«February 2014 Final United States Environmental Protection Agency Office of Solid Waste and Emergency Response Office of Resource Conservation and ...»
Room Temperature Temperature may alter the rate of mercury emanation from building materials. Mercury undergoes some volatilization at room temperature; higher temperatures can increase the kinetic energy of gases and may result in faster migration through the concrete matrix. The American Society of Heating, Refrigerating and Air Conditioning Engineers defines comfortable room temperature as being between 20 and 26ºC (68ºF and 79ºF), depending on the season (ASHRAE Standard 55). This range represents the temperatures that provide thermal comfort for approximately 80 percent of the population.
Therefore, the majority of habitable buildings will have temperatures somewhere within this range. At present, the evaluation did not identify any research directly evaluating the effects of temperature on mercury emanation from concrete. Therefore, this remains an uncertainty in the current evaluation.
The available studies on fly ash concrete (Golightly et al., 2006; 2009) evaluated mercury emanation at a constant ambient temperature of 40 ºC (104 ºF). A temperature of 40 ºC represents an upper bound of realistic outdoor temperatures in most of the United States. However, the building materials placed indoors will be cooled by shading, air conditioning, air flow, and other factors that likely prevent indoor temperatures from reaching levels this high. Based on these findings, the temperatures present in these studies likely overestimate mercury releases from fly ash concrete, but the magnitude of the overestimation is unknown.
The one available study on FGD gypsum wallboard (Shock et al., 2009) evaluated mercury emanation at a temperature between 22 and 24 ºC (71.6 and 75.2 ºF). These data represent a range of comfortable room temperatures. Although higher temperatures than these are possible, it is considered unlikely that they would occur simultaneously with low-end air turnover rates. Use of open windows, fans, or other means to cool the area would increase the air turnover rate. Based on these findings, the temperatures present in this study provide an estimate of typical mercury releases from FGD gypsum wallboard. The potential for higher indoor air temperatures may result in an underestimation of the upper bound of mercury releases. However, the impact of this underestimation on resulting exposure estimates is likely to be small.
Pressure Gradient The presence of a negative pressure gradient in a building may alter the rate of mercury emanation from building materials. Several factors can generate negative pressure gradients, including: temperature differences between the indoors and outdoors, changes in the outdoor wind or barometric pressure, and operation of mechanical fans or vents. Studies have shown that the measured negative pressures in homes range between 1 and 50 pascals [0.00015 and 0.007 pounds per square inch (psi) (MassDEP, 1995)]. It is well known that the presence of a pressure gradient can greatly influence the movement of 5-21 gases into a building through cracks in building walls and foundations (US EPA, 2008a). However, the evaluation did not identify any research that evaluates the degree to which change in pressure affects mercury emanation from intact concrete. Therefore, this remains an uncertainty in the current evaluation.
Of the available literature, only Golightly et al. (2009) reported the negative pressure applied during sample collection. However, based on the similar experimental setups used among the three available studies, the evaluation assumes that the negative pressures generated are on the same order of magnitude. Golightly et al. (2009) maintained a constant pressure drop of 3 psi across the concrete throughout the experiment with a mechanical fan that pulled air across the concrete and through the air sampler. This negative pressure is over two orders of magnitude higher than the pressure commonly found in buildings. Based on these findings, the pressure gradients present in these studies will overestimate mercury releases. However, the magnitude of this overestimation is unknown.
Relative Humidity The relative humidity may alter the rate of mercury emanation from concrete. As the relative humidity in a home decreases, the rate of movement of water from the concrete matrix may begin to increase. Lower water retention within the concrete matrix may act to remove barriers to the transport of mercury vapor. The evaluation did not identify any research directly evaluating the effects of indoor ambient humidity on mercury emanation from concrete.
Shock et al. (2009) reported the relative humidity during collection of air samples near wallboard.
The relative humidity ranged between 25 percent and 40 percent during sample collection. The two studies on concrete did not report the relative humidity (Golightly et al., 2005; 2009). However, because no reported attempts were made to alter the humidity level in the buildings, the evaluation assumes that the relative humidities fall somewhere in the standard indoor range of 30 percent to 60 percent (US EPA, 2010e). Furthermore, because of the higher air temperature maintained around the concrete during these evaluations, the relative humidity in the sampling containers in which the concrete was enclosed would be lower than that of the surrounding room. Based on these findings, the relative humidities present in these studies may over- or underestimate the amount of mercury released from concrete and wallboard on a case-by-case basis, but the magnitude of the effect on the results of the evaluation is likely to be small.
5.2.4 General Uncertainties This section discusses the uncertainties that are present throughout the evaluation and affect multiple exposure pathways.
Pollution Control Technologies The current evaluation contains information on fly ash from coal-fired power plants that use air pollution control technologies necessary to span the range of coal types and established air pollution control technology configurations addressed in US EPA (2010a). In recent years, new pollution control technologies, such as those with activated carbon or halogen additions, have been proposed and implemented in response to new regulations on coal combustion facilities. The intent of these pollution
Hexavalent Chromium Mode of Action At present, the US EPA Integrated Risk Information System (IRIS) program is in the process of developing an oral slope factor for ingested hexavalent chromium [chromium (VI)]. In the absence of a finalized Tier I or Tier II toxicity value, as described in Office of Solid Waste and Emergency Response 2003 Directive 9285.7-33 (US EPA, 2003b), the evaluation relied upon the Tier III value finalized by the State of New Jersey Department of Environmental Protection (NJDEP) (NJDEP, 2009; Stern 2010).
The NJDEP drew no conclusions about the mode of action for chromium (VI). Based on existing laboratory research, there is evidence that chromium (VI) may have a mutagenic mode of action (NTP, 2012; McCarroll et al., 2010). On the other hand, there is laboratory research indicating that the mutagenic mode of action occurs only at high exposures associated with cell death and is, therefore, not relevant to human environmental exposures (Stern, 2010). At present, neither NJDEP nor IRIS has finalized a decision on the relevance of a mutagenic mode of action for human ingestion of chromium (VI). Therefore, the lack of consensus on the interpretation of the current state of the science adds uncertainty to the quantitative estimation of the oral cancer risk from ingestion of chromium (VI). If the evaluation were to quantify a mutagenic mode of action, it would result in lower (i.e., more stringent) screening benchmarks than those currently used for chromium (VI).
Human Exposure Factors Exposure modeling relies heavily on data pertaining to population activity patterns, mobility, dietary habits, body weight, and other factors. The physical characteristics, activities, and behavior of individual receptors can vary considerably. Therefore, the single set of often high-end exposure factors used in the current evaluation is likely to overestimate potential exposures. The magnitude of this overestimation will vary on a case-by-case basis, depending on the characteristics of the individual receptor.
In instances where information on exposure factors was not available for a given receptor, this evaluation used data on similar receptors. For example, this evaluation drew child fish-consumption rates from data on adult recreational anglers. This extrapolation likely overestimates actual exposures, as the amount of food consumed by a small child is anticipated to be less than that consumed by a fullgrown adult. In instances where age-specific child exposure factors were not available, this evaluation used available child exposure data for all age cohorts. The current evaluation used soil ingestion rates reported for children between the ages of three and six for all child age cohorts. This extrapolation may under or overestimate exposures on a case-by-case basis.
This evaluation drew exposure factors from the 1997 Exposure Factors Handbook (US EPA, 1997b) and the 2008 Child-Specific Exposure Factors Handbook (US EPA, 2008b). This evaluation carefully reviewed and evaluated both documents for quality. The evaluation criteria included peer review, reproducibility, pertinence to the United States, adequacy of the data collection period, validity of the 5-23 approach, representativeness of the population, characterization of the variability, lack of bias in study design, and measurement error. EPA has also recently released the 2011 Exposure Factors Handbook (US EPA, 2011). This document has undergone the same level of peer review as the previous handbooks. However, OSWER is still assessing how best to incorporate the updated recommendations into Agency evaluations. Therefore, this evaluation only drew data from this document where they were not available in other iterations of the Exposure Factors Handbook. A review of the relevant exposure factors contained in the 2011 Exposure Factors Handbook (US EPA, 2011) found that adult water consumption rates are lower and that adult body weights are higher than those found in the 1997 Exposure Factors Handbook (US EPA, 1997b). All other exposure factors relevant to this evaluation remained the same between the 1997 and 2011 editions.
Cumulative Exposures In the current evaluation, exposures to different COPCs were considered independently. In reality, receptors are exposed to multiple constituents simultaneously. An individual COPC may interact with other constituents present, resulting in synergistic or antagonistic effects that exacerbate or diminish the health impacts predicted by evaluating each COPC independently. For this screening assessment, it is considered inappropriate to consider additive exposures. The calculations in this document represent exposures at or above a reasonable high-end. Due to the natural variability of CCRs and human behavior, it is considered unlikely that a single receptor would be simultaneously exposed to high end concentrations of every COPC. Furthermore, individual receptors are unlikely to be exposed to COPCs from multiple exposure pathways within the same timeframe. COPCs are released and transported to different media at different rates. For example, a receptor may receive ground water from an impacted well and fish from an impacted surface water body; however, the well and water body will not be the same distance away from the contaminant source. As a result, the concentrations in the well and the water body will be different. Based on these considerations, the Agency chose not to consider cumulative exposures in this evaluation. This approach may underestimate or overestimate potential exposures on a case-by-case basis.
Exposure Pathways In Step 4 (Screening Assessment), this evaluation considered the single pathway for each release that is most likely to result in the highest chronic exposures during use of fly ash concrete and FGD gypsum wallboard. Because these CCR products can be used in a variety of places and in a variety of ways, receptors can be exposed through pathways other than those evaluated in this document. However, consideration of these additional pathways is unlikely to result in a chronic time-averaged exposure any higher than those presented in this document. Some exposures, such as those arising from home renovation, may be higher in the short term, but will quickly decrease with time. The current evaluation assumes that all of the media a receptor encounters are contaminated with high-end COPC concentrations. If a receptor were to at any point leave this theoretical high-exposure environment for another, the receptor’s exposures will only decrease. Therefore, while the existence of other exposure scenarios introduces some uncertainty into the evaluation, it is unlikely to affect the results of the evaluation.
5-24 5.3 Final Conclusions The Methodology for Evaluating Encapsulated Beneficial Uses of Coal Combustion Residuals (US EPA, 2013a) is a resource to aid states, tribes, local governments, the general public, and the regulated community in evaluating the beneficial use of any encapsulated CCR. The current evaluation applied this methodology to two of the largest encapsulated beneficial uses of CCRs, fly ash used as a direct substitute for portland cement in concrete and FGD gypsum used as a replacement for mined gypsum in wallboard. The conclusions of this evaluation are applicable to the specific conditions considered in this evaluation, such as products conforming to relevant physical and performance standards established by voluntary consensus standard-setting bodies.
All COPCs were eliminated prior to Step 4 (Screening Assessment), or were found to be at or below all relevant regulatory and health-based screening benchmarks identified for this evaluation. Based on these findings, the evaluation did not proceed to Step 5 (Risk Assessment). This evaluation characterized the uncertainties present, and minimized the impact of these uncertainties to the extent practicable by biasing the evaluation in a conservative direction. The review of uncertainties conducted showed that, while the magnitude of the uncertainties is difficult to quantify, it is likely that the conservative direction of the uncertainties causes the evaluation to overestimate potential exposures. As a result, while the exposure concentrations calculated and reported in this document are considered sufficient to draw conclusions regarding the beneficial uses under evaluation, these concentrations should not be cited for purposes outside of the context of this evaluation.