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Published online 3 January 2006
Published in J Environ Qual 35:141-150 (2006)
DOI: 10.2134/jeq2005.0079
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Surface Water Quality

Specific Conductance and Ionic Characteristics of Farm Canals in the Everglades Agricultural Area

Ming Chena,*, Samira H. Daroubb, Timothy A. Langa and Orlando A. Diaza

a Everglades Research and Education Center, University of Florida, Belle Glade, FL 33430
b Soil and Water Science Department and Everglades Research and Education Center, University of Florida, Belle Glade, FL 33430

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

Received for publication March 2, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSION
 REFERENCES
 
Specific conductance in farm canals of the Everglades Agricultural Area (EAA) in south Florida is an important water quality parameter that was categorized as a parameter of concern according to an observed frequency of >5% excursions over the Class III water quality criterion and needed to be addressed as a part of the Everglades Regulatory Program. This study was conducted to evaluate specific conductance in farm canals of the EAA. Specific conductance was monitored at 10 representative farms (a total of 12 pump stations) in the EAA using multi-parameter water quality data loggers, for periods ranging from 24 to 83 mo. Cation and anion concentrations were also determined. Nonparametric Mann-Kendall trend analyses and Sen's slope analysis of specific conductance were conducted to determine specific conductance trends. Mean specific conductance ranged from 0.74 to 1.68 dS m–1 and only 2 of the 10 farms were above the State Class III water quality criterion of 1.275 dS m–1. Statistically significant downward trends were observed at 3 of the 10 farms. Determination of ion compositions in grab samples at 8 of the 10 farms indicated that the major ions contributing to the increase in specific conductance in the EAA were Cl–, HCO3–, and Na+. Mean Na/Cl ratios in most of the EAA canals ranged from 0.57 to 0.78, whereas those of SO4/Cl ranged from 0.46 to 0.98. Investigation of historical data and literature indicates that elevated specific conductance in parts of the EAA is a natural phenomenon due to entrapment of connate seawater in the Everglades formation. Sulfur contributes minor increases in specific conductance in the EAA with probable sources from organic soil mineralization, ground water, Lake Okeechobee, and S fertilizers.

Abbreviations: BMPs, best management practices • EAA, Everglades Agricultural Area • WCA, Water Conservation Area


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSION
 REFERENCES
 
THE EVERGLADES AGRICULTURAL AREA (EAA) is located south of Lake Okeechobee and north of the Water Conservation Areas (WCAs) in south Florida. The EAA is comprised of an artificially drained area of approximately 283 300 ha of organic soils, which provide a rich environment for the cultivation of sugarcane (Saccharum officinarum L.), vegetables, rice (Oryza sativa L.), and sod (Rice et al., 2002). The EAA plays an important role in the Everglades water supply, either directly through agricultural drainage runoff or indirectly by serving as a conduit for large water transfers from Lake Okeechobee to the WCAs. Water is pumped from farm canals to main canals and eventually into the Everglades Protection Areas.

In April of 1994, the Florida state legislature passed the "Everglades Forever Act," which mandated a research program, to be conducted in cooperation with the EAA landowners for evaluation and implementation of P best management practices (BMPs) in the EAA. Mandatory P load reduction BMPs in the EAA started in 1995 and had been successful in reducing P loads out of the EAA. In 1997, the Everglades Regulatory Program was revised to identify water quality parameters that are not being significantly improved by BMPs and to identify further BMP strategies needed to address these parameters" (Florida Statute Section 373.4592; Everglades Forever Act, 1994). Specific conductance is among these important water quality parameters in surface waters of the EAA that was categorized as a parameter of concern for Refuge inflows of the Everglades Protection Area. Additionally, specific conductance was categorized as a potential concern for the Refuge rim canal, interior and outflows, WCA-2 interior, and WCA-2 and WCA-3 inflows (Weaver et al., 2002). Use of the 5% break point between parameters classified as potential concerns and concerns of excursions parallels the common scientific practice of allowing a 5% rejection limit in statistical analyses (Weaver et al., 2001). All the excursions were likely associated with the pumping or seepage of high ionic strength ground water into the surface water in the canals (Weaver et al., 2002). Therefore, specific conductance and ionic characteristics were targeted by the Everglades Regulatory Program.

Specific conductance is an indirect measure of the total concentration of ions (e.g., Ca2+, Mg2+, Na+, K+, Cl–, HCO3–, SO42–, and F–) in the water. Although all ions contribute to specific conductance, their valences and mobilities differ, so their actual and relative concentrations affect specific conductance. When the concentration of ions is high, specific conductance is high, and the resistance to electrical passage is low.

Changes in specific conductance beyond natural background variability can result in potentially deleterious efforts to aquatic life. Several states have adopted different numeric water quality standards for specific conductance to protect cold-water aquatic life. For example, the recommended maximum contaminant level for specific conductance in drinking water is 0.90 dS m–1 in California (Brandstetter et al., 1997). In Florida, the current state Class III water (surface waters for recreation, propagation, and maintenance of a healthy, well balanced population of fish and wild life) quality criterion, which allows a 50% increase in the specific conductance over background or 1.275 dS m–1, whichever is greater, is meant to preserve natural background conditions and thus protect aquatic organisms from stressful ion concentrations (Weaver et al., 2002).

Given natural conditions, the specific conductance of a water body is generally based on the geology of the watershed through which the water flows. Water coming in contact with soils and erodible source rock material will dissolve salts, especially when soil drainage is poor. Some rocks and soils release ions easily when water flows over them. Concentrations generally are greatest in streams draining basins with rocks and soils that contain easily dissolved minerals (Risey and Doyle, 1997). The chemistry of the surface water can be modified by precipitation and evapotranspiration, weathering of geological formation, and chemical changes brought about by biological organisms and chemical equilibria (Flora and Rosendahl, 1981).

Historical water quality data on the EAA and surrounding areas provides important information in evaluating current specific conductance status of farm canals in the EAA. Parker et al. (1955) conducted a field study from 1941 to 1943 on surface and ground water in south Florida. The Hillsboro Canal and the North New River Canal (Fig. 1 ) showed wide fluctuations in specific conductance. For example, the Hillsboro Canal specific conductance ranged from 0.22 to 1.44 dS m–1 from July through August. The North New River Canal specific conductance ranged from 0.28 to 1.04 dS m–1 during the same time period.



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  		Fig. 1. Monitoring sites for specific conductance and ionic characteristics at farm canals of the Everglades Agricultural Area, Florida.

 
The U.S. Army Corps of Engineers (1971) reported specific conductance from 1950 to 1970 in surface water in south Florida. The Hillsboro Canal showed specific conductance ranging from 0.49 to 1.11 dS m–1 for its 10th and 90th percentiles, with a median value of 0.78 dS m–1. The percentile values are consistent with results of Parker et al. (1955), but are lower than those of Gleason (1974), who reported that the Hillsboro Canal specific conductance ranged from 1.10 to 1.50 dS m–1 on 31 July 1973.

A collaborative study by the University of Florida and the USDA Soil Conservation Service, conducted in the 1940s, described an isolated area of fairly permeable rocks underlying about half of Lake Okeechobee and nearby lands to the southeast, perhaps 8 m thick and encountered at a depth of 4 to 10 m (Jones, 1948). The water from this area contained total solids concentration of 4000 to 5000 mg L–1 (Jones, 1948). In a later study conducted jointly by the U.S. Geological Survey and the U.S. Army Corps of Engineers, higher specific conductance values were found in the northern half of this area than that from the southern half (Waller and Earle, 1975).

Water quality data possess unique characteristics that may exhibit seasonal variation, which may include a seasonal fluctuant, as well as a yearly trend. This variation may be the result of a diversity of conditions, including specific agricultural land-use practices, biological activity, or sources of stream flow or sediment. Investigations on specific conductance and ionic characteristics have been conducted in certain parts of south Florida (Flora and Rosendahl, 1982; Lietz, 2000), but in general, the information on ionic characteristics of farm canals in the EAA is limited.

The objectives of this research were: (i) to characterize specific conductance and ionic concentration in farm canals that are representatives of the EAA, (ii) to determine the time-serial trend of specific conductance, and (iii) to evaluate geographical distribution of specific conductance in the EAA. These characteristics are good indicators of water sources that are potentially informative about the effectiveness of BMPs in improving the quality of water entering the Everglades Protection Areas from the EAA.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSION
 REFERENCES
 
Specific Conductance Monitoring Program in the Everglades Agricultural Area
A monitoring program was established on 10 farms (12 discharge structures) in the EAA (Fig. 1). Cropping systems, farm size, and monitoring period are presented in Table 1. Hydrolab DataSonde (series 3, 4, 4a) multi-sensor water quality data loggers (Hydrolab Corp., Austin, TX) were used to measure and record specific conductance in situ. The DataSonde units were calibrated according to instrument specifications and programmed for a 6-d run recording measurements every hour. The units were deployed at a depth of 1.0 m beneath the canal water surface. DataSonde units were replaced every 6 d and data stored electronically for future analysis. A post-run assessment for drift of the instrument's sensors was subsequently conducted. The instruments were then cleaned, maintained, and recalibrated in the laboratory and returned to the field for deployment during the subsequent monitoring cycle. All field and laboratory activities strictly followed relevant Standard Operating Procedures (Chen, 2001). Quality control criteria for specific conductance regarding sensor drift and biofouling were <±0.1 dS m–1 at 1.413 dS m–1 (0.01 M KCl).


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  		Table 1. Background information on size, cropping system, and specific conductance monitoring dates of 10 representative farms in the Everglades Agricultural Area, Florida.

 
Ion concentrations in grab samples were determined to identify the major ions that are associated with specific conductance in the EAA. Grab samples were taken during 2001–2002 adjacent to DataSonde locations. Analyses for Na+, Ca2+, Mg2+, and K+ concentrations were conducted via an atomic absorption spectrometer (SpectrAA 220 FS, Varian, Springvale, Australia) using USEPA Method 200.0 (USEPA, 1983). Analyses for Cl–, SO42–, and F– were conducted using ion chromatography specified by USEPA Method 300.0 (USEPA, 1983). A Dionex DX-500 ion chromatograph (Dionex Co., Sunnyvale, CA), equipped with an injection valve, a sample loop, guard column, and ion separator columns, was chosen to measure anion concentrations. Bicarbonate anion was not analyzed in our monitoring program, since it was used as the eluent in the ion chromatography determinations. However, we did calculate HCO3– ion concentrations as the difference between the sum of the measured cations and anions.

Statistical Data Analysis
Histogram analyses and goodness-of-fit tests were conducted to check the distribution patterns of specific conductance and other parameters measured (Gilbert, 1987). Summary statistics were conducted to assess significant differences of parameters (SAS Institute, 1999). A box and whisker plot was then used to display visual summaries (site by site and aggregates) of: (i) the center of the data (the median = the centerline of the box), (ii) the variation or spread (interquartile range = the box height), (iii) the skewness (quartile skew = the relative size of box halves), and (iv) presence or absence of unusual values (outliers and extreme values).

Monthly averages of specific conductance for 10 farms were used for time series trend analysis. The upward and downward trends with time were evaluated using a nonparametric Mann-Kendall test using the ChemStat 6.0 software (Starpoint Software, 2005). The Sen's slope test was also used to detect yearly trends (Lietz, 1996; Gilbert, 1987).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSION
 REFERENCES
 
General Characteristics of Specific Conductance in Farm Canals of the Everglades Agricultural Area
Specific conductance in farm canals of the EAA varied from 0.32 dS m–1 at UF9205A to 2.75 dS m–1 at UF9208A, with overall mean value of 1.15 dS m–1 (Table 2). This mean value is close to the high end of the data reported by Parker et al. (1955), although at the low end of the data reported by Gleason (1974). The median is close to the mean and the coefficients of the kurtosis and the skewness are close to zero, indicating that the specific conductance data are normally distributed (Gilbert, 1987).


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  		Table 2. Summary statistics for specific conductance at farms canals of the Everglades Agricultural Area, Florida.

 
Summary statistics of specific conductance in canal water at the 12 discharge structures in the EAA based on the monthly data show that the mean of specific conductance at the 12 discharge structures decreased in the following order (in dS m–1): UF9208A (1.68) > UF9206B (1.57), UF9206A (1.51) > UF9201A (1.17), UF9207A (1.09), UF9207B (1.09), UF9204A (1.08) ≥ UF9202A (0.95) ≥ UF9200A (0.89), UF9203A (0.86), UF9209A (0.84) > UF9205A (0.74) (Table 2). Specific conductance data from two farms (UF9206A and UF9206B, and UF9208A) show averages above 1.275 dS m–1, which is the State Class III water criterion for specific conductance.

Historical data have indicated monthly fluctuation in dissolved salts in the Everglades canals (Parker et al., 1955). A box and whisker plot of the monthly specific conductance data indicates that specific conductance shows monthly fluctuation (Fig. 2 ). Analysis of variance of the data (p = 0.05) shows the overall monthly trend of specific conductance in canal water of EAA farms decreased as follows (arithmetic mean in dS m–1): September (1.30), August (1.28), and October (1.26) > November (1.20), February (1.20), July (1.19), January (1.17), December (1.17), and March (1.14) > April (1.03), June (0.96), and May (0.89). The three highest months (August, September, and October) also had the highest average rainfall (206, 166, and 147 mm, respectively) during 1997–2003. This pattern is consistent with the observation by Parker et al. (1955) and Volk and Sartain (1976), who found separately that Cl– and Na+ concentrations in the wet season (June–October) samples were considerably higher than concentrations in the drier months.



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  		Fig. 2. Plot of mean specific conductance (dS m–1) from all monitoring sites in the Everglades Agricultural Area combined against month of the year.

 
Ion Composition of Specific Conductance in Farm Canals of the Everglades Agricultural Area
The marsh water in the Everglades was reported to be Ca2+–HCO3– dominated (Flora and Rosendahl, 1982). Many of the surface waters in southeastern Florida had <15 mg L–1 of Cl–, but ground waters with 100 mg L–1 or more were not uncommon (Parker et al., 1955). It was also believed that in the pH range of 7.0 to 7.5, the principal ion responsible for alkalinity is HCO3– (Hem, 1985; Gleason, 1974). In this study, ion composition in grab samples at 10 discharge structures indicated that HCO3–, Cl–, and SO42– are the major anions and Na+ and Ca2+ are the major cations in farm canal water of the EAA farms (Table 3). The mean concentrations of HCO3– varied from 190.3 mg L–1 at UF9209A to 399.8 mg L–1 at UF9208A. The mean concentrations of Cl– varied from 71.6 mg L–1 at UF9202A to 174.2 mg L–1 at UF9208A, whereas the mean concentrations of SO42– ranged from 45.2 mg L–1 at UF9203A to 118.8 mg L–1 at UF9208A.


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  		Table 3. Mean ion concentrations in grab water samples at farm canals of the Everglades Agricultural Area, Florida.

 
The quantity of Na+ in ordinary surface or ground water has been reported to be <30 mg L–1, and considerable quantities of Na+ would be found in waters contaminated with sea water or in waters with salts enclosed in the older marine deposits (Parker et al., 1955). Typically, Ca2+ comprises 69.4% and Na+ accounts for 23.7% of the total major cations concentration in the Everglades marsh water (Flora and Rosendahl, 1982), which approximates the ionic character of the shallow ground water that was found in the east region of the Everglades (Flora and Rosendahl, 1981). The chemical character of the canal water differed significantly, with Ca2+ comprising 46.6% and Na+ supplying 41.0% of the total cation concentration (Flora and Rosendahl, 1981; Flora and Rosendahl, 1982). In the current study, mean concentration of Na+ ranged from 27.5% (41.7 mg L–1) of the total cation concentration at UF9202A to 55.8% (110.7 mg L–1) of the total cation concentration at UF9206A, while those of Ca2+ ranged from <26.0% (51.0 mg L–1) at UF9206A to 51.1% (77.7 mg L–1) at UF9202A. In addition, mean concentrations of Mg2+ accounted for 13.0% (25.8 mg L–1) of the total cation concentration at UF9206A to 16.9% (46.1 mg L–1) at UF9208A, whereas those of K+ ranged from 3.8% (5.8 mg L–1) at UF9202A to 5.5% (11.4 mg L–1) at UF9206B (Table 3).

The Na/Cl weight ratio for seawater is 0.55, whereas that of SO4/Cl is 0.14 (Stumm and Morgan, 1981; Smart et al., 2001). The Na/Cl weight ratio for Hillsboro Canal water and WCA-2A marsh water varied from 0.76 to 0.85 (Gleason, 1974). In this study, the mean Na/Cl weight ratio for farm canal water in the EAA ranged between 0.57 and 0.78, higher than that of the seawater and close to that of the marsh water (Gleason, 1974). However, the mean SO4/Cl weight ratio for farm canal water in the EAA ranged between 0.47 and 0.98 (Table 3), much higher than that of the seawater but close to that of shallow well water in western Palm Beach County and in the EAA, which were 0.79 (Parker et al., 1955) and 0.43 (Miller and Lietz, 1976), respectively.

Correlation coefficient analysis shows a strong correlation coefficient (r = 0.95, n = 464) between concentrations of Na+ and Cl– in canal water near the EAA farms. This would reflect, predominately, that sea salt is the dominant source of both Na+ and Cl– (Neal and Kirchner, 2000; Cruz and Amaral, 2004), and secondly, the enrichment of Na+ relative to Cl– may occur during the process of biogeochemical weathering (Smart et al., 2001; Cruz and Amaral, 2004).

The relatively high Na+ content of canal water in the EAA may have resulted partly from the remnants of saline residues that have not been completely flushed out of the ground and partly from cation-exchange processes (Parker et al., 1955). Because the muck and rock of the EAA are much less permeable than sandstones and limestones, saline residues have not been entirely flushed out, and organic colloids are still partly saturated with Na+, presumably adsorbed from ancient seawater. When brought in contact with Na+ bearing clays, the Ca2+ in solution is exchanged for Na+ on the clays. The water then comes in contact with more calcite, which dissolves to form more Ca2+ and HCO3–.

[1]

[2]

From this reaction, we know that dissolution of 1 mol of calcite by 1 mol of H2CO3 or other acid will result in an equivalent amount of Na+ and HCO3– (Banks et al., 1998). Repetition of the process increases HCO3– and Na+ to higher values as measured in the EAA farm canals (Table 3).

Relatively high content of SO42– in canal water in the EAA has been reported by the USGS (Bates et al., 2002; Orem, 2004). These studies concluded that elevated S concentrations in the EAA canals are the result of high agricultural S use. Ground water was also a possible source (Bates et al., 2002). However, Schueneman (2001) investigated the S sources in the EAA and found out that natural sources from organic soil mineralization in the EAA produces >15 times as much as SO42– to the environment as growers' practices. Other potential contributors of SO42– in the EAA are solution of concentrated deposits of sodium sulfate or calcium sulfate (Parker et al., 1955), surface water from Lake Okeechobee (Schueneman, 2001), and ground water discharge (Bates et al., 2002). Rainfall and fertilizer carriers are relatively minor sources (Schueneman, 2001).

Sulfate in our study was not the predominant ion contributing to the specific conductance of canal water in the EAA. Average contribution of SO42– concentration to the total ion concentration in the EAA farm canals is 7.9%. Specific conductance increased from a mean of 0.84 dS m–1 at UF9209A to a mean of 1.51 dS m–1 at UF9206A (Table 2). Approximately 87.6% of the increase in specific conductance between these two sampling sites was due to increases in concentrations of Na+, Cl–, and HCO3– ions. Potassium, Ca2+, Mg2+, and F– were responsible for approximately 4.9% of the increase in specific conductance, whereas SO42– had a 7.6% effect on specific conductance (Table 3).

Time Trends of Specific Conductance in Farm Canals of the Everglades Agricultural Area
A trend in water quality is defined as a monotonic change in a particular constituent with time (Lietz, 2000). Historically, specific conductance at the Everglades National Park has increased 149% since the early 1960s (Flora and Rosendahl, 1982), whereas canal waters in the EAA exhibit widely fluctuating levels rather than strong trends (Gleason, 1974). In the current study, a box and whisker plot of the yearly specific conductance data from 1997 shows a generally decreasing trend until 2000, then a slight increasing trend afterward (Fig. 3 ). Correlation coefficient analysis (p = 0.05) indicated that annual mean specific conductance follows the order (in dS m–1): 1997 (1.46) > 2003 (1.25) > 2002 (1.18), 1998 (1.18), 2001 (1.12) > 1999 (1.08), 2000 (1.08). The time period of minimum mean specific conductance appears to line up with the drought, which reached its peak in 2000 with only 848 mm of annual precipitation, following the order (in mm yr–1): 1997 (1269 mm) > 2003 (1229 mm) > 2001 (1189 mm) > 1998 (1059 mm) > 1999 (1040 mm) > 2002 (971 mm). Processes that contribute to decreased specific conductance in canal waters in the EAA during the dry months (Fig. 2) and drought year (Fig. 3) could increase by irrigation water from Lake Okeechobee, which has a lower mean specific conductance (Table 4) and decrease in stormwater inflow by rainfall. Since water was moving from fields to secondary and primary canals during the wet season, it can be assumed that the water was solubilizing certain elements found in the Histosols and/or was displacing the soil solution (Volk and Sartain, 1976).



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  		Fig. 3. Plot of average monthly specific conductance (dS m–1) from all monitoring sites in the Everglades Agricultural Area combined by year.

 

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  		Table 4. Summary statistics for historical specific conductance of Lake Okeechobee, ground water, and canal water in the Everglades Agricultural Area, Florida.

 
The nonparametric Mann-Kendall trend analysis (Table 5) confirmed statistically significant downward trends (p ≤ 0.05) in specific conductance at UF9202A, UF9205A, and UF9207B. Sen's estimator of slope analysis indicated reduction rates of specific conductance at UF9202A, UF9205A, and UF9207B are at 0.0043, 0.0034, and 0.0050 dS m–1 yr–1, respectively (Table 5). A statistically significant upward trend in specific conductance at UF9208A was detected using the nonparametric Mann-Kendall trend analysis (p ≤ 0.05). However, the upward trend was not significant using the Sen's estimator of slope analysis.


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  		Table 5. Nonparametric Mann-Kendall trend analysis and Sen's slope analysis of specific conductance by site.

 
The significance of the downward trends for these three structures would appear to be indeterminate. Structure UF0202A is the drainage pump station for a small sugarcane farm (130 ha) that rarely operated its drainage pumps, since it is surrounded by much larger farms that effectively provided drainage through subsurface flow (Table 1). Structure UF9205A stopped production in late 1999, after only 2 yr of specific conductance monitoring, when it was incorporated into a Stormwater Treatment Area. Structure UF9207B is an auxiliary pump station for its main farm UF9207A, and as such, the station pumps only after very high rainfall events, so it does not adequately represent the majority of EAA farms. These three structures that showed downward trends in specific conductance also had low mean specific conductance (lower than the Class III water quality criterion of 1.275 dS m–1 for specific conductance).

One farm, UF9208A, had an upward trend for specific conductance, which was significant using only one of two trend analyses utilized in the study, the nonparametric Mann-Kendall test. This small farm of 106 ha is positioned between the Ocean Canal on its north border and a large feeder canal on its south border (the feeder canal joins the Hillsboro canal approximately 1.6 km east of the farm). This farm is located over an area of known high specific conductance ground water (Fig. 4 ).



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  		Fig. 4. Study sites with mean specific conductance superimposed upon a map of chloride concentration of shallow wells (6.7–16.7 m depth) in the Everglades Agricultural Area (chloride map recreated from Parker et al., 1955).

 
Geographical Distribution of Specific Conductance in the Everglades Agricultural Area
Historical information indicates that higher specific conductance water in certain areas in the EAA is a natural phenomenon (Waller and Earle, 1975). Geologically, underlying the organic soils of the Lake Okeechobee–Everglades depression is the Fort Thompson formation, a series of alternating beds of limestone, shells, sand, and marl of marine, brackish, and fresh water origin (Jones, 1948; Parker et al., 1955). The ma rine beds represent times when the area was flooded by the sea; the freshwater beds record times when sea level was below the present level and freshwater lakes and marshes occupied the sea; and the brackish water beds may represent either times of rising or falling sea levels when the water in the area was neither salt nor fresh but was a mixture of the two. The water in most of the Everglades region comes mostly from precipitation within the basin (~1350 mm yr–1) and that upon Lake Okeechobee drainage basin (~1275 mm yr–1). The quantity of flow into the region through subsurface aquifers is negligible. However, in the EAA, ditches or wells penetrating the underlying rock may release sufficient flow to increase materially the amount of pumping required for drainage (Parker et al., 1955). The water yielded by the occasional solution holes and lenses of permeable material in the Fort Thompson formation under the upper Everglades is usually so highly charged with minerals that it cannot be used for household purposes or irrigation (Jones, 1948).

Ground water in the Everglades is highly mineralized and dissolved solids increase with depth. Saline waters and residues left by Pleistocene invasions of the area by the sea have never been completely flushed out of the formations in much of the Everglades, particularly in areas near Lake Okeechobee (Jones, 1948; Parker et al., 1955). Some wells in the EAA <16.7 m deep yield water high in specific conductance, SO42– and Cl– (Parker et al., 1955, Scott, 1977).

We superimposed the average specific conductance of the monitored discharge structures on the map of the shallow ground water (6.7–16.7 m well depths) Cl– levels from Parker et al. (1955) (Fig. 4). The current specific conductance data points show that the elevated conductance observed at UF9208A is in the area of wells that have a Cl– concentration >500 mg L–1 and those of UF9206A and UF9206B are in the area of wells that have a Cl– concentration of 101–200 mg L–1. Chloride concentrations >100 mg L–1 were considered to be evidence of saltwater mixing with freshwater in the surficial aquifer system and indicate the presence of saltwater interface (Hittle, 1999). Up to 400 mg L–1 of Cl– and 390 mg L–1 of Na+ were reported in shallow ground waters close to UF9206A and UF9206B in two separate investigations conducted by Miller and Lietz (1976) (Fig. 5 ) and Scott (1977). Specific conductance in three farms in the vicinity of UF9206A and UF9206B averaged 1.56, 2.15, and 2.29 dS m–1, respectively, and the shallow ground water had a mean specific conductance of 1.54 dS m–1 (CH2M Hill, 1978).



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  		Fig. 5. Water chemical characteristics of shallow wells (5.3–6.0 m depth) in the Everglades Agricultural Area and current study farm locations (well data from Miller and Lietz, 1976).

 
High specific conductance values and ion concentrations were found in shallow wells (5.3–6.0 m) in an area just south of Lake Okeechobee, between West Palm Beach Canal and Hillsboro Canal (Miller and Lietz, 1976) (Fig. 5). It is most likely that these two canals (the depth of the canals is between 5.0 and 6.0 m) intersect this highly mineralized area permitting saltwater intrusion by cutting into the saltwater interface. The current specific conductance data also correspond well with documented specific conductance data of the South Florida Water Management District (Table 4). Summary statistics of specific conductance data obtained from the South Florida Water Management District indicated variable and significantly high values of specific conductance in wells of the EAA, with a mean value of 2.45 dS m–1 (Table 4). During 1990–1992, mean specific conductance in main canals decreased in the order of: Ocean Canal (1.63 dS m–1), West Palm Beach Canal (1.49 dS m–1), Hillsboro Canal (1.35 dS m–1), North New River Canal (1.09 dS m–1), and Miami Canal (1.01 dS m–1). Lake Okeechobee had an average specific conductance of 0.62 dS m–1 during the time period of 1978–1999 (Table 4).


   SUMMARY AND CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSION
REFERENCES
 
Out of the 10 farms that were monitored, only 2 had an average specific conductance higher than the State Class III water quality criterion of 1.275 dS m–1. High levels of Na+ and Cl– were observed at the same two farms. Seasonal trend analysis confirmed that elevated specific conductance was associated with relatively high rainfall in August, September, and October. Yearly trend analysis, conducted on each site throughout the monitoring period, showed no trend of specific conductance in most farms, but lined up with the drought year in 2000. Ion composition data confirmed that the ions determining specific conductance in the EAA waters are Na+, HCO3–, and Cl–, which account for 87.6% of the increase in specific conductance; whereas SO42– contributed only 7.6% of the increase in specific conductance. The Na/Cl ratio in most of the EAA farm canals is above the ratio of seawater (0.55), and the SO4/Cl ratio ranged from 0.47 to 0.98, higher than that of seawater but close to the shallow well water. Influence of the connate seawater entrapped in ground water and other natural sources explains the source of the salts in shallow ground water and elevated specific conductance values found in this study. It is the conclusion of this study that high specific conductance in some of the EAA farm canals is a natural phenomenon. The majority of the EAA farm canals fell below Class III water standard criterion. The study also provides a baseline for future conductance measurements within the EAA system.


   ACKNOWLEDGMENTS
 
This project was sponsored by a grant from the Everglades Agricultural Area–Environmental Protection District and the Florida Department of Environmental Protection. The research was also supported by the Florida Agricultural Experimental Station and approved for publication as Journal Series no. R-10947.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSION
 REFERENCES
 




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H. Wang, M. G. Waldon, E. A. Meselhe, J. C. Arceneaux, C. Chen, and M. C. Harwell
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