Geochemical survey to determine water-quality characteristics of the Big Sioux Aquifer in eastern South Dakota

A geochemical survey of the Big Sioux aquifer, South Dakota was under taken to: (1) Estimate the quality of the water in the aquifer, (2) determine if there are any water-quality problems in the aquifer, and (3) design a water-quality monitoring network to monitor changes in present and potential water-quality problems. For most agricultural uses, the water in the Big Sioux aquifer is of acceptable quality. However, in some locations, the ground water is either marginally acceptable or unfit for human use. The major water-quality problem is the high nitrate concentrations (geometric mean of 4 and up to 120 milligrams per liter as N) found in many domestic water supplies. The fact that samples taken from observation wells located by rooads away from houses and barnyards show lower concentrations of nitrate (geometric means of 0.4 and up to 22 milligrams per liter as N) indicates that the source of nitrate pollution is probably localized and due to barnyard wastes. A monitoring network is proposed consisting of at least: (1) Three wells adjacent to domestic wells, (2) three wells about 1,000 feet away and down the potentiometric gradient from domestic wells, and (3) three wells at least 3,000 feet from any source of pollution. This monitoring network would provide information on changes in both localized nitrate pollution and other selected water-quality parameters. (USGS)