Water Quality along Selected Flowpaths in the Prairie du Chien-Jordan Aquifer, Southeastern Minnesota

The Prairie du Chien-Jordan aquifer in southeastern Minnesota consists of the dolomitic Prairie du Chien Group and the underlying Jordan Sandstone. A study was conducted to characterize water-quality differences between the Prairie du Chien and Jordan parts of the aquifer, to identify the variables that appear to be responsible for these differences, and to describe the susceptibility of the Jordan part of the aquifer to contamination. Differences in water quality between the Prairie du Chien and Jordan parts of the aquifer were investigated by sampling existing wells in six study areas in Hennepin, Dakota, Scott, and Olmsted Counties. On-site field measurements and samples for water-quality analysis were collected from 139 wells completed in the Prairie du Chien and Jordan parts of the aquifer and from overlying aquifers. Lithologic differences between the two parts of the Prairie du Chien-Jordan aquifer did not have a major effect on major-ion chemistry of water in the aquifer. Analysis of the distribution of indicator-constituent concentrations (nitrite-plus-nitrate nitrogen, chloride, and tritium) was useful in assessing the actual and potential extent of human-related contaminants in the aquifer. In general, ground water from the Prairie du Chien part of the aquifer had higher indicator-constituent concentrations, a predominance of areas with oxidizing conditions, and shorter residence time compared to ground water from the Jordan part of the aquifer, which had ground water with lower indicator-constituent concentrations, a predominance of areas with reducing conditions, and longer residence time. Statistical analyses indicated a significant relation between the distribution of indicator-constituent concentrations and the following variables: well grouting, total depth of the well, and the presence of the overlying Decorah Shale. The thickness and composition of the overlying glacial drift was shown to be significantly related to nitrite-plus-nitrate nitrogen concentrations but not to chloride and tritium concentrations. Land use was not significantly related to the distribution of indicator constituent concentrations in the Prairie du Chien-Jordan aquifer.