Karst Hydrogeologic Investigation of Trout Brook, Dakota County, Minnesota

Abstract

Trout Brook in the Miesville Ravine County Park of Dakota County is the trout stream with the highest nitrate concentration in the karst region of southeastern Minnesota. Water quality data from 1985 and 1995 (Spong, 1995) and from 2001, 2002, 2006 and 2010 by the Dakota County Soil and Water Conservation District (SWCD) (2010) document an increasing level of nitrate in Trout Brook. A karst hydrogeologic investigation was designed to measure nitrate levels at sampling points along the stream and to increase our understanding of the source and movement of nitrates throughout the length of Trout Brook. Eighteen springs and seeps have been located in the Main Branch and tributaries of Trout Brook. A previously unreported flowing section and stream sieve, Weber Sieve, were found above what had been thought to be the head of perennial flow in the East Branch of Trout Brook. Two new sinkholes developed after the 14-15 June 2012 flood in a field northeast of the East Branch of Trout Brook. This investigation included regular monitoring of major anions in the streams and springs, synoptic stream flow measurements, a dye trace of a sinking stream in the Trout Brook drainage, and continuous temperature monitoring at two springs.

The initial assumption was that the majority of the baseflow of Trout Brook was from discrete springs. However, synoptic baseflow and nitrate measurements show that only 30-40 percent of the total flow in Trout Brook is from discrete springs, and the rest appears to be from distributed groundwater discharge directly into the stream. Both the discrete springs and the distributed recharge occur along reaches of Trout Brook that drain the significant high transmissivity zone near the bottom of the regionally important Shakopee aquifer. Dye traces have confirmed flow-paths from Weber Sieve to LeDuc and Bridgestone Springs and have begun to define springsheds for these head water springs. The temperatures of two springs were monitored for 7.5 months. The observed small, seasonal temperature fluctuations at the springs seem to be due to the air temperature while storms that result in flooding and surface runoff cause larger, short-term temperature fluctuations.

Nitrate concentrations and chloride/bromide ratios decreased systematically from the upstream springs to the downstream springs. The nitrate concentrations have been increasing at four springs from 1985 to 2012 and at two surface sampling points from 2001 to 2012. The nitrate concentration of another surface sampling point increased from 2001 to 2006 but decreased from 2006 to 2012. Snowmelt and rainfall runoff was sampled on 2 March 2012 and showed no detectable nitrate in the runoff from a watershed with no row-crop agriculture, but elevated nitrate was detected in an adjacent watershed with row-crop agriculture. All of these trends illustrate the dominance of agricultural sources of nitrate in Trout Brook.