Extreme low-flow conditions in a dual-chamber denitrification bioreactor contribute to pollution swapping with low landscape-scale impact

Abstract

Denitrification bioreactors are an effective edge-of-field conservation practice for nitrate (NO3) reduction from subsurface drainage. However, these systems may produce other pollutants and greenhouse gases during NO3 removal. Here a dual-chamber woodchip bioreactor system experiencing extreme low-flow conditions was monitored for its spatiotemporal NO3 and total organic carbon dynamics in the drainage water. Near complete removal of NO3 was observed in both bioreactor chambers in the first two years of monitoring (2019–2020) and in the third year of monitoring in chamber A, with significant (p < 0.01) reduction of the NO3-N each year in both chambers with 8.6–11.4 mg NO3-N L−1 removed on average. Based on the NO3 removal observed, spatial monitoring of sulfate (SO4), dissolved methane (CH4), and dissolved nitrous oxide (N2O) gases was added in the third year of monitoring (2021). In 2021, chambers A and B had median hydraulic residence times (HRTs) of 64 h and 39 h, respectively, due to varying elevations of the chambers, with drought conditions making the differences more pronounced. In 2021, significant production of dissolved CH4 was observed at rates of 0.54 g CH4-C m−3 d−1 and 0.07 g CH4-C m−3 d−1 in chambers A and B, respectively. In chamber A, significant removal (p < 0.01) of SO4 (0.23 g SO4 m−3 d−1) and dissolved N2O (0.21 mg N2O-N m−2 d−1) were observed, whereas chamber B produced N2O (0.36 mg N2O-N m−2 d−1). Considering the carbon dioxide equivalents (CO2e) on an annual basis, chamber A had loads (~12,000 kg CO2e ha−1 y−1) greater than comparable poorly drained agricultural soils; however, the landscape-scale impact was small (<1 % change in CO2e) when expressed over the drainage area treated by the bioreactor. Under low-flow conditions, pollution swapping in woodchip bioreactors can be reduced at HRTs <50 h and NO3 concentrations >2 mg N L−1.