TRACKING NITRATE LOSS THROUGH THE HYPORHEIC ZONE OF A LOW GRADIENT STREAM THROUGH THE USE OF CONSERVATIVE TRACERS

The goal of this project was to delineate the hyporheic zone of a third-order, low gradient stream and quantify nitrate loss within the steam bed through the processes of denitrification and plant uptake. Little Kickapoo Creek in Illinois is a meandering stream in a glaciated, agricultural area with nitrate concentrations ranging from 5-50 mg/l (as NO3-). Understanding how hyporheic flow might reduce these nitrate concentrations through natural remediation processes is not only important for this locality but for any area where nitrate are high. The parameters of pH, chloride, dissolved oxygen (DO) and oxygen isotopes were used to delineate the hyporheic zone. At a minimum this study observed how mixing of surface water and groundwater occurred within the sand-gravel streambed of this low gradient stream. It was also observed the environment created by this mixing zone, allows for two separate processes to remove nitrate. The low gradient stable subsurface, along with ample sunlight allows for algae and other aquatic plants to thrive. The plants remove nitrate as a nutrient from the top ten centimeters of the hyporheic zone where aerobic conditions persist. Seasonal changes in vegetation and temperature controlled the amount of nitrate removed. Within the first 10 cm of the streambed the conditions are aerobic and plant up-take appears to be the dominate method of nitrate removal. Below the first 10-12 centimeters streambed conditions become anaerobic. Bacteria within these conditions use nitrate as an electron acceptor during the process of organic decomposition. This bacterial denitrification acts as a second means of removing the nitrate from the water. During the late summer and fall nitrate is completely removed in the hyporheic zone. During the winter 50%-80% is removed within the first 45 cm. Evidence of these two processes is found from observing major anion chemistry, tracking DO and pH within the system and through column studies to demonstrate the presence of denitrifying bacteria. Using chloride as a conservative tracer to delineate water sources, a conceptual flow model was produced. The original conceptual model accounted for only two distinct water types. However by using mixing models to calculate percent surface water a possible third water source discharging from the meander lobe was identified.