MODEL OF RESIDENCE TIME AND ANALYSIS OF NITROGEN REMOVAL FOR TWO CONSTRUCTED WETLANDS AT THE FRANKLIN DEMONSTRATION FARM IN LEXINGTON, ILLINOIS

The main objectives of this research are to compare the size and gradient of two experimental wetlands, to model water’s residence time within each wetland, and to determine the groundwater flow paths within the site and assess whether or not they influence the fate of nitrogen. The two constructed wetlands chosen are the West and Gully located on a 250-acre research farm. Of the two, Gully is about half the size and has a higher head gradient. A tracer test has been completed, providing the water’s residence time, areas of high denitrification, and nitrogen removal rates. A regional 2-D model of groundwater flow was calculated in GFLOW using estimates of groundwater head gradients as groundwater usually follows topography and the measured head values for two main streams serving as constant head boundaries for the model. Estimating aquifer thickness, base elevation, hydraulic conductivity, and other parameters, the model produced a potentiometric contour surface map of the site area with the head elevations shown in one meter contour intervals. The generated flow lines proved that the groundwater flow came from the northeast and flowed southwest toward the two streams. This regional scale agreed with the local scale of the farm where tile drainage generally flows southerly through each wetland. Well locations have been uploaded into ArcMap GIS and presently a regional-scale model that includes both wetlands and the tile drainage using GMS MODFLOW is being configured and modified. This will soon help to determine how groundwater influences the fate of nitrogen and the effectiveness of wetland construction parameters. If there is a difference between the West and Gully wetlands, what are the groundwater parameters that influence how nitrogen is being removed? This study seeks to test the hypothesis that as groundwater does in fact impact denitrification within and around each wetland as well as prove West has a lower residence time than Gully due to differences in construction. The results of this research will be a 3-D model of water residence time showing denitrification most likely occurring outside of the wetlands’ cells and in the groundwater surrounding the cells. These results will be beneficial when considering optimal wetland size to drainage area ratio, monitoring procedures, and wetland management.