CONTROLLING EFFECTS OF PREEXISTING FRACTURES ON GROUNDWATER FLOW THROUGH CRYSTALLINE ROCKS IN CENTRAL WISCONSIN

Groundwater availability is limited in the northern two-thirds of Wood County, Wisconsin because low-permeability Precambrian rocks are the only available aquifer there. This area underwent deformation and high grade metamorphism during the Penokean Orogeny about two billion years ago. Groundwater flow within these rocks depends on presence of shallow fracture networks. To understand the fracture networks, we conducted structural analyses on fracture orientation data collected from three boreholes drilled near Pittsville, Wisconsin, in Wood County.

Previous work showed that these borehole are hydraulically connected by fractures. Results from our structural analyses show that each of the three bore holes have unique fracture orientation patterns with little or no similarity to each other. Also, the dominant fracture orientations change with depth in all three boreholes. The depths at which the fracture orientation patterns change roughly corresponds with borehole gamma log spikes, implying that orientations of fractures might be controlled by changes in lithology.

We have mathematically determined possible locations where the fractures might intersect underground, thus providing possible pathways for groundwater flow. Drilling wells at those locations might optimize water yield. We plan to create a three dimensional model of the fracture orientations using Arc scene software package to better understand this complex fracture network.