USING A TRACER TEST TO ASSESS THE TRANSPORT AND FATE OF NITRATE IN A SATURATED BUFFER ZONE

The Upper Mississippi Basin (UMB), which includes Illinois, has highly fertile soils and therefore, experiences intensive agricultural practices. While fertile, the soils do not drain well, resulting in the installation of drainage-tile systems. Agricultural practices within the UMB include the application of nitrogen (N)-rich fertilizers. The tile systems coupled with the application of N have led to the excessive export of nitrates (NO₃^-) from the agricultural fields into surface and subsurface waters through subsurface tile drainage systems. Excess NO₃^- contributes to eutrophication and to development of hypoxic zones in aquatic environments. One method that has exhibited success in lowering nitrate (NO_3^-) concentration is the diversion of tile drained waters from the agricultural fields into a saturated buffer zone (SBZ) before the water enters a stream. A SBZ is an area of perennial vegetation between agricultural fields and water ways where a tile-outlets drain. The SBZ serves as a sink where NO₃^- is reduced through natural processes such as plant uptake, denitrification, and dilution with groundwater. Previous works have shown a reduction in the NO_3^- content in the SBZ, but the extent to which this removal occurs cannot be quantified without knowing the residence time of the water through the SBZ. Our goal was to use sodium bromide (NaBr) and sodium chloride (NaCl) as tracers to determine the residence time of the tile waters in a SBZ at the T3 site in Hudson, Illinois and to quantify the amount of reduction or dilution of the NO₃-N in the SBZ using a mixing model. Results from the tracer test show an average groundwater velocity of 0.36 m/day with a standard deviation of 0.18 m/day using the arrival time of the chloride tracer and 0.61 m/day with a standard deviation of 0.24 m/day using the arrival times from the bromide tracer. The residence time of the NO₃- is estimated to be between 40 days to 50 days. The average horizontal hydraulic conductivity from the tracer test was calculated to be 6.62×10^-5 m/s, which conforms with results obtained from slug tests performed on the site (3.03×10^-5 m/s). A residence time of 40 days is expected to reduce the NO₃- significantly and, our mixing model will confirm this otherwise.