Ground-water resources of Audrain County, Missouri

The deep (principal) aquifer in Audrain County has an average thickness of about 1,300 feet and is composed of dolomite and minor quantities of sandstone of Cambrian and Ordovician age. The deep aquifer is the source of water for all public-supply and irrigation wells in Audrain County. Pumpage from the deep aquifer has caused a decrease in hydraulic head of more than 200 feet since 1900 in the vicinity of the city of Mexico. Calculations from a two-dimensional digital model of the deep aquifer indicate that the drawdown would increase 10 to 25 feet from May 1979 levels in Audrain County by May 2000 in the absence of irrigation pumpage and if public-supply wells continue to pump at the 1980 rate. If the additional stress due to seasonal irrigation is continued at 1980 pumping rates, 60 +/- 20 feet of drawdown is predicted by May 2000. Audrain County is the northernmost extent of freshwater in this aquifer in Missouri. In Audrain County the dissolved-solids concentration of water from this aquifer varies from 1,200 milligrams per liter in the north to less than 400 milligrams per liter in the south. Lowered water levels in the aquifer may allow water with a larger dissolved-solids concentration to move into the area. (USGS)