Comparison of long-term phosphorus removal performance of two large-scale constructed wetlands in South Florida, U.S.A

Abstract

The Everglades Stormwater Treatment Areas (STAs) have been constructed as a component of the Everglades restoration in southern Florida, U.S.A. These large-scale treatment wetlands function to remove excess phosphorus (P) from agricultural and urban runoff. The STAs can receive variable inflow hydraulic and P loading in response to storm events or regional droughts and are operated under state and federal permits. An evaluation of the long-term P removal performance for two of the best performing STAs, STA-2 and STA-3/4 is presented along with the challenges in managing these large wetlands.The trends in the inflow loadings and P retention over the period of record for STA treatment flow-ways were examined on an annual and wet/dry seasonal basis. Daily water depths and weekly outflow P concentrations were also examined. The period of record average annual inflow hydraulic loading rates were relatively low (2.0–3.8cmd−1) and the average annual P loading rates were low to moderate (0.6–1.6gm2yr−1) compared to the other STAs, although variable seasonal and annual loadings were observed. Despite receiving variable hydraulic and P loads, these STAs have reduced surface water TP concentrations from 100 and 87μgPL−1 at inflow structures to 23 and 18μgPL−1 at outflow structures in STA-2 and STA-3/4, respectively. The outflow concentrations did not consistently respond to changes in the inflow loading rates, suggesting that the P assimilative capacity of the wetlands may not be exceeded on a long-term basis. The occasional elevated outflow TP concentrations indicate that internal physical processes, such as sediment transport or resuspension, may have occurred or the short-term phosphorus assimilative capacity of the wetland was temporarily exceeded, but it appears as though the long-term mechanisms related to biogeochemical processes were not affected.