MEASUREMENT OF HG METHYLATION IN SEDIMENTS USING HIGH SPECIFIC-ACTIVITY HG-203 AND AMBIENT INCUBATION

Two methods were developed for estimating the rate of in situ methylmercury (MeHg) formation in sediments. One method is based on incubation of intact sediment cores without added Hg over a period of days. The second method uses (HgCl2)-Hg-203 with a specific activity high enough to be used as a tracer (relative to bulk Hg). Use of high-specific activity (HgCl2)-Hg-203 allowed measurement of methylation rate in hours at ambient total Hg concentrations. (HgCl2)-Hg-203 was pre-equilibrated with pore water before injection into intact cores, to allow complexation with dissolved ligands. Methylation rates were measured with (HgCl2)-Hg-203 additions as low as 0.02 mu Ci and 1.2 ng Hg per g wet weight sediment. These methods were tested in epilimnetic and littoral sediments of two pristine seepage lakes in Northern Wisconsin, and found to compare well. In situ methylation rates in Pallette and Little Rock Lake sediments ranged from 0.1 to 0.4 ug/m(2) d. Use of Hg-203 gave lower errors with shorter incubation times than the ambient incubation method. A method for extraction of Me(203)Hg from bulk sediments is given.