Pumping Tests and Hydraulic Tomography in Fractured Granite, Blair Wallis, Laramie Range, Wyoming

Abstract

We investigated the hydraulic properties of a fractured granite aquifer in the Laramie Range, Wyoming, using cross-hole pumping tests among six boreholes ranging from 16-76 m deep. Results of four cross-hole pumping tests were interpreted using analytical conceptual models and using SimSLE, a two-dimensional transient hydraulic tomography (HT) algorithm (Xiang et al., 2009). A set of field-scale vertically averaged horizontal hydraulic conductivities (Kh) was interpreted first using analytical models, i.e., Theis (1935), Cooper and Jacob (1946), Neuman (1974), and Moench (1997). The estimated Kh values are very close despite the difference in the assumptions of these analytical models. HT is a useful technique for imaging inter-well permeability heterogeneity. The SimSLE algorithm was able to image, between these boreholes, continuous high Kh and low specific storage (Ss) zones, thus areas with high hydraulic diffusivity. These inter-well zones allow fast preferential flows that reflect connectivity of fractures between the boreholes. In addition, the same algorithm identified a low Kh barrier between one borehole and the rest of the well field, which is consistent with what is known from site geological, drilling, and geophysical data. Finally, scale effect is observed as the pumping-test-inferred Kh is more than a factor of 5 greater than the slug-test-inferred Kh for several boreholes, suggesting fractures with greater permeability were encountered when the well test volume was expanded.