Removal of nutrients from urban stormwater runoff by storm-pulsed and seasonally pulsed created wetlands in the subtropics

Few studies have documented urban stormwater nutrient retention by subtropical and tropical wetlands. Freedom Park, in Naples, Florida, USA, is a 4.6-ha created wetland system designed to treat urban stormwater runoff before it impacts downstream ecosystems. The wetland system was specifically designed for nitrogen and phosphorus removal from the water column, especially during the wet season, when the hydraulic loading rate (HLR) averages 80 cm week−1 compared to the dry season average of 9 cm week−1. Water samples were taken every other week at 12 locations along these treatment basins over the period March 2, 2016-February 28, 2017 and analyzed for nutrients [total phosphorus, soluble reactive phosphorus (SRP), nitrates, total Kjeldahl nitrogen, and total nitrogen] and turbidity. Additionally, storm-event samples were taken every 6 h through and after a series of major rain-event in September 2016 to determine the wetlands' effect during short-term pulsing events. From May 2016 through February 2017, 948,000 m3 were pumped in from the stormwater ditch and from the downstream Gordon River, resulting in an average hydraulic loading rate (HLR) to the wetland/pond system of 51.3 cm week−1. Overall, the Freedom Park wetlands are significant sinks of nutrients with phosphorus concentrations decreasing by 55% and nitrogen concentrations by 26%. There are statistically significant decreases between the inflow and outflow of total phosphorus (inflow = 0.111 ± 0.023 mg-P L−1; outflow = 0.051 ± 0.010 mg-P L−1 p < 0.01) and total nitrogen (inflow = 1.165 ± 0.088 mg-N L−1; outflow = 0.862 ± 0.051 mg-N L−1 p < 0.01). Six hours after the September 2016 storm event, there was a temporary increase of total phosphorus (p = 0.01) to 0.614 mg-P L−1 and total nitrogen (p < 0.01) to 3.106 mg-N L−1 at the inflow of the system followed by a decrease to ambient levels 12 h afterwards. The wetland system appears to be working efficiently in removing nutrients for both high and low input concentrations and has been doing so for almost a decade. Processes that may be responsible for the decreases in nutrients-specifically sedimentation and vegetation uptake-and the difference between wet and dry season concentrations are discussed including ways to enhance these processes and the hydrology to optimize future stormwater nutrient retention.