Phosphorus Retention Capacity of a Pilot Stormwater Treatment Area in the Lake Okeechobee Basin.

Stormwater Treatment Areas (STAs) located strategically in the Lake Okeechobee Basin (LOB) are currently used to reduce phosphorus (P) loads to the lake.  The overall objective of this study was to document the existing soil conditions in a pilot-scale STA following construction but prior to operation. Specific objectives were to: (1) determine the maximum P retention capacity of soils under aerobic and anaerobic conditions, (2) establish the relationship between P retention and selected physico-chemical properties, and (3) estimate the maximum P storage in a pilot-scale STA within the LOB. Single point isotherms were conducted on 140 soil samples (42 locations at 0-10 and 10-30 cm depths)  to assess spatial distribution of soil P retention capability. Batch P sorption isotherms were conducted under aerobic and anaerobic conditions for selected soils along an inflow-outflow transect.  Phosphorus sorption maxima (Smax) calculated from Langmuir isotherms showed no significant differences under aerobic (mean 172 ± 89 mg/kg) and anaerobic conditions (mean 178±120 mg/kg), suggesting that the P retention capacity of the soil would not be affected under flooded conditions. The equilibrium P concentration (EPC_o), and k, a constant related to the P bonding energy were also not affected by flooding. Significant correlations were observed between Smax and extractable Ca, Fe, Al, and Mg under aerobic and anaerobic conditions. Low EPC_o values suggest that these soils are potential P sinks at low P concentrations. Using either the single point isotherm or Smax under aerobic conditions, maximum P retention capacity was estimated to be in the range 52-60 g P m^-2.   If one assumes that the STA is loaded at rate 2-5 g P m^-2 per year, these soils represent a life expectancy of over 10-30 years. Once the STA starts accreting organic matter and other particulate matter, the newly accreted material dictates the exchange of P between soil and the water column.