In 2005, the St. Johns River Water Management District constructed a regional stormwater treatment facility (RST), consisting of a pond and subsurface wetland, to treat nutrient-enriched agricultural runoff within a priority sub-basin of the lower St. Johns River. This effort was in support of the 37% nitrogen and phosphorus reductions required to meet the Total Maximum Daily Loads as mandated by the Clean Water Act for the Florida Department of Environmental Protection’s 303 D listed impaired waters. The proposed RST wetland site was formerly used in agricultural production and the sandy, mineral soils were found to have elevated phosphorus levels.  This was of particular concern, due to the potential for release of stored phosphorus under anaerobic conditions upon wetland flooding.  A soil phosphorus storage capacity (SPSC), typically used in agronomic settings, was calculated to estimate the amount of phosphorus stored in the soil and its remaining phosphorus absorption capacity.  Results of the SPSC value indicated the soil would likely be a source of P rather than a sink within the top 12” of the soil profile.  To bind existing P, as well as provide additional binding sites within the soil profile, approximately 25 tons/acre of alum water treatment plant residual was surface applied and incorporated into the top 12” of soil.  Results from the first soil sampling event 27 days after alum application indicated the soils had not reached equilibrium, and subsequently there was still a potential to serve as a source of P; however, there was a 95% reduction in the amount of P that could be released from the top 6” surface and an overall 83% average reduction in the top 12”.  Interpretation of results and implications for use of the SPSC in this regard will be discussed.