The increasing need for good-quality ground water in St. Croix County, caused by a steadily increasing population, can be met from the sand-and-gravel and sandstone aquifers. As much as 15 gallons per minute (0.95 litres per second) can be obtained from wells almost everywhere in the county. Yields of more than 1,000 gallons per minute (63 litres per second) are available from glacial deposits and alluvium where there is sufficient thickness of saturated sand and gravel. The sandstone aquifer underlies the entire county and is more than 1,000 feet (300 metres) thick in the southwest near River Falls. In St. Croix County the sandstone aquifer is all bedrock younger than Precambrian age and includes, in ascending order, from oldest to youngest, sandstones of Cambrian age (in ascending order, the Mount Simon, Eau Claire, Galesville, and Franconia Sandstones, and the Trempealeau Formation); and the Prairie du Chien Group, St. Peter Sandstone, and Galena-Platteville unit (Platteville and Decorah Formations and Galena Dolomite, undifferentiated), all of Ordovician age. This aquifer is capable of yielding more than 1,000 gallons per minute (63 litres per second) to wells in much of the county and is the principal source of municipal water.

The chemical quality of water from the two aquifers is similar. The water from both aquifers is hard to very hard, having a median hardness between 180 and 190 milligrams per litre. Iron and manganese are locally present in botherSome amounts (combined total exceeding 0.3 milligrams per litre). Nitrate is a minor problem in both aquifers, with median concentrations more than 10 milligrams per litre, possibly indicating some contamination of ground water by organic matter.

About 3.83 million gallons per day (0.168 cubic metres per second) of ground water was pumped in the county in 1974, of which 94 percent was from the sandstone aquifer. About 44 percent of the total water pumped was for industrial and commercial use, 42 percent for residential