Phosphorus retention in soils from a prospective constructed wetland site: Environmental implications

The phosphorus (P) retention characteristics of soil layers are critical for predicting the effectiveness of a constructed wetland, built in high water table soils, in reducing P transport to the receiving water body. Soil samples were collected from surface and subsurface horizons for P sorption and mineralogical studies directed to the assessment of a proposed site for stormwater treatment area (STA) construction in the Okeechobee Basin, Florida. The P sorption maxima were positively correlated with oxalate-extractable aluminum (ox-Al) and citrate dithionite bicarbonate-extractable aluminum (CDB-Al) under anaerobic conditions, but there was no significant correlation with either ox- or CDB-extractable iron (Fe). The ox/CDB ratio for extractable Fe was relatively high (>0.5), indicating that most of the Fe was in noncrystalline form, i.e., highly reactive and prone to rapid dissolution under reducing conditions. The X-ray Diffraction (XRD) patterns indicated that smectite was the dominant mineral in the clay-size fraction. The permanent negative charge of smectite limits contribution to P sorption. The equilibrium P concentrations (EPC0) of the soils were significantly correlated to CDB-Al but not to CDB-Fe, suggesting that Al-associated P is involved primarily in elevating EPC0 in these soils. Both P sorption capacity and EPC0 were fairly low for soils from most locations, indicating that the proposed STA cannot remove P from the rerouted water to a significant extent under ambient conditions and would not also cause significant P loading to the water table aquifer. Thus, alternatives for effective P sequestration, such as establishment of vegetative communities or chemical amendment, would be required.