The following information is based on the book Iowa’s Groundwater Basics, by Jean C. Prior, published by the State of Iowa, Iowa City, in 2003. The book was designed and illustrated by Patricia J. Lohmann.”
About: The Mississippian aquifer primarily consists of limestones and dolomites that formed when Iowa was covered with a warm, tropical sea. Corals, crinoids, brachiopods, and snails were common in the sea, and can often be found as fossils across Iowa.
Rock Age: 320 to 350 million years.
Water Movement: Along cracks and fractures in the dolomites.
Production: Yields 50 to 100 gallons per minutes to wells where aquifer is near the surface and fractures are abundant. Yields drop as fractures decrease.
Water Quality: Best quality in north-central Iowa where the aquifer is near the surface. Quality deteriorates as materials that overlie the aquifer increase.
Other Fact: Flowing wells, where water flows freely to the surface in wells, are common in areas.
Productive wells from the Mississippian aquifer supply private and public water supplies for much of the north-central part of the state (map above). The water quality is generally good. In contrast, the same aquifer produces much smaller yields of poorer quality water in central and southeastern Iowa. Throughout its range the aquifer consists of a thick sequence of limestone and dolomite, with thinner deposits of sandstone, shale, the silica-rich mineral chert (flint), and gypsum.
Along the outcrop belt of these rocks, the Mississippian aquifer is overlain by alluvium, loess, and glacial drift, while elsewhere the aquifer is first overlain by thick shale and lesser sandstone units (Pennsylvanian). Since recharge to the aquifer is primarily by infiltration from the land surface, this contrast in materials has a direct effect on the aquifer’s water quality. For example, groundwater quality is very good in the north-central subcrop area, where dissolved solids concentrations are less than 500 mg/L. Less favorable conditions are present in the central and southeast parts of the area where Mississippian strata have fewer creviced openings and downward recharge is limited by the less permeable Warsaw shale. In these areas, domestic wells may produce only 5 to 10 gpm, and often less than that.
Like the bedrock aquifers beneath it, the entire Mississippian aquifer has a gradual overall slope to the southwest, so that hundreds of feet of younger Pennsylvanian strata and glacial clays overlie the aquifer in south-central and southwestern Iowa. Where these thick Pennsylvanian shales overlie the Mississippian aquifer, high concentrations of sodium, fluoride, and sulfate limit the aquifer’s use as a drinking water source, even though yields of 10 to 50 gpm are sometimes possible. In western and southern Iowa the total dissolved solids content ranges from 1,500 to 5,000 mg/L, making the water unsuitable for human and livestock use.
The most productive wells from the Mississippian aquifer are concentrated in north-central Iowa where excellent yields of 500 to 900 gpm are known from municipal wells in Marshall, Story, Hardin, and Wright counties. These high yields coincide with an area of well-developed karst, where the limestone is highly dissolved. Typical well depths range from 100 to 300 feet and commonly yield from 50 to 100 gpm. In central and southeast Iowa, however, yields are as low as 2 to 3 gpm, owing to the scarcity of crevices in the rock formations. Well depths here extend about 100 to 400 feet below land surface.
Regional flow in the Mississippian aquifer is in a southerly direction, and it discharges into the Des Moines and Skunk rivers and their tributaries.
The Mississippian aquifer can be observed as picturesque limestone bluffs along the Mississippi River valley (its namesake), especially between Burlington and Keokuk in southeastern Iowa. In addition, the shallow carbonate aquifer here displays good examples of karst features, including sinkholes within the Burlington city limits, a well~developed solutional cavern at nearby Starr’s Cave State Preserve, and Blackhawk Spring at Crapo Park. Other natural exposures of Mississippian aquifer rocks are seen along river valleys and creek banks as well as roadsides and quarries from Lee, Des Moines, and Louisa counties at the southeast corner of the state to Humboldt and Hancock counties in north-central Iowa. Scenic Lacey-Keosauqua State Park in Van Buren County and Three Bridges County Park along the Iowa River in Marshall County, as well as limestone quarries near Le Grand (famous for their fossil crinoids) are in Mississippian carbonate rocks. Visitors can drink from a flowing artesian well at Benson Park, along Hwy. 3 just west of Clarion in Wright County. This well taps the Mississippian aquifer at a location of considerable artesian pressure. Mineralized groundwater from the Mississippian aquifer at Colfax in Jasper County was thought to have therapeutic effects, and so the ornate Hotel Colfax was built nearby in 1884 as a health spa.
Edited by: Drew Hutchinson