Graduation Term

Summer 2025

Degree Name

Master of Science (MS)

Department

Department of Geography-Geology: Hydrogeology

Committee Chair

Seyoum, Wondwosen

Committee Member

Eric Peterson

Committee Member

Lisa Tranel

Abstract

Saturated Riparian Buffers (SRBs) are engineered systems designed to remove nitrate and other pollutants from subsurface agricultural drainage water before it reaches a stream. This study utilizes a high-resolution coupled flow and reactive transport model to explore nitrate reduction and transformation in an SRB in glacial sediment. The influence of material heterogeneity on nitrate reduction and the effectiveness of the SRB was investigated. The study was conducted in an SRB located next to and receiving tile drainage water that ultimately drains into a 3rd order stream (T3 stream) in central Illinois, USA. A three-dimensional steady groundwater flow model was developed using MODFLOW-USG code to simulate the groundwater flow. The model consists of two layers: clay loam with organic topsoil and weathered diamicton, which consists of clays, sand, and gravel. The calibrated and validated root-mean-squared-error (RMSE) values were 0.33 and 0.35 m, respectively. The calibrated hydraulic conductivity for the weathered diamicton ranges from 0.2-16.8 m/day, confirming the existence of heterogeneity. The calibrated flow model was then converted into a transient MODFLOW-USG transport model to simulate advection, dispersion, and decay processes. Sensitivity analysis of the model suggests that recharge is the most sensitive parameter. Transient simulation from March 2021 to June 2021 shows significant (70-90%) NO₃-N reduction occurred in low hydraulic conductivity zones, while low to average (17-54%) reduction occurred in moderate-high hydraulic conductivity zones. The NO₃ load was estimated at 0.17 kg per day reaching the stream compared to the nitrate load of 5.14 kg/day received by the SRB from the distribution tile. About 4.90 kg/d of nitrate load was lost for the entire stress period, which accounts for 95% nitrate reduction, while a storage of 0.007 kg/day remained in the system, indicating the effectiveness of the saturated riparian buffer. This research provides insight into improving the design of SRBs by optimizing tile flow and redirecting tile flow to interact with low hydraulic conductivity zones. Thus, increasing SRB efficiencies in the reduction of NO₃⁻N contamination and ultimately improving stream health.

Access Type

Thesis-Open Access

DOI

https://doi.org/10.30707/ETD.1763755358.855611

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