Phosphate Retention in a Constructed Stormwater Wetland with Low Total Suspended Solids Concentrations

Abstract

Although constructed stormwater wetlands (CSWs) have been shown to treat stormwater runoff, the mechanisms controlling these water-quality improvements need further study. This research explores the fate of phosphate through a .3 ha CSW. Results show that the median phosphate concentration progressively decreases as the water travels from the inlet (.66 mg/L) to the outlet (.22 mg/L) under baseflow conditions but no decrease in median phosphate concentration is observed during storm events. No correlation between phosphate and total suspended solids (TSS) concentrations is observed, presumably because of the relatively low TSS concentrations entering the CSW (median inlet TSS concentration <12 mg/L). Soil samples show elevated phosphate concentrations in the submerged region of the CSW compared with the unsubmerged area. Phosphate accumulation on soils does not correlate with travel path through the CSW or with depth of water, but phosphate concentrations positively correlate with increasing soil organic content. Although batch sorption experiments indicate that the surface sediment in the submerged regions of the CSW is at or near saturation with respect to phosphate, no decrease in phosphate removal is observed. It is hypothesized that continued phosphate removal is at least partially a function of decaying vegetation in the CSW causing a continuous amount of the organic matter to deposit on the surface of the submerged sediment layer (and thus providing a continuous reservoir for phosphate removal from the water column).