Cambrian-Ordovician Aquifer (Overview)
The Cambrian-Ordovician aquifer, or more commonly the “Jordan aquifer,” is one of the most dependable sources of groundwater in Iowa and is widespread across Iowa. The Cambrian-Ordovician aquifer is comprised of three separate water-bearing units, the St. Peter Sandstone, the Prairie du Chien Group, and the Jordan Sandstone. In 2008, an intensive one-year investigation was initiated to provide a comprehensive assessment of groundwater resources in the Cambrian-Ordovician aquifer.
As part of this investigation, the hydrologic characteristics of the Cambrian-Ordovician aquifer were examined. A network of 51 wells was used to evaluate water levels in Cambrian-Ordovician aquifer. Eleven (11) wells had water level data collected over a span of time and were used to calibrate the aquifer model. Forty-nine (49) aquifer pump or recovery tests and 38 specific capacity tests were used to calculate the hydraulic conductivity and transmissivity values of the aquifer.
With this information, a three layered groundwater flow model was developed for the Cambrian-Ordovician aquifer (Figure 1). Layer 1 consists of regional confining beds and aquifers above the Cambrian-Ordovician aquifer. Layer 2 is the Cambrian-Ordovician aquifer. Layer 3 consists of regional confining beds and aquifers below the Cambrian-Ordovician aquifer. The model was created using Visual MODFLOW version 4.3.
Figure 1. Hydrogeologic conceptual model of the Cambrian-Ordovician aquifer.
Developing the groundwater flow model involved:
The Cambrian-Ordovician aquifer is 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. Water levels in the Cambrian-Ordovician aquifer have declined over much of Iowa from the 1975 baseline when compared to simulated levels in 2008 (Figure 2). Increased pumping rates have resulted in declines that exceed 140 feet in the Johnson/Linn County area in eastern Iowa and Fort Dodge/Webster City area in north-central Iowa.
- Calibrating a steady-state model to represent the conditions in the aquifer prior to the installation and use of wells in the aquifer.
- Calibrating a transient model to represent changing conditions in the aquifer through time as wells were installed and water removed from the aquifer.
- Using the calibrated transient model to predict additional drawdown through 2029 for various future water use scenarios.
Figure 2. Declines in water levels from Horick and Steinhilber (1978) baseline map to computer simulated results in 2008.
The groundwater flow model was utilized to predict future impacts to the Cambrian-Ordovician aquifer based on various pumping scenarios. If groundwater pumping rates increase by 25% above 2007 rates, the 200-foot regulatory limit will likely be exceeded in the Marion-Cedar Rapids and Des Moines areas in 20 years (Figure 3).
Figure 3. Declines in water levels from Horick and Steinhilber (1978) baseline map to computer simulated results in 2029 with a 25% increase in pumping rates.
For more information,