INFLUENCE OF COPPER RECOVERY ON THE LIMNOLOGY AND GEOCHEMISTRY OF THE BERKELEY PIT LAKE, BUTTE, MONTANA

The Berkeley Pit is one of the largest (1.5 x 10¹¹ L as of June 2010) and most acidic (pH < 2.8) mining lakes in the world. In recent years, elevated concentrations of dissolved Cu coupled with high metal prices have prompted a large scale copper-recovery project. Between January 2004 and the time of this writing, over 1.3 x 10¹¹ L of lake water have been pumped to a nearby plant for precipitation of copper onto scrap iron via the following reaction: Cu²⁺ + Fe(s) → Fe²⁺ + Cu(s). The Cu-depleted and Fe-enriched water is then discharged back to the surface of the lake. This project has radically changed the limnology of the lake from being meromictic prior to 2004 to being holomictic today. Average concentrations of dissolved Cu in the lake have dropped from ≈ 150 mg/L prior to 2004 to ≈ 65 mg/L today, but are considerably higher than Cu-depleted water leaving the cementation plant (typically < 20 mg/L). We postulate the existence of a replenishable supply of acid-soluble copper on the walls of the lake. Thermodynamic calculations suggest that the acidic lake waters are near local equilibrium with the assemblage pyrite + chalcocite, implying that the latter phase may exert a control on Cu-solubility in the pit lake. The Cu recovery project has also caused an increase in the rate of precipitation of secondary ferric oxy-hydroxy-sulfate compounds (e.g., schwertmannite, jarosite) by bringing deep, Fe²⁺-rich water to the surface where it is subsequently oxidized. Co-precipitation and/or adsorption of trace elements onto these fresh precipitates has depleted the lake in arsenic and phosphate, but has had a negligible effect on the concentrations of other solutes, such as Al (≈ 270 mg/L), Cd (≈ 2.2 mg/L), Mn (≈ 250 mg/L), and Zn (≈ 600 mg/L). Because of the substantial decreases in dissolved Fe and Cu concentrations, total acidity values (calculated) of the lake waters have decreased significantly. This is a beneficial but unforeseen consequence of the ongoing copper recovery project, and may result in decreased lime consumption rates when chemical treatment of the still-flooding lake begins, most likely sometime after 2020.