Stability of phosphorus within a wetland soil following ferric chloride treatment to control eutrophication

The addition of iron and aluminum compounds has become an increasingly popular method to regulate phosphorus eutrophication in lakes and reservoirs. It has been proposed that addition of ferric chloride to wetlands, such as the northern Everglades, could provide a means for enhancing the natural mechanisms of phosphorus removal from agricultural runoff entering the wetland. The objectives of this study were; (1) to evaluate the potential for phosphorus retention by ferric chloride addition in eutrophic areas of the northern Everglades; (2) to determine if the precipitated ferric phosphate or phosphate adsorbed to iron oxyhydroxide complexes is subsequently released by iron reduction over a 139-day study period; and (3) to determine whether the addition of ferric chloride would prove a viable method for reducing phosphorus concentrations in the Everglades in conjunction with natural phosphorus removal methods. To accomplish these objectives, ferric chloride was added to Everglades water spiked with radiolabeled P, and the resulting precipitates were incubated in microcosms simulating the natural Everglades ecosystem. Throughout the course of the experiment, 32P activity in the overlying water column and reduction-oxidation potentials were monitored as precipitates were buried to determine if 32P was released due to iron reduction. The results of redox potential measurements and 32P activity indicate that although iron reducing conditions exist below the first few millimeters of sediment, on average less than 1% of the added 32P was measured in the water column during the 139-day incubation. The blanket of ferric oxyhydroxides thus proves effective in preventing release of phosphorus into the overlying water column, suggesting that ferric chloride addition could serve as a means of long-term phosphorus retention in the northern Everglades, and perhaps other wetland systems as well.