Evaluation of a Denitrification Wall for Surface Water Nitrate Load Reduction.

Permeable reactive barriers (PRBs) to stimulate denitrification, often termed denitrification walls, have been proven as an effective, maintenance-free means to remove nitrate from groundwater for decades. The denitrification process is stimulated through the addition of a woody material vertically in to the soil to intercept shallow groundwater, which reduces dissolved oxygen levels and serves as an electron donor for denitrification. Because nitrate contamination in groundwater underneath agricultural fields is often diffuse, the objective of this study was to reduce stream nitrogen loading by targeting a denitrification wall where a large groundwatershed was funneled to a stream headwaters. The denitrification wall removed all nitrate from at least 45,000 liters of groundwater per day. By targeting the denitrification wall adjacent to a stream headwaters, we were able to reduce annual surface water nitrogen load by 361 kg (0.4 tons) for a 68% reduction. For the next stage of the research we sought to determine the impact on nitrogen reductions if the denitrification wall had been constructed with media of different types, concentrations and sizes. Utilizing mesocosms experiments we were able to determine that temperature as well as wood surface area, carbon concentration, type and quality all influence denitrification rate and the longevity of the carbon source. This study indicates that the efficacy of denitrification walls can be improved by maximizing treatment volume and choosing the appropriate media type and concentration.