DETERMINATION OF MERCURY METHYLATION RATES USING A 203-HG RADIOTRACERTECHNIQUE

A radiotracer method for the determination of mercury (Hg) methylation rates in bulk water and water overlying intact sediment cores has bee n developed. A sediment core with overlying water is collected in a co re tube, the overlying water is spiked with high specific activity 203 -Hg radiotracer, and the core is incubated at ambient temperature. Ali quots of the overlying water are removed, the Hg is extracted from the sample, and the activity in the extract is measured. A 10-25 fold sam ple preconcentration is achieved using a dithizone-chloroform extracti on technique and a sodium nitrite back extraction step to separate ino rganic Hg(II) from monomethylmercury (MMHg). The use of this technique , in conjunction with high specific activity 203-Hg and has allowed fo r spiking concentrations in the overlying water of approximately 1 ng Hg/L. This spiking level is about the same concentration as the ambien t water overlying the core, thus not significantly perturbing the syst em. Our technique is a significant improvement over previous methodolo gies which used 203-Hg spike additions of 1 mu g Hg/L or higher. The t echnique was used to measure Hg methylation rates at the Experimental Lakes Area (ELA) in Ontario, Canada during August of 1983 and at an ex tensively studied estuarine site in Gulf Breeze, Florida, USA during S eptember, 1993 and June, 1994. Multiple cores were collected and spike d with a range of 1 to 11,800 ng Hg (as 203-Hg) into the overlying wat er. MMHg production at the ELA site indicated rates of 0.25 to 3.7 pg/ cm(2)/day (0.08 to 2.5% methylation/day). Results from Gulf Breeze wer e significantly higher at 1.5 to 425 pg/cm(2)/day or 0.06 to 18% methy lation/day. These rates are one to three orders of magnitude greater t han previously measured ''specific rates'' in bulk water samples and s ediments. A direct comparison of rates with previous sediment methylat ion assay techniques is not possible, however, because of the signific ant differences between our methodology and previous assay protocols.