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Another source of uncertainty stems from the method used to calculate BCFs. BCFs are based on whole body concentrations for fish. However, the amount of bioconcentration often varies among 5-16 different organs and muscle tissues in the fish (du Preez et al., 1993). For example, fish muscle tissues often have the lowest accumulation of metals. Because muscle is the bulk of the edible part of the fish, a whole body concentration that includes other organs (with higher concentrations of contaminants) may overstate potential human exposures from fish ingestion (CalEPA, 2012). In addition, BCFs may be measured from juvenile fish. Studies have shown an inverse relationship between metal accumulation and weight or size of the fish, with the metal concentrations in tissues decreasing as fish size or weight increases (Liao et al., 2003, as cited by CalEPA, 2012). Scientists have attributed this effect to a number of factors, including growth dilution, increased metabolic rate in juvenile fish, and increased ability to depurate the metals as the fish matures. As a result, metal uptake studies in fingerlings or juvenile fish may overestimate BCFs in mature fish (CalEPA, 2012). Finally, BCFs are based only on the bioconcentration of metals from environmental media. Ingestion of sediment and sediment-dwelling invertebrates by bottom-dwelling fish species may also contribute to metal uptake by these fish (CalEPA, 2012).
The final source of uncertainty is the application of the BCFs. In most cases, the evaluation used BCF data for a single fish species to represent all fish at that trophic level. This may or may not accurately represent the fish species commonly caught and consumed, introducing some uncertainty into the analysis. Overall, it is unknown whether use of BCFs overestimates or underestimates concentrations present in fish tissue.
5.2.3 Uncertainties for Air Exposures The uncertainties addressed in this section pertain to the evaluation of mercury releases from fly ash concrete and FGD gypsum wallboard during consumer use, and the resulting receptor exposure.
Linear Increase of Mercury Emanation The current evaluation assumes that the rate at which fly ash concrete and FGD gypsum wallboard emit mercury is a linear function of the amount of mercury present in the CCR products. Vaporized mercury moves through porous solids (e.g., concrete and wallboard) by diffusion, which is controlled by the physical characteristics of the material (e.g., porosity) and ambient environmental conditions (e.g., temperature and pressure). These physical characteristics and environmental conditions may change with time, independent of the mercury concentration present, and may have non-linear impacts on the rate of mercury diffusion. However, the available literature identifies emanation rates measured under high temperatures, negative pressure conditions, and short cure times (i.e., higher concrete porosity).
Therefore, the assumption that these extreme physical characteristics and environmental conditions remain constant likely overestimates mercury releases under typical room conditions.
Elemental mercury is the form of mercury that is available to vaporize from concrete and wallboard.
The remainder of the mercury is generally present in various oxidized compounds (e.g., mercuric chloride) that do not vaporize under standard environmental conditions. A linear increase in mercury emanation assumes that the ratio of elemental mercury to total mercury remains constant with increasing mercury concentrations. Elemental mercury is difficult to capture during coal combustion and is more 5-17 likely to escape into the atmosphere, while oxidized mercury is more water soluble and is more effectively captured by pollution control devices (Wilcox et al., 2012). As a result, the ratio of elemental and oxidized mercury present in fly ash is likely to decrease as total mercury concentrations increase.
The presence of higher carbon content does not alter this fact. Most of the retention and oxidation of mercury associated with fly ash involves carbon content. Studies have shown that organic carbon with the highest oxidation capacity results in some of the highest mercury concentrations in fly ash (AbadValle et al., 2011). Therefore, the assumption of a constant ratio of elemental and oxidized mercury in fly ashes and FGD gypsum when extrapolating emanation rates will likely overestimate releases. This agrees with the finding of Golightly et al. (2009) that fly ashes with higher organic carbon content result in fly ash concrete with lower mercury emanation rates relative to total mercury content.
Air Exchange Rates Air exchange rates can vary considerably based on geography due to the different climates across the United States, and based on the season due to different building heating and cooling requirements. When conducting the survey in 1995 that forms the basis for these air exchange rates, the authors examined air exchange rates in homes across the country during all four seasons. However, in recent years, attention has been focused on reducing heat loss and, consequently, air loss from buildings. Several building construction codes currently require a minimal air exchange rate of 0.35 ACH for newly constructed habitable structures. 12 However, the actual air exchange rate of a completed building is rarely measured and is still subject to seasonal changes (US EPA, 2010b). Therefore, the measured air exchange rates presented in Koontz and Rector (1995) still represent the best available estimates of average and highend exposures for the country. Use of this data may overestimate exposures, but the magnitude of this overestimation is unknown.
Organic Carbon Content Golightly et al. (2009) found that fly ash with higher organic carbon content results in fly ash concrete with lower mercury emanation rates relative to total mercury content. As a result, the linear increase of mercury emanation assumed in this evaluation causes the fly ash concrete with the lowest organic content to have the highest adjusted emanation rates. Yet, a low organic content fly ash is unlikely to contain the highest mercury concentrations, because a strong positive correlation exists between the mercury concentration and organic content of fly ash (Wilcox et al., 2012 and Abad-Valle et al., 2011). Data on the organic carbon content was not available for the majority of fly ash samples.
Therefore, to remain conservative, all available samples were incorporated into the evaluation. As a result, the fly ashes with the highest mercury concentrations likely represent organic content higher than the limit of 6 percent for concrete production (Golightly et al., 2009). Extrapolating mercury emanation from fly ash concrete using an upper bound mercury concentration that does not consider the limit on fly ash organic carbon content is likely to overestimate exposures, but the magnitude of this overestimation is unknown.
Examples of these building codes include American Society of Heating, Refrigerating, and Air-Conditioning Engineers Standard 90.1 (ASHRAE, 2007), International Residential Code (ICC, 2006a), and International Mechanical Code (ICC, 2009).
5-18 FGD gypsum is composed predominantly of calcium sulfate. Fly ash is considered a contaminant in FGD gypsum because it decreases the quality of the wallboard produced. As a result, industry standards are in place limiting the amount of fly ash allowed in raw FGD gypsum intended for wallboard production (Henkels and Gaynor, 1996). However, the trace amounts of fly ash that are present may contribute to mercury emanation from the FGD gypsum wallboard. The FGD gypsum samples available did not report fly ash content. However, the examination of numerous FGD gypsum and FGD gypsum wallboard samples produced across the United States provides a high degree of confidence that the evaluation captured the range of fly ash and mercury concentrations.
Chemical Admixtures in Fly Ash Concrete The majority of fly ash concrete evaluated in the literature consists of cement, fly ash, water, and some form of aggregate. However, Golightly et al. (2007; 2009) also included an air entrainment admixture (AEA). AEAs are chemicals added to concrete to control the size and spacing of air pockets (i.e., voids) within the concrete matrix. These voids connect smaller capillary pores, which are the spaces within the concrete matrix filled with unreacted water. The primary purpose of introducing air voids in the concrete matrix is to provide the unreacted water present in capillary pores a place to expand when exposed to freezing temperatures. This reduces the amount of strain placed on the concrete matrix and reduces internal cracking. Once the ice thaws, the water in the voids is drawn back into the narrower pore network through capillary forces. Although AEAs alter the number and spacing of air voids, they do not directly impact the size and spacing of capillary pores and smaller gel pores. This network of capillary and gel pores throughout the concrete matrix serves as the primary pathway through which gases can diffuse through the concrete and into indoor air. This evaluation found no indication that inclusion of AEAs in concrete will reduce the rate of mercury emanation from concretes during use. Instead, AEAs may actually promote transport of gases through the concrete by providing a direct connection for the capillary pores within the concrete and increasing the volume of internal air spaces. This greater air space and interconnectivity could allow easier migration of mercury through the concrete, resulting in an overestimation of releases from concrete without chemical additives.
Surface Covering Both concrete and wallboard are typically coated by some combination of paint, glue, carpet, laminate, or other substance prior to use. Concrete surfaces that remain uncovered are often polished to a smooth finish. All of these coatings will impede the migration of mercury into the indoor air by either directly covering interstitial pore spaces or minimizing the surface area that can emit mercury. However, it is unknown to what extent different combinations of surface coatings inhibit mercury emanation rates.
All available literature on mercury emanation from fly ash concrete and FGD gypsum wallboard measured releases from uncoated products. Consistent with the available data, the current evaluation assumed that the building materials were placed bare in a building. The assumption of no surface covering prior to use overestimates the calculated exposures, but the magnitude of the overestimation is unknown.
5-19 Combined Use of Fly Ash Concrete and FGD Gypsum Wallboard The current evaluation did not consider a scenario where fly ash concrete and FGD gypsum wallboard were used in the same residence. While it is possible that both CCR products could be used in the same building, wallboard and concrete are unlikely to be used in the same wall. Because FGD gypsum wallboard emits mercury at a lower rate than concrete, even after adjustment, use of wallboard would only reduce the mercury concentrations estimated for a building with all floors, ceilings, and walls made with concrete. Therefore, this uncertainty is small and unlikely to affect the results of the evaluation.
Sorption of Mercury to Building Surfaces Mercury vapor has the potential to sorb to various surfaces within a building. This sorption will reduce the concentration of mercury present in the indoor air, but it will introduce an additional potential exposure pathway. The sorption of mercury onto indoor surfaces may result in higher exposures than those predicted through consideration of indoor air concentrations alone as a result of hand to mouth contact with dust and with various household surfaces. However, the calculated 90th percentile raw fly ash mercury concentration of 0.65 mg/kg is over an order of magnitude below the incidental ingestion screening benchmark of 14.4 mg/kg recalculated for an infant. Therefore, the impact of this uncertainty on the findings of the current evaluation is likely to be minimal.
Steady-State Mercury Concentration “Steady state” is the condition of equilibrium when the rates of mercury entering and leaving a building are equal and the resulting mercury concentration inside the building is constant. The current evaluation makes the conservative assumption that mercury concentrations in the air are at a steady state for the duration of receptor exposure. In reality, mercury concentrations will fluctuate with time. The opening and closing of doors and windows will alter the air exchange rate of the building, disrupt steady-state conditions by allowing mercury to leave the building faster than it enters, and reduce mercury concentrations in the air. On the whole, the assumption of steady state will overestimate exposures, but the magnitude of the overestimation is unknown.
Complete Mixing of Indoor Air The current evaluation conducted the evaluation of mercury emanation under the simplifying assumption that indoor air is completely mixed.
The 1997 Exposure Factors Handbook (US EPA, 1997b) supports this assumption with the following statement:
“…for an instantaneous release from a point source in a room, fairly complete mixing is achieved within 10 minutes when convective flow is induced by solar radiation. However, up to 100 minutes may be required for complete mixing under quiescent (nearly isothermal) conditions.” The studies relied upon in the 1997 Exposure Factors Handbook (US EPA, 1997b) used a low air exchange rates of less than 0.1 hr-1. This is lower than the conservative 10th percentile estimate of 0.18 hr-1, and much lower than the central tendency estimate of 0.45 hr-1 recommended by US EPA (1997b).
Thus, these studies seem to support the assumption of complete mixing. Other studies summarized by 5-20 US EPA (1997b) were conducted at more typical room ventilation rates, which found that “the ratio of source-proximate to slightly-removed concentration was on the order of 2:1.” This indicates that mercury concentrations may increase directly adjacent to the ceiling, wall, or floor that emits mercury vapor. However, most building occupants are not stationary and move around a building throughout the day. Therefore, while the assumption of complete mixing may underestimate some short-term exposures, it is a reasonable representation of typical long-term, chronic exposures.