Stability of Phosphorus within a Wetland Soil following Ferric

The South Florida Water Management District (SFWMD) has constructed a ... It has been proposed that addition of ferric chloride or aluminum sulfate to...
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Environ. Sci. Technol. 2001, 35, 4126-4131

Stability of Phosphorus within a Wetland Soil following Ferric Chloride Treatment To Control Eutrophication LINDSAY J. SHERWOOD AND ROBERT G. QUALLS* Department of Environmental and Resource Sciences, MS/370, University of Nevada, Reno, Reno, Nevada 89557

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 ferric chloride addition to agricultural runoff entering the northern Everglades could provide a means for enhancing natural mechanisms of phosphorus removal from the wetland. In this study we added ferric chloride to Everglades water spiked with 32PO4, incubating the resulting precipitates in microcosms simulating the Everglades ecosystem. 32P activity and reduction-oxidation (redox) potentials were monitored to determine if the 32P was released into the overlying water column due to iron reduction. Results of redox potential measurements and 32P activity indicate that although reducing conditions exist in the soil, on average less than 1% of the added 32P was measured in the water column during the 139-day incubation. Ferric chloride addition thus might prove an effective means of long-term phosphorus retention in the Florida Everglades and perhaps other wetland systems.

Introduction Ferric chloride and aluminum sulfate (alum) have been widely used in wastewater treatment processes to reduce phosphorus concentrations (1). The addition of these iron and aluminum compounds has also become an increasingly popular method to regulate phosphorus availability and control eutrophication in lakes and reservoirs (2). Alum and ferric chloride have been added to river water or stormwater entering lakes (3, 4) or directly to the lake (5, 6), leading to a decrease in phosphorus concentrations. The main mechanism of phosphorus removal upon addition of ferric chloride and alum involves the precipitation of metal oxyhydroxides and subsequent adsorption of phosphorus by ligand exchange (7). In the cases of lakes and reservoirs, the water body itself serves as a settling basin, not only removing phosphorus from the water column but also forming a blanket of precipitated metal oxyhydroxides covering the top layer of sediment, blocking the release of phosphorus from the sediment. Natural water in the Everglades ecosystem characteristically contained relatively low nutrient levels, with especially low levels of phosphorus (8). Since the late 1960s, nutrients in water draining from the Everglades Agricultural Area have periodically flowed into the water conservation areas of the northern Everglades, creating a nutrient enrichment gradient * Corresponding author phone: (775)327-5014; fax: (775)784-4789; e-mail: [email protected]. 4126

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in Water Conservation Area 2A (WCA-2A) extending roughly 8-10 km downstream of inflow structures (9-11). The South Florida Water Management District (SFWMD) has constructed a demonstration-scale wetland, the 1545 ha Everglades Nutrient Removal Project, to address the problem of nutrient loading from stormwater runoff associated with the Everglades Agricultural Area (12). Based on the first 3 years of operation, the Everglades Nutrient Removal Project achieved approximately an 80% reduction in total P levels to an average of about 50 µg/L total P (13). However, much lower concentrations (planning target of 10 µg/L total P) may be necessary to meet future restoration goals for the Florida Everglades (South Florida Water Management District, personal communication). It has been proposed that addition of ferric chloride or aluminum sulfate to water within the marshes of the stormwater treatment areas will improve performance and reduce phosphorus concentrations in the effluent to the desired threshold levels (14). In this scheme, ferric chloride or alum would be added to water traversing a control structure and the marsh itself would serve as the settling basin, greatly increasing the retention time and enabling smaller doses to be applied. The synergistic effect between chemical and wetland methods for phosphorus removal would provide a relatively low cost solution, and the settling and potential burial of the phosphorus complexes in the soil could provide a mechanism for long-term phosphorus removal from the water column. However, the reducing conditions present just below the soil surface of the northern Everglades could present a significant limitation on the long-term effectiveness of iron addition. It has been shown empirically that Fe3+ is reduced to the Fe2+ form in the 100-200 mV range of measured redox potential in a variety of soils (15, 16). Native phosphorus has been shown to be released as soils are reduced under anaerobic conditions after flooding (17-19). The reduction of Fe3+ to Fe2+ has been demonstrated as the mechanism for the release of phosphorus and reduction in the phosphorus sorption capacity of the soil (17, 18). The objectives of this study were to evaluate the potential for phosphorus retention by ferric chloride addition in eutrophic areas of the northern Everglades and to determine if the precipitated ferric phosphate or phosphate adsorbed to iron oxyhydroxides complexes was subsequently released by iron reduction over the course of the study period. To accomplish these objectives, radiolabeled precipitates formed by the addition of ferric chloride to Everglades water spiked with 32PO4 were incubated in microcosms simulating the natural Everglades ecosystem. Consequently, we ensured that any 32P activity appearing in the water column of the microcosms must have originated from P previously bound to the iron precipitate. Throughout the course of the experiment we measured redox potentials at fixed depths within the microcosms and monitored 32P activity to determine if reducing conditions existed, and if a significant amount of phosphorus was released into the overlying water column due to iron reduction. At the end of the experiment, saturated soil samples were withdrawn from replicate one of each treatment microcosm and subjected to a sequential fractionation procedure to determine the fate of the phosphorus not released into the water column.

Materials and Methods Sampling Location. The 1545 ha Everglades Nutrient Removal project is located near West Palm Beach, FL, bordering the northwest corner of Water Conservation Area 1. Water 10.1021/es0106366 CCC: $20.00

 2001 American Chemical Society Published on Web 09/06/2001

FIGURE 1. Diagram of treatments used in microcosm experiments. from the Everglades Agricultural Area runoff is diverted from the West Palm Beach Canal and enters into the Everglades Nutrient Removal (ENR) portion of Water Conservation Area 1. Total phosphorus concentrations of influent water into the ENR over a 3-year study period (August 1994August 1997) ranged from 66 to 201 µg/L total P, with an average concentration of 108 µg/L total P (13). Soil and water samples were taken from a location in the western portion of the ENR (longitude 26°38′44′′ north/latitude 80°26′00′′ west) in May of 1999. Composite soil samples were collected from the top 6-12 in., sealed in coolers, and shipped to the laboratory. Water was collected at the same time from approximately the same locations. Before addition to the microcosms, the soil was homogenized to ensure uniform composition. Microcosm Construction. The objective of the microcosm experiments was to determine what proportion of radiolabeled phosphorus was released back into the water column under reducing conditions. Microcosms were constructed from PVC pipe sections 15 cm in diameter and 60 cm in length and were filled with a 20 cm depth of Everglades soil collected from the sampling site (0.34 mg Fe/cm3 soil, approximately 44% C content, soil pH ) 7.2). Four treatment levels were investigated in triplicate to examine 32P release under different levels of burial (Figure 1). Ferric oxyhydroxide with adsorbed phosphate precipitates deposited on the soil surface might be subject to various fates. Precipitates might simply be buried under more iron precipitates (Fe-precipitate treatment), or they might be buried in peat soil, which accumulates at the rate of approximately 6 mm per year in the P enriched portions of the Everglades (9). Three and 6 cm burial depths were chosen to ensure that at least one treatment would be below any aerobic zone. Radiolabeled precipitates were prepared in 250 mL bottles prior to addition to the microcosms. Everglades water (pH ) 7.5-8.2, DOC ) 35 mg/L) was adjusted to an initial orthophosphate concentration of 50 µg/L and then spiked with 92.6 µCi of H332PO4 (omitted for the three control microcosms). Then 800 µL of 0.1 M FeCl3 were added to each bottle, and the solutions were stirred for 2 min and then allowed to sit for 24 h as ferric oxyhydroxide-phosphate precipitates formed. After the settling period, the contents of each bottle were filtered using Gelman GN-6 0.45 µm membrane filters, and the precipitates were washed with deionized water to remove soluble unbound phosphate. Precipitates were carefully scraped off the filter paper and added to small beakers. Radioactivity of the filtrate and that remaining on the filter paper was measured to calculate by mass balance the 32P activity actually added to each microcosm. The 32P activity remaining in the dissolved phase was only a small percentage

of the total activity added to each bottle, on average 0.14% for all treatment microcosms. To make the radiolabeled precipitate part of a larger layer of precipitate, a 46 mL aliquot of nonradiolabeled bulk precipitate (50 µg/L PO4 previously precipitated with 0.1 M FeCl3 and centrifuged to concentrate the precipitate flock) was added to the radiolabeled precipitate. The combined precipitate was then added over the top of the 20 cm of Everglades soil already present in each microcosm. Precipitates were then covered by an additional layer of soil simulating burial by cattail litter and soil deposition (controls, 3 and 6 cm burial depth treatments), or in the case of the Fe-precipitate treatment, an additional layer of nonradiolabeled precipitate to simulate the proposed continuous addition of ferric chloride to the stormwater treatment areas. Measurements of Redox Potential. Redox potentials were measured in the soil using platinum electrodes (20) permanently installed in each microcosm and a portable voltmeter. Prior to installation, redox probes were tested in a standard Eh buffer (21). Redox potential measurements were taken using a portable Orion model 250A mV/pH meter and an Orion double junction Ag/AgCl reference electrode that was inserted in the top few centimeters of the water column. All mV readings were corrected to the standard H electrode by adding +244 mV. Redox potentials for each day were averaged over the three replicates of each treatment level. Three redox probes were installed in each microcosm (except in control microcosms which contained two redox probes per microcosm) at appropriate levels for the burial depths (Figure 1). After addition of the redox probes, a 25 cm depth of Everglades water (ortho-P ) 27 µg/L, pH ) 7.5-8.2, DOC ) 35 mg/L) was added to each microcosm. The top of each microcosm was covered with aluminum foil and allowed to settle for 24 h before aeration began. The water was aerated via airstones for 10 hours a day, simulating the natural diurnal dissolved O2 cycle of the Everglades surface water (22). Microcosms were maintained at approximately 23 °C. Water Sampling and Analysis. Over the course of the experiment, water samples were withdrawn close to the soilwater interface from each treatment microcosm to determine 32P activity, P concentration (see Figure 1, Supporting Information), and Fe3+/Fe2+ concentration (see Figure 2, Supporting Information) in the water column. Radioactivity. Unfiltered and 0.45 µm filtered water samples in Ecolite (+) scintillation fluid (ICN) were counted using a Beckman LS60001C liquid scintillation counter to analyze for 32P activity. Background activity in a water/ scintillation fluid blank was subtracted from both the filtered and unfiltered sample 32P activity, and the resulting activity corrected for decay to the time it was added to each VOL. 35, NO. 20, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Average particulate (A) and dissolved (B) 32P released into the overlying water column in microcosms. Activities were averaged over the three replicates of each treatment level for each sampling period and are represented as a percentage of the total 32P added to each microcosm with standard error bars. microcosm. The final filtered and particulate (filtered unfiltered activity) percentages of the added 32P were averaged over the three replicates of each treatment level during each sampling period. Soil Sampling and Analysis. Saturated soil samples were taken from replicate one of each treatment microcosm on day 164, 165, or 166 from treatment addition and placed into plastic bottles that were immediately purged with N2 gas. Sampling increments for the three treatment microcosms were as follows: Fe-precipitate treatment: every 0.5 cm from the surface to 3 cm, then every 1 cm from 3 to 6 cm; 3 cm burial treatment: every 0.5 cm from the surface to 4 cm, then a 4-6 cm increment; 6 cm burial treatment: a surface to 0.5 cm and a 0.5 cm to 1 cm increment, then every 1 cm from 1 to 8 cm, giving a total of nine sampling increments for each treatment. Determination of Fe2+ and Fe3+ in Soil Extracts. Oxalate extractions of soil samples were performed in a glovebag under anaerobic conditions to determine concentrations of Fe3+ and Fe2+ (23). 32P Extraction of Soil Samples. Sequential extraction of P with NaHCO3 and NaOH in 5 mL soil samples was performed in a glovebag under anaerobic conditions using techniques modified from Chang and Jackson (24). To extract any 32P still bound to iron in the soil samples, an additional extraction procedure was performed in which 30 mL of ammonium oxalate/oxalic acid extract solution was added to each of the soil pellets remaining after the NaOH extraction procedure. Samples were shaken for 12 h, centrifuged, and filtered. The supernatant of each of the NaHCO3, NaOH, and oxalate extract were analyzed for 32P activity. An additional extraction was performed for selected depth increments with high 32P activity in which hydroxylamine hydrochloride was added to unextracted soil samples to 4128

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FIGURE 3. Average redox potentials for control microcosms. Redox potential measurements were averaged over the three replicates and depth level for each sampling date and bars indicate standard error. *One malfunctioning probe was excluded form the average of the 0-0.9 cm below peat surface data between day 103 and day 110. convert any Fe3+ to Fe2+ (26) and indicate the 32P potentially released by complete Fe3+ reduction. The supernatant was later analyzed for 32P activity. Analysis of 32P in Ashed Residues. Total 32P activity of wet soil from the original sampling bottles containing each depth increment were determined by ashing at 500 °C and solubilizing the residues in 0.1 M HCl (25). A 200 µL aliquot of a 100 g L-1 hydroxylamine hydrochloride solution was also added to the residue to facilitate reduction of any Fe3+ present in the ash. Ash residues were heated for 1 h at 90 °C and analyzed for 32P activity.

Results and Discussion 32P

Released into the Water Column. Results from the analysis of filtered and unfiltered water samples from each treatment microcosm indicate that on average less than 1% of the total 32P activity added to each microcosm appeared in the overlying water column at any time throughout the 139-day study (Figure 2). The activity of each water sample on a particular sampling date was corrected for decay to the activity at the day of 32P addition to each microcosm and then expressed as a percentage of actual 32P added to each microcosm. Results of this radio-tracer experiment indicate that phosphorus mobilization did not occur to any significant extent regardless of the burial depth of the radiolabeled precipitates. Note that values after day 100 were not significantly different from zero due to the extremely low percentage value and the increasing variability as readings approached background levels. This evidence could indicate that ferric chloride addition provides an effective barrier against phosphorus release. Redox Potential Measurements at Fixed Depths. Redox potential measurements at fixed depths indicate that conditions favorable for Fe3+ reduction to Fe2+ do exist both at the 3 and 6 cm depths as well as in the first few millimeters below the soil surface (Figures 3-6). Reducing conditions

FIGURE 4. Average redox potentials for Fe precipitate treatment microcosms. Redox potential measurements were averaged over the three replicates for each sampling date and bars indicate standard error. *Average is for all probes, except one which was removed after day 86, one which was removed after day 60 and returned after day 106, and one which was removed after day 100 and returned after day 114 due to loss of function.

FIGURE 6. Average redox potentials for 6 cm burial treatment microcosms. Redox potential measurements were averaged over the three replicates and depth level for each sampling date and bars indicate standard error. *One malfunctioning probe was excluded from the 0-0.9 cm below peat surface average over the course of the experiment.

FIGURE 5. Average redox potentials for 3 cm burial treatment microcosms. Redox potential measurements were averaged over the three replicates and depth level for each sampling date and bars indicate standard error. *One malfunctioning probe was excluded from the 0-0.9 cm below peat surface average over the course of the experiment. were also found in the top 1 cm of Everglades soil in a ferric chloride application experiment performed in the field at the same site from which the soil samples were taken (27). On average, redox potentials at the 3 cm depth in both control and treatment microcosms as well as at the 6 cm depth in treatment microcosms were found to be at or slightly below -150 mV, well into the range at which iron reduction might occur. Average redox potentials a few millimeters below the soil surface for control, Fe-precipitate treatment, and 3 cm burial treatment microcosms were also at or slightly below -150 mV. Average redox potentials at the surface of the 6 cm burial treatment were slightly more variable; however, the individual measurements were less than +120 mV (with seven exceptions), indicating that reducing conditions were present at the 6 cm depth level. Iron hydroxides have been shown to be very sensitive to changes in redox potential (17, 28). Under anaerobic condi-

tions, phosphate adsorbed to iron oxyhydroxide complexes or precipitated ferric phosphate complexes may redissolve as Fe3+ to Fe2+ reduction occurs. However, the potential release of phosphorus under reducing conditions depends not only on redox conditions under the surface of the soil but also on the solubility of the various iron oxyhydroxidephosphate complexes formed. Lack of easily mineralized organic matter could retard the development of reduced conditions and subsequent Fe reduction; however, Everglades soil is easily mineralized (29), and redox potentials indicated reducing conditions were achieved within days in the microcosms. Fe2+ has been shown to predominate at redox potentials below +120 mV, the boundary of Fe3+ to Fe2+ reduction (30). Classic experiments by Mortimer (31, 32) have shown an increase in the release of phosphorus and iron with a decrease in redox potential as oxygen is reduced near the soil-water interface in lakes. With the reduction of ferric hydroxidephosphate complexes in Mortimer’s experiments, mobilization of phosphate was seen in the water column. However, in water treatment plants where ferric chloride treatments are used to reduce phosphorus concentrations of effluent in anaerobic digesters, phosphate release has not been shown (33). A possible explanation for the lack of phosphorus release in some studies is that under anaerobic conditions, a form of ferrous phosphate (vivianite) with very low water solubility was formed (34). Consideration of the solubility product of vivianite and the observed concentrations of Fe2+ and inorganic P and the pH suggest that precipitation of vivianite was possible (35). However, the unknown extent of complexation of Fe2+ by humic acids means the concentration of free Fe2+ cannot be known exactly. Analysis of 32P in Soil Extracts. Results from the analysis of 32P activity in the water samples and redox potential measurements indicate that conditions favorable for Fe3+ reduction to Fe2+ did occur in the soil contained within the microcosms, and no significant 32P release was observed in the overlying water column. Therefore, the next question we VOL. 35, NO. 20, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. 32P Activity Remaining in the Soil (pCi/cm3 Soil) at the End of the Incubationsa sequential extraction procedure treatment

depth increment

NaHCO3 extract

NaOH extract

oxalate extract

total P (ash)

hydroxylamine hydrochloride

Fe ppt Fe ppt Fe ppt Fe ppt Fe ppt Fe ppt Fe ppt Fe ppt Fe ppt 3 cm burial 3 cm burial 3 cm burial 3 cm burial 3 cm burial 3 cm burial 3 cm burial 3 cm burial 3 cm burial 6 cm burial 6 cm burial 6 cm burial 6 cm burial 6 cm burial 6 cm burial 6 cm burial 6 cm burial 6 cm burial

surf-0.5 cm 0.5-1 cm 1-1.5 cm 1.5-2 cm 2-2.5 cm 2.5-3 cm 3-4 cm 4-5 cm 5-6 cm surf-0.5 cm 0.5-1 cm 1-1.5 cm 1.5-2 cm 2-2.5 cm 2.5-3 cm 3-3.5 cm 3.5-4 cm 4-6 cm surf-0.5 cm 0.5-1 cm 1-2 cm 2-3 cm 3-4 cm 4-5 cm 5-6 cm 6-7 cm 7-8 cm

14 ndc ndc ndc ndc ndc b ndc ndc 6 8 1 0.3 1 ndc 2 ndc 1 ndc ndc 5 ndc ndc ndc ndc ndc 2

23 0.2 0.9 ndc ndc ndc ndc ndc 2 9 4 ndc 2 ndc 0.1 8 1 1 0.8 3 ndc 0.7 ndc 0.8 ndc 0.2 ndc

23 2 ndc ndc ndc ndc ndc ndc ndc 8 6 2 0.3 ndc 2 ndc ndc ndc ndc 2 ndc ndc 3 ndc 2 6 ndc

109 27 7 4 b 2 ndc b b 60 26 13 14 8 10 16 8 7 10 10 b b b ndc 12 10 2

88 b b b b b 3 b b 25 b b b b 15 b 5 b 4 b b b b b 8 b b

a NaHCO , NaOH, and oxalate extractions were performed sequentially on the same soil sample, while total P and hydroxylamine hydrochloride 3 extractions were both performed on fresh soil samples. b Indicates analysis not performed. c nd indicates activity not detected above background.

addressed was whether the 32P was mobilized and diffused through the soil in the 3 and 6 cm depth increments, or whether ferric chloride addition formed an effective barrier, trapping the radiolabeled precipitates at the depth of treatment addition. The activities of the soil extracts (reported in pCi/cm3 soil) indicate that there was some translocation of 32P into depth increments surrounding the site of treatment addition (Table 1). For the Fe-precipitate treatment, the majority of the activity was found in the surface to 0.5 cm depth increment, although some activity diffused into lower depth increments. The amount of activity in the NaHCO3 extract sample represents the amount of readily exchangeable 32P that might be available for plant uptake or recycling and was generally only between 0 and 30% of the total 32P activity in ashed samples. The activity obtained from the ashing of soil samples represents the amount of total 32P activity in the soil, giving the largest activity in the surface to 0.5 cm depth increment. Results of 32P activity in the 3 and 6 cm depth increments indicate movement of phosphorus toward the surface from the site of treatment application. The translocation of 32P from the depth at which it was placed could have been caused by two processes: (1) release of 32P to the dissolved phase followed by diffusion to the surface where it was again bound or (2) mass flow of the low-density colloidal precipitates through the soil. The first process is the classical mechanism proposed by Mortimer for release and trapping of phosphorus at the surface of reduced lake sediments. Although this migration of 32P activity raises the question of whether phosphorus would be mobilized into the water column over a longer period of time, the results from the Fe-precipitate treatment seem to indicate that even at the surface to 0.5 cm depth increment, 32P is not mobilized into the overlying water column to any significant extent. In addition, only a small 4130

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portion of 32P in the soil was readily exchangeable, which seems to indicate little was released to the dissolved phase for diffusion. Bioturbation, which can be a means of turning over surface layers of sediments in lakes, is unlikely to occur in this soil. Sharply defined 137Cs profiles in the soils of Water Conservation Area 2A suggest bioturbation has an insignificant role (9). X-ray diffraction analysis of soil samples from field applications in the same soil, using the same ferric chloride dose, indicates that the iron was in an amorphous state, and there was no evidence of any crystalline iron compounds (such as goethite) in the iron precipitates (27). Further study could determine if the iron was reduced and then reprecipitated with phosphate to form an insoluble complex, or if the iron remained in the ferric form in a stable complex even under reducing conditions. The 32P that was released by iron reduction and dissolution using hydroxylamine hydrochloride averaged about 74% of the total P activity obtained by ashing (Table 1). Forms of Fe in Soil Samples. Results of oxalate extractions of soil from the 3 and 6 cm treatment microcosms indicate that the iron added to each microcosm (in the form of ferric chloride) was not distinguishable from the iron native to Everglades soil. It was expected that an observable change in the ratio of Fe3+ to Fe2+ would be seen at the depth of treatment addition for the 3 and 6 cm treatments; however, this change was only observed in the Fe-precipitate treatment microcosm where the additional nonradiolabeled iron precipitate was added over the top of the radiolabeled precipitate. In the surface to 0.5 cm depth increment of the Fe-precipitate treatment microcosm, the total oxalate extractable iron (Fe3+ and Fe2+) was 2.43 mg/cm3 soil and the Fe3+ to Fe2+ ratio was 26. In the depth increments far below the treatment additions, the average oxalate extractable iron was 0.34 mg/cm3 soil and the average Fe3+ to Fe2+ ratio was

7, which can be taken to approximate native iron levels in the Everglades soil of the sampling area. Fe3+ was also found to persist in the soil for at least several weeks after ferric chloride addition in an Everglades field study using the same site from which our soil was taken (27). The main objective of this experiment was to test the stability of iron oxyhydroxide with adsorbed phosphate precipitates that were buried under a layer of soil or under more iron precipitate. In this experiment, iron precipitates were not exposed to light after burial; however, during the settling process of the application of ferric chloride to water within the stormwater treatment areas, photochemical redox reactions might occur. This variable was omitted in our experiments by covering microcosms to exclude light; however, further study would be necessary to determine the effect of these potential reactions on iron speciation and phosphorus retention in the proposed application of ferric chloride to stormwater treatment areas within the Everglades. Based on behavior in laboratory microcosms simulating the Everglades ecosystem, it is believed that the addition of ferric chloride to water within the STAs would prove a viable method of reducing phosphorus concentrations. Ferric chloride addition has the ability to reduce PO4 (32P) concentrations to less than a few µg/L in conjunction with natural mechanisms of phosphorus removal, such as uptake by microbial communities and aquatic vegetation, and seems to provide an effective barrier preventing the release of phosphorus into the overlying water column even after exposure to several months of reducing conditions occurring after burial of the precipitates. Results of this research support the initiation of a larger scale longer-term field trial to test the viability of long-term P storage and retention in the STAs of the northern Everglades. This research also presents a method that might be used to study the fate of phosphorus bound under specific conditions in lake sediments. Although this research was concerned with the potential application of ferric chloride to reduce phosphorus levels in the northern Everglades, addition of aluminum sulfate (alum) is also a potential alternative. Unlike iron, aluminum compounds do not undergo reduction-oxidation reactions under anaerobic conditions. However, there is some concern regarding the effect of aluminum and sulfate ions on the plant and animal communities. Further investigation would be required to test the potential use of alum for phosphorus reduction and retention in the northern Everglades.

Acknowledgments This project was funded by a grant from the Everglades Agricultural Area Environmental Protection District and the Florida Department of Environmental Protection. We would like to thank Curtis Richardson and Phillip Bachand and for valuable suggestions regarding this project, Lea Karppi and Sean Cimilluca for collecting soil and water samples from the ENR, Jeff Johnson for information regarding the sampling site, Myun Chul Jo for the use of the scintillation counter, and Ilka Dinkelman for laboratory assistance.

Supporting Information Available Methods and data for ortho-P, dissolved organic P + particulate P, and Fe2+ and Fe3+ concentrations in the water column of the microcosms. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review February 12, 2001. Revised manuscript received July 18, 2001. Accepted July 23, 2001. ES0106366

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