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

Denitrification bioreactors are an effective edge-of-field conservation practice for nitrate (NO) reduction from subsurface drainage. However, these systems may produce other pollutants and greenhouse gases during NO removal. Here a dual-chamber woodchip bioreactor system experiencing extreme low-flow conditions was monitored for its spatiotemporal NO and total organic carbon dynamics in the drainage water. Near complete removal of NO 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 NO-N each year in both chambers with 8.6-11.4 mg NO-N L removed on average. Based on the NO removal observed, spatial monitoring of sulfate (SO), dissolved methane (CH), and dissolved nitrous oxide (NO) 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 CH was observed at rates of 0.54 g CH-C m d and 0.07 g CH-C m d in chambers A and B, respectively. In chamber A, significant removal (p < 0.01) of SO (0.23 g SO m d) and dissolved NO (0.21 mg NO-N m d) were observed, whereas chamber B produced NO (0.36 mg NO-N m d). Considering the carbon dioxide equivalents (COe) on an annual basis, chamber A had loads (~12,000 kg COe ha y) greater than comparable poorly drained agricultural soils; however, the landscape-scale impact was small (<1 % change in COe) 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 NO concentrations >2 mg N L.