Geology in support of groundwater management for the northwestern Twin Cities Metropolitan area

Abstract

The Paleozoic bedrock in the northwest part of the Twin Cities Metropolitan area (Fig. 1) has been increasingly relied upon as a source of potable water. Most municipal wells in the area are now constructed to draw water from Paleozoic bedrock because productive drift aquifers are uncommon and are more susceptible to contamination. Increased reliance on Paleozoic strata as a source of groundwater led to a request by environmental managers, led by the Metropolitan Council, for improved geologic maps of the area as well as a hydrogeologic characterization of the most heavily used aquifers and their intervening confining units, those in the siliciclastic-dominated, Upper Cambrian part of the Paleozoic section. Hydrogeologic characterization combined with mapping provides a geologic framework for the northwest metro area that is of greater usefulness to environmental managers because it will increase the accuracy of groundwater protection plans, as well as predictions of aquifer productivity. Our hydrogeologic investigation is focussed on detailed characterization of the Upper Cambrian aquifer commonly referred to as the “Franconia-Ironton/Galesville” or “FIG” aquifer, using recently developed borehole geophysical techniques (Paillet and others 2000; Runkel and others, 2003). The tentative strategy for communities in the northwest metro area is to use the FIG aquifer as the principle source of potable water. Information was also collected and synthesized, to a lesser extent, on the Eau Claire and St Lawrence Formations, which are generally considered confining units above and below the FIG aquifer, and on the Mt Simon Sandstone, which is considered a less desirable alternative to the FIG as a source of water.