THE INFLUENCE OF TILE FLOW ON THE TRANSPORT - FATE OF NITRATE IN THE VADOSE ZONE OF A SATURATED RIPARIAN BUFFER: A CASE STUDY IN CENTRAL ILLINOIS

Illinois has fertile but poorly drained soils, resulting in the installation of tile drainage systems. Tiles drain soil water from agricultural fields directly into streams, short-circuiting the soil’s role in nutrient cycling and contributing to elevated nitrate as nitrogen (NO₃-N) concentrations in surface waters. Most NO₃-N exports from tile-drained watersheds in the Midwest occur from January to June, corresponding to periods of tile flow. The diversion of tile-drained water into a saturated riparian buffer (SRB) has been shown to reduce NO₃-N concentrations, but what happens in the vadose zone near the tile in terms of water flow and nitrate transport - fate is unknown. The vadose zone is a critical nitrogen storage location and foci for biogeochemical processes utilizing nitrate. This study examined the influence of tile flow on water movement in the vadose zone and the transport and fate of nitrate in an SRB. Nitrate sampling began in April 2023 and will continue until November 2023, representing conditions when the tiles are running versus periods when the tiles are not running. During the period of tile flow, mean NO₃-N concentrations in the vadose zone waters at depths 0.3 m and 0.60 m below the surface were 0.68 mg/L and 0.50 mg/L, respectively. The tile water had a mean of 10.44 mg/L, while in the groundwater, drawn from wells screened 1.5 m below the land surface, concentrations were 0.80 mg/L upgradient of the diversion tile and 6.97 mg/L downgradient of the tile. A one-way ANOVA indicated that the NO₃-N concentrations within the waters of the vadose zone and upgradient of the tile were significantly different from the downgradient and tile waters. The data suggest the tile has an influence on the downgradient of the tile compared to the vadose zone waters and the upgradient of the diversion tile. By the end of May, tile flow had ceased corresponding with a lowering of the groundwater table. During the drier conditions, no tile flow, and the lysimeters did not yield water even after heavy rains. The inability to draw water from the vadose zone suggests any available water is being withdrawn by the plants. Sampling will continue into the fall to assess whether the nitrate accumulates within the vadose as the plants go dormant.