AERMOD Modeling for Health Impact Assessment of Dimethyl Mercury

摘要

Perma-Fix Northwest (PFNW), located in Richland, Washington, processes and stabilizes low-level radioactive waste and mixed waste. PFNW is proposing to receive and stabilize supernatant tank waste from the U.S. Department of Energy's Hanford Site that is classified by the U.S. Nuclear Regulatory Commission (NRC) as "low-level mixed waste". Due to the origins of the waste, the Washington State Department of Ecology (Ecology) has expressed concerns that the waste may contain dimethyl mercury (DMM) and that emissions of DMM may occur as a result of processing the waste. DMM is listed as a toxic air pollutant (TAP) under Ecology regulations for sources of TAPs in Washington Administrative Code (WAC) 173-460. Ecology and the local air quality agency, Benton Clean Air Agency (BCAA), regulate emissions of TAPs through a series of review processes known as "first, second, and third tier review." First tier review includes an analysis of potential TAP emissions and comparison to regulatory emission rate thresholds. If potential emissions of a TAP exceed the regulatory emission rate threshold, an air dispersion modeling analysis is required to determine if the maximum off-site concentration is above the ambient concentration threshold. If the maximum off-site concentration is above the ambient concentration threshold, then second tier review is required. Second tier review includes a health impact assessment of the potential emissions. If health risk or hazard from the potential TAP emissions exceeds the regulatory threshold, third tier review could be required. The ambient concentration threshold ("acceptable source impact level", or ASIL) for DMM in WAC 173-460-150 is 1 × 10~(-99) micrograms per cubic meter (μg/m~3), which is essentially a "zero threshold" and represents less than one molecule of DMM. In the first tier review, PFNW quantified potential TAP emissions from the processing of the waste, including DMM. The waste was analyzed for DMM content and the results were below the detection threshold. Ecology therefore required PFNW to use conservative assumptions about the concentration of DMM and the results of the air dispersion modeling analysis showed that the maximum off-site concentration of DMM was above the regulatory threshold. PFNW was thus required to submit a second tier review for the potential emissions of DMM.