Groundwater Availability Modeling of the Cambrian-Ordovician Aquifer in Iowa

Increased demand for groundwater by agriculture, industries, and municipalities has raised concerns for the long-term sustainability of the resource. However, the information necessary for decision makers to answer basic questions regarding how much water can be withdrawn from Iowa’s aquifers on a sustainable basis was not available. The 2007 Iowa General Assembly, recognizing this lack of information, began funding a multi-year evaluation and modeling of Iowa’s major bedrock aquifers by the Iowa Geological and Water Survey (IGWS). This report documents an intensive one-year investigation of the hydrogeology of the Cambrian-Ordovician aquifer and construction of a groundwater fl ow model that can be used as a planning tool for future water resource development. Iowa Administrative Code Chapter 52.4(3) states that water levels are not to decline more than 200 feet from the 1975 baseline in any high-use area (State Of Iowa, 1998). The potentiometric surface map of the Cambrian-Ordovician aquifer prepared by Horick and Steinhilber (1978) is currently used as the baseline. The hydrologic characteristics of the geologic layers included in the modeling of the Cambrian-Ordovician aquifer were investigated. An important component of this study was a network of approximately 51 wells, which were used to evaluate water levels. Key to the investigation were 11 observation wells which had time series data. These data were used for the transient model development. A total of 49 aquifer pump tests and recovery tests and 38 specifi c capacity tests were used to calculated the aquifer parameters. The majority of the recovery tests were evaluated for the first time. The hydraulic properties of the Cambrian-Ordovician aquifer were shown to vary considerably in both the lateral and vertical direction. The hydraulic conductivity of the aquifer ranges from 0.3 to 20.9 feet per day, with an arithmetic mean of 4.6 feet per day. Transmissivity values range from 150 to 8,500 feet squared per day. The storage coeffi cient of the Cambrian-Ordovician aquifer ranges from 10-6 to 10-3. The arithmetic mean storage coeffi cient is 3.3 x 10-4. Recharge to most of the Cambrian-Ordovician aquifer is through relatively thick confining beds that include glacial till and various shale units. Due to the relatively thick confining units, the rate of recharge to the Cambrian-Ordovician aquifer is very small. Calibrated recharge rates range from 10-5 inches per year to 0.02 inches per year over the study area. With this information a numerical groundwater fl ow model of the Cambrian-Ordovician aquifer was developed using three hydrogeologic layers. The model was created using Visual MODFLOW version 4.3. Hydrologic processes examined in the model include net recharge, hydraulic conductivity, specifi c storage, flow through boundaries, no flow boundaries, well discharge, and groundwater upwelling. The modeling approach involved the following components: 1. Calibrating a pre-development steady-state model using water level data from historic records. 2. Calibrating a transient model using water-use data from 1901 through 2007. Simulated water levels were compared to observed time-series water level measurements. 3. The calibrated model was used to predict additional drawdowns through 2029 for future water usage simulations. The calibrated model provided good correlation for both steady-state and transient conditions. Root mean square errors of 18.3 and 34.8 feet were relatively small errors for an aquifer that covers most of the state of Iowa. Simulated water level changes are most sensitive to recharge in the steady-state model, and hydraulic conductivity in the transient model. Based on this model, and maintaining current withdrawal rates, the Cambrian-Ordovician aquifer will likely exceed the 200-foot regulatory limit in the Fort Dodge-Webster City area by 2029. If 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.