APPLICATION OF AN ANALYTIC ELEMENT MODEL TO UNDERSTANDING GROUNDWATER FLOW AND NITRATE FLUX IN THE BEAR CREEK WATERSHED IN CENTRAL IOWA

Since 1990, the former Agroecology Issue Team at Iowa State University has studied the hydrogeology of multi-species riparian buffers and their efficiency to reduce nitrate transport in the Bear Creek watershed, a 7656 ha agricultural watershed in central Iowa. Most of the research on groundwater has been conducted on three small sections of the creek. In order to assess the effectiveness of riparian buffers in regulating nutrient flux at the watershed scale, a regional, two-dimensional analytic element model (GFLOW 2000), which simulates groundwater and surface water conjunctively, was developed to represent the region. The constructed model represents approximately 2200 km² of central Iowa (with a near-field of about 600 km²) and is centered on Bear Creek, a second order stream that is a tributary to the South Skunk River. The geology consists of up to 30 m of late Wisconsinan till overlying Mississippian limestone. Unconfined conditions were assumed for the model simulations. Model input parameters include hydraulic conductivity (K=1.7 x 10^-5 m/s), aquifer thickness (137 m), areal recharge (81 mm/yr), porosity (0.2), pumping from municipal wells, and discharge from drainage tiles into Bear Creek. Inhomogeneities along the creeks and rivers, which consist of alluvial and outwash deposits, were assigned K values of 3.5 x 10^-4 m/s. Model calibration utilized up to six years of hydraulic head data from 41 piezometers along Bear Creek and stream discharge data from 3 gaging stations. Preliminary simulations show a deviation of 5 to 11 ft between actual and modeled heads (mean squared difference=8.5), which suggests that K and recharge need to be further optimized in the model to more closely match observed heads. Once an optimized solution is obtained, the model results can assist in calculating potential nitrate flux and removal through buffered areas of the watershed.