Impacts of Variable Hydrologic Regimes upon Denitrifying Bioreactor Performance

Denitrifying woodchip bioreactors (DNBs), low-cost passive systems engineered for reactive nitrogen (N) removal, are an increasingly popular best management practice for mitigating N export in agricultural runoff and other nonpoint sources. Dynamic hydrologic flows cause highly variable hydraulic retention times (HRTs) within bioreactors, leading to variations in redox conditions and biogeochemical processes that impact nitrate (NO₃^-) removal efficiency as well as the export of undesirable products including nitrous oxide (N₂O) and methane (CH₄) to the atmosphere and/or receiving waters.

This study examines the influence of variable hydrologic flows upon the performance of a DNB intercepting tile drainage flow from a farm in Central New York State during the summer of 2018. We test the appropriateness of zero-order biokinetic models (Halaburka et al., 2017) for predicting NO₃^- removal as a function of HRT through frequent monitoring of longitudinal NO₃^- profiles as well as in situ ¹⁵NO₃^- push-pull tests (PPTs). ¹⁵NO₃^- PPTs conducted before and after bioreactor drainage and re-flooding will quantify the N₂O product ratio of denitrification and evaluate the impact of drying-wetting cycles on accumulation of N₂O. Total nitrogen analysis coupled with ion and gas chromatography results will enable detailed accounting of sources and sinks of dissolved inorganic N, organic N, and N₂O within the bioreactor. Co-injection of the dissolved gas tracers helium and sulfur hexafluoride in PPTs were used to quantify the extent of immobile gas phases within the bioreactor porous media and evaluate the role of trapped bubbles in retarding transport of N₂O and CH₄ through the reactor. Monitoring of porewater chemistry revealed significant accumulation of CH₄ during low- to no-flow conditions, which predominated for much of the summer. This study will provide insight into the ideal design and operation of future DNBs to ensure adequate water quality protection while minimizing the production of the undesirable products N₂O and CH₄.