Fate of nitrate in seepage from a restored wetland receiving agricultural tailwater

Abstract

A B S T R A C TConstructed and restored wetlands are a common practice to filter agricultural runoff, which often contains high levels of pollutants, including nitrate. Seepage waters from wetlands have potential to contaminate groundwater. This study used soil and water monitoring and hydrologic and nitrogen mass balances to document the fate and transport of nitrate in seepage and surface waters from a restored flow-through wetland adjacent to the San Joaquin River, California. A 39% reduction in NO 3 -N concentration was observed between wetland surface water inflows (12.87 AE 6.43 mg L À1 ; mean AE SD) and outflows (7.87 AE 4.69 mg L À1 ). Redox potentials were consistently below the nitrate reduction threshold ($250 mV) at most sites throughout the irrigation season. In the upper 10 cm of the main flowpath, denitrification potential (DNP) for soil incubations significantly increased from 151 to 2437 mg NO 3 -N m À2 d À1 when nitrate was added, but showed no response to carbon additions indicating that denitrification was primarily limited by nitrate. Approximately 72% of the water entering the wetland became deep seepage, water that percolated beyond 1-m depth. The wetland was highly effective at removing nitrate (3866 kg NO 3 -N) with an estimated 75% NO 3 -N removal efficiency calculated from a combined water and nitrate mass balance. The mass balance results were consistent with estimates of NO 3 -N removed (5085 kg NO 3 -N) via denitrification potential. Results indicate that allowing seepage from wetlands does not necessarily pose an appreciable risk for groundwater nitrate contamination and seepage can facilitate greater nitrate removal via denitrification in soil compared to surface water transport alone.

Recovering Nitrogen from Farm-Scale Drainage: Mechanism and Economics

“…Given that organic N usually has a shortterm retention potential in soil (McDowell, 2003), the subsurface movement of organic N from the SDA could be an important source of N to downstream waterbodies. Previous studies have suggested that during subsurface movement of N, a large part of the NO x -N is lost through denitrification under the anoxic conditions, thereby enhancing N treatment (Koron, 1992;Kliewer and Gilliam, 1995;Brauer et al, 2015). This is especially true for the warm, wet environment of the study area (Spalding and Exner, 1993).…”

Laboratory study on nitrate removal and nitrous oxide emission in intact soil columns collected from nitrogenous loaded riparian wetland, Northeast China

“…Previous studies have revealed plant uptake, soil retention, denitrification, microbial immobilization and nitrification as the major process of nitrogen retention in wetlands [24–26]. In wetlands which receive high nitrate loading from agricultural runoff or sewage treatment plant discharge, denitrification has been found as the most dominant process of reducing [26, 27]. The results of our experiment revealed remarkable reductions in concentrations from between 59.32 and 59.37 mg N/L at the inlet (5cm above soil-water surface) to concentrations below 1 mg N/L at the lower outlet of the soil columns.…”

Effectiveness of a Natural Headwater Wetland for Reducing Agricultural Nitrogen Loads

“…Xu et al [9] also detected decline in Total-N and nitrate-N concentrations along the wetland but not in the concentrations of ammonium, which may be removed by transient sorption, nitrification, anammox and plant uptake, but also generated through mineralization of organic N, and dissimilatory nitrate reduction to Ammonium [4]. Ackerman et al [41], Maxwell et al [42], and Brauer et al [43] detected significant pollutant reduction along the groundwater flow paths associated with seepage from a wetland. They illustrate that subsurface flux away from wetlands provides another mechanism for N removal.…”