Saturated buffer performance under alternative weir settings: Implications for design and management

Saturated buffers are important edge-of-field conservation practices to reduce nitrate-nitrogen (NO₃-N) loading from subsurface (tile) drainage systems to downstream waters. The impact of seasonal management of weir elevations in the water control structure on NO₃-N removal has not been well studied. This study evaluated the effect of control box weir elevation management on nitrate removal and compared in situ flow treatment with design predictions from the USDA Natural Resources Conservation Service conservation practice standard 604. A 253 m long saturated buffer draining approximately 6 ha was monitored for flow and NO₃-N load from 2022 to 2024. The weir elevation was adjusted to "full drainage" (no treatment), "growing season" (reduced treatment capacity), and "fallow season" (full treatment capacity) settings according to weather conditions and field operations. During a 29-day full drainage period in 2022, the saturated buffer bypassed 25% of the annual drainage flow and 28% of the annual NO₃-N load. However, the fraction of flow treated and NO₃-N load removal efficiency was greater in 2022 than 2023 and 2024. Treated flow within the saturated buffer was greater than predicted, while peak drainage system flow was less, resulting in a greater percentage of drainage system capacity treated by the saturated buffer than designed. These discrepancies suggest that alternative design methods should be explored. While the saturated buffer removed substantial NO₃-N in the year with alternative weir management, careful consideration should be given for potential sites that may require extended full drainage periods, as large NO₃-N losses can bypass during such conditions.