INTRODUCTION:

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The Everglades region of south Florida is the subject of investigations to determine the effects of agricultural and water management practices, and of urban development on the geochemistry of the ecosystem. The geochemistry of sulfur is of particular interest because of the link between the reduction of sulfate to sulfide and the production of toxic methylmercury (Hurley et al., 1998; Lambou et al., 1991), which is known to be a problem in some areas of the Everglades. Our purposes have been to determine if sulfur content has increased in recent times, to find the sources of sulfur contamination to the northern Everglades, and to determine its relationship with methylmercury content in sediments. To this end, sediment cores were collected and analyzed for sulfur speciation and sulfur stable isotopic ratios (³⁴S/³²S, expressed as ³⁴S in per mil units). We also collected water (surface, ground, and rainwaters) to determine sulfate content and ³⁴S values.

Figure 1. Study areas in the Northern Everglades of South Florida. Click for larger image.

The Everglades ecosystem encompasses a large area, including the Kissimmee River basin, Lake Okeechobee, the freshwater northern Everglades, the Everglades National Park, and Florida Bay (Figs. 1 and 2). Most of our sampling for sulfate in water was conducted in the northern Everglades, with emphasis on the Water Conservation Areas (WCA 1A, 2A, 2B and 3A), the Nutrient Removal Area (ENR), the Everglades Agricultural Area (EAA), Lake Okeechobee, and the Kissimmee River. Solid sediment was collected in the EAA, WCA 1A and 2A, and Lake Okeechobee. To a lesser extent, sediment and water was also collected from the southern Everglades in Taylor Slough (part of the Everglades National Park) and from Florida Bay (Fig. 2).

There is widespread sulfur contamination in the northern Everglades. Marsh areas near to canal discharge have surface water sulfate concentrations that average about 0.50 meq/L and often exceed 1.0 meq/L, in contrast to background sites which typically have surface water sulfate concentrations of about 0.05 meq/L or less. The sources of water that are potentially major contributors of this sulfur contamination include groundwater, rainwater, and water channeled from Lake Okeechobee through canals traversing the Everglades Agricultural Area and released into the Water Conservation Areas at pumping stations and spillways (Fig. 1). Sulfur enters the wetlands as sulfate (SO₄⁼) contained in groundwater, rainwater, and canal water. The canal water consists of both irrigation drainage from the EAA and water from Lake Okeechobee. Since 1995, we have collected surface water from the following areas: the Hillsboro, North New River, and Miami Canals in the EAA, a buffer wetland constructed on former agricultural land (the Everglades Nutrient Removal Area or ENR), from WCA 1A, 2A, 2B, 3A, and from the canals bordering or within these areas (Fig. 1). Nutrient-impacted WCA 2A was intensely investigated because it receives direct discharge from the Hillsboro Canal that drains the EAA. More recently (since May 1997), we collected rainwater in the ENR, groundwater in WCA 2A and in the ENR, and surface water from Lake Okeechobee and the Kissimmee River near where it empties into the lake (Fig. 1).

The interpretation of stable isotope values (³⁴S) of sulfate is complicated by isotopic fractionation during bacterial reduction of sulfate to sulfide under anoxic conditions, primarily in sediments. The sulfide products are enriched in the isotopically lighter ³²S, relative to sulfate (Goldhaber and Kaplan, 1974), and the ³⁴S values of residual sulfate increase (Nakai and Jensen, 1964). Negative sulfide ³⁴S values are usually obtained where there is an essentially unlimited amount of sulfate available (as in seawater); the ³⁴S values in freshwater are usually positive. The ³⁴S values of the residual sulfate can become very high when the sulfate reservoir is limited. The amount of sulfide produced and the rate of its production through bacterial reduction are controlled by the availability of sulfate and biodegradable organic matter (Berner, 1980; Berner and Raiswell, 1984; Boudreau and Westrich, 1984; Canfield, 1991). Another complicating factor is that oxidation of isotopically light sulfide to sulfate will add isotopically light sulfate to a reservoir, thus decreasing the ³⁴S value of the sulfate in that reservoir (without changing the ³⁴S values of the residual sulfide). The formation of disulfide minerals (mostly pyrite) from sulfidic sulfur is limited by reactive iron availability, assuming excess precursor sulfide availability. Sulfidic sulfur can also react with organic matter, forming organic sulfur compounds, or it can diffuse out of the sediments into the water column where it can become oxidized to sulfate. If this is the case, the sulfate reservoir in the water column will increase and its ³⁴S values will become lighter.