EXPLORING NITRATE REDUCTION IN A SATURATED RIPARIAN BUFFER THROUGH A THREE-DIMENSIONAL REACTIVE CONTAMINANT TRANSPORT MODEL: IMPLICATIONS FOR DESIGNING BETTER RIPARIAN BUFFERS

Sub-surface tile drainage systems are predominantly used in Midwest agricultural farmlands to improve soil aeration and crop yields. However, it also leads to nutrient export from fields, contaminating streams that drain into the Mississippi River and contributing to a hypoxic zone in the Gulf of Mexico, posing a major environmental concern. Saturated Riparian Buffers are promising management practices to mitigate this issue. Studies show their effectiveness in reducing nitrate within glacial till formations, but knowledge gaps remain. Specifically, the influence of sub-surface heterogeneities on tile drainage discharge to stream is not fully understood. These variations significantly impact nitrate transport and reduction within the SRB. This study aims to develop a coupled flow-reactive transport model to assess the impact of nitrate loads from sub-surface tile drainage and precipitation on stream health. A three-dimensional static geological model was developed, incorporating data from 21 wells installed at the study site and three cone penetration test logs obtained from the Illinois State Geological Survey. The static model consists of three units: organic-rich soils, clays with sand and gravel, and unweathered diamicton. Field estimates of the hydraulic conductivities for the unsaturated zone (organic-rich soil) and saturated zone (clays with sand and gravel unit) were obtained from the infiltrometer test and slug test, respectively. The results of the unsaturated and saturated hydraulic conductivity were 0.154 m/day and 0.621 m/day, which are consistent with typical till formation. The water table information obtained during the spring within the study area shows that groundwater flow is from north to west, and discharges into the stream. Stream gauging experiment indicated a gaining stream system, with downstream discharge exceeding upstream discharge (16,851 m³/day vs. 9297 m³/day). Field measurements will be incorporated and used to convert the static geological model into a dynamic groundwater flow model using the MODFLOW-USG code to simulate steady hydraulic head conditions in the clay with sand and gravel unit and calibrated with observation data from years 2021 and 2024. The results of the flow model will drive the reactive transport model, analyzing nitrate transformation and reduction with fluxes from anthropogenic and natural sources.