Bioavailability of mercury in power plant wastewater and ambient river samples: Evidence that the regulation of total mercury is not appropriate

Mercury (Hg) is a neurotoxin that can cause debilitating effects to human and environmental receptors under high exposure conditions. For industrial and municipal point sources that discharge Hg, wastewater limitations on total Hg (THg) concentrations or loads are typical. While this regulatory practice provides simplicity for regulated industry and water resource agencies (i.e., for analytical detection and reporting purposes), it ignores the important considerations of speciation and bioavailability. In this study, water samples were collected from multiple power plant wastewater, simulated mixing zone, and ambient river locations (N = 10 to 20) and were analyzed for bioavailable Hg forms (methylmercury and acid-labile Hg, or BHg), THg, and dissolved Hg. The median concentration of THg in wastewater, mixing zone, and ambient river samples was 7.1, 5.3, and 2.3 ng/L, respectively. The percentages of THg as BHg (median values) were 18.7%, 29.3%, and 8.5% for wastewater, mixing zone, and ambient river samples. The percentages of methylmercury (MeHg) as THg were not statistically different between paired ambient and mixing zone samples (P > 0.05); this result indicates that wastewater did not increase the MeHg fraction when mixed with ambient water. Multiple regression analysis indicated that variation in THg for combined wastewater and mixing zone samples could be adequately explained by pooled water quality parameters (total suspended solids, total dissolved solids, sulfate, total organic carbon, pH, specific conductivity; r(2) = 0.51; P < 0.05); however, no significant regression relationships were apparent for the percentage of BHg. These results, at least for the wastewater samples evaluated, indicate that regulating THg is likely overly conservative, and mechanisms to regulate the bioavailable forms of Hg are needed. If Hg fish tissue monitoring data indicate that concentrations are less than consumption thresholds, metal translator methodologies or bioavailability-based criterion techniques (as currently used for non-Hg trace elements) should be allowed for Hg. Integr Environ Assess Manag 2019;15:142-147. (c) 2018 SETAC