SPECIATION AND COLLOID TRANSPORT OF MERCURY FROM GOLD AND MERCURY MINES

Mercury mining and the use of Hg to recover placer gold has resulted in widespread contamination of ecologically sensitive environments in California. This presentation focuses on Hg speciation and transport in two settings:

(1) Placer gold mining in the Clear Creek tributary to the Sacramento River has impacted the hydrology and ecology of a salmonid spawning stream. Restoration of the watershed utilized dredge tailings contaminated with Hg introduced during gold mining, posing the possibility of persistent Hg release to the surrounding environment. Column experiments have been performed to evaluate the extent of Hg transport under chemical conditions potentially similar to those in river restoration projects utilizing dredge tailings. Physicochemical perturbations, in the form of shifts in column influent ionic strength and the presence of a low molecular weight organic acid, were applied to coarse and fine sand placer tailings containing 69-194 ng-Hg/g, respectively. Significant concentrations of mercury, up to 16 µg/L, leach from these sediments. Sequential chemical extractions of the tailings indicate that elemental Hg introduced during gold mining has been transformed to readily soluble species such as mercury oxides and chlorides (3-4%), intermediately extractable phases that likely include (in)organic sorption complexes and amalgams (75-87%), and fractions of sparingly soluble Hg sulfides (6-20%). EXAFS analysis of colloids obtained from column effluent identified cinnabar particles as the dominant mobile mercury-bearing phase.

(2) Hg mine tailings, Sulphur Bank, CA. In this study, columns packed with a calcined Hg ore have been leached with organic acid solutions in an attempt to simulate infiltration through the rooting zone of revegetated mine waste and so address (1) the physicochemical conditions under which Hg-bearing colloids may be mobilized, (2) the speciation of Hg in mobilized colloids, and (3) the extent of colloidal Hg transport. For all organic solution concentrations investigated, colloid-associated Hg was mobilized. Chemical analyses of eluted colloids and filtrates of column effluent indicated that 98% of Hg mobilized from the column was in particulate forms. Hg speciation was determined using EXAFS spectroscopy, indicating that HgS minerals dominate the Hg species in the mobilized colloids.