FRACTURE-FLOW HYDROGEOLOGY OF THE BUNKER HILL MINE, KELLOGG, IDAHO: OR WHAT I DID AT THE UNIVERSITY OF IDAHO

Groundwater flow through Precambrian metaquartzite rocks in the New East Reed drift of the Bunker Hill Mine near Kellogg, Idaho depends on the properties of fractures such as faults, joints and relict bedding planes. Water flowing into the mine via fractures is acidic and contaminated by heavy metals. A better understanding of how the fractures affect the groundwater flow system is necessary in evaluating alternatives to reduce the acid-mine drainage problem.

Fracture mapping techniques were used to obtain information on the fracture properties, including type, orientation, trace length, number of visible terminations, roughness, waviness, infilling material, and a qualitative measure of the amount of flow. Hydrogeologic data were also collected, including discharge measurements from four structural features and four areas where large quantities of water flow from vertical rock bolts, water levels in three piezometer nests at the surface, and pressure measurements in four horizontal drillholes as well as constant-discharge flow tests on three of them.

Relict bedding planes appear to be the primary conduits for groundwater flow. Two major joint sets are present, and appear to connect water flowing along the discontinuous bedding planes. Although three minor joint sets are also present, they do not seem to impact groundwater flow, but they, along with the two major joint sets, may store relatively large quantities of water. Water from the surface appears to be the primary source of recharge, and large seasonal fluctuations in the potentiometric surface probably are responsible for the temporal variations in mine inflow. The hydraulic conductivities of faults filled with breccia appear to be relatively high, while those filled with gouge are low. In addition, one of the faults may act as a constant head/recharge boundary. Finally, grouting of relict bedding planes and breccia-filled faults may offer the best hope for minimizing recharge and acid-mine drainage.