Characterizing the performance of denitrifying bioreactors during simulated subsurface drainage events

The need to mitigate nitrate export from corn and soybean fields with subsurface (tile) drainage systems, a major environmental issue in the midwestern United States, has made the efficacy of field-edge, subsurface bioreactors an active subject of research. This study of three such bioreactors located on the University of Illinois South Farms during their first 6 mo of operation (July- Dec. 2012) focused on the interactions of seasonal temperature changes and hydraulic retention times (HRTs), which were subject to experimental manipulation. Changes in nitrate, phosphate, oxygen, and dissolved organic carbon were monitored in influent and effluent to assess the benefits and the potential harmful effects of bioreactors for nearby aquatic ecosystems. On average, bioreactors reduced nitrate loads by 63%, with minimum and maximum reductions of 20 and 98% at low and high HRTs, respectively. The removal rate per unit reactor volume averaged 11.6 g NO₃-N m^-3 d^-1 (range, 5-30 g NO₃-N m^-3 d^-1). Multiple regression models with exponential dependencies on influent water temperature and on HRT explained 73% of the variance in NO₃-N load reduction and 43% of the variance in its removal rate. Although concentrations of dissolved reactive phosphorus and dissolved organic carbon in the bioreactor effluent increased relative to the influent by an order of magnitude during initial tests, within 1 mo of operation they stabilized at nearly equal values.