Nutrient Retention Performance of Advanced Bioretention Systems Results from Three Years of Mesocosm Studies

摘要

Current bioretention designs are not very effective at nutrient retention for a variety of reasons. Negatively charged organic amendments compete for the few phosphorus (P) binding sites available in the sand matrix. The rapid rate of flow through the systems does not permit enough time for effective nitrogen (N) retention to occur. To improve bioretention designs for nutrient retention, it is necessary to understand the processes involved in nutrient transformations. Given this understanding, it is then possible to develop approaches that improve nutrient retentions. Our mesocosm studies have shown that advances in bioretention system design substantially improve retention of N and P from runoff. Using advanced media, our research over three years has confirmed our initial findings on P retention trends. After accumulating 190 g-m~2 P (the equivalent of nearly five decades of stormwater runoff), the advanced media had a median TP discharge concentration of 0.077 mg/1. The median Ortho-phosphate discharge concentration was 0.015 mg/1. N retention by the outlet regulated treatment was 66% in even quite large (~6 month ARI) events, and 62% for oxides of N. The corresponding free discharge treatment was significantly less effective, at 27% and 19% retention respectively. A year later, total N retention in small storm events exceeded 70%, while nitrate retention exceeded 90%. These results confirm how advanced media combined with outlet controls substantially raise the bar for treating runoff nutrients. This is become ever more important with the adoption of strict numeric criteria in TMDLs in affected waterbodies throughout the US.