A Meta-Analysis of Denitrifying Bioreactors: Implications for Nitrate Abatement.

Denitrifying bioreactors are increasingly being used to reduce edge-of-field nitrate export. Bioreactors typically consist of walls or beds filled with a carbon substrate, often wood chips. Walls are placed within the upper 2 meters of groundwater and receive inputs from natural groundwater flow. Beds intercept subsurface tile drainage and generally treat much higher discharge volumes than walls because of the larger area that contributes to the tile drainage network. We report the results of a meta-analysis (using MetaWin) that was conducted to identify the potential effects of various factors, including temperature, inflow nitrate concentration, retention time and design, on bioreactor nitrate removal rates (g m^-3 d^-1). The data set associated within factors was then coded into two to three categories representing low, moderate and high levels. We calculated mean effect sizes and bias-corrected 95% confidence intervals (using a bootstrapping procedure) for each category. The analyses drew from a number of published mass balance studies of individual bioreactor walls and beds. In some instances, we were able to obtain more detailed results from the authors. The denitrification rates associated with walls frequently appeared to be nitrate limited and much lower than bed rates, likely due to the lower flux per unit volume associated with walls. Higher inflow nitrate concentrations were also associated with higher nitrate removal rates, suggesting nitrate limitations and/or first order kinetics. As expected from many denitrification studies, higher temperatures were associated with higher nitrate removal rates; however a number of bioreactor studies found substantial nitrate removal at temperatures less than 7 degree C, suggesting that bioreactors can be of value in more northern climates where considerable nitrate flux occurs during early spring. The results of this meta-analysis can assist managers as they seek guidance on the range of nitrate removal associated with different design and site characteristics.