INFLUENCE OF TILE WATER ON NITRATE CONCENTRATIONS IN THE GROUNDWATER: A CASE STUDY OF A SATURATED RIPARIAN BUFFER ZONE, MCLEAN COUNTY, CENTRAL ILLINOIS

Agricultural activities, particularly the use of inorganic fertilizers and the installation of tile drainage networks, are major causes of increased nitrate levels in water bodies throughout the world. These agricultural practices, prevalent in the U.S. Midwest, alter water quality, leading to eutrophication, to the development of harmful algal blooms, and to hypoxic conditions in surface water bodies. Saturated riparian buffer (SRB) zones use a diversion system to redirect tile drainage water into a riparian buffer. As a best management practice, SRBs have been shown to reduce the nitrate load discharged to surface water. Examining six years (2015-2021) of data from a SRB, this research investigated trends in the nitrate as nitrogen (NO₃-N) concentrations of the groundwater upgradient, independent, of the diversion system as compared to waters downgradient, dependent, of the system during different tile conditions. A mixed effect model analysis of the data identified a global linear trend that indicates a positive relationship between NO₃-N concentrations and the volume of tile discharge. The waters in the downgradient wells followed the general trend but the change in NO₃-N concentration with increased discharge was variable among the wells. Overall, the observed increase in concentration aligned with the higher NO₃-N contributed by the diverted tile waters. However, the general trend was not applicable to all waters in the system, the NO₃-N concentrations of the upgradient waters remained constant or decreased as the tile discharge increased. Analyses to be completed will examine the duration of the tile flow influence on the SRB to demonstrate how tiles regulate nitrate concentrations in the system.