Groundwater Availability Modeling Under Drought Conditions Lower Racoon River Aquifer Dallas and Polk Counties, Iowa: Drought Assessment

Increased demands 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, which was 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?” A groundwater study was initiated to understand the shallow groundwater resources in the Lower Raccoon River aquifer. The primary objective of this study was to evaluate the aquifer for future water supply development under drought conditions. A groundwater flow model of the Lower Raccoon River aquifer was created using Visual MODFLOW 2011.1. The model was used to generate source water capture zones, evaluate surface water and groundwater interaction, and estimate maximum sustainable pumping rates. Water level data during the summer of 2012 were used to help calibrate the model. Based on the mass balance calculations in the model, the percentage of water production supplied by the Raccoon River (from Dallas Center to West Des Moines) was 52 percent, and 29 percent was supplied by induced recharge from the numerous sand and gravel quarries. The remaining 19 percent of the water production is supplied by precipitation recharge and groundwater inflow into the model area. The percentage of induced recharge varied from 0 percent at Van Meter to 89 percent at Des Moines Water Works radial wellfield. Induced recharge from the Raccoon River (from Dallas Center to West Des Moines) allows public wells to maintain water production during prolonged dry periods. Limitations in water production exist when streamflow along the Raccoon River drops below 17.1 cubic feet per second (43.4 cfs if the Fleur Drive infiltration gallery is included). Based on available pumping records, an average of 14.6 billion gallons of water are pumped from the Lower Raccoon River aquifer each year. Additional water production is available from the aquifer, but limitations exist during extremely dry years. Additional pumping capacity ranges from 10 percent in the West Des Moines wellfield, to over 1,000 percent at Adel. Potential well yields greater than 500 gallons per minute (gpm) are found near Adel, Van Meter, and West Des Moines. The highest potential well yields occur east of Adel, and are the result of the cobble and boulder zone found at the base of the aquifer, and the abundance of induced recharge from the nearby sand and gravel quarries and the Raccoon River.