Abstract
The Berkeley Pit Lake has been filling with acid rock drainage since dewatering ended in 1982. The Pit contains 200 gigaliters of acid water and continued to fill until pumping began in 2019 to maintain a water level below the regulatory limit. Water management practices, including copper recovery, surface water diversion, lime treatment sludge disposal, and RO brine disposal, have substantially reduced the overall pit acidity by 30-40%. These practices have also reduced the iron from >1,000 mg/L to <10 mg/L, increased the pH from <2.5 to >4, increased the magnesium from <300 mg/L to >600 mg/L, changed the pit limnology from meromictic to holomictic, and transitioned the hypolimnion from an anoxic to an oxic state. With continued in-pit disposal of alkaline sludge, copious amounts of aluminum-hydroxy-sulfate are predicted to precipitate beginning at a pH of 4.2±0.2. As predicted, aluminum concentrations have reduced from 300-350 mg/L in 2017 to 225-250 mg/L in 2023. Sediment cones deployed for up to 6 months at a depth of 20 meters indicate a consistent sedimentation rate of 8-10 g/m2/day comprised mostly of iron, aluminum, silicon, calcium, and sulfur. We postulate that mineral transformations within the sediment bed (e.g., conversion of schwertmannite and jarosite to goethite) are releasing acid and potentially trace elements (e.g., arsenic, phosphorous) that were co-precipitated and/or adsorbed to the initial mineral phase. This hypothesis is currently being tested in a set of laboratory experiments and field sampling events, the latter employing unmanned vehicles.