ETHANOL PRODUCTION AND AQUIFER SUSTAINABILITY IN AN ALLUVIAL/BURIED-VALLEY AQUIFER SYSTEM IN CENTRAL IOWA

Anticipated production of nearly 5.6 Ggal/yr of ethanol at 61 operating or proposed ethanol plants in Iowa has raised concerns about the impact to groundwater of the 4 to 6 gallons of water required for every gallon of ethanol produced. Because many ethanol plants have strategically located themselves near alluvial/buried valley aquifers and because these aquifers also provide about 30 percent of the drinking water in Iowa, the impact of increased pumping needs to be assessed in order to make informed decisions about regional water use and future well field expansion by various users. In this study, the City of Ames, Iowa, (population~50,000), which relies on an alluvial/buried valley aquifer for drinking water (the Ames aquifer), is being used as a test case to study the long-term sustainability of a intensively used, multi-use aquifer system. Initiated in 2005 and funded by the City, the 3-year project is using coring, piezometer installation, geochemistry, microbiology, groundwater dating, and groundwater simulation/optimization modeling to accomplish this task. To help establish regional boundary conditions and estimate aquifer parameters prior to detailed 3-D simulations, a steady-state, 2-D, analytic element model (GFLOW) was used to simulate the water table in the entire central Iowa region (about 2,112 km²). The simulation focused on a near-field area of 1,125 km² and utilized hydraulic parameters from pumping/slug test and hydraulic head data developed as part of this investigation and during the past 25 years of aquifer/aquitard investigations in the region. History matching of the water table under pre-pumping conditions was successfully achieved using 140 hydraulic head targets, 3 flux targets derived from USGS discharge data, a recharge rate of 83 mm/yr. Simulations suggest that groundwater flow in the aquifer is nearly parallel to the South Skunk River, requiring a K value of 6100 m/d for calibration. Calibration in upland till areas was achieved using a K value of 1.5 m/d. UCODE and PEST are being used to refine parameter estimates prior to simulation of pumping in the aquifer.