A Study of Subsurface Water Flow in a Southeastern Minnesota Karst Drainage Basin

Abstract

The Root River drainage basin in Fillmore county, southeastern Minnesota has well developed karst topography and karst groundwater flow in carbonate sedimentary rocks of upper Ordovician age. In the upper carbonate aquifer subsurface water flows rapidly through solution-enlarged fractures and conduits, and is intimately connected to surface water. As such it is very subsceptible to pollution.

An area was chosen in the drainage of the South Branch of the Root River, southeast of the town of Spring Valley, for detailed hydrologic study. The area has one of the highest densities of karst features in Minnesota.

The first part of the study involved quantitative fluorometric tracing of subsurface water using the fluorescent dye Rhodamine WT and a field fluorometer. The tracing studies delineated subsurface flowpaths, revealed travel times and dispersion along the flowpaths, and permitted mass balance calculations of inflowing and outflowing water. The traces defined the recharge area of Moth and Grabau Springs at the head of Forestville Creek, and important trout stream.

A gauging station was installed to measure the discharge of these springs, and so far has produced two years of continuous record. A network of rain gauges was installed to measure precipitation over the recharge area. Data from these installations describe the way the karst system responds to recharge events. Several sub-environments of flow exist within the aquifer. Initial estimates of transmissivity and aquifer diffusivity can be derived from the data.