portland cement concrete. Therefore, the evaluation did not carry leaching of arsenic, cadmium, lead, molybdenum, and thallium forward for further consideration.
This comparison shows that, during the first 28 days of curing, average mercury emanation rates from portland cement concrete may range between 2.8 and 4.4 ng/m -hr. These rates are lower than the
11.4 ng/m -hr measured for 33 percent fly ash concrete; they are also below the range of 14.9 to 34.9 ng/m2-hr measured for 55 percent fly ash concrete. At around 56 days of curing, the reported emanation rate from portland cement concrete of 3.5 ng/m2-hr was within the range of rates measured after 28 days.
The range of emanation rates from 55 percent fly ash concrete decreased considerably to between 4.4 and 15.6 ng/m2-hr after 56 days of curing, but remained higher than the portland cement concrete emanation rate. No data were available for 33 percent fly ash concrete at 56 days of curing. These results indicate that fly ash concrete may emit mercury at higher rates during the first 56 days than portland cement concrete. Therefore, this evaluation retained mercury as a COPC for further consideration.
2.2 Releases from FGD Gypsum Wallboard and Mined Gypsum Wallboard This subsection presents comparisons of potential releases from FGD gypsum wallboard and the analogous product, mined gypsum wallboard, during use. The type and amount of data available determined the type of comparison conducted. These comparisons considered all available lines of evidence to determine whether releases from FGD gypsum wallboard are comparable to or lower than those from mined gypsum wallboard. The evaluation retained COPCs with the potential to be released from FGD gypsum wallboard at rates that are higher than from mined gypsum wallboard for further consideration in subsequent steps of the evaluation.
The results indicate that mercury emanation rates from FGD gypsum wallboard, which ranged between 0.14 and 0.34 ng/m2-hr, are higher than emanation rates from mined gypsum wallboard, which ranged between 0.03 and 0.04 ng/m2-hr. The lowest mercury emanation rate measured for FGD gypsum wallboard is nearly three times higher than the highest measured emanation rate from mined gypsum wallboard, indicating that FGD gypsum wallboard may emit mercury at higher rates than mined gypsum wallboard. Therefore, the evaluation retained mercury as a COPC for further consideration.
2.3 Conclusions of Step 2 Based on the results of the comparisons conducted in this step, the evaluation found that releases of several COPCs were comparable to or lower than those from analogous products. For the generation of dust from fly ash concrete, the evaluation eliminated manganese and silver from further consideration.
For leaching from fly ash concrete, the evaluation eliminated arsenic, cadmium, lead, molybdenum, and thallium from further consideration. For releases to the air, the evaluation could not eliminate mercury for either fly ash concrete or FGD gypsum wallboard. Table 2-4 provides a list of the remaining COPCs following this step of the evaluation.
2-11 3 Step 3: Exposure Review The purpose of this section is to apply the third 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 2 (Comparison of Available Data). This section identifies the high-end chronic exposure pathways through which highly exposed individuals (HEIs) may potentially come in contact with COPCs in each identified type of release. This evaluation used this information to identify the complete exposure pathways requiring further evaluation and to develop a conceptual exposure model.
3.1 Fly Ash Concrete This subsection describes the potential exposure pathways and receptors for each COPC that may be released from fly ash concrete during use. All of the COPCs identified in this evaluation are inorganic metals. While these constituents may change valence states, form complexes with other ions and compounds, or undergo other reactions that reduce their mobility or bioavailability, they will never naturally degrade. Once released into the environment, these constituents will persist indefinitely.
Therefore, when a release occurs, exposures are theoretically possible.
3.1.1 Potential Exposure Pathways for Fly Ash Concrete Exposure to Dust Because of the high strength of finished concrete, releases of dust from solid concrete are generally negligible during use. However, some high abrasion environments may result in the generation of nontrivial amounts of concrete dust. The current evaluation identified the most relevant pathway as roadways exposed to traffic with studded tires. Any of the COPCs present in the concrete may be present in the resulting dust.
Ingestion of generated dust that has settled on various surfaces may occur through incidental hand to mouth contact. Wind or overland runoff may carry the dust from roadways to downgradient soils. This dust has the potential to accumulate in the surface soil over time. Based on these findings, this evaluation retained ingestion of concrete dust as an exposure pathway of potential concern for both human and ecological receptors.
Inhalation of dust may occur when wind or physical disturbances suspend the dust in air. However, studded tires are seasonal and limited to winter months in the states that permit their use. Over the course of seconds to hours, suspended dust will either disperse in the air or settle out on the ground.
Significant wearing of the roadway would need to occur to generate the amount of dust necessary to sustain chronic, elevated dust concentrations in the air. A review of the air quality around Alaskan roads found that, even in the presence of traffic with studded tires, it was natural sources (e.g., wildfires) or vehicle exhaust that were the primary drivers of National Ambient Air Quality Standards exceedances for fine particulates (Zubeck et al., 2004). In addition, most roadway dust identified by this study originated from dirt and asphalt roads, rather than roads made with concrete. Because inhalation of 3-1 concrete dust is not a pathway that may drive exposures, this evaluation did not consider it in Step 4 (Screening Assessment).
Dermal contact may occur through direct contact with concrete dust that has settled on various surfaces. However, absorption through the skin is limited compared to other exposure pathways because of the relatively low lipid solubility of most metals (Paustenbach, 2000 and Hostynek et al., 1998 as cited in US EPA, 2007a). Because dermal contact with concrete dust is not a pathway that may drive exposures, this evaluation did not consider it in Step 4 (Screening Assessment).
Exposure to Ground Water Some fraction of the precipitation that falls on concrete may infiltrate directly through any cracks present in the concrete matrix and into underlying soil. The remaining fraction of the precipitation that does not infiltrate directly through the concrete matrix may run off and infiltrate through adjacent soil.
Any of the COPCs identified may be released into water that comes into contact with the concrete. All COPCs dissolved are assumed to migrate vertically through the soil column and enter the ground water table.
Ingestion of COPCs present in concrete leachate may occur if ground water is used as a source of potable water for human receptors. Leaching can occur during each precipitation event and can result in a ground water plume that contaminates downgradient private wells for multiple years. Therefore, this evaluation retained ingestion of ground water impacted by concrete leachate as an exposure pathway of potential concern for human receptors. Ecological receptors are not anticipated to have any appreciable direct contact with ground water.
None of the COPCs identified for ground water are volatile under standard environmental conditions. Therefore, inhalation of COPCs from ground water used as a source of drinking water is not a complete exposure pathway and this evaluation did not consider it in Step 4 (Screening Assessment).
Dermal contact may occur through direct contact with ground water while bathing. However, absorption through the skin is limited compared to other exposure pathways because of the relatively low lipid solubility of most metals (Paustenbach, 2000 and Hostynek et al., 1998 as cited in US EPA, 2007a). Because dermal contact with ground water is not a pathway that may drive exposures, this evaluation did not consider it in Step 4 (Screening Assessment).
Exposure to Surface Water When precipitation falls on concrete, some fraction of this precipitation may run off overland and into downgradient water bodies. This overland runoff may also infiltrate into underlying ground water before discharging to downgradient water bodies. Some concrete structures, such as bridges and dams, may have frequent direct contact with water bodies and leach directly into surface water. Any of the COPCs identified may be released into water that comes into contact with the concrete.
Surface water used as a source of potable water is assumed to be a negligible exposure pathway for human receptors. Surface water is assumed to be routed through a municipal water treatment facility prior to consumption, reducing the levels of any COPCs present. Incidental ingestion of COPCs in surface water may occur during swimming or other activities near a water body. For human receptors, it 3-2 is assumed that these exposures are infrequent and small in comparison to exposures from intentional ingestion of ground water. However, ingestion may be a significant exposure pathway for ecological receptors that live in and around the water body. Therefore, the evaluation retained the ingestion of and direct contact with surface water as an exposure pathway of potential concern for ecological receptors.
In turn, some of the fish present in these water bodies can represent a sizable portion of the diet for some human receptors. Therefore, the evaluation retained ingestion of fish as an exposure pathway of potential concern for human receptors.
As discussed for ground water, none of the COPCs identified in concrete leachate are volatile under standard environmental conditions. Therefore, inhalation of COPCs from surface water is not a complete exposure pathway and this evaluation did not consider it in Step 4 (Screening Assessment).
Dermal contact may occur through direct contact with surface water while swimming. However, absorption through the skin is limited compared to other exposure pathways because of the relatively low lipid solubility of most metals (Paustenbach, 2000 and Hostynek et al., 1998 as cited in US EPA, 2007a). Because dermal contact with surface water is not a pathway that may drive exposures, this evaluation did not consider it in Step 4 (Screening Assessment).
Exposure to Air Concrete 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.
This 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).
3.1.2 Potential Receptors for Fly Ash Concrete Human Receptors Due to the prevalence of concrete as a building material, human receptors may be exposed to COPCs in industrial, commercial, or residential settings. Of these receptor types, residential receptors are the most likely to be HEIs, due to the longer duration of time spent indoors, as well as the generally smaller ratio of air volume to wall surface area in residential buildings compared to offices or industrial workspaces. Residential receptors are also the only human receptors anticipated to be exposed to COPCs through ingestion of untreated ground water as a source of potable water. Commercial and industrial workspaces are generally connected to a municipal drinking water source that is treated, regulated, and monitored prior to distribution. Finally, recreational fishers may be exposed to COPCs through ingestion 3-3 of fish that have been exposed to and accumulated the COPCs from contact with surface water, sediment, and biota. Figure 3-1 shows the conceptual exposure model developed for human receptors.
Dashed lines represent exposures 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 3-1: Human conceptual exposure model for fly ash concrete.
Ecological Receptors The current evaluation identified plants, invertebrates, amphibians, fish, birds, and mammals as the relevant classes of ecological receptors. On a national scale, any of these receptor types may be present downgradient of a concrete source. The evaluation did not select specific ecological receptors at this step because the most sensitive receptor may differ on a case-by-case basis, depending on both the species and COPC present in a given environment. Instead, sensitive ecological receptors for each COPC were determined during development of screening benchmarks based on available toxicological data (see Appendix B). Figure 3-2 shows the conceptual exposure model developed for ecological receptors.