Combined Tools for Springshed Mapping

Springs in the Paleozoic karst lands of southeastern Minnesota provide water to trout streams, whose existence depends on a steady supply of clear, cool, and constant temperature water. Twenty-four Minnesota springsheds, recharge areas of the source springs, have been mapped by dye tracing over the past three decades. This mapping has shown that the boundaries of these springsheds bear little resemblance to surface watershed boundaries. However, dye traces are time and labor intensive, and the delineated springsheds feed just twelve of the 173 designated trout streams in southeastern Minnesota. Water management associated with the increasing human impacts of intensive agriculture, new water demands, climate change, and landscape alteration requires more efficient means of defining the remaining springsheds that support and define trout fisheries.

Alternative methods are being investigated to expedite the springshed delineation process. We are combining gamma logs of a few water wells with conventional driller's logs to construct detailed structural contour maps. Preliminary mapping indicates structurally and hydraulically controlled springsheds. Structurally controlled springsheds are thinly saturated aquifer systems. Hydraulically controlled springsheds typically have greater saturated aquifer thickness. A supplemental method uses temperature and conductance loggers to augment conventional spring hydrographs. Comparison of spring thermo- and chemo-graphs allows better hydrograph separation. This refined hydrograph analysis allows improved estimates of basin size and connectivity. Dye trace, structural, and hydrograph analysis may be further combined with major element chemistry and stable isotopes of hydrogen and oxygen in combination with pre-existing water quality data to look for seasonal influxes of recharge water. The results of these methods can then be compared with new and pre-existing dye trace results to gauge their effectiveness in delineating springsheds. The integrated use of a variety of hydrogeologic tools allows a better understanding of these highly dynamic karst systems.