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Bioaccumulation of methylmercury in the aquatic food web is governed in part by the methylation of inorganic divalent mercury (Hg(II)) by anaerobic microorganisms. In sulfidic settings, a small fraction of total Hg(II) is typically bioavailable to methylating microorganisms. Quantification of this fraction is difficult due to uncertainties in the speciation of Hg(II) and the mechanisms of uptake by methylating microbes. However, recent studies have shown that the bioavailable fraction is likely to include a portion of Hg(II) associated with solid phases, that is, nanostructured mercuric sulfides. Moreover, addition of thiols to suspensions of methylating cultures coincides with increased uptake into cells and methylmercury production. Here, we present a thiol-based selective extraction assay to provide information on the bioavailable Hg fraction in sediments. In the procedure, sediment samples were exposed to a nitrogen-purged solution of glutathione (GSH) for 30 min and the amount of GSH-leachable mercury was quantified. In nine sediment samples from a marine location, the relative GSH-leachable mercury concentration was strongly correlated to the relative amount of methylmercury in the sediments (=0.91, <0.0001) across an order of magnitude of methylmercury concentration values. The approach was further applied to anaerobic sediment slurry microcosm experiments in which sediments were cultured under the same microbial growth conditions but were amended with multiple forms of Hg with a known spectrum of bioavailability. GSH-leachable Hg concentrations increased with observed methylmercury concentrations in the microcosms, matching the trend of species bioavailability in our previous work. Results suggest that a thiol-based selective leaching approach is an improvement compared with other proposed methods to assess Hg bioavailability in sediment and that this approach could provide a basis for comparison of sites where Hg methylation is a concern.

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Chlorinated ethenes are among the most common environmental contaminants and are known or suspected carcinogens. This class of compounds includes perchloroethene (PCE), trichloroethene (TCE), and their breakdown products, including dichloroethene (DCE) isomers and vinyl chloride (VC). Engineers and scientists must be able to measure concentrations of these chemicals in water samples to assess site contamination, monitor clean-up progress, and test possible remediation technologies. Gas chromatography with flame ionization detection (GC/ FID) is a common method for measuring these contaminants in environmental samples. In this study, we tested the hypothesis that FID response factors are equal for all chlorinated ethene compounds. The rationale for the investigation was that if the hypothesis is correct, a single calibration curve can be used for GC/FID analysis of all chlorinated ethene compounds, saving time and money during sample analysis. Based on our measurements, a single calibration curve (FID response versus mass of analyte injected) is applicable to analysis of PCE, TCE, and all three DCE isomers (r 2 = 0.990, n = 50 measurements), allowing for simplified quantification of those chemicals. However, the apparent FID response factor for VC was lower by *40%, indicating that a separate calibration curve would need to be used to accurately estimate the VC concentration in water samples. The difference in the apparent VC response factor is caused predominantly by losses of VC to volatilization during the analysis.