Methylmercury input to the Mississippi River from a large metropolitan wastewater treatment plant

Methylmercury (MeHg) and total mercury (THg) inputs to the Mississippi River from a large metropolitan wastewater treatment plant were measured to characterize the relative contribution of the treatment plant to in-stream loads of these contaminants. Concentrations of MeHg and THg were determined in filtered and unfiltered whole water samples collected weekly from the treatment plant effluent stream and from the river upstream of the plant discharge. Unfiltered MeHg concentrations in the plant effluent ranged from 0.034 to 0.062 ng L(-1) and were always less than those in the river (range: 0.083-0.227 ng L(-1)). The MeHg loading to the river from the treatment plant ranged from 0.026 to 0.051 g d(-1) and averaged 0.037 g d(-1) over the 13-week sampling period. The in-stream MeHg load in the river upstream varied widely depending on hydrologic conditions, ranging from 0.91 to 18.8 g d(-1) and averaging 4.79 g d(-1). The treatment plant discharge represented 1.6%, on average, of the in-stream MeHg load, ranging from 0.2 to 3.5% depending on flow conditions in the river. MeHg in treatment plant effluent was primarily in the filtered phase (mean: 57%, <0.2 microm), but in the river the filtered/unfiltered ratio (F/UF) was typically less than 30% except during a major precipitation runoff event, when F/UF increased to 78%. The MeHg/THg ratio in unfiltered treatment plant effluent varied little (range: 1.6-1.9%), suggesting that THg concentration can serve as a relatively accurate proxy for MeHg concentration in this effluent stream. Supplemental sampling of the treatment plant influent stream showed that removals of MeHg and THg across the treatment process averaged 97% and 99%, respectively. These results show the treatment plant to be effective in removing MeHg and THg from wastewater and in minimizing its impact on Hg levels in the receiving water.