Performance of Regenerative Stormwater Conveyance on the Removal of Dissolved Pollutants: Field Scale Simulation Study

Regenerative stormwater conveyance (RSC) is a stormwater control measure (SCM) built in a channel that uses a series of riffles, grade control structures, and pools with an underlying sand media layer to detain and treat stormwater runoff. A purpose-built experimental field RSC was subjected to simulated hydrographs by mixing dissolved nitrogen and phosphorus into inflow from a managed retention pond. The system storage capacity of 30.5m2 within the pools (14.2m2) and sand layer (16.3m2) was sized to store the runoff volume from the 25-mm event. Twelve storm simulations were conducted under varying conditions of storm depth (25 and 38mm) and antecedent dry periods (1day and 3days). Simulations confirmed that RSC can convert large portions of flow volumes from surface to subsurface flow; mean reductions in surface flow volumes were 80% for the 25-mm event and 40% for the 38-mm event. Reductions in dissolved pollutant concentrations in surface flow were insignificant or very minor for all analytes. However, mean reductions in subsurface flow concentrations were high for both total Kjeldahl nitrogen (TKN, 79%) and orthophosphate (81%). Because of well-drained subsurface conditions, nitrification of TKN to nitrate was observed, leading to a 250% increase in nitrate concentration. Results of the simulation study indicate that microbial transformation and sorption mechanisms are present in RSC subsurface outflow. Subsurface flow contributed 66% of the total nitrogen load and 42% of the total phosphorus load for the 25-mm event. When adequately sized, RSCs can provide substantial hydrologic mitigation for urban drainage areas. As implementation of RSC continues, regulators and designers are encouraged to consider RSC subsurface flow as an important contribution to downstream pollutant load. Proper filter media selection and differentiated aerobic/anaerobic conditions are essential to optimize subsurface flow treatment.