Modeling Aquatic Mercury Fate in Clear Lake, Calif.

This paper describes the construction and application of a numerical model representing mercury transformations and bioavailability in the aquatic environment of Clear Lake, Calif. The initial application described here provides the basis for more detailed fate and transport modeling of the lake. Results show that total Hg and MeHg concentrations in the lake may be reasonably modeled as functions of sediment total Hg. Calibrated rates of methylation and benthic exchange compare favorably with rates observed in the field and at other lakes. The model suggests the importance of accurately defining a biochemically "available" fraction of total Hg, and demonstrates that even a relatively small amount of available Hg in lake sediments can account for high levels of potentially toxic methylmercury in the water and sediment bed. Indications are that Clear Lake is experiencing a net loss of Hg at a rate of less than 2%/year. At such a remediation rate, mercury concentrations in the lake would meet current water quality criteria in a little over 200 years.