Nitrate reduction in a reconstructed floodplain oxbow fed by tile drainage

Conservation practices are needed to reduce the loss of nitrate via subsurface tile drainage systems and in this study we evaluated nitrate retention in a reconstructed oxbow in central Iowa that was engineered to receive inputs from two drainage tiles. Our objectives were to evaluate the hydrogeology and nitrate loading patterns and quantify the average and seasonal nitrate retention efficiency in the reconstructed oxbow. Over a two-year period, water and nitrate concentrations and loads into the oxbow were dominated by tile drainage inputs compared to groundwater seepage. Nitrate concentrations were highest in tile drainage water (9-17 mg/l), similar in upgradient groundwater and in the oxbow itself (4-8 mg/l) and lowest in downgradient groundwater (0.2 mg/l). Using N:Cl ratios, we estimated nitrate retention efficiency from May to September to range from 44% to 47% in 2014 and 2015, respectively, and found that on a monthly basis, greater retention efficiencies were measured in late summer and early fall. The nitrate retention efficiency was similar to other practices such as bioreactors, wetlands and saturated buffers. Given ecosystem benefits of oxbows and similar costs compared to bioreactors, we believe that reconstructing oxbows to receive tile drainage water should be considered a viable practice for tile drainage treatment in agricultural areas.