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

Pore water geochemistry

Concentrations of dissolved chemical species in pore water from 12 cores collected at different sites in south Florida during 1994 and 1995 are presented in Tables 3 and 4, and Figures 4A to 4L. In the figures, pore water profiles are arranged with sites in the north (WCA 1A) in the upper left hand corner of the figures, and sites in the south (ENP) in the lower right hand corner. No pore water samples were collected from Big Cypress Preserve in 1994 or 1995 due to the high sand content of the organic soils there. Data points plotted in the figures at a depth = 0 are surface water concentrations.

pH and alkalinity - Values of pH in pore waters typically ranged from 6.5 to 8.0, with slightly more acidic conditions (as low as 5.8) at site 1A-7 located in the center of rainfall-dominated WCA 1A (Table 3 and Fig. 4A). Vertical profiles of pH in many of the cores exhibited an initial decrease in pH from the surface water to the near-surface pore waters, followed by a gradual increase or levelling of pH values below this. The initial decline in pH likely reflects accumulation of protolytic chemical species (e.g. dissolved organic acids, protonated carbonate species, and hydrogen sulfide) in pore waters of near-surface peats where bacterial activity and decomposition of organic matter are expected to be highest.

Titration alkalinity values in pore waters ranged from 4 to 22 meq/l (Table 3 and Fig. 4B). Higher titration alkalinities were observed at sites in WCA 2A compared to sites in WCA 3A and freshwater areas of ENP. This likely reflects higher heterotrophic activity and production of dissolved carbonate species in sediments at sites in eutrophic WCA 2A compared to other areas. Within WCA 2A, sites in the heavily nutrient-impacted cattail areas near the Hillsboro Canal (2A-E1 and 2A-F1) have somewhat higher pore water titration alkalinities compared to a sawgrass-dominated site near the center of WCA 2A (2A-U3). A cattail-dominated site near the Hillsboro Canal in WCA 1A (1A-1) also had relatively high alkalinities (up to 12 meq/l), as did a brackish water site in ENP along Taylor Creek (ENP-TC2). Vertical profiles for titration alkalinity typically exhibited gradual increases with depth to some maximum values (10-22 meq/l in WCA 2A, 4-10 meq/l in WCA 3A, and 4-7 meq/l in freshwater areas of ENP), and a levelling-off to constant or slightly decreasing values below the maximum.

Dissolved organic carbon - Concentrations of dissolved organic carbon (DOC) in pore waters (Table 3 and Fig. 4C) ranged from 12 to 160 ppm C (1 to 13 mmoles/l). Note that this concentration range is similar in magnitude to that for titration alkalinity (4 to 22 meq/l), indicating that cycling of DOC is a very important mechanism for carbon transport between reservoirs in the Everglades. Generally higher concentrations of DOC were observed in pore water at sites in WCA 2A (40-160 ppm C), especially at site F1 near the Hillsboro Canal compared to sites in other areas. High concentrations of DOC (up to 153 ppm C) were also observed in pore water from the brackish water site in ENP along Taylor Creek (ENP-TC2). At sites in WCA 3A and in the freshwater areas of ENP DOC concentrations in pore water did not exceed 40 ppm C, consistent with previous studies from this area (Orem et al., 1987). DOC concentrations in pore water from WCA 1A generally ranged from 40-80 ppm C, with generally higher concentrations at the site near the Hillsboro Canal (1A-1). Profiles of DOC concentration versus depth in pore water were somewhat variable, but often exhibited increasing concentration with depth to some maximum value, and then constant to decreasing concentrations below the maximum. In most cores the maximum concentration of DOC was attained at a depth of 20 cm or less that corresponds to the zone of freshest organic matter and highest bacterial activity. Several cores (e.g. 1A-7 and 2A-U3) appeared to exhibit multiple DOC maxima downcore.

Sulfide and sulfate - Concentrations of sulfide and sulfate in pore water from our series of sites are shown in Table 3 and Figs. 4D and 4E, respectively. In natural waters, sulfate is the major oxidized form of sulfur and sulfide is the major reduced form (usually present as HS^- at circumneutral pH values). Concentrations of these two forms of sulfur in wetlands are regulated by several different processes: (1) sulfate input from runoff and precipitation, (2) sulfate reduction under anoxic conditions in sediments, (3) oxidation of sulfide to sulfate after diffusion back into oxic surface waters, (4) reaction of sulfide with sedimentary and dissolved organic matter to form organic sulfur compounds, and (5) the reaction of sulfide with dissolved metals to form insoluble metal sulfide phases in sediments.

Dissolved sulfide concentrations (Fig. 4D) vary by more than 5 orders of magnitude in pore waters from sites sampled in 1995 (sulfide was not measured during 1994), from nearly 2,000 µg/l to below the detection limit of about 0.01 µg/l. The highest sulfide concentrations were observed in WCA 2A. At a cattail-dominated site near the Hillsboro Canal in WCA 2A (2A-F1) sulfide concentrations were actually higher in the surface water (@125 µg/l) than in the pore water (25 to 50 µg/l), suggesting rapid rates of sulfate reduction near the sediment/water interface, and/or rapid diffusion of sulfide into the surface water here. At depth in this core, sulfide concentrations reached an asymptotic value of about 25 µg/l down to the bottom of the core (@75 cm). In contrast, the site near the center of WCA 2A (2A-U3) exhibited very low surface water sulfide concentration (0.01 µg/l), a dramatic increase to a peak sulfide concentration of 1780 µg/l at a depth of 6 cm, and then a decline to concentrations < 20 µg/l below 60 cm. In WCA 1A, a site near the Hillsboro Canal (1A-1) also had relatively high sulfide concentrations, with near-surface pore water values approaching 200 µg/l. In the center of WCA 1A (site 1A-7), however, sulfide in pore water was below the detection limit throughout the core (<0.01 µg/l). Sulfide concentrations were also generally low (but detectable) at both sites in WCA 3A (3A-3 and 3A-15). At the brackish water mangrove site along Taylor Creek in ENP (ENP-TC2) sulfide levels gradually increased with depth to values near 100 µg/l.

Vertical profiles of dissolved sulfate in pore water from our series of sites in south Florida are shown in Fig. 4E. At most sites, sulfate concentrations show an exponential decrease with depth, reflecting bacterially-mediated reduction of sulfate to sulfide. Surface water sulfate concentrations were highest at sites in WCA 2A (2A-F1, 2A-E1, and 2A-U3), ranging from 20 to 60 mg/l. Inspection of the profiles at sites 2A-U3 and 2A-F1 suggests that rates of sulfate reduction are higher at the latter site. This may reflect differences in the organic matter composition of the sediments, mostly sawgrass at 2A-U3 and mostly cattails at 2A-F1, with cattail detritus perhaps being more easily degraded under anaerobic conditions (Davis 1991). The sulfate profile at 2A-F1 shows an interesting pattern with depth; an initial exponential decrease with depth followed by a slight increase in sulfate concentration in the 10 to 20 cm range. This may reflect pumping of oxygen into the root zone by cattails at this site, resulting in some oxidation of sulfide or peat organic sulfur to sulfate. Cattails may be more efficient at moving oxygen into their roots and thus at aerating the surrounding sediments, compared to sawgrass (Grace 1988; Koch and Rawlik 1993; Davis 1994), with cattails having large internal air spaces for oxygen and employing pressurized bulk flow ventilation, and sawgrass using much slower diffusive gas exchange (Chanton et al. 1993). There is also some indication of this effect at a cattail site near the Hillsboro Canal in WCA 1A (1A-1). South of WCA 2A (in WCA 3A and ENP) sulfate concentrations in surface waters are significantly lower (7 mg/l at 3A-3, 2 mg/l at 3A-15, and about 1 mg/l at freshwater marsh sites in ENP), but vertical profiles of sulfate concentration still show evidence of sulfate reduction. At the brackish water site in ENP (ENP-TC2), sulfate concentrations decrease exponentially with depth to about 20 cm, but than sharply increase. This reflects intrusion of denser/saltier seawater from Florida Bay underlying a fresher lens of water flowing down Taylor Creek.

Nutrients - Concentrations of reactive phosphate and ammonium in pore waters are presented in Table 3 and Figs. 4F and 4G, respectively. Reactive phosphate concentrations in pore waters range from <0.9 µg/l to nearly 3,000 µg/l. The highest concentrations of reactive phosphate were observed in pore water from the sites nearest the canal in WCA 2A (2A-F1 and 2A-E1), with peak values in the near-surface pore water of 1,000 to 3,000 µg/l. Phosphate concentrations at the central marsh site in WCA 2A (2A-U3) are about 10 times lower (peak values of 100 to 200 µg/l) than those at the sites nearest to the canal. This is consistent with the phosphate gradient observed in solid phase sediments in WCA 2A, which shows much higher concentrations at the canal sites (Koch and Reddy, 1992). The lowest reactive phosphate concentrations in pore water that we observed were from the center marsh site in WCA 1A (1A-7), with a peak concentration of only about 6 µg/l. This site is rainfall-dominated and likely receives little or no canal discharge water. In contrast, the site in WCA 1A near the Hillsboro Canal (1A-1), which likely receives some canal seepage, has peak phosphate concentrations about 10x higher. Peak phosphate concentrations in pore water from the freshwater marshes of WCA 3A and ENP were generally less than 60 µg/l, except for a peak value of about 120 µg/l at the Pa-Hay-Okee Lookout site near the eastern edge of Shark River Slough. The mangrove site in Taylor Creek (ENP-TC2) had peak phosphate concentrations of about 350 µg/l.

Many of the profiles of phosphate concentration in pore waters versus depth (Fig. 4F) are characterized by a large concentration gradient from surface water values to much higher near-surface pore water concentrations. Peak concentrations of phosphate in pore waters usually occur in the upper 20 cm. Below the peak value, phosphate concentrations gradually decrease. This pattern is consistent with a zone of maximal microbial activity, organic biodegradation, and recycling of phosphorus (sedimentary organic phosphorus to inorganic dissolved phosphate) within the near-surface sediments. Several profiles, however, deviate from this general vertical pattern, exhibiting phosphate maxima at depths well below 20 cm (3A-15, ENP-TT, and ENP-GH). Such patterns which deviate from the norm may reflect local anomalies in substarte characteristics, microbial community structure in the sediments, or the DRP concentration of groundwater infiltrating the sediments.

Ammonium concentrations in pore waters (Fig. 4G) range from about 4 µg/l to 3,600 µg/l. At most sites, ammonium concentrations in pore waters are in the 100 to 400 µg/l range. Very high ammonium concentrations (1,000 to 3,600 µg/l) were observed in pore water at sites F1 and U3 in WCA 2A during 1995 sampling. During 1994 sampling, however, ammonium concentrations at sites U3 and E1 (near F1) were approximately 10x lower. Thus, ammonium concentrations appear to exhibit a marked temporal variability. Temporal studies of pore water geochemistry at selected sites (including F1 and U3 in WCA 2A) are currently underway and will be discussed in future reports. Other sites with peak ammonium concentrations exceeding 1,000 µg/l include 1A-7, 3A-15, and ENP-TC2. The 1A-7 and 3A-15 sites are considered to be relatively "pristine", with little discharge from canal water draining agricultural areas. Thus, high ammonium concentrations in pore waters are not only associated with areas contaminated by canal runoff. Nitrogen fixation by periphyton in surface waters (Browder et al. 1994), and subsequent microbial recycling of organic N from periphyton detritus in sediments may also produce high ammonium concentrations in pore waters. Excluding the brackish water Taylor Creek site (ENP-TC2), sites in and near ENP had ammonium concentrations in pore water generally in the range of 100 to 400 µg/l. Vertical profiles of ammonium in pore water had somewhat different characteristics at each site, but often exhibited a sharp initial increase to a maximum concentration in the near-surface pore water, followed by a gradual decrease to somewhat lower concentrations at depth.

Chloride, fluoride, and bromide - Concentrations of major anions in pore water (chloride, fluoride, and bromide) are presented in Table 3 and Figs. 4H and 4I. In general, concentrations of these anions in pore water are somewhat higher than in surface water. This could be due to evaporative concentration of pore water during dry periods. Alternatively, higher concentrations of anions in pore water could reflect equilibrium with salts or mineral phases in the peat, or upwelling of higher ionic strength groundwater. The range of concentrations of these anions in pore waters from the freshwater marshes are about 40-700 mg/l for chloride, 0.1-1.3 mg/l for fluoride, and <0.2-2.6 mg/l for bromide. Overall, Chloride, fluoride, and bromide concentrations in pore waters were somewhat higher at sites in WCA 2A compared to areas in WCA 3A and ENP (excluding the brackish water site ENP-TC2). This likely reflects influx of excess anions into WCA 2A from canal discharge. Concentrations of chloride are also higher at the site near the Hillsboro Canal in WCA 1A (1A-1), compared to sites in WCA 3A and ENP. This may be due to some seepage of water from the canal at this site. Very high chloride concentrations at the ENP-TC2 site reflect brackish water conditions. Note the change to higher chloride concentrations in pore water below 25 cm at the ENP-TC2 site. This is consistent with the sulfate data and reflects the presence of fresher (lower density) water in the surface sediments and saltier Florida Bay water in deeper sediments.

Major cations - Concentrations of major cations in pore water (sodium, potassium, calcium, magnesium, and strontium) are presented in Table 4 and Figs. 4J, 4K, and 4L. The pore water geochemistry of cations at these sites follows a pattern similar to that observed for the major anions (chloride, fluoride, and bromide): (1) generally higher concentrations in pore water compared to surface water, (2) highest concentrations at sites in WCA 2A, and the near-canal site in WCA 1A (1A-1) compared to sites in WCA 3A and ENP (Ca is an exception to this pattern), and (3) fresher water overlying saltier water in sediments at the mangrove site along Taylor Creek (ENP-TC2). Note that the shapes of the profiles for sodium, calcium, magnesium, and strontium are similar at individual sites, suggesting similar controls on pore water concentrations. Potassium concentrations are generally at the detection limit (10 ppm) by the analytical method used here (ICP-AES), and show little change as a function of depth. Sodium concentrations in the freshwater marshes range from 100 to 500 ppm at sites in WCA 1A and 2A, and typically 25 to 50 ppm at sites in WCA 3A and ENP. Magnesium concentrations ranged from 24 to 94 ppm at sites in WCA 1A and 2A, but only 5 to 24 ppm at freshwater marsh sites in WCA 3A and ENP. Strontium concentrations ranged from 2 to 7 ppm at sites in WCA 1A and 2A, but only 0.5 to 2 ppm at freshwater marsh sites in WCA 3A and ENP. In contrast, calcium concentrations showed little regional variation (range of 70 to 170 ppm downcore at most sites), except for somewhat elevated concentrations at site F1 in WCA 2A (range of 66 to 265 ppm).

Concentrations of iron and silica in pore water from these sites are also listed in Table 4. In general, concentrations of these two elements were below the detection limits by ICP-AES (0.1 ppm for iron and 1.0 ppm for silica. At a few sites, however, detectable levels of these elements were observed. Silica was observed in pore water only at site F1 in WCA 2A (range of 1.3 to 4.4 ppm), possibly suggesting a source from canal water discharge although other explanations are possible. For iron, concentrations ranging from 0.1 to 2.3 ppm were observed below 36 cm at site WCA 2A-U3, and throughout the core at sites WCA 3A-15 and ENP-GH. The presence of dissolved iron at these sites implies a source of iron and low levels of dissolved sulfide. Note the low concentrations of dissolved sulfide in pore water below 25 cm at site WCA 2A-U3, and throughout the core at site WCA 3A-15 (Table 3 and Fig. 4E). The sources of iron for these sites are unclear, but the biogeochemical cycling of iron in the iron-starved environment of south Florida is likely of importance to primary productivity and deserves further study.