APPROACHES TO UNDERSTANDING THE EFFECTS OF ACID MINE DRAINAGE ON GROUND-WATER SYSTEMS

Site investigations and computer simulations focused on understanding hydrogeologic and geochemical conditions in an aquifer can help researchers understand and predict the mobility of dissolved constituents in ground water at sites affected by acid mine drainage. Knowledge of the hydrogeologic system aids understanding of dissolved constituent mobility because hydrogeologic conditions can influence aquifer geochemical conditions. Characterization of aquifer geology, recharge and discharge areas, flow conditions, potentiometric surfaces, and hydraulic properties through field investigation and computer simulation can provide insight to hydrologic controls on geochemical conditions. Standard hydrologic field-investigations techniques include drilling and installation of monitoring wells, geologic logging and sampling of aquifer materials, slug testing of wells, ground-water-level measurements, and stream-flow measurements. Computer simulations of ground-water flow can provide a framework for water-budget calculations and flow-condition interpretations. Aquifer redox and pH conditions are influenced by aquifer hydrogeology and geochemistry and help determine the chemical behavior and physical transport of dissolved constituents. Field measurement of pH, alkalinity, and redox-sensitive dissolved constituents such as dissolved oxygen, ferrous iron, sulfide, and methane can be used to delineate geochemical zonation in an aquifer. Results of field and laboratory analysis of ground-water samples can be used in geochemical equilibrium calculations to compute speciation of redox-sensitive dissolved constituents not directly measured, mineral-saturation indices, and dissolved constituent adsorption. A case study is presented to demonstrate the application of field and computer simulation techniques to a sand and gravel aquifer affected by acid mine drainage near Leadville, Colorado.