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The chemistry and limnology of the Berkeley Pit lake, located in Butte, Montana, have substantially changed during the past 15 years. These changes are due to: (1) a large-scale Cu recovery project in which deep water was pumped to a Cu precipitation plant and returned to the surface of the lake; and (2) in-pit disposal of lime-treatment sludge. As a consequence, the lake has shifted from a meromictic to a holomictic state, over 99% of the dissolved Fe has precipitated as schwertmannite and other ferric solids, the total acidity of the water column has dropped by roughly 35%, and the pH has risen from about 2.5 to near 4.1. Whereas most of the lake was anoxic prior to 2013, the water column now contains &gt; 6 mg/L dissolved oxygen from the lake surface to the deepest levels sampled (&gt; 150 m). Concentrations of several other solutes, including As, P, Cr, and V, have fallen sharply due to adsorption or co-precipitation with Fe, while other species, such as Al, Mg, and Mn, have increased due to dissolution of solids in the sludge. Although the Cu recovery circuit was discontinued in 2013, in-pit disposal of sludge is expected to continue. The pit water is now close to saturation with basaluminite, and the pH should remain poised in the range of 4.1–4.5 for many years as Al precipitates out of the water column. Pumping and treating of pit water is set to commence in 2019, and will continue for the foreseeable future. Continued in-pit disposal of sludge should translate into long term water-quality improvement and a decrease in annual lime-consumption costs. Although the Berkeley Pit still has a long ways to go before it will meet water quality standards, it is heartening to mine-water specialists that the pit lake may not be a problem that is “too big to fix”.

Improvements to the Water Quality of the Acidic Berkeley Pit Lake due to Copper Recovery and Sludge Disposal