Field evaluation of four denitrifying woodchip bioreactors for nitrogen removal in eastern South Dakota, United States.

Edge-of-field practices such as denitrifying woodchip bioreactors can be used to improve the water quality of agricultural effluents. This study evaluated the effectiveness of four field-scale woodchip bioreactors in removing nitrate‑nitrogen (nitrate-N) from subsurface drainage in eastern South Dakota. Four woodchip bioreactors were installed and monitored between 2014 and 2016 near Arlington, Baltic, Hartford, and Montrose, South Dakota. Results showed that reduction in nitrate-N concentration for the four bioreactors ranged from 7 % to 100 %, corresponding to removal rates of 5 to 27 g N/m3/day for the four bioreactors during the study period. Average Nitrate-N load reduction in the four bioreactors studied ranged from 39 % to 89 % during the study period. Reduction of nitrate-N in the four bioreactors decreased, on average, by 30 % when temperature dropped below 12 °C during the study period. Flow rate and hydraulic retention time (HRT) also influenced nitrate-N removal in the bioreactors as samples collected immediately following rainfall events showed high nitrate-N load removal compared to samples collected later after the rainfall events during the study period.