Measurement of Hg methylation in sediments using high specific-activity203Hg and ambient incubation

Two methods were developed for estimating the rate ofin 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 uses²⁰³HgCl₂ with a specific activity high enough to be used as a tracer (relative to bulk Hg). Use of high-specific activity²⁰³HgCl₂ allowed measurement of methylation rate in hours at ambient total Hg concentrations.²⁰³HgCl₂ was pre-equilibrated with pore water before injection into intact cores, to allow complexation with dissolved ligands. Methylation rates were measured with²⁰³HgCl₂ additions as low as 0.02 μCi 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/m² d. Use of²⁰³Hg gave lower errors with shorter incubation times than the ambient incubation method. A method for extraction of Me²⁰³Hg from bulk sediments is given.