Development of the Sleeper Pit Lake

Abstract.

The Sleeper open pit gold mine operated from the mid-1980s through the mid-1990s. Operations were mostly sub-water table and extensive dewatering was required to lower groundwater levels by 180 m. Dewatering flows peaked at 930 L/s, with most flow contributed from an alluvial aquifer. After completion of mining, the pit was rapidly filled with water pumped from the alluvial aquifer to reduce the exposure time of sulfide wall rocks and waste rocks in the ultimate pit. The pumped alluvial groundwater provided a large volume of low total dissolved solids (TDS), high alkalinity water that controlled the early chemistry. The rising lake waters were amended with lime to buffer excess acidity contributed to the lake from reactive pit wall rocks during submergence. The pore water contained in submerged waste rock at the base of the pit was elevated in TDS and subsequently of higher density that the lake water. The density contrast and waste rock location limited contributions of waste rock pore water to the main body of the lake. Some stratification of the early lake occurred, with shallow water characterized by higher pH, low dissolved metals, and sulfate; deeper water had lower pH and higher dissolved metals and sulfate. The reservoir of alkalinity in the shallow layer mixed with the deeper waters and created a stabilized lake with a homogenized column that exceeded water quality expectations. Current water quality meets all Nevada primary drinking water standards with the exception of sulfate, TDS, and manganese, which are slightly elevated, as predicted. Chemistry has remained stable since development of the initial lake.