Ground-water recharge and flowpaths near the edge of the Decorah-Platteville-Glenwood confining unit, Rochester, Minnesota

The primary source of ground water for the city of Rochester, Olmsted County, southeastern Minnesota is the St. Peter-Prairie du Chien-Jordan aquifer. Based on results of a previous U.S. Geological Survey investigation in the Rochester area, relatively high rates of areal recharge to the St. Peter-Prairie du Chien-Jordan aquifer occur along the edge of the overlying Decorah-Platteville-Glenwood confining unit. The primary source of water to the zone of increased recharge along the edge of the confining unit is the upper carbonate aquifer.

Ground-water recharge rates to the St. Peter-Prairie du Chien-Jordan aquifer during all of 1998 ranged from 1.9 to 25.5 in./yr (inches per year). Recharge rates were greatest near the edge of the Decorah-Platteville-Glenwood confining unit and least where the confining unit is thick and is overlain by the upper carbonate aquifer (mean of 2.0 in./yr). Recharge rates downslope from the edge of the confining unit were greatest to the St. Peter on the slope entering the main South Fork Zumbro River Valley. Results of groundwater age dating using chlorofluorocarbons (CFCs) indicated recharge dates ranging from (1) the mid-1950’s to the early 1990’s for the St. Peter, (2) the late 1960’s to approximately 1990 for the Prairie du Chien, and (3) the early to mid-1950’s for the Jordan.

Cross-sectional model simulations indicated that most of the areal recharge entering the aquifer system through the upper carbonate aquifer discharges from springs and seeps. Of the 2.28 ft3/s (cubic feet per second) of areal recharge that enters the upper carbonate aquifer, 2.23 ft3/s is discharged from the aquifer by springs and seeps. Results indicate that areal recharge to the upper Prairie du Chien moves primarily westward and discharges to Bear Creek.

Areal recharge rates derived from hydrograph analysis, CFC age-dating, and cross-sectional model analysis were much greater to the St. Peter downslope from the edge of the Decorah-Platteville-Glenwood confining unit (25.5, 35.3, and 23.75 in./yr, respectively) than occurs to the Prairie du Chien in any hydrogeologic setting. The model-simulated discharge from springs and seeps in the lower part of the upper carbonate aquifer represents a potential source of water of 33 in./yr to the St. Peter unit, similar to the estimated areal recharge rates derived from hydrograph analysis and CFC age-dating.

The water withdrawn by pumped wells or discharged to Bear Creek is derived predominantly from areal recharge near the edge of the Decorah-Platteville-Glenwood confining unit (0.47 ft³/s), rather than from water that has leaked downward through the Decorah unit (0.03 ft³/s). Model simulated discharge through springs and seeps in the lower part of the upper carbonate aquifer (0.21 ft³/s) represents a potential source of water to the St. Peter-Prairie du Chien-Jordan aquifer.