Observations of methylmercury in precipitation.
Using a newly developed technique, methylmercury species have been quantified in several precipitation and lake water samples. Mercury species are determined by aqueous-phase ethylation to the volatile dialkyl analogs, followed by cryogenic gas chromatographic (GC) separation. Mercury-specific detection by cold vapour atomic fluorescence spectrometry permits detection limits of about 0.1 pg as Hg. Snow samples collected from north-central Wisconsin contained monomethylmercury levels of about 0.25 pM (0.05 ng l-1 Hg) and total mercury concentrations of 20 pM (4 ng l-1 Hg). A time series of rain samples collected during a storm passing over the North Olympic Peninsula in western Washington State showed average monomethylmercury levels of 0.75 pM (0.15 ng l-1 Hg), with total mercury concentrations from 10 to 25 pM (2–5 ng l-1 Hg). Total mercury showed a strong washout effect over the course of the storm, while methylmercury appeared to show a diurnal pattern, with elevated levels during the daylight hours. No dimethylmercury was observed in any precipitation sample. Methylmercury was observed in most lakes studied, with a high of 3.1 pM (0.64 ng l-1 Hg) in Onadaga Lake, New York, and a low of \textless 0.015 pM in Lake Crescent, located in the Olympic Mountains of Washington State.