HYDROSTRATIGRAPHIC AND CLIMATE CONTROLS ON SPRING FLOW AND PERCHED GROUNDWATER IN THE DRIFTLESS AREA OF WISCONSIN

Wisconsin’s Driftless Area is home to thousands of springs that help support the region’s world-class trout streams and sustain critical habitat for endangered and threatened species. The relationship of springs to the groundwater flow system is poorly understood as is the response of spring flow to changes in climate. Some springs appear to be supplied by perched groundwater, while others appear to be sourced by deeper flow systems. Two tufa-depositing springs may provide insights into the stratigraphic position of perched groundwater as well as the effects of climate change and variability on perched groundwater levels. Both tufa-depositing springs discharge at a mid-slope position, well above the level of the receiving stream and form mounds that are 9 to 12 meters high. Perched groundwater in the Driftless Area has recently been shown to be laterally-extensive and broadly associated with an Ordovician carbonate-siliciclastic sequence (Decorah, Platteville, and Glenwood Formations). Groundwater levels are thought to be stable over decadal time scales. A 2009-2010 Keck Geology Consortium project provides further evidence that many springs in the region may be supplied by a perched aquifer that sits atop shale interbeds (Spechts Ferry Member of the Decorah Formation). Correlation of detailed outcrop stratigraphy and spring position with borehole geophysical logs provides a refined understanding of the flow system that supplies these springs. The tufa-depositing springs, as well as other springs in the Driftless Area, may have been flowing soon after glaciers retreated from the rest of the upper midwestern United States. Therefore, geochemical patterns in the tufa sequences have potential to reveal changes in paleo-temperature and paleo-moisture conditions. Where tufa records correlate with other proxy climate data in the region, this would indicate the existence of conditions that allowed springs to flow continuously throughout, for example, warmer or dryer periods in the Holocene. Alternatively, a lack of correlation may indicate a lapse in tufa deposition and spring flow, and thus lower regional groundwater levels. Used in association with the hydrogeologic data, the tufa records could lead to new insights into perched groundwater levels under climate regimes that differ from the present.