Aspects of Uncertainty in Mercury Risk Assessment. L. Levin, D. Porcella, C. Seigneur, and C. Whipple, Electric Power Research Institute, 3412 Hillview Ave., Palo Alto, CA 94303; AER Inc., 2682 Bishop Drive, San Ramon, CA 94583; ICF Kaiser Engineers, 1800 Harrison Street, Oakland, CA 94612-3430
Exposure to mercury via fish ingestion is distributed throughout the U.S. population based on a number of key factors, some poorly quantified to date. Deposition of mercury to receiving waters and hydrologic drainage systems appears to be dominated by background global concentrations and transport from point sources to mesoscale (100-1000 km) distances. The dominance of non-local transport is strongly determined by atmospheric chemistry of mercury, and the ratio of wet to dry deposition. For reduction-dominant aqueous-phase chemistry, local-scale (to 50 km) deposition is typically about 5% of mercury mass emitted from elevated point sources; for oxidation-dominant gaseous-phase chemistry, local deposition is six-fold as great. Calculation of mercury cycling through local ecosystems requires computation of fish-tissue concentrations of methylmercury (typically 95% of total mercury in foodfish). Site-independent "bioaccumulation factors" (ratios of fish-tissue concentrations to waterway concentrations of mercury) are not reliable estimators of measured concentrations; use of site-specific dynamic models of mercury aquatic cycling show BAFs are poorly correlated with hydrologic variables over a wide range of lake environments. Information on consumption of fish from domestic waterways is incomplete, typically based on diary studies and market and creel surveys. Since the diary studies cover limited time periods, statistical biases in data analysis can lead to three-fold variability in high-end fish consumption estimates. The non-linear distribution of fish consumers implies that the number of individuals exposed above a given level (the Reference Dose) drops by about an order of magnitude when fish consumption information is given this alternative interpretation.