Risk-based decision making: The east fork poplar creek case study

A Probabilistic risk assessment revealed that methylmercury released from the U.S. Department of Energy(DOE) Y-12 weapons facility at Oak Ridge, Tennessee, USA, poses moderate risks to mink and kingfishers residing near the receiving waters of East Fork Poplar Creek. Polychlorinated biphenyls (PCBs) released from this facility pose severe risks to mink but little risk to kingfishers. The objective of this study was to use a risk-based decision-making approach to select remedial cleanup levels for each of these contaminants. We conducted Monte Carlo simulations to estimate total daily intakes of each contaminant by mink (mercury and PCBs) and kingfishers (PCBs only) for a range of exposure-reduction scenarios. The resulting exposure distributions were then integrated with their respective dose–response curves to estimate postremediation risks. The results indicated that total mercury levels in surface water would need to be reduced from current levels (mean = 0.225 μg/L) to 0.03 to 0.05 μg/L to reduce risks to very low levels (<5% probability of ≥10% mortality) for both mink and kingfishers. If interested parties define acceptable risk as, for example, a 20% probability of ≥10% mortality, then mercury levels would need to be reduced to 0.14 μg/L. The PCBs analysis indicated that reducing water-borne exposures would produce only a modest reduction in risk to mink because much of the current exposure is through terrestrial exposure pathways.