Forthcoming Research Paper

Evaluating and Enhancing the Design of Borrow Pit Wetlands for Managing Nutrients in Agricultural Runoff

by Mohamed S. Gaballah, David Crane, Sarah Copertino, Matthew Chambers, and Roderick Lammers

Abstract

Agricultural runoff delivers substantial nutrient loads to the Missouri River. Recent levee realignments present new opportunities to integrate constructed wetlands (CWs), particularly by repurposing U.S. Army Corps of Engineers borrow pits (areas excavated to provide soil material for levee construction) to intercept nutrient-laden runoff. This study evaluates the efficacy of a 21-hectare borrow pit CW that was engineered to receive surface runoff from an agricultural drainage ditch before reaching the Missouri River. Nutrient concentrations and water levels were monitored over one year and the data were used to calibrate models to predict total nitrogen (TN) and total phosphorus (TP) removal over a 12-year period (2013–2024). A hydrologic model was used to simulate surface inflow and nutrient concentrations. A wetland water and nutrient mass balance model was used to simulate wetland dynamics. Surface inflow was the largest hydrologic input, but groundwater exchange with the Missouri River was critical for controlling wetland water depth. TN inputs were primarily groundwater-driven, whereas TP inputs came mainly from surface runoff. The CW demonstrated average removal efficiencies of 36.6 ± 14.5% for TN and 62.6 ± 16.8% for TP. Removal efficiencies increased as nutrient loads increased, indicating the wetland has the capacity to manage higher nutrient loads. We modeled several design modifications to determine how these CWs could be optimized for nutrient removal. For example, adding an outflow culvert allowed for passive depth control but slightly reduced nutrient removal. Decreasing wetland area increased hydraulic loading rate and decreased nutrient removal efficiency. Results highlight the importance of considering hydrologic interactions between wetlands and the Missouri River in wetland design and evaluation. We show the potential of using borrow pit CWs to manage agricultural runoff in this region through relatively inexpensive modifications to existing agricultural drainage networks.