Influence of Copper Recovery on the Water Quality of the Acidic Berkeley Pit Lake, Montana, USA

The Berkeley Pit lake: in Butte, Montana, formed by flooding of an open-pit COpper mine, i one of the world's largest accumulations of acidic, metal-rich water. Between 2003 and 2012, approximately 2 x 10(11) L of pit water, representing 1.3 lake volumes, were pumped from the bottom of the lake to a copper recovetyrplant, where dissolved Cu2+ was precipitated on scrap iron, releasing Fe2+ back to solution and thence back to the pit. Artificial mixing caused by this continubus pumping changed the lake from a meromittic to holomictic state, induced oxidation of dissolved Fe2+, and caused subsequent precipitation of more than 2 x 10(8) kg of secondary ferric compounds, mainly schwertmannite and jarosite, which settled to the bottom of the lake. A large mass of As, P, and sulfate was also lost from solution. These unforeseen changes in chemistry resulted in a roughly 25-30% reduction in the lake's calculated and measured total acidity, which represents a significant potential savings in the cost of lime treatment, which is not expected to commence until 2023. Future monitoring is needed to verify that schwertmannite and jarosite in the pit sediment do not convert to goethite, a process which would release stored acidity back to the water column.