Dye Tracing Within the St. Lawrence Confining Unit in Southeastern Minnesota

Abstract

Southeastern Minnesota’s karst lands support numerous trout streams. These trout streams are formed by springs discharging from Paleozoic bedrock. Dye tracing has been the tool of choice for mapping the springsheds (karst groundwater basins) that feed these springs. Previous work was focused on the Galena limestone karst. In order to accelerate springshed mapping, a two-year study was funded by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative- Citizen Commission on Minnesota Resources (LCCMR). Across southeastern Minnesota, numerous springs discharge from the Cambrian St. Lawrence formation. The St. Lawrence is considered to be a confining unit under the Minnesota well code. A dye trace was initiated when a stream sink was discovered in the upper St. Lawrence Formation. The sampling points included springs, stream crossings, and a municipal well that may be at risk for surface contamination. Dye was recovered at one spring in less than two weeks and at two other springs in less than three weeks. This translates into travel times of 200- 300 meters/day. The springs all discharge from the lower St. Lawrence Formation. The St. Lawrence contains beds of dolostone; the dye trace demonstrates that there is a karst conduit flow component in this formation. This is evidence that these springs are significantly more susceptible to degradation than previously thought.