SIMULATING NITRATE TRANSPORT IN FRACTURED TILL INCLUDING TILE DRAINAGE: PRELIMINARY RESULTS (Invited Presentation)

In till deposits of the Dows Formation in the Des Moines Lobe in central Iowa, tile drains are used to lower the water table and promote intensive row-crop agriculture. High nitrate concentrations found in tile drainage effluent and large drainage ditches have been shown to degrade water quality in Iowa streams and drinking water in alluvial aquifers. The till deposits are fractured; however, the effect of fracture flow in the presence of tile drains is not well understood. We hypothesize that fracture flow could increase the transport velocity and decrease the travel time of nitrate to tile drains. To test this hypothesis, we obtained tile drain discharge, fertilizer application, nitrate concentration, and precipitation data from 1996 to 2013 for a 50-acre, row-cropped field in central Iowa. We then implemented a fully-coupled, 3-D, finite-element code, HydroGeoSphere (HGS), to simulate groundwater flow and tile discharge in that field. Hydraulic properties of the till and fractures were estimated using stochastically-generated fracture networks, FracKfinder, and previous field and laboratory data. Soil properties were obtained from the USDA SSURGO database. Groundwater flow and nitrate transport in fractures were simulated using a dual-continuum model. Tile drains were mapped to the model mesh using GPS coordinates and represented in the model as 1-D line elements. The calibration period used 1996-2002 field data, and model predictions were compared to 10 years of field data (2003-2013) from the site. Results to date have been promising and will be compared to simulations in unfractured till in order to test our original hypothesis.