Application of fracture-flow hydrogeology to acid-mine drainage at the Bunker Hill Mine, Kellogg, Idaho

Abstract

The mechanics of groundwater flow through fractured rock has become an object of major research interest during recent years. This project has investigated the flow of groundwater through fractured Precambrian metaquartzite rocks in a portion of the Bunker Hill Mine near Kellogg, Idaho. Groundwater flow through these types of rocks is largely dependent upon the properties of fractures such as faults, joints and relict bedding planes. Groundwater that flows into the mine via the fractures is acidic and is contaminated by heavy metals, which results in a severe acid mine drainage problem. A more complete understanding of how the fractures influence the groundwater flow system is a prerequisite of the evaluation of reclamation alternatives to reduce acid drainage from the mine. Fracture mapping techniques were used to obtain detailed information on the fracture properties observed in the New East Reed drift of the Bunker Hill Mine. The information obtained includes fracture type, orientation, trace length, the number of visible terminations, roughness, waviness, infilling material, and a qualitative measure of the amount of water flowing through each fracture. The hydrogeologic field data collected include routine measurements of the discharge from four individual structural features and four areas where large quantities of water are discharging from vertical rock bolts, the depths to water in three piezometer nests at the ground surface, the pressure variations in four diamond drillholes, and constant discharge flow tests conducted on three of the diamond drillholes. The field data indicate that relict bedding planes are the primary conduits for groundwater flow, and suggest that the two major joint sets that are present connect water flowing through the discontinuous bedding planes. The three minor joint sets that are present do not seem to have a significant impact on groundwater flow, but along with the two major joint sets may store relatively large quantities of water. It appears that rock-bolt holes in the central portion of the drift primarily intersect relict bedding planes, whereas rock-bolt holes in the southeastern portion of the drift primarily intersect joints; this probably is related to the shallower angle of dip of the bedding planes in the central portion of the drift. It also appears that recharge from the surface directly above the mined-out openings is the primary source of water in the upper workings of the mine, and that the large seasonal head variations in the potentiometric surface are primarily responsible for the observed temporal variations in mine inflow. Infilling material may control the hydrogeologic character of the faults, with those filled with gouge having low hydraulic conductivities and those filled with breccia having relatively high hydraulic conductivities. In addition, one of the faults may act as a positive (constant head-recharge) hydrogeologic boundary. A double-porosity approach probably is the most appropriate for simulating the groundwater flow system in the vicinity of the New East Reed drift. Finally, grouting of a combination of breccia-filled faults and relict bedding planes may offer the best hope for minimizing mine-water inflow or recharge.