Abstract
Grouting to reduce fracture permeability is one option for minimizing ground water inflow to a large, acid-producing lead-zinc mine in fractured metamorphic rock in north Idaho. For grouting to reduce mine water inflow effectively, the hydrogeologic characteristics of the various scales of structurally controlled fracturing must be identified and a conceptual model of the ground water flow system must be developed. This paper is the first of two papers which use fracture mapping, geologic structural mapping, and a series of underground hydraulic stress tests to develop a conceptual model of structurally controlled ground water flow in the vicinity of the mine. These data were collected in an effort to identify order within the structurally controlled spatial permeability distribution. Geologic structural discontinuities, ranging from joints to faults that extend for several miles, form a geologic structural hierarchy in the rock surrounding the mine. The tiers of the hierarchy control ground water flow into the mine at different scales. The most prominent faults control ground water inflow on the scale of the entire mine. Various levels of hydraulic continuity are evident within the rock mass bounded by two of the most prominent faults. The highest level of hydraulic continuity appears to be associated with a set of sub-parallel, steeply dipping faults. Minor faults, joints, and relict bedding planes to a limited extent connect the fractures of this set and form a lower level of hydraulic continuity. The next lower level of hydraulic continuity within the hierarchy is related to a major fault that is characterized in drill core by abundant gouge. The hydraulic continuity of the matrix within the unfractured quartzite is the lowest level within the hierarchy. These levels constitute the components of the order within the spatial permeability distribution that we have interpreted from the structural and hydraulic stress test data.
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