Fourteen aquifers, ranging from Quaternary to Precambrian in age, have been identified as the principal sources of water to wells in Minnesota. Half the municipal population anc nearly all the rural population depend on water from these aquifers. Buried and surficial sand and gravel aquifers of Quaternary age occur in nearly all areas of the State and are composed of outwash, beach-ridge, valley-train, and ice-contact deposits. Cretaceous aquifers, absent in the northeast, are nearly continuous in the western half, and are thin or discontinuous in the central and southeast areas. Sandstone and carbonate rocks of Paleozoic and late Precambrian age in southeastern and northwestern Minnesota comprise the Cedar Valley-Maquoketa-Dubuque-Galena, Red River-Winnipeg, St. Peter, Prairie du Chien-Jordan, Franconia-Ironton-Galesville, and Mount Simon-Hinckley-Fond du Lac aquifers. Aquifers of early Precambrian age occur in all but southeastern Minnesota and include the North Shore Volcanic Group, Sioux Quartzite, Proterozoic metasedimentary rocks, Biwabik Iron-formatIon, and undifferentiated Precambrian rocks.
The State's ground water generally contains less than 1,000 milligrams per liter of dissolved solids, except in the extreme southwest, northeast, and western areas. Mineralized water is present at depth throughout the State. Freshwater extends to depths of about 1,000 feet in the center of the Hollandale embayment and in the Twin Cities basin. Six principal water-quality types are present in the .aquifers. Calcium magnesium bicarbonate type water, the most common, is generally present throughout the upper part of the ground-water system.
