Phosphorus retention within a relic agricultural ditch in a constructed wetland

Stormwater treatment areas (STAs) are constructed wetlands established to capture phosphorus (P) from agricultural runoff before reaching the Florida Everglades. Retained P is primarily stored in wetland soils and sediments generated through a collection of interrelated physical, chemical, and biological processes. The amount of P and other elements (Al, Ca, Cu, Fe, Mg, Mn, Pb, and Zn) retained in the flocculent (floc) and recently accreted soil (RAS) horizons from a relic agricultural ditch within Cell 4S of STA-1E were compared with the surrounding marsh soils (upstream and downstream sites of the ditch). The amount of P retained in the ditch was significantly greater than the surrounding marsh soils and for all the elements in the floc horizon and five of the nine elements in the RAS horizon, suggesting that different processes or process rates influenced accumulation. Phosphorus species in the floc and RAS sediment horizons were identified and quantified using 31 P nuclear magnetic resonance (NMR) spectroscopy and total P determined by microwave plasma-optical emission spectroscopy. In general, P forms were dominated by orthophosphate, sugar phosphates, nucleotides, DNA, and pyrophosphate, with varying relative abundances of species. Total P concentration significantly decreased from upstream to downstream of the ditch by an average of 28 and 35% for floc and RAS soils, respectively. The relatively high P accrual rate within the ditch suggested that relic ditches perpendicular to flow could reduce P transport to downstream soils and sediments and, in turn, help maintain low P levels in overlying water.