Abstract
Stannous-chloride-reducible divalent mercury, or ‘reactive’ mercury (Hg(II)R), is a methodologically defined Hg fraction that is increasingly being used as a surrogate measure of the Hg pool available for microbial Hg(II)-methylation. A critical examination of Hg(II)R stability was conducted in a set of experiments that examined techniques of sediment preservation and storage over time (0, 1, 7, 30, 90, 180, and in some cases 360 days), temperature (-80°C, -15°C, and 5°C), and with/without a glove bag that excluded atmospheric oxygen. A second set of experiments examined effects of field preservation, sediment homogenization and thaw time associated with samples previously preserved frozen. On the basis of experimental results, a recommended protocol is presented here for studies where Hg(II)R is a targeted analyte. (1) An oxidation-reduction probe should be used in the field to ensure the sampling area has similar redox conditions. (2) Samples may be collected in the field and transported to the lab in one of two ways; in Mason jars with no head space and placed on wet ice or collected in individual vials and placed on dry ice. (3) Based on statistical results, chilled samples should be analyzed immediately (within 1-2 days) or stored at -80ºC for up to three months and frozen samples should be analyzed immediately (within 1-2 days). (4) Prior to analysis, frozen sample may be thawed sample 1-4 hours. (5) Compositing samples of variable redox status is not recommended. Sediment for the experiments was chosen from four distinct Hg-contaminated sites surrounding the Sacramento–San Joaquin Delta and San Francisco Bay. Sites were chosen based on legacy Hg contamination in the form of cinnabar (HgS) from past Hg mining impacted sites (Alviso Slough and Cache Creek sites), and elemental Hg from past gold mining sites (Cosumnes River and Humbug Creek (South Yuba River watershed). Samples were chosen with a wide range of organic content, oxidation-reduction conditions, and total reduced sulfur concentration because these properties are known to influence Hg(II)R concentration. Geochemical modeling conducted with PHREEQC indicated that the oxidation of aqueous sulfide plays an important role in controlling the saturation index of cinnabar, which has a direct effect on the solubility of Hg(II), and by extension an influence on the sediment Hg(II)R assay.