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Wetland Soils

Sediment Inventory and Phosphorus Fractions for Water Conservation Area Canals in the Everglades

^a Univ. of Florida, Everglades Research and Education Center, Institute of Food and Agric. Sciences, 3200 E. Palm Beach Rd., Belle Glade, FL 33430
^b Univ. of Florida, Soil and Water Science Dep., Institute of Food and Agric. Sciences, P.O. Box 110510, Gainesville, FL 32611

^* Corresponding author (oadiaz{at}ifas.ufl.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Nutrient loading from the Everglades Agricultural Area and nearby urban communities plus water flow rate and canal size have significantly influenced the amount of sediment and phosphorus (P) pools stored in the Water Conservation Area (WCA) canals in the Everglades. A study was conducted to characterize the potential impact that sediments might have on the overlying water column by conducting an inventory of total P (TP) and major P forms in sediments of all major canals in the WCAs. Sediment samples and sediment depth measurements were taken at transects every 1.6-km along all canals reaches. A total sediment volume of about 6.8 million m³, with a P mass of approximately 1808 Mg was estimated to be stored within all WCA canals, with the eastern canal accounting for 71% of the total sediment volume and about half of TP mass. Phosphorus fractions associated with Ca- and Mg-compounds and residual organic P (Po) were the dominant forms stored in these canals, with the greatest P mass observed in the western side of the WCAs. These results indicates that >80% of the TP mass stored in surface sediments in the WCAs is fairly stable, and represent an important long-term sink for P. Canal sediments from the eastern side of the WCAs were low in bulk density, highly organic and more susceptible to resuspension and transport during strong drainage events. These sediments showed higher Fe- and Al-bound P and organic-bound P fractions, making them more susceptible to changes in the redox potential of the sediments that could result in the long-term release of Fe-bound P to the overlying water column.

Abbreviations: MCS, Miami Canal South • MCN, Miami Canal North • Pi, inorganic phosphorus • Po, organic phosphorus • STA, Stormwater Treatment Area • TP, total phosphorus • WCA, Water Conservation Area

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THE FLORIDA EVERGLADES is a peat-base subtropical wetland system that is characteristically oligotrophic (Noe et al., 2001) and P limited (Koch and Reddy, 1992; McCormick et al., 1996). Phosphorus concentrations in the water column of the least impacted Everglades are typically <10 µg L^–1 and soluble reactive P concentrations are often less than detection limits (Walker, 1999). However, major hydrologic modifications during the last 100 yr, such as, the construction of more than 2500 km of canals and levees and hundreds of water control structures have disrupted the natural sheet flow and significantly altered the hydroperiod in large areas of the Everglades (Light and Dineen, 1994). Increased nutrient loading from anthropogenic sources has significantly impacted many areas of this nutrient-limited ecosystem, especially the northern Everglades (Newman et al., 1997; SFWMD, 1992).

In an attempt to correct these environmental changes, restoration efforts have focused on reducing nutrient loading, especially P, and restoring a more natural hydroperiod to sensitive wetland areas such as the WCAs and the Everglades National Park. Part of this effort has included the implementation of Best Management Practices in the Everglades Agricultural Area since 1995 that resulted in average annual P load reductions of >50% compared with baseline concentrations (Daroub et al., 2003; Sievers et al., 2003). To reduce P loads even further, a total of six Stormwater Treatment Areas (STAs) encompassing about 16 000 ha were built, with some already in operation. The main purpose of the STAs is to filter water coming from agricultural and urban areas to remove excess P before discharge into WCA canals (Guardo et al., 1995; Chimney and Moustafa, 1999). Treated water from the STAs discharges into an extensive network of canals for distribution throughout the WCAs. The ultimate goal of farm Best Management Practices and these constructed wetlands is to deliver water of low P concentration to the downstream ecosystems. However, historic P loading to canals and resulting P flux from accumulated sediments to the water column has come into question as a potential new source of P to canal waters and ultimately the downstream ecosystems.

Sediment P has been the subject of a number of studies due to its role in eutrophication, particularly in shallow water bodies (Bostrom et al., 1982). An understanding of the physical and chemical properties of sediments stored within a system is important for a number of environmental issues, including internal nutrient cycling and release to the overlying water column, substrate stability for macrophyte establishment, and potential for resuspension that can affect light availability for primary production and eventual transport to downstream ecosystems (Eyre and McConchie, 1993). The quantity of P stored in sediments is great compared with P in the water column. This means that even if only a very small amount of P is released from sediments, it can have a significant impact on P concentrations in the water column (Bostrom et al., 1982). One approach to characterize the potential impact that sediments may have on a particular system is to conduct a quantitative inventory of TP and major P forms stored in the sediments.

Sediments play an important role in P cycling of shallow water bodies such as lakes, wetlands, and streams. Studies have shown that sediments can function as sink or source for P depending on water column and sediment physicochemical properties (Reddy et al., 1995; Richardson, 1985). The distribution of P forms in sediments has been investigated since the 1950s when Chang and Jackson (1957) presented an extraction scheme for soils that was later adapted to investigations of lake sediments (Williams et al., 1971). With this method as a base, a number of other modifications have been presented (Hieltjes and Lijklema, 1980; Psenner et al., 1988). In recent years, a modification of this fractionation scheme has been adopted for wetland soils (Qualls and Richardson, 1995; Reddy et al., 1998). These methods involve sequential extractions of soils or sediments with KCl, NaOH, and HCl, with each chemical removing discrete forms of P. Major P pools identified in these schemes are: loosely adsorbed P, Fe- and Al-bound P, Ca-and Mg-bound P, alkali-extractable Po, and residual Po.

Previous studies in the EAA have demonstrated that farm canals have a significant impact on TP loads discharged from agricultural farms (Stuck et al., 2001). Phosphorus studies in the EAA have suggested that the bulk of the exported particulate P is sourced from biotic material growing in farm canals (Stuck et al., 2001, Daroub et al., 2003). We hypothesize that canals in the WCA distribution network downstream of STAs could have a similar impact, in view of the low P (10 µg L^–1) water concentrations that will ultimately be discharged into them. This study was undertaken in an effort to evaluate dominant P forms and potential impact of existing sediment material in the major WCA canals on low-P water discharged from existing and future STAs. Objectives of this study were to: (i) determine the current bottom topography (bathymetry) of major canals in the WCAs, (ii) conduct a total inventory of sediment material stored in each canal, and (iii) quantify forms of inorganic (Pi) and Po-pools in sediments from major canals in the WCAs.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Sediment Inventory
Site Description and Sampling Techniques
The canal reaches selected for this study characterize approximately 196 km of canals that are or will be used to transport treated water discharged from current and proposed STAs (Fig. 1 ; Table 1). Canals were grouped by location into eastern (L7, L39, and L40), central (L5, L6, and L38), and western (Miami Canal North [MCN] and Miami Canal South [MCS]) to evaluate general trends across the WCAs. Transect locations at each study site were marked by GPS coordinates using a Trimble Unit Pro-XR DGPS Unit (Trimble Navigation Limited Mapping & GIS Systems, Sunnyvale, CA). The bathymetry of each canal cross-section was surveyed at a spacing of 1.6 km down from the upstream end of each canal reach (Fig. 1). At each transect location, two steel rebars were installed at the edge of the water on each side of the canal. At the time of sediment sampling or measurement, a calibrated steel cable was attached to the anchor rebars. A boat used during measurements was anchored to the cable by clamps in a position directed into the canal flow. Distance from the reference anchor rebar was determined using the calibrated steel cable. Sediment samples were taken using an Ogeechee sediment core sampler at the canal centerline and at locations 25% of the canal width from each bank. Sediment cores were transported to the University of Florida Everglades Research and Education Center for description, sectioning, and analysis. Sediment surface elevation and sediment depth at each transect were measured with a calibrated submersible footpad and a calibrated steel rod. The footpad was lowered until it rested on the sediment surface to determine the depth of the sediment surface below the water surface. The calibrated steel rod was pushed into the sediment manually. Sediments accumulated in these canals are fairly soft, opposing little resistance to the calibrated steel rod. The depth of penetration of the steel rod into the sediment was determined until the steel rod reached the bedrock at the bottom of the canal. Sediment depth readings were taken at 1.5-m increment across all transects. Reported sediment volume from L5 corresponds to about one third of the canal length as the canal was not measured in its entirety due to some work being done as part of the STAs 3 and 4-canal network. All WCA canals were originally dug to the bedrock during the late 1950s (Light and Dineen, 1994). Thus, sediment material in the inventory is defined as particles of different size, shape, and chemical composition that have been deposited through time on top of the bedrock at the bottom of the canals.

View larger version (90K): [in this window] [in a new window]   Fig. 1. Canal reaches and sampling locations in the Water Conservation Areas in south Florida.

Sediment Phosphorus Forms
Sampling and Analysis
Twenty transects from seven major canal systems in the WCAs were selected and sampled during the summer of 2001 (Fig. 1). Transects used for the P fractionation analysis were different from transects used for sediment inventory to avoid any disturbance of the surface sediment layer caused during depth measurements. At each transect location triplicate intact sediment cores were collected (as described above) within the middle two thirds of the canal cross-sectional area. However, due to differences in water depths, bathymetry of the bottom, and variation in sediment thickness, core collection was often skewed toward one bank or another. Sediment cores were collected using a piston core sampler (Fisher et al., 1992). Cores were sectioned on site into 0- to 10- and 10- to 30-cm increments, stored in polyethylene bags on ice, and transported to the University of Florida Wetland Biogeochemistry Laboratory in Gainesville for further analysis. Data from the 0- to 10-cm depth of the P fractionation study is presented in this paper.

Phosphorus Fractionation
The P fractionation procedure was a modification of the procedure described by Hieltjes and Lijklema (1980), and adopted for wetland soils (Reddy et al., 1998). Field-wet subsamples (0.5 g dry weight equivalent) were sequentially extracted with 1 M KCl (labile P), 0.1 M NaOH (Fe and Al-bound P, and alkali extractable Po), and 0.5 M HCl (Ca and Mg-bound P). The residue from the above extraction was combusted at 550°C for 4 h and the ash dissolved in 6 M HCl and analyzed for TP. Total P from the ash procedure is assumed to be residual Po. The NaOH extracts were analyzed for both TP (Method 4500-P, APHA, 1998) and Pi after a 0.45-µm filtration (Method 365.1, U.S. Environmental Protection Agency, 1983). These fractions are referred to as NaOH-TP and NaOH-Pi, respectively, with NaOH-Pi representing the Fe- and Al-bound P. The difference between NaOH-TP and NaOH-Pi was assumed to be Po (NaOH-Po) associated with humic and fulvic acid (Qualls and Richardson, 1995; Reddy et al., 1998). Phosphorus analyses were conducted using an auto analyzer (Method 365.1, U.S. Environmental Protection Agency, 1983).

Data Analysis
Descriptive statistics of means, standard deviation and standard errors (proc MEANS), regression (proc REG), and correlation analyses (proc CORR) were performed using the SAS statistical program (SAS Institute, 1999).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Sediment Physicochemical Properties
Canal sediments showed a wide range in selected physicochemical properties throughout the WCAs (Table 2). Bulk density values from these canal sediments are much less than typical bulk density from mineral soils. In general, bulk density increased with depth at all locations, with the least values observed in the sediments of the L7N and L40N canals. Low bulk density values in surface sediments in these two canals were probably due to the contribution of detrital matter from aquatic vegetation. In contrast, the greatest bulk density values were observed in sediments from the MCN that were characterized by lower organic matter content and higher mineral matter. In general, organic matter and TP decreased with depth at all locations (Table 2). Organic matter and bulk density were inversely correlated, so that sediments with more mineral content and less organic matter had greater bulk density. Average TP concentrations from surface sediments ranged from 258 mg kg^–1 in sediment samples from the L6 canal to 1700 mg kg^–1 in samples from the MCS.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Sediment depth profiles of main canals in the Water Conservation Areas of the Everglades.

Higher sediment accumulation in the eastern canals is probably the result of a combination of factors such as flow, canal size, and nutrient loading from drainage waters coming from the Everglades Agricultural Area and adjacent urban areas (Walker, 1999). Historical data shows that TP concentrations of inflow water coming into the Everglades Protection Areas generally shows a decreasing north to south gradient, with the highest concentrations measured in the inflow to canals around WCA-1, with concentrations decreasing in the canals from the central and western side of the WCAs (Noe et al., 2001; Payne and Weaver, 2004). Higher TP loads increase the productivity of these canals resulting in higher amounts of particulate P that is being deposited through mechanisms such as biological uptake, sorption, and precipitation (Stuck et al., 2001). In addition, these canals have a larger cross-sectional area, allowing a longer flow residence time, thus, increasing the probability of particulate matter accumulation.

Sediment depth and accumulation in main WCA canals also showed variability from north to south (Fig. 2). Sediment accumulation in the eastern canals was considerable larger in the northern sections, where the cross-sectional area of these canals (L7N and L40N) was larger and exposed to few drainage events during the year (Table 3, Fig. 2A and 2B). The opposite was observed in the L38 canal where the greatest sediment accumulation was observed in the wider southern section of the canal (Fig. 2C). Total sediment accumulation and physical and chemical properties from the Miami Canal were notably different from north to south. The Miami Canal section north of structure S-8 was deepened in the late 1950s to provide better flow conveyance from the EAA to what is now WCA-3 (Light and Dinnen, 1994). This section of the Miami Canal has a considerable larger cross-sectional area than the southern section allowing longer flow residence time, thus, increasing sediment deposition (Table 3). Sediments from this canal section were high in TP and mineral matter as reflected in the higher bulk densities and TP storage when compared with the rest of the canals. On the other hand, sediment accumulation in the MCS was less and variable along the entire canal. The cross-sectional area of the MCS is considerably smaller than the north section, which reduces the probability of sediment accumulation, especially for the first 9 km south of structure S-8 (Fig. 2D). Sediment accumulation was considerable higher north of structure S-339 and south of structure S-340, which are two gate-sheetpile barrier dams constructed to hold water in the adjacent marshes during low flow periods (U.S. Army Corps of Engineers, 1992).

Phosphorus Storage in Canal Sediments
Sediments from the major canals in the WCAs play a major role in P storage from agricultural drainage waters imported into the conservation areas. It has been estimated that more than 90% of P in drainage waters entering the Everglades Protection Areas from the EAA is removed within the WCAs and associated canals before entering the Everglades National Park (Rudnick et al., 1999). Estimates from this study shows that the surface 12-cm sediment layer of all canal reaches downstream of the STAs contains a TP mass of 217 Mg (Table 3). Canal reaches upstream of the STAs have an additional 18 Mg of P. Detailed methods and results related to these data are presented in Daroub et al. (2003). Total P mass calculated for the entire sediment profile in all canal reaches downstream and upstream of all STAs is estimated to be in excess of 1800 Mg P (Table 3). The importance of total sediment P accumulated in major WCA canals and the potential of P flux into the water column becomes more critical as the deadline approaches to achieve the goal set by the Everglades Forever Act that proposes a TP criterion of 10 µg L^–1 for the Everglades Protection Areas by Year 2016 (Payne and Weaver, 2004).

Inorganic Phosphorus Fractions
Phosphorus stored in the labile pool is of concern because it represents the readily available pool of P. However, the KCl-Pi fraction represented <1% of the TP in surface sediments at all locations (Table 4). Other studies in South Florida have also reported low labile P concentrations, like those in the stream sediments (0.1–2.3% of TP) of the Okeechobee watershed (Reddy et al., 1995, 1996), and in the organic soils (0.3–3% of TP) of selected hydrologic units in the Everglades (Reddy et al., 1998). Low P recovery in KCl extractions was probably due to some precipitation of P as Ca-P compound during the equilibration period (Reddy et al., 1998).

View larger version (36K): [in this window] [in a new window]   Fig. 3. Distribution of sediment P-forms in transects located from north to south in the (A) L40 canal, (B) L7/L39 canal system, and (C) Miami canal. KCL-Pi = labile inorganic P; NaOH-Pi = Fe- and Al-bound P; HCl-Pi = Ca- and Mg-bound P; NaOH-Po = alkali organic P; Residue Po = residual organic P.

Organic Phosphorus Fractions
Organic P extracted by NaOH is associated with humic and fulvic acids, which is biologically reactive and can be hydrolyzed to bioavailable forms (Bowman and Cole, 1978; Ivanoff et al., 1998). The NaOH-Po fraction represented 1 to 17% of TP in all canal sediments, with the greatest concentrations measured in the L40 canal and the least concentrations observed in the sediments from the MCN (Table 4). This P fraction accounted for 22 532 kg of the calculated TP mass from all surface sediments (Table 5). The NaOH-Po fraction along the eastern canals gradually increased with distance from the inflow structures, with the greatest increases observed in the L7/L39 canal (Fig. 3B). This P fraction accounted for 7% of TP in sediments around the inflow structure G-310, gradually increasing up to 19% in sediments from the southern section of L39. In contrast, sediments from the Miami Canal were more variable with NaOH-Po values ranging from <1% of TP in sediment collected north of structure S-8, to 14% of TP in sediments in the southern section of the canal (Fig. 3C).

These results show that a considerable proportion of the TP in WCA sediments is stored in the NaOH-Po pool, especially in the eastern canals of the WCAs, in particular in the L40 canal sediments. Some studies in the Everglades have suggested that Po storage in the impacted areas of WCA-2A appears to be through vegetative uptake and subsequent accumulation via detrital tissue deposition (Qualls and Richardson, 1995; Reddy et al., 1998). The NaOH-Po fraction from this study was significantly correlated with organic matter content (r² = 0.62, p = 0.0001). The relative stability of the NaOH-Po fraction is unknown and dependent on environmental conditions regulating the rate of Po mineralization. Under hypoxic conditions, this pool is relatively stable as the rate of organic decomposition is significantly regulated by the availability of oxygen. However, some studies have measured significant organic matter decomposition under anaerobic conditions where the process is supported by other redox couples (Racz, 1979).

Residual Po at all locations represented between 21 and 35% of TP, with the greatest concentrations observed in sediments from the MCN (Table 4). This P fraction accounted for 54 491 kg of the calculated TP mass of all surface sediments (Table 5). Residual Po is considered to be highly resistant and biologically unavailable (Hieltjes and Lijklema, 1980). In eastern canals, residual Po represented up to 30% of TP in the L40 (Fig. 3A). In the Miami Canal, residual Po represented more than half of TP in sediments north of structure S-8, and an average of 23% of TP in the southern section (Fig. 3C). Phosphorus stored in this pool fraction represents a stable and long-term storage pool in this ecosystem. The residual Po fraction was the second largest storage pool, in these canal sediments after the Ca-P pool. These results indicate that >80% of the TP mass in the surface 10-cm sediment layer of all canals in the WCAs is fairly stable. However, there is still a considerable P fraction, especially in sediments stored in canals bordering the Loxahatchee National Wildlife Refuge that are more susceptible to be released with changes in redox potential and can become a source of P to the overlying water column.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Several decades of continual loading of nutrient-laden agricultural and urban drainage water to major WCA canals are reflected in the large amount of sediment P stored in these canals. The surface 12-cm sediment layer of all canals downstream of active and future STAs stores a sediment volume of about 706 000 m³ and a P mass of 217 Mg, which may play a significant role in achieving the 10 µg P L^–1 recommended by the Everglades Forever Act in 2016. The light organic sediments from the L40 canal are of special concern, because of the potential of sediment resuspension and transport by the expected higher flows velocities from a fully operational STA-1E. This could potentially mobilize P inside the least impacted areas of the Loxahatchee National Wildlife Refuge and protected areas downstream of STA-1E.

The Ca- and Mg-bound P pool and residual Po fraction were the dominant forms of P stored in these canals, with the greatest concentrations measured in sediments from the Miami Canal. The large amounts of P stored in these P fractions indicates that >80% of the TP mass in surface sediments of all canals in the WCAs is fairly stable, with these pools representing an important long-term sink for P in these canals. Moderately available P represented by P stored in the Fe/Al and humic bound Po pools were a considerable fraction of the TP mass particularly in surface sediments from canals in the eastern side of the WCAs, where it accounted for 22% of TP mass. The higher Fe-bound P fraction in the eastern canals make them more susceptible to changes in redox state that can result in potential long-term slow P release to the overlying water column. Results from this study and the concern of higher flows as STAs in the eastern side of the WCAs becomes fully operational, present a challenge for drainage flow management in these canals. The significance of the potential P load from these sediments will become even more critical as the acceptable TP concentrations in drainage water released to the Everglades Protection Areas and Everglades National Park are reduced.

	ACKNOWLEDGMENTS

 
This research was supported by the Florida Agricultural Experiment Station and a grant from the Everglades Agricultural Area-Environmental Protection District (EAA-EPD), and approved for publication as Journal Series No. R-10290.

Received for publication February 24, 2005.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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