Groundwater Availability Modeling, Silurian Aquifer, East-Central Iowa

Increased demand for groundwater by agriculture, industries, and municipalities have raised concerns about the future availability of groundwater in Iowa. 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 Silurian aquifer in East-Central Iowa. The hydrologic characteristics of the geologic layers included in the modeling of the Silurian aquifer were investigated. Additionally, a groundwater flow model was developed and can be used as a planning tool for future water resource development. A total of 43 aquifer pump tests and recovery tests and 261 specifi c capacity tests were used to calculate the aquifer parameters. The hydraulic properties of the Silurian aquifer were shown to vary considerably in both the lateral and vertical direction. The hydraulic conductivity of the aquifer ranges from 0.3 to 20.2 feet per day, with an arithmetic mean of 3.5 feet per day. Transmissivity values range from 43 to 21,500 ft2/day. The storage coefficient of the Silurian aquifer ranges from 10-6 to 10-3. Recharge to most of the Silurian aquifer is through confi ning beds that include glacial till and various shale units. Due to the highly variable thickness and coverage of these confi ning units, the rate of recharge ranges from 10-4 inches per year over the southwestern half of the study area to 1.8 inches per year over eastern portions of the study area where the confining beds are thin or absent. With this information, a numerical groundwater fl ow model of the Silurian aquifer was developed using three hydrogeologic layers. The model was created using Visual MODFLOW version 10.1. 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 2006 to 2010. Simulated water levels were compared to observed water level measurements. 3. The calibrated model was used to predict future drawdowns in the cities of Marion and Coralville. The calibrated model provided good correlation for transient conditions. A root mean square error of 11.9 feet was calculated for July 2010. This is a relatively small error for an aquifer that covers most of eastern Iowa. Simulated water level changes are most sensitive to recharge. The groundwater fl ow model was applied to two locations where new groundwater withdrawals are being proposed. Based on the groundwater flow model, increasing the withdrawal from the Silurian aquifer to 0.6 billion gallons per year (bgy) in the City of Marion would create an additional 30 to 33 feet of drawdown near the proposed Marion wells, and 2 to 10 feet of additional drawdown would occur near the City of Hiawatha wells and at the Ralston hazardous waste site. The Silurian aquifer within a one-mile radius beneath the Ralston hazardous waste site is a protective groundwater source as defi ned by Iowa Administrative Code Chapter 53.7(1). Marion well 1 is within the one-mile radius and may need to be used as a standby well or properly abandoned. The proposed Silurian well locations are approximately two miles from the Ralston site and are outside the protected groundwater source area. Additional hydrologic data will be necessary to evaluate the potential impact the proposed wells may have on the Ralston site. The additional pumping stress caused by the proposed Silurian wells may cause the contaminant plumes to migrate toward the proposed Silurian wells. Based on the groundwater model, increasing the daily pumping of City of Coralville wells by 3 percent per year for 10 years would add an additional 10 to 30 feet of drawdown near existing Coralville wells 9 and 11, and approximately 48 to 56 feet of additional drawdown near proposed wells P-13 and P-14 compared to 2010 values. Actual drawdown at each well may be slightly higher due to well loss and a higher gallon per minute (gpm) value. These predictive drawdowns would place additional stress on the Silurian aquifer in the Coralville area. The current groundwater pumping level at wells 9 and 11 is at the same elevation as the top of the Silurian aquifer. Lowering the groundwater elevation further may have negative impacts on the production rate and the water quality of the Silurian aquifer. The City of Coralville may have to limit its future withdrawal of water from the Silurian aquifer to maintain the sustainability of the resource. Contingency plans should be prepared by the City of Coralville to evaluate alternative water sources.