Ground-water flow in the karst-terrane aquifers of southeastern Minnesota is not well defined. Variable fracture patterns in the bedrock affect permeability. Techniques to predict the effects of fracture patterns on ground-water flow in the karst-terrane aquifers of southeastern Minnesota are unavailable. The use of such techniques may be useful to officials responsible for the management and protection of ground water in these aquifers, which have a high susceptibility to contamination. The U.S. Geological Survey, in cooperation with the Minnesota Department of Natural Resources and the Legislative Commission on Minnesota Resources, investigated fracture patterns, anisotropic transmissivity, and seepage to streams from the Prairie du Chien Group, which is the karst portion of the St. Peter-Prairie du Chien-Jordan aquifer, to improve the understanding of ground-water flow through karst-terrane aquifers in southeastern Minnesota.

This report presents the results of testing hypotheses that (1) the major axes of linear terrain features correlate with the major axes of subsurface fractures in the Prairie du Chien Group, and that (2) the major axes of subsurface fractures in the Prairie du Chien Group correlate with seepage from the Prairie du Chien Group.

The first hypothesis was tested by comparison of linear terrain features to fracture orientation measurements. Fracture orientations in 10 exposures of the Prairie du Chien Group at quarries, road cuts, and natural outcrops showed statistically significant directional trends at 8 of 10 sites. Directional trends of linear terrain features identified from 1:80,000 aerial photographs were significant in four of the ten 60-square mile areas that surround these sites. The fracture orientation measurements correlate with the local linear terrain features in 2 of the 10 sites.

The second hypothesis was tested by analyzing the correlation between seepage rates into streams hydraulically connected to the Prairie du Chien Group and surrounding linear terrain features that were mapped in approximately 300 square mile areas. Data from Riceford Creek support this hypothesis; data from Crow Creek and Middle Fork of the Whitewater River and from Duschee Creek are inconclusive. This hypothesis could not be tested by the data from the Middle Fork of the Zumbro River, the South Branch of the Root River, and the South Branch of the Middle Fork of the Zumbro River because the surrounding linear terrain features lack directional trends.

The transmissivity of the karst portion of the St. Peter-Prairie du Chien-Jordan aquifer is anisotropic at an aquifertest site in the study area. Results of the aquifer test indicate that the major axis of transmissivity is along a line N95°E. The aquifer-test results indicate that the principal axis of joint fractures at the test site is slightly clockwise from an east-west line because this axis is assumed to correlate with the major axis of horizontal transmissivity.