Does It Pay to be Mature? Evaluation of Bioretention Cell Performance Seven Years Postconstruction

Bioretention cells (BRCs) are stormwater control measures that integrate water quantity and quality management. Previous research primarily focused on the performance of cells less than two years old, before potential clogging and sorption site saturation. To assess long-term effectiveness, this project evaluated the performance of a BRC after seven years to determine reductions in flow rate, flow volume, suspended solids, nitrogen, phosphorus, and fecal indicator bacteria (FIB). These observations were compared with analyses conducted immediately postconstruction. Inflow and outflow water samples were collected for sediment, nutrients, and FIB. Soil media samples were collected from multiple depths and analyzed for denitrifying bacteria. Results indicate the BRC remains effective at reducing storm flows, nutrients, and FIB. These reductions were correlated to outflow volume, rather than storm characteristics. The majority of denitrifying bacteria were present in the top media layers, despite the presence of an internal water storage layer for denitrification. This finding suggests denitrification primarily occurs at anoxic microsites in the upper layers and that a lack of carbon at deeper depths may limit significant microbial growth. (C) 2017 American Society of Civil Engineers.