LAUSR

Abstract The Everglades Stormwater Treatment Areas (STAs) are large-scale freshwater wetlands constructed to reduce total phosphorus (TP) concentrations in runoff to support restoration of the Everglades. TP reduction and retention in STAs occurs through settling, plant growth and accumulation of dead plant material in a layer of peat. This study evaluated the performance of six treatment flow-ways in STA − 2 and STA-3/4, two of the best performing Everglades STAs, through water and TP budget analyses based on over a decade of hydrological, meteorological, and water quality data. STA performance was evaluated by comparing annual and long-term average annual flow-way and treatment cell TP flow-weighted mean outflow (FWMC) concentrations, TP load retention percentage, and TP FWMC reduction percentage. The effects of annual hydraulic loading rate, annual phosphorus loading rate, annual average hydraulic residence time, inflow TP FWMC and annual average water depth on annual flow-way TP FWMC were evaluated. For the six flow-ways studied, a higher frequency of low-level outflow TP annual FWMC was achieved when the annual hydraulic residence time was longer than 14 d, annual hydraulic loading rate was less than or equal to 3.5 cm day−1, the annual water depth was shallower than or equal to 0.65 m, the annual phosphorus loading rate was smaller than or equal to 1.2 g m2 yr−1, or the annual inflow TP FWMC was less than or equal to 100 μg L−1. This study demonstrated that TP settling rate alone is not adequate to evaluate STA treatment performance especially when hydraulic loading rates differ. This study also helped improve understanding of the factors that affected the treatment performance of large-scale constructed wetland flow-ways which consistently retained TP and reduced TP concentrations over a long-term operational period.