Summary Abbreviations > Introduction Review of Sulfur Contamination Reducing Sulfate Sources Bio. Mitigation Natural Minerals Mitigation Chem. Treatment Mitigation Active Removal Mitigation Conclusions Acknowledgments References Tables and Figures PDF

Sulfate contamination of the Everglades is a serious water quality issue facing restoration of this ecosystem. Sulfate concentrations in some marsh areas are more than 60 times background concentrations, and sulfate in excess of background levels covers an estimated 60% of the freshwater Everglades (Orem et al., 1997; Stober et al., 1996 and 2001; Orem et al., 2004). The excess sulfate enters the Everglades in the discharge of canal water from the Everglades Agricultural Area (EAA). Excess phosphorus also enters the ecosystem in EAA canal water discharge (Koch and Reddy, 1992; Craft and Richardson, 1993; DeBusk et al. 1994; Zielinski et al., 1999). Existing data suggest that sulfur in fertilizer and soil amendments used in the EAA (both new additions and legacy sulfur in the soil) is a major source of excess sulfate entering the ecosystem (Bates et al., 2001 and 2002). Other potential sources of sulfate (including groundwater), however, need further investigation. The report by Gilmour et al. (2007b) in the 2007 South Florida Environmental Report provides a complete examination of the current state of knowledge of the sulfur contamination issue in the Everglades.

Sulfate discharged from canals or leaking through levees into the ecosystem spreads out over a large area since, unlike phosphorus, it is not removed to any great extent by plant uptake. Sulfate slowly diffuses into the anoxic soils (peats) underlying the Everglades and stimulates microbial sulfate reduction (MSR), producing toxic hydrogen sulfide as a byproduct (Goldhaber and Kaplan, 1974; Berner, 1980; Rheinheimer, 1994). Hydrogen sulfide at contaminated sites may build up in sediments to concentrations thousands of times background levels (Gilmour et al., 2007b).

The excess sulfate and sulfide has numerous deleterious impacts on the Everglades. One of the more environmentally important impacts is the link between sulfate contamination and methylmercury (MeHg) production in the ecosystem (Gilmour et al., 1998; Benoit et al., 1998, 1999a, b; Axelrad et al., 2007; Gilmour et al., 2007a). MeHg, a bioaccumulative neurotoxin, is produced primarily by methylation of ambient inorganic mercury during MSR (Compeau and Bartha, 1985; Gilmour et al., 1992; Munthe et al., 1995; Branfireun et al., 1999). Contamination of fish with MeHg is the most significant environmental contaminant issue in the USA in terms of number of locations impacted (Krabbenhoft and Wiener, 1999; USEPA, 1998). Neurotoxic MeHg represents a serious threat to wildlife (Bouton et al., 1999; Frederick et al., 1999; Heath and Frederick, 2005), and is a human health issue, with human exposure through fish consumption (Gilbert and Grant-Webster, 1995; Schober et al., 2003). In addition to its neurotoxic effects, MeHg may also be an endocrine disruptor that affects successful reproduction in fish and fish-eating wildlife (Klaper et al., 2006). South Florida has among the highest levels of MeHg in fish in the USA (Lambou et al., 1991). Experimental chamber (mesocosm) studies conducted in the Everglades have shown that sulfate addition stimulates the production and bioaccumulation of MeHg (Gilmour et al., 2007b). Inorganic mercury enters the Everglades primarily in rainfall, and most of the inorganic mercury in the rainfall appears to originate from outside of the USA (Hanisch, 1998). The origin of most inorganic mercury from outside of the USA severely limits the ability of state and Federal officials to limit MeHg production and bioaccumulation in fish in the Everglades by controlling emissions of inorganic mercury from various anthropogenic sources (e.g. coal-fired power plants, medical waste incinerators, cement manufacture). Thus, controlling sulfate inputs to the Everglades may represent the most effective way of minimizing MeHg production and bioaccumulation here.

In addition to impacts on MeHg production and bioaccumulation, sulfur contamination has also dramatically altered redox patterns in the Everglades. Unnaturally low (negative) redox and highly sulfidic conditions occur in large swaths of the northern Everglades heavily impacted by sulfate from canal discharge (Gilmour et al., 1997b). The lower redox conditions and high concentrations of toxic sulfide in soils may impact macrophytes and soil infauna (Koch et al., 1990; Bradley and Morris, 1990; Kludze and Delaune, 1996). A recent greenhouse study suggests that sawgrass (Cladium) is more sensitive to sulfide toxicity than cattail (Typha) at sulfide concentrations greater than 9 mg/L (Gilmour et al., 2007b). Sulfide concentrations exceeding 9 mg/L are routinely exceeded at heavily sulfate- and phosphorus-contaminated sites in the northern Everglades where cattail has displaced sawgrass. Various studies have suggested that excess phosphorus stimulates the growth of cattail over sawgrass in heavily impacted parts of the Everglades (Davis, 1991; Craft et al., 1995; Newman et al., 1996; Craft and Richardson, 1997; Miao and DeBusk, 1999; Childers et al., 2003). It is hypothesized, however, that sulfide buildup in soil also plays a key role in the displacement of sawgrass by cattail in the Everglades, probably in combination with phosphorus eutrophication (Gilmour et al., 2007b).

High levels of sulfide and low redox conditions may also impact trace metal cycling, and increase remobilization of nutrients from soils through a process referred to as internal eutrophication. Stimulation of MSR by excess sulfate has been shown to increase remobilization of nutrients from freshwater marshes in the Netherlands (Lamers et al., 1998; Smolders et al. 2006). Preliminary mesocosm studies have shown that sulfate can also enhance remobilization of ammonium, phosphorus, and dissolved organic carbon and nitrogen from Everglades' peats (Gilmour et al., 2007b). Thus, sulfate contamination of the Everglades may limit the ability of marsh soils to effectively sequester phosphorus contamination from EAA runoff. Similarly, high levels of sulfate in stormwater treatment areas (STAs) will limit their ability to effectively sequester phosphorus from EAA runoff.

Current plans to decompartmentalize and restore sheet flow are likely to increase sulfate loads to areas such as Loxahatchee National Wildlife Refuge (LOX), Everglades National Park (ENP), and Big Cypress National Preserve (BCNP). Rerouting of water will impact different areas in different ways with some areas receiving greater sulfate loads and some areas receiving lower sulfate loads. In the central Everglades, recently reported declines of MeHg levels in fish appear to be linked to declines in sulfate concentration, not to declines in inorganic mercury deposition (Axelrad et al., 2007). In contrast, monitoring data suggest that sulfate-contaminated water has been rerouted down canals from the central Everglades to ENP, where recent increases in MeHg concentrations in fish have been reported (Gilmour et al., 2007a). Plans to move sulfate-contaminated water from the L-28 canal into BCNP may also result in increased MeHg levels in biota here. Elevated levels of MeHg in fish have been observed in the northeastern part of BCNP, in the area near the L-28 canal (D. Rumbold, personal communication). Other unwanted impacts of sulfate contamination may also occur in these areas. Unfortunately, STAs as currently designed do not significantly reduce sulfate loads discharged into the ecosystem (South Florida Water Management District, unpublished data). Land and water managers need to carefully assess the costs versus benefits of using sulfate-contaminated water in Everglades' restoration. Restoration efforts will not be considered successful if sulfidic soils and MeHg-contaminated fish persist in the ecosystem.

In this report we examine potential strategies for reducing sulfate loads to the Everglades (i.e. mitigation strategies). Any effort to mitigate sulfate loads to the Everglades might begin with considering reducing the principal sources of the contamination. This certainly would involve an evaluation of sulfur use in agriculture in the EAA (sugarcane and vegetable cultivation) and areas north of Lake Okeechobee (cattle and citrus) that drain into the lake and ultimately the Everglades. It may be possible to reduce sulfur use without seriously reducing crop yields. Alternatives to some soil amendments containing sulfur may be available, and unnecessary sulfur in some fertilizers or soil amendments could be eliminated by reformulation at the manufacturing level. This would require cooperation among agricultural scientists, the agricultural industry, fertilizer manufacturers, and government officials. Sulfate loads delivered to the Everglades will likely not be able to be reduced to pre-development levels. However, it cannot be emphasized too strongly that any reduction in sulfate load would benefit the ecosystem, especially with regard to the issue of MeHg production and bioaccumulation. Monitoring and mesocosm studies conducted in the central Everglades have shown how quickly MeHg production and bioaccumulation respond to reductions in sulfate loads.

In addition to reducing sulfate loads to the ecosystem, approaches for active reduction of sulfate concentrations in canal water are also considered. A number of approaches are presented, including biological removal strategies, passive mineral removal, chemical treatment approaches, and active removal using various technologies. Many of these approaches will be impractical for removal of sulfate from Everglades' water due to scaling issues, cost, or other factors. The most effective approach to reducing sulfate loads to the Everglades will likely be multifaceted, involving reductions in sulfate sources, biological sequestration of sulfur, passive removal processes, and the use of already impacted marsh area to reduce sulfate loads to unimpacted parts of the ecosystem.