RECENT ADVANCES IN SPRINGSHED MAPPING USING DYE TRACING, GIS AND STRUCTURAL GEOLOGY TOOLS

Dye tracing techniques combined with sinkhole distribution and structural contour mapping are proving to be an efficient technique for defining springsheds in the sinkhole plains of Orion Twp., Olmsted Co., Minnesota, which are developed in the relatively flat-lying Ordovician Galena Group of Southeastern Minnesota. Standard tracing techniques use Rhodamine WT, uranine and eosin dye, carbon samplers and direct water samples and scanning spectrofluorophotometry with spectral peak analyses to document selected sinkhole to spring connections. The sinkhole distributions are mapped from digital orthoquads and then supplemented with features not visible on the orthoquads with fieldwork using GPS techniques. Structural contour maps of the bottom of the carbonate units are constructed using a critical reevaluation of water well logs in Minnesota's County Well Index by comparison to gamma logs and outcrops.

The initial results show springsheds with a few km2 area. Mapping of springsheds can help encourage implementation of Best Management Practices to protect and improve the water quality in the springs. The springs are used as livestock water supplies, and recharge deeper aquifers in nearby settings, which are used for municipal water supplies. The structural contour maps and refined geologic maps demonstrate that the dense arrays of sinkholes have formed where the Stewartville Formation is the first bedrock and in the areas of highest local dip. The regional dip is roughly 1:1,000 but the sinkholes are concentrated in the areas where the dips are close to 1:100.