ESTIMATING THE LONG-TERM SUSTAINABILITY OF AN ALLUVIAL/BURIED-VALLEY AQUIFER SYSTEM IN CENTRAL IOWA

Alluvial and buried-valley aquifers in Iowa are a major source of high quality groundwater. But, can high quality, municipal drinking water derived from such an aquifer in central Iowa be sustained into the future? The city of Ames, Iowa (pop. 50,000), is located on an alluvial/buried-valley aquifer system – the Ames aquifer – that coincides with the South Skunk River and Squaw Creek. The city presently draws water from 4 well fields where the depths are generally 95 to 150 ft and pumping yields in the coarse outwash gravel range from 250 to 1,500 gpm. The confined and unconfined parts of the aquifer are connected to surface water. In 2005 and 2006, declines in water levels and threats of water rationing raised concerns about future sustainability of the aquifer system. The aquifer now supplies almost 10 Mgal/day to the city during peak periods in the summer. For this project, we proposed a more holistic approach to the aquifer than in previous studies. Objectives included: 1) production of a common ArcGIS database; 2) investigation of the city's emergency water supply; 3) simulation of groundwater flow using a 2-D regional screening model (GFLOW) and a smaller scale, 3-D MODFLOW model (calibrated using groundwater age dating); 4) characterization of the redox geochemical environment; and 5) optimization of the pumping schemes, siting of future well locations, and estimating sustainability using MODOFC. Unfortunately, despite nearly 4 decades of previous work on the aquifer system, its hydrogeology is still poorly understood. Specifically, the emergency water supply lakes (where surface water is pumped out and into the streambed of the South Skunk River to recharge the aquifer) are flow-through lakes that are already connected to the aquifer. The 2-D groundwater flow model also suggests that pumping wells are inducing infiltration from surface water. The redox geochemistry of the aquifer is conducive to clogging of well screens and specific capacities in some wells have declined as much as 80 percent in 13 years, thereby increasing pumping lifts and decreasing the yield of the aquifer. In short, our objective of estimating the sustainability of the aquifer will be delayed until the critical geological, hydrogeological, and geochemical data are analyzed, previous conceptual models are corrected, and groundwater flow is simulated under realistic (stream) boundary conditions.