GEOCHEMISTRY OF ACID MINE DRAINAGE IN THE PINE CREEK BASIN, NORTHERN IDAHO

Owing to nearly a century of mining and related activities in the Coeur d’Alene River Basin of northern Idaho, elevated concentrations of metals, potentially hazardous to both human and ecological health, contaminate water resources in the area. The present study examines the effects of contaminated water issuing from a number of adits, seeps, and tailings piles on general water quality of the Pine Creek watershed, located 10 km south of Smelterville and the Bunker Hill Superfund site. We have sampled and analyzed mine waters periodically for more than eight years. Of the waters sampled, only one sample site was found to be acidic (pH 2-3), while all others were found to be slightly acidic to slightly alkaline. The dominant anions in solution were sulfate in the one acidic water, and bicarbonate and sulfate in the near-neutral to alkaline waters. Magnesium and Ca are generally the dominant cations. Despite the near-neutral pH of the samples, elevated levels of Zn, Cd, Mn, and Pb were determined. Using collected analytical data the controls on metal concentrations were investigated. The near-neutral waters were generally near saturation with respect to calcite, dolomite, smithsonite, zincite, pyrolusite, rhodochrosite, malachite, tenorite, and barite. The acidic water was near saturation with respect to barite and anglesite. All the waters were strongly supersaturated with respect to quartz and ferric oxyhydroxides, and the latter could be seen precipitating from a number of seeps. Our results suggest that the variations in water chemistry from site to site within the study area are a result of varying relative extents of sulfide oxidation and carbonate dissolution reactions. Finally, our results demonstrate relatively constant water chemistry over the duration of the study, except in years of unusually high discharge. Unusually high discharge rates result in anomalously high metal concentrations, and excess cations relative to anions. This finding suggests that metals may be mobilized as colloids when discharge is anomalously high.