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Geochemistry of Sulfur in the Florida Everglades:
Surface waters were collected in the EAA, the ENR, WCA 1A, 2A, 2B, and 3A, Lake Okeechobee, and Taylor Slough (Figs. 1 and 2). Groundwater and rainwater were collected in WCA 2A and in the ENR. The concentrations and sulfur isotopic ratio data were determined for samples from each area (Tables 6, 7, 8; Figs. 13 and 14).
Regional patterns in sulfate concentration and 34S values obtained from March 1995 through July 1998 are shown in (Fig. 13a-g). Surface waters collected from the canals in the EAA tend to have higher sulfate concentrations with lower 34S values compared to water samples from the Hillsboro Canal where it is adjacent to WCA 2A. Sulfate concentrations of water from the Kissimmee River and Lake Okeechobee (Fig. 13a) are low compared to water collected from the EAA canals. Surface water in WCA 2A, which receives discharge from the Hillsboro Canal through the S-10 spillways, tends to be somewhat lower in sulfate concentration with higher 34S values than water collected from the canal. These changes are probably due to progressive reduction of sulfate and to dilution with rainwater or groundwater. Surface waters from WCA 3A and 2B have relatively low sulfate content. WCA 2B receives water from the southern part of WCA 2A but not directly from the canals, and WCA 3A is a very large area, most of which is far from canal discharge sites. The Loxahatchee National Wildlife Refuge (WCA 1A), with little direct input of water from the Hillsboro Canal, also has very low sulfate content although the 34S values are in the same range as the canals in the EAA. In contrast, the 34S values of water from WCAs 3A and 2B are in the same range as water from WCA 2A, suggesting that similar sulfate processing is occurring in these areas.
In the Taylor Slough of the southern Everglades (Fig. 2) sulfate concentrations are low and 34S values are high in the northern part of the Slough (Table 6 ). The mangrove swamps of the southern part of Taylor Slough have very high sulfate concentrations, reflecting marine influence from Florida Bay. Interestingly, Taylor Creek, in the mangrove swamps, had high sulfate concentrations in 1996 and relatively low sulfate concentrations in 1998 and 1999, probably reflecting differences in freshwater discharge.
Bay waters near small islands (keys) in Florida Bay have sulfate concentrations near seawater levels (56 meq/L) and 34S values at the seawater value (~20 per mil) (Table 6 ; Fig 2). Sulfate concentrations slightly greater than that of seawater are probably caused by evaporation of the shallow waters of the Bay, while sulfate concentrations lower than that of seawater are caused by dilution with water from Taylor Slough. Proximity to the coastline determines the amount of dilution (Pass Key>Russell Key>Bob Allen Key>Whipray Basin).
Rainwater could cause dilution trends in surface water, but certainly not trends of increase in 34S values. Analysis of rainwater collected in the ENR from January through March 1998 gives a 34S value of 10.7 per mil (Table 7 ); rainwater samples collected from the same site in approximately two month intervals from March through September 1998 show a range of 34S values from 2.1 to 3.2 per mil. Rainwater collected in WCA 2A in July 1998 had a 34S value 5.9 per mil. These values are much lower than 34S values for sulfate from surface water that we have analyzed in any part of the northern Everglades. The rainwater that we collected had sulfate concentrations (0.04 to 0.09 meq/L), that are low in comparison to sulfate concentrations in surface water and quite similar to sulfate concentrations found in rainwater from the northern Everglades region in the early to mid 1970's (Waller and Earl, 1975).
Surface and groundwater were collected in WCA 2A (Table 8 ; Fig. 14a, b) and at the head and tail of the S-10C spillway (Table 8 ; Fig. 13b) on the Hillsboro Canal in September, 1997. The sulfate concentration and 34S values from all of the surface water samples collected at this time fall in range typical of surface water in WCA 2A (Figs. 13c and 14a). Groundwater beneath WCA 2A (Fig. 14b) is generally much lower in sulfate concentration compared to surface water (particularly at 4.5 m depth, and is variable with respect to 34S values. Groundwater collected at S-10C at 30.5 m depth (Fig. 14b) and at F1 at 9 m depth (Fig. 14b) have sulfate concentrations as high or higher than surface water. However, their 34S values are significantly different from surface water; this is also true of groundwater collected at 9 m depth at all sites. All of these results suggest that groundwater was not the major source of sulfate to surface water in WCA 2A at the time of collection, although there does appear to be a greater potential for at least some groundwater influence near the Hillsboro Canal (sites E1, F1, and S-10C; see figure 1) than at sites away from the canal (Harvey et al., 1998).
Groundwater collected in WCA 2A (Fig. 14c) and at S-10C (Fig. 13b) in June 1998 displays a similar pattern, with sulfate concentrations low in comparison to sulfate concentrations in surface water except for a very high sulfate concentration for groundwater at 9 m at site F1 in WCA 2A. The 34S values for groundwater in June 1998 have a greater range of values in comparison to the 34S values obtained from samples collected in September 1997 (five of the groundwater samples collected at that time are not included in figure 14c because they had sulfate concentrations insufficient for isotopic analysis). The very high 34S values in three of the water samples (40 per mil or greater) are probably due to nearly complete reduction of a limited sulfate reservoir in groundwater at these sites, possibly the result of drought conditions during this season (summer of 1998). The 34S values in groundwater collected at this time are all very different than those obtained in surface water (Fig. 14a).
Groundwater hydrology in the ENR appears to be more complex, and definite conclusions cannot be drawn concerning its influence on surface water. Groundwater was collected at five sites along a transect across the ENR (see figure 1), parallel to the direction of groundwater flow (south-east to north-west). In September 1997, the near-surface groundwaters (at or above 8 m depth) at these sites fall in the same [SO4--]--34S field as the surface water samples in the ENR (Figs. 14d,e,f). In June 1998, the concentrations of sulfate in groundwater at these same sites (Fig. 14f) were not much changed compared to 1997, but the 34S values of sulfate tended to be higher. Groundwater collected at greater depths at these sites tends to have higher sulfate concentrations, similar to values found in water from the EAA canals. Groundwater taken at 58 m at the northernmost site on the transect in the ENR (Fig. 1) had sulfate concentrations of 31.99 meq/L (1997) and 33.58 meq/L (1998) with 34S values of 24.68 and 25.11 per mil, respectively (not shown in figures 14e or 14f).
U.S. Department of the Interior, U.S. Geological Survey
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