Low-Grade Weirs: An Innovative Best Management Practice for nitrate-N Mitigation
Author(s): Littlejohn, K.; Kröger, R.; Moore, M.; Farris, J.
Best management practices (BMPs) generally refer to measures that provide some level of environmental protection for downstream aquatic systems. In agricultural watersheds, BMPs aim to improve the water quality of runoff from the landscape by controlling or trapping pollutants that can potentially degrade downstream aquatic ecosystems. Installation of low-grade weirs in agricultural drainage ditches is being evaluated as an innovative, yet cost effective, management practice that decreases nutrient concentrations and loads by increasing the water volume and hydraulic residence time of the ditch. The objective of our study was to assess the nutrient mitigation capabilities of low-grade weirs in artificially constructed ditches (four ditches with weirs and four without weirs). A replicated nutrient runoff event was simulated using a calculated 11-15 mg/L nitrate (NO3-) concentration applied continuously to each ditch for eight hours. Inflow and outflow concentrations and loads were quantified to determine overall NO3- reduction. A significant difference in weir and non-weir ditch volumes (P= 0.006) was observed, as well as a significant difference in observed hydraulic residence times between weir and non-weir ditches (P= 0.029). Similarly, ditches with weirs demonstrated a statistically lower (P= < 0.001) median outflow load (47.9 mg/min) than ditches without weirs (63.2 mg/min). An enhancement of the biogeochemical environment within the ditch was also observed, with a significantly greater NO3- concentration reduction (P= 0.029) during the hypothesized biogeochemical reduction phase of the experiment for those ditches containing weirs. These results highlight the dynamics of low-grade weirs in reducing nutrient concentrations and loads from agricultural landscapes, potentially establishing low-grade weirs as an additional, innovative BMP for nutrient reduction.
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