The Cambrian - Ordovician Aquifer and the Northeast Iowa Landscape
by Robert M. McKay
The aquifer's name is derived from the geologic age of its constituent rocks -- Cambrian and Ordovician (480 to 510 million years old). This widespread source of groundwater, also referred to as the "Jordan aquifer," is one of the most dependable sources for large capacity wells in Iowa. The name "Jordan" comes from the sandstone in its lower portion, but the aquifer actually includes the dolostone and sandstone units in the overlying Oneota, Shakopee, and St. Peter formations. Across the eastern two-thirds of Iowa, the "Jordan aquifer" provides water for food processing, manufacturing, irrigation, and cooling, in addition to drinking water for both municipal and non-municipal users. Properly developed wells generally yield several hundred to over 1000 gallons per minute (gpm) and occasionally may produce in excess of 2000 gpm.
From its outcrop belt in the north-east corner of Iowa, the Cambrian-Ordovician aquifer dips southwestward beneath the landscape at about 18 feet per mile. This gradual tilt buries the aquifer to depths greater than 2500 feet in south-central Iowa, where the town of Murray, in Clarke County, is the last community along the aquifer's slope to withdraw water for drinking use. Along the slope of the aquifer, or downdip, the dissolved mineral content of the water gradually increases to undesirable levels. In the western third of Iowa, elevated concentrations of sulfate, sodium, magnesium, chloride, and bicarbonate prevent the aquifer's use as a water supply. Downdip, water yields are also reduced significantly because of an increase in the mineral cement holding the rock grains together.
Additional uses for the rocks of the Cambrian-Ordovician aquifer include high-quality concrete aggregate in northeast Iowa, reservoir rock for the storage of natural gas in Dallas, Louisa, and Washington counties, and the potential for purified freshwater storage for the City of Des Moines.
Photo by Robert McKay.
Reprinted from Iowa Geology 1995, Iowa Department of Natural Resources