Evaluating Edge-of-Field and in-Ditch Stratagems to Reduce Nutrient Losses in the Mid-Atlantic Region.

Interest in adapting novel and existing treatment technologies (e.g., permeable reactive barriers, edge-of-field and in-ditch bioreactors) to reduce nutrient losses in mid-Atlantic drainage systems is based on a desire to address regional water quality goals and improve the overall health of the Chesapeake Bay. This project evaluated the design and installation challenges and early nitrate removal performance of four categories of denitrifying bioreactors. Two ditch diversion bioreactors, a tile drain bioreactor, a novel in-ditch bioreactor, and two sawdust-amended denitrification groundwater walls were constructed between fall 2014 and winter 2015 for edge-of-field monitoring. The sawdust walls were the easiest to install, whereas the in-ditch bioreactor was the most demanding to design and install due to its novelty. All four bioreactors exhibited nitrate removal during relatively short monitoring periods, and showed promise for bioreactor applications in the region. Two ditch diversion bioreactors installed according to Natural Resources Conservation Service design standards averaged greater than 65% nitrate load reduction. Lower than expected nitrate removal by the tile-drain bioreactor was likely due to a combination of site/design constraints, potential internal short-circuiting, and water chemistry (5.0% annual N load reduction; 0.49 g NO₃-N removed m^-3 d^-1, 0.00003 lb N ft^-3 d^-1). The in-ditch bioreactor averaged 65% nitrate concentration reduction, but flow monitoring will be a critical part of future assessment of this design. Observed sedimentation of the in-ditch bioreactor is expected to be one of the biggest practical challenges for in-ditch denitrification practices. The N removal rates for the two sawdust walls centered around 0.90 and 0.30 g NO₃-N removed m^-3 d^-1 (0.00006 and 0.00002 lb N ft^-3 d^-1). Obtaining a water balance by monitoring inflow, outflow, and by-pass flow for these types of practices is critical for future assessment of their overall contributions to N load reduction and water quality improvement in the Mid-Atlantic.