A steadily increasing population in Jefferson County, Wisconsin, is expanding the need for good-quality ground water. This need can be met by good-quality water available from the sand-and-gravel, Galena-Platteville, and sandstone aquifers. As much as 15 gallons per minute (0.95 liters per second) can be obtained from wells almost everywhere in the county. Yields of more than 1,000 gallons per minute (63 liters per second) are available from glacial drift where it contains a sufficient thickness of saturated sand and gravel. The Galena-Platteville aquifer is a dolomite that occurs mainly in the eastern one-half of the county and is locally more than 300 feet (90 meters) thick. Estimated well yields from this aquifer exceed 500 gallons per minute (32 liters per second). The sandstone aquifer underlies nearly the entire county except for small areas in the northwest corner. It is more than 1,100 feet (330 meters) thick in the southwest along the Dane-Jefferson County line. This aquifer is capable of yielding more than 1,000 gallons per minute (63 liters per second) to wells in much of the county and is the principal source of municipal water. The chemical quality of water from the three aquifers is similar. The water is very hard, having a median hardness between 315 and 325 milligrams per liter. Median values for dissolved solids range between 325 and 349 milligrams per liter. Iron and manganese commonly are present in bothersome amounts (combined total exceeding 0.3 milligrams per liter. About 13.0 million gallons per day (0.570 cubic meters per second) of ground water was pumped in the county in 1972, 87 percent from the sandstone aquifer. About 62 percent of the total water pumped was for industrial and commercial purposes, 26 percent for residential use, and 12 percent for municipal, irrigation, and institutional use.