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 Silurian-Devonian aquifer consists of limestones and dolomites deposited when Iowa was covered by tropical seas during two major systems of geologic time — the Silurian and Devonian. Corals, crinoids, brachiopods, and the first fish were found in the Devonian sea, long before dinosaurs existed.
Rock Age: 375 to 440 million years.
Water Movement: Along cracks, fractures, and caverns in the limestones and dolomites.
Production: Typically 150 to 400 gallons per minute (gpm) to city wells and 10 to 30 gpm for private wells.
Water Quality: Best quality in eastern and northern Iowa. Deteriorates as the aquifer is found under progressively thicker rock layers. High levels of arsenic are found in parts of the aquifer.
Other Facts: Sinkholes, caves, and springs, which form a topography called karst, are common in areas.
This aquifer is composed of bedrock units from two major systems of geologic time, the Silurian and the Devonian. They are usually described as a single aquifer package because, regionally, they are composed of similar carbonate rocks that are hydraulically connected and yield similar water quality. Locally, however, there are differences in terms of rock types and water quality, and thus these two units are sometimes considered independently. For example, in eastern Iowa they are two discrete aquifers. In some places, there is no underlying Silurian, and in other places there is no overlying Devonian. And locally, shale and clayey dolomites act as aquicludes, further separating parts of the aquifer system.
This aquifer underlies the entire state except for the northwest and northeast corners. It is an important source of water primarily in eastern and northern Iowa, serving rural, public, and industrial users. The aquifer is composed mainly of porous dolomites of Silurian age, and limestone and thick shales of Devonian age. As is typical of carbonate rocks, porosity and permeability are dependent on such natural openings as fractures, brecciated zones, bedding planes, and solution caverns. These features vary in size, extent, and frequency of occurrence. This aquifer’s porosity and permeability are best developed in a broad band about 65 to 70 miles wide and about 200 miles long across the northeastern part of the state.
The typical thickness of the Silurian-Devonian aquifer in eastern and northern Iowa, its principal area of use, ranges from 200 to 400 feet. The aquifer reaches its greatest thickness, 500 to 700 feet, in the southwest part of the state. Most wells tapping the Silurian-Devonian aquifer are 100 to 700 feet deep, and many municipal and industrial wells yield 150 to 400 gpm. Domestic wells can deliver from 10 to 30 gpm, as long as the wells intercept a good fracture system. Interestingly, one of the most productive zones of any bedrock aquifer in Iowa occurs in a narrow zone along the Cedar River valley from Charles City to Waterloo, an area underlain by highly creviced and cavernous Devonian limestones. Yields of 2,000 to 4,000 gpm are known from wells at Cedar Falls, Waterloo, Waverly, and Charles City. The Silurian-Devonian aquifer in parts of this area receives induced recharge from the Cedar River, infiltrating first through a thin layer of alluvial sand and then through cavernous limestone.
Water quality from this aquifer is generally good in eastern and northern areas of the state, with a dissolved solids concentration between 300 and 500 mg/L. This good natural quality, however, decreases rapidly over short distances to the southwest, especially where covered by overlying shales. The aquifer is used much less in the central and southern parts of the state, where it includes thick deposits of gypsum and anhydrite in the Cedar Valley and Wapsipinicon rock units, as well as thick shales. Yields of 20 to 150 gpm can be obtained in these parts of Iowa, but high sulfate concentrations and total dissolved solids as high as 5,000 mg/L make the water unsuitable for human use. In western and southwestern Iowa, the aquifer is deeply buried beneath younger rocks, and the welldeveloped fracture systems so common in the eastern subcrop area are proportionately fewer and the yields smaller.
The regional flow of groundwater in the Silurian-Devonian aquifer is to the southeast. Natural discharge from the aquifer toward valleys contributes significantly to the amount of water flowing in the Iowa, Winnebago, Shell Rock, Cedar, and Maquoketa rivers. The aquifer thus serves as an important source of baseflow to these streams.
These river valleys are also good places to observe numerous scenic outcroppings of this aquifer. Palisades-Kepler State Park in Linn County, Maquoketa Caves State Park in Jones County, and Backbone State Park in Delaware County are excellent and diverse examples of the Silurian aquifer. Fossiliferous Devonian rocks rim the margins of Coralville Lake near Iowa City, and are especially well exposed for visitors at Devonian Fossil Gorge in Johnson County. A portion of the Rockford Fossil and Prairie Center in Floyd County was the former site of the Rockford Brick and Tile quarry, which utilized the thick shale of the Upper Devonian aquitard.
Edited by: Drew Hutchinson