The development of ground water for irrigation in northwestern Big Stone County has not kept up with development in other irrigable areas of the State. This is due, in part, to the absence of extensive surficial aquifers and the difficulty in locating buried aquifers.
Test augering south of Beardsley outlined a small surficial aquifer that consists of outwash deposits of sand and gravel. The aquifer is elongate, trending northeast-southwest, with a maximum saturated thickness of 42 feet and an areal extent of about 5 square miles.
Deep test drilling showed that small buried outwash aquifers occur over much of the project area. The 2-mile spacing between test holes, however, was too great to permit correlation of the buried aquifers.
A major buried outwash aquifer, east of Beardsley, consists of 130 feet of fine, uniform quartz sand, The aquifer lies in a steep-sided northwest-southeast trending valley eroded in bedrock and is overlain by 130 feet of clayey till. Pumping-test results indicate that its transmissivity ranges from 10,800 to 13,400 feet squared per day, and that the storage coefficient ranges from 0.0001 to 0.002. Recharge to the aquifer was negligible during 1979.
Two Cretaceous aquifers, the Codell Sandstone Member of the Carlile Shale and the Greenhorn Formation, underlie the area, but little is known about these aquifers.
Samples of water were collected for chemical analysis from wells in the surficial outwash, buried outwash, and Cretaceous aquifers. With the exception of nitrate, the greatest difference in chemical quality was between samples from the buried outwash and Cretaceous aquifers. Water from Cretaceous aquifers is softer than water from the outwash aquifers, but contains concentrations of sodium and boron that are high enough to damage soils and crops if used for irrigation. Nitrate concentrations exceeded the Minnesota Pollution Control Agency's recommended limits for drinking water in one sample from the surficial aquifer.
