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Assessing the Impact of Ethanol Production and Sustainability of Alluvial/Buried Valley Aquifers with Groundwater Models: A Test Case for the Ames Aquifer
YEAR: 2007
INVESTIGATORS: William Simpkins, Laurie Achenbach

Concern about the impact of ethanol production on Iowas water resources has increased due to the large increase in statewide ethanol production. Water use at ethanol plants could soon reach 22 billion gallons per year (Ggal/yr). Groundwater is preferred for these operations because of its high quality and stable quantity; however, placement of ethanol plants is determined by access to corn, not necessarily to groundwater supply. Permits for ethanol-related groundwater withdrawals are being granted in rural areas where determination of local pumping impact is often performed by a well driller or consultant. Tools for assessing the larger-scale impacts of ethanol production in aquifers (including water quality and ecological impacts) and for evaluating the sustainability of aquifers in the State need to be developed in order to provide a scientific basis to strengthen administrative oversight of groundwater use. The proposed research will compare the ability of three different types of groundwater models to assess the potential impact of ethanol production and determine aquifer sustainability using a test case of the Ames aquifer a regional, alluvial/buried valley aquifer in central Iowa. The grant will help fund Phase III of the project Water Supply for Ames in the 21st Century: A Comprehensive Reassessment of the Ames Aquifer and will address the following objectives: 1. Simulate steady-state and transient groundwater flow in the Ames aquifer under non-pumping and pumping conditions using a 3-D, finite-difference, groundwater flow model; 2. Compare output of 2-D and 3-D groundwater models to determine which would best identify impacts of ethanol production in aquifers and the appropriate scale of investigation; 3. Apply simulation-optimization modeling to the calibrated 3-D model to investigate the impact of large-scale ethanol development on drawdown, optimize well pumping schedules, manage future well field expansion, and assess the long-term sustainability of the aquifer. Modeling will take advantage of three USGS stream gages in Squaw Creek and the Skunk River, 65 hydraulic head targets, and nearly 100 boring logs in the Ames aquifer that have been used to construct a 3-D framework for the aquifer. In order to further understand the geology, the 3-D distribution of hydraulic head, and estimate recharge rates for the model, funds are requested for the installation of two new piezometer nests. Nest 1 in the Downtown Well Field would be designed specifically to estimate vertical K (Kv) and recharge (R) in the aquitard. Nest 2 in the Skunk River alluvium will specifically test the hypothesis that the Skunk River is a losing stream and can develop an unsaturated zone beneath the river. Both piezometer nests will be installed in fall 2007 and instrumented with combination pressure transducers/ dataloggers (corrected for barometric pressure) in order to gather real-time data that can be related to pumping schedules and USGS stream gaging data. Modeling with MODFLOW will begin in July 2007 and the simulation will be calibrated to hydraulic head measurements in the model domain and streamflow under pre-development conditions. Transient simulations will commence once the model is calibrated. Results of a regional 2-D, steady-state, analytic element, groundwater/surface water model (GFLOW) and a local-scale, 3-D, transient, groundwater flow model (MODFLOW), will be compared to determine which approach is best suited to evaluating the impacts of ethanol production at different scales. A simulation-optimization model (the Groundwater Management process for MODFLOW 2005) will be applied in spring 2008 to evaluate the sustainability of the Ames aquifer, taking into account limits on groundwater withdrawals, streamflow depletion under multiple uses (pumping) of the aquifer, and alternative climatic scenarios. By using models to assess the impact of ethanol production and to address sustainability for the Ames aquifer, the results of this study will provide a template to guide management and regulation of similar aquifers under pumping stresses from ethanol production in Iowa and the Midwest.