USE OF THE MERCURY CYCLING MODEL (MCM) TO PREDICT THE FATE OF MERCURYIN THE GREAT-LAKES

In response to U.S. EPA's proposed Great Lakes water quality criteria for mercury (Hg),a field-validated Hg cycling model (MCM) was used to predict Hg levels in the abiotic and biotic components of Lake Superio r and Lake Erie. The U.S. EPA criteria are based on water column Hg co ncentrations and simple trophic level transfer and, thus, do not consi der sediment interactions and water chemistry factors. The model, usin g data from published reports, was run to simulate a 25 year steady st ate period. For these simulations, methylmercury (MeHg) represented 5% of total Hg in Lake Erie and 8% of total Hg in Lake Superior. These p roportions are roughly 3-5 times lower than U.S. EPA's estimate that M eHg contributes about 25% of total Hg in the water column of the Great Lakes. The predicted median concentrations of total Hg in top-carnivo re fish were 0.13 mg/kg in Lake Superior and 0.16 mg/kg in Lake Erie. Predicted median MeHg concentrations in Lake Superior and Lake Erie (w ater column) were 0.019 and 0.075 ng/L, respectively. For both lakes, most (>55% of the Hg was partitioned to sediments. Although the MCM si mulation does have practical limitations (e.g., lakes are treated as f ully-mixed open systems), the results demonstrate that generic assumpt ions of Hg behavior in all Great Lakes waterbodies are too simplistic.