Groundwater Availability Modeling of the Mississippian Aquifer North-Central Iowa

Increased demand for groundwater by agriculture, industries, and municipalities has raised concerns for the long-term sustainability of the resource. In 2007, the Iowa legislature began funding a comprehensive Water Resources Management program to be implemented by the Iowa Department of Natural Resources. A key aspect of the program is to evaluate and quantify the groundwater resources across the state using computer simulation models. These models help answer questions such as: How much water can be pumped from an aquifer over 10, 20, or 100 years? or Will my well go dry? This report documents an intensive one-year investigation of the hydrogeology of the Mississippian aquifer in north-central Iowa, and the construction of a groundwater flow model that can be used as a planning tool for future water resource development. The hydrologic characteristics of the geologic layers included in the modeling of the Mississippian aquifer were also investigated. A total of 19 aquifer pump tests and recovery tests and 140 specific capacity tests were used to calculate the aquifer parameters. The hydraulic properties of the Mississippian aquifer were shown to vary considerably in both the lateral and vertical direction. The hydraulic conductivity of the aquifer ranges from 0.14 to 1,510 feet per day, with an arithmetic mean of 123 feet per day. Transmissivity values range from 17 to 93,000 ft.2/day. The storage coefficient of the Mississippian aquifer ranges from 10-4 to 10-1. Recharge to most of the Mississippian aquifer is through confining beds that include glacial till and various shale units. Due to the highly variable thickness and coverage of these confining units, the rate of recharge ranges from 10-3 inches per year over the southwestern half of the study area to 1.5 inches per year over north-central portions of the study area, and along the major river valleys, where the confining beds are thin or absent. With this information, a numerical groundwater flow model of the Mississippian aquifer was developed using three hydrogeologic layers. The model was created using Visual MODFLOW version 11.1. Hydrologic processes examined in the model include net recharge, hydraulic conductivity, specific storage, general head boundaries, constant head boundaries, well discharge, river boundaries, and well interference. 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 2003 to 2012. Simulated water levels were compared to observed water level measurements. The calibrated model provided good correlation for transient conditions. A root mean square error of 16.8 feet was calculated. This is a relatively small error for an aquifer that covers most of north-central Iowa. Simulated water level changes are most sensitive to changes in hydraulic conductivity. Based on the groundwater flow modeling results, an additional 1 billion gallons per year (bgy) of groundwater could be withdrawn from the Mississippian aquifer using precipitation recharge alone. A much higher withdrawal rate is possible based on the relatively large volume of groundwater (10.6 bgy) that is discharged into the major river systems. Not all of this water could be withdrawn without potentially impacting the baseflow conditions of these rivers. A conservative estimate of 50 percent of the river recharge might be available for new or amended water use permits. This would be an additional 6.3 bgy of additional groundwater availability (5.3 bgy from converted river recharge and 1 bgy from available precipitation recharge).