NITROGEN CYCLING WITHIN THE HYPORHEIC ZONE OF A LOW GRADIENT, AGRICULTURAL STREAM

Current studies of the hyporheic zone have demonstrated a relationship between hyporheic flow and nitrate reduction. Within the Midwestern United States high nitrate concentrations in surface water resulting from fertilizer use, is a serious pollution concern. Little Kickapoo Creek in Illinois is a meandering stream in a glaciated, agricultural area, a setting that has been the focus of little hyporheic research. Nitrate concentrations range from 5-50 mg/l (as NO₃^-). Understanding how hyporheic flow might reduce these nitrate concentrations through natural remediation processes is important for this and similar localities. The first objective is to delineate the hyporheic zone by locating where surface water and groundwater mix through the use of the conservative tracers chloride and δ¹⁸O. Streambed samplers have been installed along a short section (>100m) of the creek. Chloride concentrations in the stream range from 50-100 mg/l whereas, the chloride in groundwater ranges from 10-15 mg/L. Preliminary sampling indicates that water within the first 20 cm of the streambed has chloride concentrations similar to stream water, whereas nitrate concentrations decrease 30-50%. Dissolved oxygen concentrations also decrease from around 9 mg/L to 1-3 mg/L, indicating that the decrease in nitrate is due to uptake. These streambed samplers have been configured so that a 3-dimensional numerical model of the hyporheic zone can be built. Hydraulic conductivity data for the streambed will be gathered using an injected tracer test and grain size analysis. The conservative tracer and injected tracer test data will be used in model calibration. The goal of the numerical model will be to quantify the flux of water through the hyporheic zone and quantify the rate of nitrate uptake/denitrification.