Abstract

The Bunker Hill Mine in northern Idaho is a large underground lead-zinc mine located in Precambrian metaquartzite rocks with virtually no primary porosity. Ground-water flow through these types of rocks is largely dependent upon the properties of fractures such as joints, faults and relict bedding planes. Ground water that flows into the mine via the fractures is contaminated by heavy metals and by the production of acid water, which results in a severe acid mine drainage problem. A more complete understanding of how the fractures influence the ground-water flow system is a prerequisite to 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 data obtained include: a) fracture type, b) orientation, c) trace length, d) the number of visible terminations, e) roughness (small-scale asperities), f) waviness (larger-scale undulations), g) infilling material, and h) a qualitative measure of the amount of water flowing through each fracture.

On a large scale of analysis, zones of distinct orientation patterns, or structural domains, were compared to zones of different discharge rates in order to evaluate any potential effect of structural domains on groundwater flow to the mine. The results of this analysis indicate that: a) three structural domains exist in the New East Reed drift, and b) a change in structural domains alone is not causing the relatively large fluctuations in discharge rates observed in the drift. However, structural domains may be influencing smaller discharge rate fluctuations observed in the drift.

Individual fracture properties were characterized on a smaller scale of analysis for each set of subparallel fractures. Analysis of the relationships between the properties of the fracture sets and the proportion of waterbearing fractures in each set indicates that: a) relict bedding planes appear to be the primary conduits for ground-water flow, b) two major joint sets are present and appear to connect water flowing through the discontinuous bedding planes, c) three minor joint sets are present which do not seem to have a significant impact on ground-water flow, but may store relatively large quantities of ground water, and d) 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.

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