GEOLOGIC CONTROLS ON GROUNDWATER AND SURFACE WATER NITRATE CONCENTRATIONS IN THE PALEOZOIC BEDROCK-DOMINATED LANDSCAPE OF SOUTHEASTERN MINNESOTA

The Minnesota Geological Survey conducted a three-year investigation evaluating geologic controls on nitrate transport in the Paleozoic bedrock-dominated landscape of southeastern Minnesota. This study identified geologic characteristics accounting for variability in surface and groundwater nitrate concentrations that cannot be adequately explained by land use practices alone. Paleozoic bedrock in the region acts as a highly anisotropic aquifer system where aquitards with limited fracture connectivity restrict the volume and velocity of vertical flow and promote rapid lateral flow along bed parallel partings that discharges as baseflow to streams and rivers. Groundwater in uppermost bedrock units across the landscape is typically nitrate-enriched (5-15 ppm nitrate-N) and concentrations diminish (<2 ppm) at depth across aquitards in a stratified manner, corresponding to groundwater age. The magnitude and variability of nitrate concentrations in stream baseflow is directly affected by the proportion of regionally sourced, older, nitrate-poor groundwater contributed from deep aquifers. Variability in the correlation between agricultural land use (percent row crop) and baseflow nitrate concentrations can be accounted for by varying proportions of dilution from older, less-impacted water relative to more locally sourced, younger, nitrate-enriched water from largely unconfined, shallow aquifers. Our results also showed that the response time of nitrate concentrations to changes at the land surface will vary depending on hydrogeologic setting. The most significantly lagged response in baseflow nitrate concentrations is expected to occur in deeply incised valleys, where substantial flow is derived from deeply confined, regionally sourced water. In contrast, more rapid changes can be expected to occur where the geologic setting permits only a relatively minimal contribution of water from these deeper sources. Future nitrate studies should consider this dilution effect. For example, efforts to evaluate the surface and groundwater response to current changes in agricultural practices should aim to monitor springs, streams, and wells positioned in hydrogeologic settings favoring short lag time responses.