Abstract Introduction Study Area Coring & Pore Water Extraction Analytical Methods Results & Discussion - Surface water geochemistry - Pore water geochemistry   Summary Acknowledgments References PDF Version

Trends in surface water geochemistry

Surface water concentrations of dissolved chemical species at selected sites in south Florida during March 1994 and April 1995 are shown in Figs. 3A to 3F. It is important to understand that the analyses reported here represent mere snapshots of the regional surface water geochemistry. Concentrations of chemical species at a site could show large temporal variability compared to the values reported here. Closely timed, synoptic sampling of surface waters, such as that carried out by the SFWMD, is necessary to thoroughly document temporal trends in surface water geochemistry. Nevertheless, trends reported here are generally consistent with more extensive databases of surface water chemistry from south Florida wetlands.

Fig. 3A. Surface Water pH | Titration Alkalinity (meq/l) [larger image]

pH and alkalinity - Surface water pH values (Fig. 3A) ranged from slightly acidic values in WCA 1A (6.6 to 6.9) to slightly basic values elsewhere (7.1 to 7.8). The slightly acidic conditions in WCA 1A likely reflect the dominance of rainfall to freshwater input in this area, and the acidity of the Loxahatchee peat comprising the substrate in WCA 1A. Titration alkalinity values (Fig. 3A) ranged from 0.2 to 8.6 meq/l, with the lowest alkalinities observed in WCA 1A (0.2 to 1.6 meq/l) and the highest in WCA 2A (5.1 to 8.6 meq/l). Relatively low alkalinities were also generally observed in surface waters from Big Cypress Preserve, with most values around 2.0 to 2.2 meq/l. Titration alkalinity in most natural waters largely represents a measure of total dissolved carbonate species (primarily HCO₃^- at the pH values reported here) in the water. Processes controlling total dissolved carbonate species concentrations include dissolution or precipitation of carbonates, and heterotrophic respiratory activity. The relatively high levels of alkalinity in WCA 2A compared to other sites may reflect higher rates of respiration. The high nutrient load into WCA 2A from canal discharge supports high rates of primary production and high heterotrophic activity (respiration). This is in contrast to more oligotrophic wetland areas in WCA 1A and ENP. Titration alkalinities at a site in the center of WCA 2A dominated by sawgrass (2A-U3) were only marginally lower than sites near the Hillsboro Canal dominated by cattail (Sites 2A-E1 and 2A-F1). The exceptionally low alkalinities in surface waters from WCA 1A reflect the dominance of rainwater input to this wetland area. Low alkalinities in Big Cypress Preserve likely reflect both low heterotrophic activity in this oligotrophic environment, and low carbonate content of the quartz sand sediments.

Fig. 3B. Surface Water Dissolved Organic Carbon (mg/l C) [larger image]

Dissolved organic carbon - Concentrations of dissolved organic carbon (DOC) in surface waters (Fig. 3B) ranged from 6.2 to 48.2 ppm C. DOC concentrations were highest in WCA 2A and showed a general trend of decreasing values to the south and west. The high levels of DOC in WCA 2A is consistent with a pattern of high concentrations for most dissolved chemical species in surface water from WCA 2A. The high concentrations of DOC in WCA 2A likely result from decomposition of organic debris from the artificially elevated biomass produced in response to input of excess nutrients. Note that the sites in the center of WCA 2A (2A-U3) dominated by sawgrass have about the same surface water DOC values as sites near the Hillsboro Canal dominated by cattail (2A-E1 and 2A-F1). This may reflect thorough mixing of DOC in surface water from WCA 2A. The relatively high DOC concentrations in WCA 1A are surprising considering the low concentrations of most other dissolved chemical species here. The efficient production of dissolved organic acids from the Loxahatchee peat in WCA 1A, which also produces the slightly acidic conditions, may contribute to the higher than expected DOC concentrations.

Fig. 3C. Surface Water Phosphate | Ammonium (ug/l) [larger image]

Nutrients - Concentrations of dissolved reactive phosphate (DRP) and ammonium in marsh surface waters (Fig. 3C) were much more variable than for most other dissolved chemical species. DRP concentrations ranged from 146 to <0.9 µg/l over all our sampling sites, with most areas higher than the 8 to 10 µg/l background level expected for pristine areas. High DRP concentrations were observed in WCA 2A, as anticipated. Note the high degree of variability of DRP concentrations in WCA 2A. Sites sampled in 1994 in WCA 2A range from 22.8 to 31.3 µg/l, while samples collected in 1995 ranged from 110 to 114 µg/l. No significant differences in surface water DRP concentrations were observed along the nutrient gradient in WCA 2A (sites 2A-E1 and 2A-F1 near the Hillsboro Canal to site 2A-U3 in the center of WCA 2A). It is likely that our sampling occurred after a period of lower canal water discharge to the marsh in 1994, and after a period of higher canal water discharge in 1995. This result emphasizes the importance of high periodicity of surface water sampling, especially for evaluating nutrient input to the marshes. An unexpected result in the surface water DRP data is the high concentrations at the two most northerly sites in Big Cypress Preserve BC-1 and BC-2, 129 and 146 µg/l, respectively. This result could be anomalous, but deserves further attention. Exceptionally low surface water DRP concentrations were observed in the rainfall-dominated and relatively pristine WCA 1A, and in the tidal Taylor Creek area in the far southeast of ENP.

Ammonium concentrations in surface waters range over two orders of magnitude from 227 to <3 µg/l (Fig. 3C). No general regional trends were observed in the ammonium data, probably reflecting the multiple sources of ammonium to surface waters (e.g. canal discharge, nitrogen fixation by periphyton, and rainfall). The highest concentration by far (227 µg/l) was observed in WCA 2A at site 2A-F1 near the Hillsboro Canal. Other sites in WCA 2A, however, had much lower concentrations, particularly samples collected in 1994. Moderate levels of ammonium were observed in rainfall-dominated WCA 1A (15 to 39 µg/l), with the highest concentration at the center of WCA 1A. Low to moderate ammonium concentrations were observed at sites in WCA 3A and ENP.

Fig. 3D. Surface Water Chloride | Fluoride | Sulfate (mg/l) [larger image]

Major anions - Concentrations of chloride, fluoride, and sulfate in surface waters (Fig. 3D) followed the general regional pattern of highest values in WCA 2A, and lower concentrations in WCA 1A and to the south in WCA 3A and ENP (excluding the brackish water sites in the far southeast along Taylor Creek, ENP-TC1 and ENP-TC2). Chloride concentrations ranged from 152 to 22 mg/l in the freshwater areas of the Everglades. The highest values were observed at the two sites near the Hillsboro Canal in WCA 2A (E1 and F1), and the lowest value was observed in the center of rainfall dominated WCA 1A. Chloride concentrations in the center of WCA 2A ranged from 64 to 97 mg/l, somewhat lower than sites near the Hillsboro Canal and similar to concentrations in the portion of WCA 3A north of Alligator Alley. Chloride concentrations in the freshwater areas of ENP ranged from 42 to 66 mg/l.

Fluoride concentrations ranged from 0.60 to 0.06 mg/l, with the highest concentrations at sites along the Hillsboro Canal in WCA 2A (E1 and F1), and at a disturbed site in the far southeast along the C111 canal (ENP-C111). In WCA 1A, 3A and in the freshwater areas of ENP fluoride concentrations ranged from 0.06 to 0.16 mg/l. Fluoride concentrations in the center of WCA 2A were quite variable, ranging from 0.09 to 0.41 mg/l. The poor sensitivity of the fluoride data from the brackish water Taylor Creek sites (ENP-TC1 and ENP-TC2) reflects interference in the fluoride measurement from a large chloride peak using ion chromatography.

Sulfate concentrations in the freshwater Everglades ranged from 55 to 0.5 mg/l. The highest sulfate concentrations were observed in WCA 2A. Sites near the Hillsboro Canal in WCA 2A (E1 and F1) and in the center of WCA 2A (U3) had similar concentrations in 1994 and 1995, indicating that sulfate from canal discharge can penetrate into the center of marsh areas. Sulfate concentrations outside of WCA 2A are much lower and do not exceed 2 mg/l in WCA 3A south of Alligator Alley and freshwater areas of ENP. Sulfate concentrations in WCA 1A appear to be somewhat elevated, except in the center of the marsh (1A-7).

Fig. 3E. Surface Water Sodium | Potassium (mg/l) [larger image]

Major Cations - Concentrations of major cations in surface waters are shown in Figs. 3E (Na and K) and 3F (Ca, Mg, and Sr). Transition metal cations in these waters were generally below the detection limits of ICP-AES (Lichte et al. 1987). Concentrations of sodium ranged from 9 to 138 ppm in the freshwater marshes. Again, the highest concentrations were observed in WCA 2A, with gradually lower concentrations to the south into ENP (except at the brackish water Taylor Creek sites ENP-TC1 and ENP-TC2) and to the west into Big Cypress Preserve. Sodium concentrations in WCA 1A were moderate to low compared to other areas.

Fig. 3F. Surface Water Calcium | Magnesium | Strontium (mg/ll) [larger image]

Potassium concentrations ranged from <10 to 22 ppm in WCA 2A, and were below the detection limit of 10 ppm elsewhere in the freshwater areas of the Everglades.

Concentrations of calcium in the freshwater Everglades (Fig. 3F) ranged from 10.6 to 95.2 ppm. Somewhat higher concentrations were observed in WCA 2A than elsewhere, but high concentrations were also found at sites in WCA 3A (3A-3), ENP (ENP-GH), and Big Cypress (BC-1). Calcium concentrations were notably lower in WCA 1A, with concentrations ranging from 10.6 to 27.6 ppm. Concentrations of calcium in surface waters are likely controlled by processes such as canal discharge, calcium uptake by periphyton, ion exchange onto peat surfaces, incorporation in calcium oxalate during peat decomposition, and dissolution of marl. Concentrations of magnesium ranged from 2.5 to 36.9 ppm, with highest concentrations in WCA 2A and gradually lower concentrations to the south (WCA 3A and ENP) and west (Big Cypress Preserve). Magnesium concentrations were also low in WCA 1A. Strontium concentrations ranged from <0.1 to 2.8 ppm, and generally followed the same regional distribution pattern as magnesium.