THE APPLICATION OF ALUM RESIDUAL AS A PHOSPHORUS ABATEMENT TOOL WITHIN THE LAKE APOPKA RESTORATION AREA

Lake Apopka, the fourth largest lake in Florida, is considered one of the most severely polluted lakes in the state. As part of the Lake Apopka restoration program, approximately 13,000 acres of muck (organic soil) farmland within the North Shore Restoration Area (NSRA) are being restored to marsh habitat to reduce external phosphorus (P) loading to Lake Apopka. In addition, the first 650 acres of the Lake Apopka Marsh Flow-Way (MFW), designed to filter particulate nutrients from Lake Apopka, has been constructed. The treatment wetland will be 3,400 acres when completed.High phosphorus flux from the soil is expected to occur during initial reflooding of the highly organic soils of the NSRA and MFW. Although chemical treatment has been successful in lake restoration programs, large-scale soil amendment application in wetlands for phosphorus immobilization has not been done. If successful, the initial efficiency of wetland treatment of polluted waters will be greatly improved.The St. Johns River Water Management District evaluated various chemical compounds and other materials for their ability to reduce P flux from the sediments and thus reducing water column P concentration. A variety of materials were tested in laboratory and small plot experiments. Based on these results a field scale experiment (three two-acre plots) was used to evaluate the effectiveness of calcium hydroxide (Ca(OH)2), gypsum (CaSO4), and alum residual from a potable water treatment plant (WTR) to reduce soil P flux. The amendments were surface-applied to hydrologically isolated cells. After soil treatment, the enclosures were shallowly flooded and maintained at a water depth of approximately 25 cm. WTR strongly reduced TP levels in the floodwater compared to the control cell. Gypsum and lime were not as effective in reducing TP concentrations in the water column. WTR was selected as the most cost-effective soil amendment for large-scale application. WTR was subjected to extensive tests including P adsorption capacity, Toxicity Characteristic Leaching Procedure (TCLP), Synthetic Precipitation Leaching Procedure (SPLP), chemical characterization, and biological assays prior to use.Approximately 52,610 wet tons of WTR were hauled (100 miles one-way) from Melbourne FL to the application site just north of Orlando, FL between March and May of 1999. Another 13,500 tons were hauled in 2002. Approximately 2,000 acres were amended at a rate of 6.5 wet tons per acre between March and June of 1999. During the summer of 2000 650 acres in the Marsh Flow-Way were amended at a rate of 10 wet tons per acre. Approximately 57,000 tons are currently stockpiled on site. The total cost for hauling and spreading alum residual up to this point has been 1.7 million. Initial reflooding began on a small area of the NSRA in 2002. The 650 acres of the MFW will be flooded in early 2003.