MERCURY SPECIATION AND TRANSPORT FROM HISTORIC PLACER GOLD AND MERCURY MINES, CALIFORNIA

The speciation of Hg in sediments and tailings derived from historic placer gold and mercury mining is important in determining the processes that release Hg. Elemental mercury, Hg^o, was used to recover gold in the amalgamation process in dredge and hydraulic placer mining. High concentrations of Hg^o remain in some in-stream and tunnel sluices and adjacent streams. Since the introduction of Hg^o into placer tailings and its subsequent incorporation into sedimentary environments, much of the Hg^o has been transformed to other mercury species by biotic and abiotic processes. Hg speciation studies by sequential chemical extraction (SCE) indicate that Hg extractable by 1N KOH, interpreted to be organic-bound (o-b), and aqua-regia extractable Hg, interpreted to be sulfide-bound (s-b) comprises most Hg present in dredge sluice tailings (61-97 %) and in sediments contaminated with placer tailings (51-92 %) based on data from the Trinity River and Clear Creek watersheds. Hg^o (extractable by 12M HNO3) comprises 2.5–47% of the total Hg present and water-soluble Hg species are 0.2-4.4%. At mercury mines, the Hg speciation in mine tailings, based on SCE and EXAFS, consists primarily of s-b Hg that includes residual HgS in the primary ore and nanoparticle HgS formed during and after ore processing. Hg sulfates, oxide and chloride comprise less than 20 % of the total Hg present and o-b species are a minor component (0.6 to 3.5%). In sediments contaminated with Hg mine tailings, s-b Hg species predominate (60-90%) but the s-b species decrease in abundance with distance from the mine source as the o-b Hg component increases (0.7 – 25%). Hg^o comprises from 3 –16% of the total Hg present and water-soluble Hg species are low (0.1-0.9%). Release and transport of Hg from gold placer and Hg mine tailings and contaminated sediments are primarily by erosion and physical transport of particulate Hg species. Release of dissolved Hg species is limited by the low concentration of water- soluble Hg species. Seasonal release of Hg-enriched colloids may also occur from both placer and mercury mine tailings. Hg speciation of sediments contaminated with tailings that are downstream from historic placer gold and mercury mines is comparable and indicates that the bioavailability of mercury is similar in contaminated sediments derived from both these mining sources.