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 Dakota aquifer is mainly composed of sandstones that were deposited in river systems that drained land to the east of Iowa. It was formed in the Cretaceous age — when dinosaurs were alive. However, few dinosaur fossils have been found in Iowa.
Rock Age: ~100 million years.
Water Movement: In empty spaces between the sand grains in the sandstones.
Production: Can yield as much as 1,000 to 1,500 gallons per minutes to wells.
Water Quality: Water quality tends to be fair to poor. Best quality in south-western region of the aquifer. Water quality deteriorates as minerals are dissolved from rock layers above the aquifer.
Other Fact: The aquifer is an important secondary source of water for western Iowa.
More Information:
The Dakota aquifer is composed of sandstone deposits 200 to 300 feet in thickness, and provides water for many rural and public water supplies in northwest and west-central Iowa (map, above). This aquifer is composed of the Woodbury and underlying Nishnabotna sandstones, which formed in riverine environments 100 million years ago. The sandstones range in grain-size from very coarse to fine, and they are poorly cemented, providing abundant pore space for groundwater storage. Over 80 percent of the Nishnabotna alone is composed of medium- to coarse-grained quartz sandstone. The sandstones are confined over most of their area by 200 to 400 feet of clay-rich glacial till as well as by thick shale, siltstone, thin chalky limestone, and lignite (low-grade coal).
Wells drilled into the Dakota range from 100 to 600 feet deep and yield 100 to 500 gpm. Some wells can produce as much as 800 to 1,500 gpm. Water quality tends to be fair to poor. The areas of poorer water quality result from high concentrations of dissolved solids (between 500 and 3,000 mg/L), particularly sulfate and calcium carbonate, which are common minerals picked up by groundwater in contact with the confining layers above. Areas of better water quality are found where confining layers are thin and some porous pathways allow for more rapid recharge by unmineralized surface waters. This occ urs most notably in the southern area of the aquifer’s extent.
The Dakota aquifer in Iowa is recharged by downward percolation through its confining units. Regional groundwater flow is from north to south, and natural discharge from the aquifer occurs into the lower reaches of major rivers in the region. The Dakota sandstone often displays a rust-colored coating formed by oxidation of iron-bearing minerals within the formation.
Edited by: Drew Hutchinson
