Response of mercury contamination in fish to decreased aqueous concentrations and loading of inorganic mercury in a small stream

Approximately 250 000 kg of mercury was lost to water and soils at the U.S. Dept. of Energy Y-12 Plant in Oak Ridge, Tennessee in the 1950s and early 1960s. A creek originating within the plant received continuous inputs of w aterborne mercury, predominantly as dissolved inorganic mercury, from groun dwater, streambed contamination, and sump and process water discharges to t he contaminated storm sewer network. These produce aqueous total mercury co ncentrations of 1-2 mu g L-1 in the upper reaches of the stream, decreasing to about 0.1-0.2 mu g L-1 in its lower reaches. A program to reduce mercur y concentrations in the creek identified specific sources (building sumps, contaminated springwater seeps, foundation drains, and contaminated piping) and rerouted water around contaminated portions of the drain system or col lected and treated mercury-contaminated water before discharging it. As a r esult, waterborne mercury concentrations in the creek and total mercury loa ding were reduced from 1.8 mu g L-1 to 0.6 mu g L-1 and 100 to 20 g d(-1), respectively, in the last 5 yr. Mean mercury concentrations in fish nearest source areas in the creek headwaters decreased at roughly the same rate as waterborne total mercury concentrations over the past five years, but at t he facility boundary downstream the decline in mercury bioaccumulation was much less. At sites 5-15 km farther downstream, no decrease was evident. Di ssolved methylmercury tended to increase with distance downstream in a patt ern inverse to that noted for its dissolved inorganic mercury precursor. Im provements in water quality and modification of weirs to allow the passage of fish have resulted in the establishment of large populations of fish in mercury-contaminated headwater areas previously devoid of fish. It may be t hat the accumulation, retention, and eventual downstream transport of this reservoir of biologically incorporated methylmercury has acted to buffer ag ainst expected reductions in mercury in fish at downstream sites.