The
Cambrian - Ordovician Aquifer and the Northeast Iowa Landscape
by Robert M. McKay
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Along the eastern margins of
Allamakee and Clayton counties, towering rock bluffs form
a picturesque backdrop for the Mississippi River. The
river's flow of water and commerce toward the Gulf of
Mexico is obvious, yet life-sustaining water also moves
through the rocky cliffs above. The cliffs, which form
some of the most spectacular scenery along the upper
Mississippi Valley, are outcroppings of one of Iowa's
most widely used sources of groundwater -- the
Cambrian-Ordovician aquifer. Near Waukon Junction in Allamakee County (photo left), rugged cliffs expose almost the entire sequence of strata that comprises this aquifer. The rocks at road level consist of the porous, loosely cemented Jordan Sandstone. Higher in the bluff, the better cemented, more resistant dolostones of the overlying Oneota and Shakopee formations stand out as cliff-formers. These same rock units outcrop from McGregor northward into Minnesota, and they dominate the landscape of many well-known points of interest, such as Yellow River State Forest, Effigy Mounds National Monument, and Mt. Hosmer City Park at Lansing. |
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.