DYE TRACING OBSERVATIONS FROM THE PRAIRIE DU CHIEN GROUP IN MINNESOTA
Breakthrough groundwater flow velocities are more than 100 meters per day in about 1/3 of the OPDC traces. In one instance, dye poured into a sinking stream emerged at a state fish hatchery spring 4 km away in 10 hours. The breakthrough curves of these traces are much wider, days to weeks, than correspondingly rapid karst flow in the Galena Group of Minnesota, which typically yield breakthrough curves 2 to 6 hours wide. The OPDC breakthrough curves often have complex, repeatable structure suggestive of anastomosing segments in their underground flow paths.
A second 1/3 of traces yield breakthrough flow velocities in the km/month to km/year range. These breakthrough curves are weeks to months wide. Some are relatively smooth while others are complex with multiple peaks, which correlate in some cases with recharge events. The latter subset suggests storage in the subsurface between recharge event “flushes”.
Traces in the third 1/3 “fail” because no dye is recovered, despite long term monitoring of nearby springs. We have yet to identify a discernible pattern in this range of dye trace results. All three sets of results are spread across the entire area where OPDC is the first bedrock. Dye input points a few hundred meters apart have produced results in different groups.
The groundwater flow variability has significantly complicated emergency responses to catastrophic sinkhole failures of waste water impoundments, hazardous material spills, leaking landfills, etc. in the OPDC. Resource management efforts aimed at protecting groundwater quality from pollution by human activities are similarly complicated by the variability of the OPDC’s karst hydrogeology.