Cambrian-Ordovician Aquifer
The Cambrian-Ordovician aquifer, or more commonly the “Jordan aquifer,” is one of the most dependable sources of groundwater in Iowa. 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 units are water-bearing, so they are usually treated as a single aquifer. While often called the “Jordan aquifer,” much groundwater comes from the Prairie du Chien. Thus, “Cambrian-Ordovician aquifer” is a more accurate term for the aquifer.
The Cambrian-Ordovician aquifer is widespread and is only absent in extreme northeast and northwest Iowa. Younger geologic units cover the Cambrian-Ordovician aquifer over most of Iowa (Figure 1). The Cambrian-Ordovician aquifer’s main water source is water leaking, or recharge, from these units into the aquifer. Once in the aquifer, water generally flows southeast toward Missouri and Illinois.
Figure 1. Cross-sectional view of Iowa’s major aquifers and aquitards from northwest to southeast (modified from Iowa’s Groundwater Basics by Jean C. Prior, et al., 2003, Iowa Dept. of Natural Resources,
Iowa Geological Survey Educational Series 6, 83 pages).
The Cambrian-Ordovician aquifer is a widespread and dependable source of water for high-capacity wells, and it is used extensively by municipalities and industries in the eastern half of the state. Wells that are properly developed in the Cambrian-Ordovician aquifer yield from several hundred to over 1,000 gallons per minute. As with other sedimentary bedrock aquifers, yields are related to the amount of natural cements filling the pore space in sandstone, and to the presence or absence of fracture openings through denser limestone and dolomites.
Water quality is generally good, with the best quality found in northeast Iowa, nearest the areas of outcrop and recharge (Figure 2). 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 sometime contains excessive levels of radium and is treated before use in municipal water supplies.
Figure 2. Water quality in the Cambrian-Ordovician aquifer (modified from Iowa’s Groundwater Basics by Jean C. Prior, et al., 2003, Iowa Dept. of Natural Resources,
Iowa Geological Survey Educational Series 6, 83 pages).
The Cambrian-Ordovician aquifer is one of two Iowa aquifers protected from overuse. Iowa Administrative Code Chapter 52.4(3) states that water levels are not to decline more than 200 feet from the 1975 baseline. The water level map of the Cambrian-Ordovician aquifer prepared by Horick and Steinhilber (1978) is used as the baseline.
Cambrian-Ordovician Aquifer
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