UNDERSTANDING GROUNDWATER FLOW IN A SATURATED BUFFER ZONE (SBZ) USING NUMERICAL MODELS: CASE STUDY OF THE T3 SITE, CENTRAL ILLINOIS

Loss of nutrients, such as nitrogen and phosphorous, from agricultural lands in the Midwest affects farm productivity and environmental quality. Nutrient runoff from lands discharged into streams degrades the aquatic ecosystem through eutrophication and subsequent hypoxic conditions. The presence of nitrogen also threatens the potability of numerous aquifer bodies globally due to their high solubility in water. To tackle this problem, the Environmental Protection Agency (EPA) recommends using best management practices such as Saturated Buffer Zone (SBZ) to reduce nitrogen from nitrate-laden tile water before being discharged into streams. However, groundwater flow and interactions at SBZ are not well understood. To aid the understanding of groundwater flow in the SBZ with the T3 site as a case study, this project aims to prove two hypotheses: (1) Radius of influence will increase as the rate of discharge of tile water increases. (2) There is an interaction between regional groundwater flow and local tile water flow and vice versa. To test the first hypothesis, finite difference groundwater modeling software (MODFLOW) is used to produce a three-dimensional steady-state groundwater model. After calibration using autocalibration techniques, several scenarios are run where tile discharge is increased, and the resulting change in the radius influence is measured. To test for the second hypothesis, particle tracking software (MODPATH) is used to place particles at the eastern edge of the model area and track particle flow paths. Then, flow path simulation was conducted in two different scenarios when there is a tile flow and no tile flow. It is expected that the radius of influence of tile water will increase as the discharge of tile water increases until the point the tile water reaches the stream. At that point, the radius no longer increases but flattens (takes the shape of an asymptote). It is also expected that there will be an interaction between the regional groundwater flow and local tile water flow. Thus, a buildup of the hydraulic head is expected when tiles are turned on. This leads to a reduction in the percentage of particles entering the buffer zone as compared to when the tiles are turned off. This has a direct implication on the effectiveness of the SBZ in reducing nitrate.