TOTAL MERCURY AND METHYLMERCURY MASS-BALANCE IN AN ALKALINE, HYPEREUTROPHIC URBAN LAKE (ONONDAGA LAKE, NY)

A total mercury (total Hg) and methylmercury (CH3Hg) mass balance was developed for Onondaga Lake, NY, based on sampling of tributaries, sediments, water column, and biota in 1992. The in situ flux of total Hg and CH3Hg from sediments to the overlying water and the rate of net CH3Hg production in the water column were determined experimentally. Fluxes from atmospheric deposition, groundwater, and volatilization were estimated from limited field data and the literature. Ultraclean sampling and analytical techniques developed specifically for Hg were used. Results indicate that tributaries contribute the majority of total Hg entering the lake (13.6 kg in 1992). Other sources of total Hg included groundwater flux (0.02 kg), atmospheric deposition (0.44 kg), and flux from sediments (0.056 kg). Net sedimentation(11.1 kg), outflow (2.8 kg), and volatilization (0.016 kg) were sinks for total Hg. The two major sources of CH3Hg were tributaries (0.26 kg) and net CH3Hg production in the water column (0.60 kg). Flux from Sediments accounted for only 0.017 kg CH3Hg. Net sedimentation (0.47 kg), outflow (0.24 kg), and net uptake by fish (0.20 kg) were sinks for CH3Hg. Gross sedimentation of CH3Hg exceeded net sedimentation by 90%, suggesting that release of CH3Hg from settling particles is a significant process.