Des Moines Metropolitan Area Groundwater Sustainability

Des Moines Metropolitan Area Groundwater Sustainability Project

By Mike Gannon and Greg Brennan

THREE-DIMENSIONAL IMAGE of the Cambrian-Ordovician aquifer showing the Des Moines area wells used in the Visual MODFLOW groundwater model. SEVERAL PUBLIC WATER UTILITIES in the Des Moines Metropolitan Area (DMMA) hired the Iowa Geological Survey (IGS) to investigate the long-term sustainability of the Cambrian-Ordovician (CO) aquifer. The DMMA is the fastest growing metropolitan area in Iowa, with projected population growth expected to top 1 million by 2030, and water usage expected to grow by approximately 40–50%. The IGS collaborated closely with the Iowa Department of Natural Resources (IDNR) to study the aquifer.

IGS hydrologist Mike Gannon and hydrogeologist Greg Brennan used a calibrated groundwater flow model to evaluate current and allocated water usages for the CO aquifer in the DMMA. Model results suggested that users in the DMMA would remain comfortably above regulatory levels after 20 years if current usage and infrastructure were maintained. We calculated the pumping water levels by averaging the winter and summer water levels at each well, which sometimes varied by 50–150 feet or more. This water-level variability is caused by the injection versus pumping cycles of the DMMA aquifer storage and recovery (ASR) wells. Each ASR well pumps approximately 3 million gallons per day (mgd) for three months in the summer; in the winter, each well injects 1.1 mgd for the remaining nine months. Water managers use ASR wells extensively in the DMMA during the summer peak water demand period. These wells are also used to blend with river and alluvial water sources to reduce nitrate concentrations.

In the optimum water-use scenario, the IGS team assumed that Grimes, Tyson Meats, West Des Moines, and Altoona would each add one new production well. The proposed West Metro Water District would add nine new ASR wells; Des Moines Water Works would add three; and Ankeny would add two.

Modeling results indicate that additional capacity for CO aquifer expansion exists in the DMMA. However, care should be taken in permit renewal, and water levels should be evaluated over the five-year permit period to verify that each production well remains in regulatory compliance.