Colloidal phase partitioning of mercury in fresh waters

Tangential-flow ultrafiltration techniques were rigorously evaluated for use in trace-level mercury research on colloidal phase (0.4 μm–10 kDa) partitioning of total mercury (Hg_T) and methylmercury (MeHg) within freshwater ecosystems. Ultrafiltration was then applied to a broad range of freshwater rivers to investigate the influence of water chemistry on the observed colloidal phase mercury concentration. Finally, the influence of discharge, watershed land use, and soil characteristics on the phase distribution and loading of mercury to Lake Superior was evaluated.; Ultrafiltration membranes made of regenerated cellulose were better suited for mercury research in low ionic strength waters (<100 μS cm −1) than those made of polyethersulfone. Regenerated cellulose had fewer sorption and charge rejection artifacts and a better mass balance closure (especially for MeHg). Both membranes had similar blank values, replicate precision, and minimum source-water concentrations. Ultrafiltration was best suited for freshwaters above 1.5 ng L−1 Hg_T; 0.2 ng L−1 MeHg; and 2 mg L−1 DOC.; The observed colloidal phase concentration of mercury comprised up to 72% of the unfiltered mercury concentration in 15 U.S. rivers of the upper Midwest (MN, WI, MI), Georgia, and the Florida Everglades. On average however, Hg_T and MeHg concentrations were evenly distributed between the particulate (>0.4 μm), colloidal, and dissolved (<10 kDa) pools. Expressed as a percentage of the filtered-passing fraction (<0.4 μm), the mercury pool in the colloidal phase decreased with increasing specific conductance. Experiments on fresh waters with artificially elevated specific conductance, suggest that Hg_T and MeHg may partition to different subfractions of colloidal material. The data also support the “particle concentration effect” hypothesis that the association mercury with colloids in the filter-passing fraction lowers the observed partition coefficient (K _D).; Tributary loading to Lake Superior was estimated to be 283 kg yr −1 Hg_T and 3.5 kg yr−1 MeHg (5.6 and 0.07 mg km−2 d−1, respectively). Roughly 42% of Hg_T and 57% of MeHg was delivered in the colloidal phase. Enough MeHg was delivered yearly to raise the fish mercury burden 0.02 μg g−1—about 5% of the 0.5 μg g−1 health limit. High flow conditions and clay surficial deposits were the most efficient exporters of mercury.