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 Cambrian-Ordovician aquifer consists of sandstones and dolomites that were deposited as Iowa fluctuated between tropical seas and dry land. The sediments that became the dolomites were deposited in two intervals as sea level rose to higher levels that covered Iowa with warm, tropical seas. The sands in the sandstones were deposited in three intervals as sea levels lowered across Iowa.
Rock Age: 460 to 505 million years.
Water Movement: Between the sand grains in the sandstones. Along cracks and fractures in the dolomites.
Production: Can yield from hundreds to over 1,000 gallons per minutes.
Water Quality: Best quality in northeast Iowa. Deteriorates as aquifer depth increases. Radium is a concern in areas.
Other Facts: Laws exist that limit the amount of water that can be removed from the aquifer.
This is the only aquifer in Iowa that is widely used even where it is covered by younger bedrock units. The Cambrian-Ordovician aquifer is a widespread and dependable source of water for highcapacity wells, and it is used extensively by municipalities and industries in the eastern halfof the state. The aquifer is composed of three separate formations – the Jordan Sandstone (Cambrian age) at the bottom, dolomite and sandstone of the Prairie du Chien Group in the middle (Ordovician age), and the St. Peter Sandstone at the top. All these bedrock units are water bearing, so they usually are treated as a single aquifer. Wells that require large amounts of water typically tap the full range of the Prairie du Chien and Jordan units, while other wells may tap only the St. Peter or a combination of the St. Peter and upper Prairie du Chien. While often called the “Jordan aquifer,” much groundwater also comes from the Prairie du Chien. Thus “Cambrian Ordovician aquifer” is a more accurate term for this water-producing zone.
Typical well depths in this aquifer range from 300 to 2,000 feet, with some over 3,000 feet deep. As noted earlier, bedrock aquifers are inclined in a southwesterly direction toward Kansas and Oklahoma; thus, the deepest wells are in southwest Iowa. This depth range highlights another characteristic of water from this aquifer – its temperature. Since the natural temperature gradient within the earth increases with depth, water temperatures within the Cambrian-Ordovician aquifer range from 60 to 80 degrees F, and reach 85 degrees F in the 3,000-ft wells.
Wells that are properly developed in the Cambrian-Ordovician aquifer yield from several hundred to over 1,000 gpm. As with other sedimentary bedrock aquifers, yields are related to the amount of natural cements filling the pore space in sands tone, and to the presence or absence of fracture openings through denser limestone and dolomite. Water quality is generally good, with the best quality found in northeast Iowa, nearest the areas of outcrop and recharge. In western and southwestern areas, as the aquifer inclines deeper beneath the land surface, the dissolved mineral content increases to undesirable levels, with high sulfate, chloride, and fluoride in particular. In portions of southeast, central, and western Iowa, the aquifer sometimes contains excessive levels of radium and is treated before use in municipal water supplies.
The main area of aquifer recharge is in southern Minnesota and northern Iowa, via vertical leakage from overlying aquifers. Subsurface flow is then to the southeast, with discharge to the Mississippi Valley.
Historical use of the Cambrian-Ordovician aquifer by municipal and industrial pumping centers such as Ft. Dodge, Mason City, Des Moines, Washington, Fairfield, and Ottumwa caused the potentiometric surface of the aquifer to drop between 50 and 150 feet. Sharp recoveries occurred when these wells shut down, but they did not regain their original standing water levels. Despite this drop, the available water is not expected to be exhausted in the near future as the aquifer contains an estimated 80 trillion gallons of water. Local, smaller declines also are noted in the Silurian~ Devonian aquifer.
Outcrops of Cambrian-Ordovician rocks produce some of the most spectacular scenery in Iowa, particularly from McGregor northward along the Mississippi Valley. The 480 to 510 million-year-old dolomites and sandstones that elsewhere comprise this aquifer dominate the landscape at such scenic points as Yellow River State Forest, Effigy Mounds National Monument, and Mt. Hosmer City Park at Lansing, all in Allamakee County.
Edited by: Drew Hutchinson