Nearly one-half the population of Minnesota (fig. 1) depends on wells for drinking water. Virtually all the rural population and 94 percent of the public-water systems use ground water. In general, the quality of Minnesota's ground water is satisfactory for most uses, such as for domestic, public, and industrial supplies and for irrigation. Most of the water can be classified, on the basis of predominant ions, as a calcium-magnesium-bicarbonate type with less than 1,000 mg/L (milligrams per liter) dissolved solids (Adolphson and others, 1981). However, naturally occurring saline water (exceeding 1,000 mg/L dissolved solids) is common along the western border, the northern shore of Lake Superior, and below depths of about 1,000 feet in the southeastern part of the State. Statewide concentrations of iron and manganese commonly exceed 300 /ig/L (micrograms per liter) and 50 jtg/L, respectively, which are the maximum recommended concentrations for these constituents in drinking water (U.S. Environmental Protection Agency, 1986b). Concentrations of sulfate in water from Cretaceous rocks and overlying glacial drift in southwestern Minnesota commonly exceed the U.S. Environmental Protection Agency's (EPA) secondary drinking-water standard of 250 mg/L (fig. 2). Ground-water quality in Minnesota has been degraded by contamination, with the most serious problems being in local areas. Major sources of contamination in the State, according to the Minnesota Pollution Control Agency (1986a, p. 47), include: (1) spills or improper disposal of industrial or manufacturing chemicals, (2) leachate from solid-waste landfills, (3) spills and leaks from petroleum-product storage areas and pipelines, and (4) feedlots and agricultural chemicals. A total of 132 sites have been identified, as of November 1986, by the Minnesota Pollution Control Agency (MPCA) and the EPA as priority sites for cleanup. Included in these priority sites are 36 hazardous-waste sites on the National Priorities List (NPL), and 6 sites at two U.S. Department of Defense facilities (fig. 3/4). In addition, two principal aquifers in the State the surficiaWrift aquifers, which underlie much of the State (fig. 2A2), and the upper carbonate aquifer, which underlies karst-type terrain in the southeastern part of the State (fig. 2X1) are very susceptible to contamination from land-surface sources, such as spills, leachates from landfills, infiltration of runoff from feedlots, and widespread application of agricultural chemicals. A survey of 887 community water systems, which included about 1,800 wells, was made by Minnesota Department of Health (MDH) (1985) for the purpose of detecting volatile organic compounds (voc) in drinking water. The survey showed detectable voc concentrations in 109 wells (fig. 3B). In 15 communities, the concentration of voc in water from some of the public-supply wells exceeded limits considered acceptable by the MDH. Various actions have been taken by the MDH to ensure safe drinking water for the approximately 100,000 persons in those communities (Minnesota Department of Health, 1985).