Geology and ground-water conditions of the Redwood Falls area, Redwood County, Minnesota

Abstract

The Redwood Falls area includes about 80 square miles in southwestern Minnesota and is about 100 miles west of Minneapolis and St. Paul. Its surface is a gently undulating glacial-drift plain, interrupted in part by the large Minnesota River valley and the tributary Redwood River valley. The drift plain was laid down by the Des Moines lobe of the Wisconsin Glaciation and consists chiefly of ground moraine and several subdued recessional moraines. The glacial drift rests either on Precambrian gneiss or on thin patches of Cretaceous sedimentary strata that overlie the Precambrian bedrock. The glacial drift consists principally of till and some outwash and ranges in thickness from 0 to about 260 feet. It is the chief source of ground water, yielding small supplies almost anywhere in the area and large supplies in at least two places. Where the drift is absent, thin, or impermeable, small yields commonly are obtained from the Precambrian bedrock. The most notable aquifers in the area are three buried outwash deposits that are associated with a conspicuous southeastward-trending buried bedrock valley. The tops of the aquifers occur about 70,120, and 200 feet below the land surface, and their maximum known thicknesses are about 25, 55, and 45 feet, respectively. The aquifers are confined largely by relatively impermeable till; however, owing to the movement of water through the till, the drift may be regarded as a hydraulic unit. The main source of recharge to the ground-water reservoir is local precipitation, and most of the natural discharge is by evapotranspiration. The water table ranges in depth from 0 to about 30 feet below the land surface. Aquifer tests were made on the three outwash aquifers to determine their hydraulic characteristics. The coefficient of transmissibility (T) of the 200-foot aquifer is about 126,000 gpd (gallons per day) per ft, and the coefficient of storage (8) is about 0.0002. The T and S values of the 120-foot aquifer are about the same as for the 200-foot aquifer. For the 70-foot aquifer the value of T is about 70,000 gpd per ft, and S is about 0.0007. The data suggest that when the artesian head in the aquifers is lowered by pumping, recharge is induced by leakage from the confining beds. The 120-foot aquifer is considered the best known source of ground water In the area. The city of Redwood Falls has used this aquifer for its water supply since August 1955 and, in 1960, pumped about 106 million gallons from it. The water level in the aquifer had stabilized by late 1956; considerable additional water probably can be obtained from the aquifer. Although the 70-foot aquifer is not as extensive or as thick as the one at 120 feet, large amounts of water probably can be obtained from this aquifer also. The 200-foot aquifer is narrow, elongate, and irregular in form and appears to be very permeable only locally. Chemical analyses show that water from the glacial-drift aquifers is primarily of the bicarbonate type, is hard, and contains an excessive amount of iron. In places water from the Precambrian bedrock is much softer than water from the drift.