Response of mercury contamination in fish to decreased aqueous concentratio- ns 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 waterborne mercury, predominantly as dissolved inorganic mercury, from groundwater, streambed contamination, and sump and process water discharges to the contaminated storm sewer network. These produce aqueous total mercury concentrations of 1-2 µg L-1 in the upper reaches of the stream, decreasing to about 0.1-0.2 µg L-1 in its lower reaches. A program to reduce mercury 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 collected and treated mercury-contaminated water before discharging it. As a result, waterborne mercury concentrations in the creek and total mercury loading were reduced from 1.8 µg L-1 to 0.6 µg L-1 and 100 to 20 g d-1, respectively, in the last 5 yr. Mean mercury concentration- s 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 the facility boundary downstream the decline in mercury bioaccumulation was much less. At sites 5-15 km farther downstream, no decrease was evident. Dissolved methylmercury tended to increase with distance downstream in a pattern inverse to that noted for its dissolved inorganic mercury precursor. Improvements 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-conta- minated headwater areas previously devoid of fish. It may be that the accumulation, retention, and eventual downstream transport of this reservoir of biologically incorporated methylmercury has acted to buffer against expected reductions in mercury in fish at downstream sites.