English

Abstract

There are many mature Cu heap leach facilities in the western United States that will face closure in the next decade. However, there is little published information on the response of groundwater systems to the cessation of leaching. Copper dump leaching was conducted on the Bingham Canyon Eastside waste rock dumps between 1963 and 2000. Leach water discharging from the toe of the waste rock dumps had a typical pH of 2.9 and a total dissolved solids concentration of about 90,000 mg/L. During active leaching this water was recirculated, but now it is neutralized as it enters the mine's tailings and process water circuit. With the end of leach water application, average annual flows discharging from the toe of the dumps have declined rapidly from 1500 L/s in 1998 to approximately 45 L/s in 2004. Average acidity in water discharging from the toe of the dumps declined by almost thirty percent between 2000 and 2005, and since 2003, most solute concentrations have declined by about ten percent. The only exception is Cu concentration which has increased by a factor of four since 2000. Water quality in the underlying saturated bedrock has also begun to improve. Since 2000, sulfate concentrations have declined by a third, alkalinity has increased, and Cu and Zn concentrations have declined by up to ninety percent in water from tunnels that receive at least a portion of their inflows from beneath the waste rock footprint. Water quality in bedrock and alluvium down gradient of the dump toe began to improve after the leach water collection system was upgraded between 1994 and 1996. Arithmetic mean sulfate concentrations in down gradient alluvial monitoring wells declined from a high of 6000 mg/L in 1994 to less than 1000 mg/L in early 2004. These data illustrate how the groundwater system responds to the termination of waste rock and heap leach operations and may provide a useful analogue for the closure of other Cu heap leach operations.