The Orlando Easterly Wetland is one of the oldest and largest constructed treatment wetlands in the US, located in Christmas, FL. Recently, phosphorus (P) concentrations have increased during the winter months resulting in concern over the P binding capacity of the soil. One method to increase the P binding capacity of wetland soils is through the addition of alum. A 1-year field study was performed in adjacent treatment cells to determine the effectiveness of alum in immobilizing P in a continually nutrient-loaded, Typha spp. system. Water column pH, soluble reactive P (SRP), total dissolved P, total P, and DOC were measured. Soil characteristics, including the size and activity of the microbial pool, plant biomass, and nutrient content were determined prior to alum initiation and every four months thereafter.

Mean outflow SRP from the alum-treated cell (0.12 ± 0.09 mg L^-1) was lower than the control (0.29 ± 0.14 mg L^-1) with corresponding removal efficiencies of 36 and 8%, respectively. By study completion, total Al concentrations were 9 times greater, and total P concentrations 5 times greater in surface soil of the alum-treated cell. No impact on surface soil pH was evident, averaging 6.5 ± 0.5 over the course of the study. Microbial biomass and activity also remained relatively unaffected by alum application. Typha growing in the alum-treated cell had significantly lower total P and N concentrations during the 4-month winter and 12-month summer samplings, as well as producing fewer leaves per plant than the control cell, suggesting nutrient limitation. The use of a continuous low-dosage alum system in treatment wetlands prevents P release from the soil back into the water column while continuing to bind influent P as the alum settles through the water column. However, treatment may negatively impact soil characteristics, in turn affecting the emergent macrophytes present.