RAPID RECHARGE OF A DEEPLY BURIED CARBONATE AQUIFER, RICE COUNTY, MINNESOTA

The Ordovician Prairie du Chien group is a regional carbonate aquifer in the upper Midwest. In south-central Minnesota, the aquifer is routinely overlain by more than 80 m of glacial drift. When exposed in deep river valleys, the carbonates exhibit features related to karsting and tectonic fracturing. The overburden consists of widespread clay-rich till with gravels in localized Pleistocene drainage channels.

Although buried deeply and located over 35 km from the nearest surface exposure, water properties from a test site in Rice County, Minnesota, indicate the Prairie du Chien aquifer is recharged within approximately 14 d. Principal evidence for rapid recharge includes seasonal water temperature fluctuations of 4.5°C, bimonthly conductivity variations, and two-fold weekly changes in dissolved oxygen (DO). Sulfate varies inversely with DO, and ammonia (>1.0 mg/L), total phosphorus (>0.4 mg/L), and total organic carbon (>41 mg/L) are all elevated. A recharge model for the site is complicated by three observations, including (1) significant temperature fluctuations occur while the regional ground surface is frozen, (2) large (i.e., ~100 µS/cm), short-term (i.e., 14 d) conductivity excursions are both increasing and decreasing from the long-term trend, and (3) low-angle bedrock dip, heterogeneous till composition, and hummocky surface topography obscure the positions of obvious recharge areas.

The following preliminary model honors our observations: Agricultural pumping throughout the winter sets up ground water recharge from one or more frozen eutrophic lakes. Although the lake water is fresh, it rapidly gains electrolytes while passing through carbonate-rich drift and thus causes only minor changes in the bedrock water conductivity. During spring thaw, surface water and shallow ground water with solutes from animal waste and road treatments are passed quickly downward and laterally through a network of gravels, karst features, and fractures. This water passes our test site as a front, arriving approximately two weeks following the thaw and exits the test site within two weeks. Significant rainfall events during the late spring and summer create fast-paced pulses of diluted water that pass through the gravel-karst-fracture network and cause ongoing fluctuations in basic ground water properties.