APPLICATION OF FRACTURE-FLOW HYDROGEOLOGY TO ACID-MINE DRAINAGE AT THE BUNKER-HILL-MINE, KELLOGG, IDAHO

The mechanics of groundwater flow through fractured rock has become an object of major research interest during recent years. This project h as 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 depen dent upon the properties of fractures such as faults, joints and relic t bedding planes. Groundwater that flows into the mine via the fractur es is acidic and is contaminated by heavy metals, which results in a s evere 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 detail ed information on the fracture properties observed in the New East Ree d drift of the Bunker Hill Mine. The information obtained includes fra cture type, orientation, trace length, the number of visible terminati ons, roughness, waviness, infilling material, and a qualitative measur e of the amount of water flowing through each fracture. The hydrogeolo gic field data collected include routine measurements of the discharge from four individual structural features and four areas where large q uantities of water are discharging from vertical rock bolts, the depth s to water in three piezometer nests at the ground surface, the pressu re variations in four diamond drillholes, and constant discharge flow tests conducted on three of the diamond drillholes. The field data ind icate that relict bedding planes are the primary conduits for groundwa ter flow, and suggest that the two major joint sets that are present c onnect water flowing through the discontinuous bedding planes. The thr ee 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 m ay store relatively large quantities of water. It appears that rock-bo lt holes in the central portion of the drift primarily intersect relic t bedding planes, whereas rock-bolt holes in the southeastern portion of the drift primarily intersect joints; this probably is related to t he 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 up per workings of the mine, and that the large seasonal head variations in the potentiometric surface are primarily responsible for the observ ed temporal variations in mine inflow. Infilling material may control the hydrogeologic character of the faults, with those filled with goug e having low hydraulic conductivities and those filled with breccia ha ving relatively high hydraulic conductivities. In addition, one of the faults may act as a positive (constant head recharge) hydrogeologic b oundary. A double-porosity approach probably is the most appropriate f or simulating the groundwater flow system in the vicinity of the New E ast Reed drift. Finally, grouping of a combination of breccia-filled f aults and relict bedding planes may offer the best hope for minimizing mine-water inflow or recharge.