Evolution of Water Quality in the Goathill North Rock Pile at the Questa Mine and the Impacts on Under Drain Performance

摘要

Some of the groundwater beneath Chevron Mining Inc.' s Questa Mine in northern New Mexico seeps through mineralized bedrock, the toe of the Goathill North mine-overburden rock pile, and then through drain rock located at the base of the rock pile. Rainwater and snow melt infiltrate, flow past, and partially dissolve minerals such as pyrite, calcite, chlorite, and fluorite, producing distinct geochemical signatures for each flow zone. In the low permeability, saturated bedrock, a low flux of oxygen reaches pyrite, and calcite buffers the pH, limiting acid rock drainage. In contrast, the Goathill North rock pile consists of unsaturated, granular material with high permeability and porosity. Air advecting through the rock pile oxidizes pyrite at higher rates than occurs in the bedrock. Water chemistry at the toe of the Goathill North rock pile indicates that buffering capacity has been consumed and shows increased concentrations of total dissolved solids and most metals between the bedrock water and rock pile water. Iron and potassium concentrations decrease, and sulfate and calcium concentrations remain constant. Precipitation of goethite, jarosite, and gypsum occurs in the rock pile material and, particularly, in the drain rock, where the secondary minerals have reduced the drain flow capacity.