Sulfate enters the Everglades almost entirely via discharge of contaminated canal water. The source of the sulfate in the canal water remains somewhat controversial, but currently available data suggest that a significant portion of it originates from recent and historic uses of sulfur in agriculture in the EAA. Strategies for mitigation of sulfate contamination of the Everglades could logically start with minimizing anthropogenic uses of sulfur that could enter the ecosystem. This would likely require the cooperation of the agricultural community in the EAA and north of Lake Okeechobee, agricultural researchers, manufacturers of agricultural fertilizers and soil amendments, water and land managers, wetlands scientists, and government officials. Further research to refine the amounts of sulfate coming from the EAA, groundwater, and elsewhere would certainly be needed. Agricultural research to determine the minimum sulfur requirements of crops grown in the EAA, and of grasslands for cattle north of Lake Okeechobee could be considered (data on this may already exist to a large extent). Anthropogenic use of sulfur-containing chemicals in agriculture needs to be evaluated in order to balance adequate sulfur nutrition for crops (sugarcane and vegetables) and grass with minimizing sulfate runoff and resulting impacts on the Everglades. It may be that sulfur already bioavailable in agricultural soils is sufficient to fulfill the needs of various crops. In some instances, fertilizers are added with sulfur in the mixture as an addition to the main element, or as a counter-ion in the mixture (e.g. ammonium added as ammonium sulfate to grazing fields, or metals added as metal sulfates to EAA crops; see Table 1). If these sulfur additions are determined to be unnecessary for plant nutrition, then substitution of more inert chloride as a counter-ion could be implemented at the manufacturing level. Fungicides are another source of sulfur to the ecosystem, especially the use of elemental sulfur as a broad spectrum fungicide and copper sulfate as a fungicide in citrus production. More environmentally innocuous alternatives could be considered, especially bioengineered fungicides that are specific for a particular fungus type. In the case of copper sulfate, cupric chloride could be considered as an alternative, although Cu itself has unwanted environmental effects (Flemming and Trevors, 1989; Leslie, 1990). As noted earlier, any reductions in anthropogenic sulfur use will benefit the ecosystem.

Currently available data do not support groundwater as a major source of sulfate to the ecosystem. This is excluding possible sulfur sources from aquifer storage and recovery. If groundwater is shown to be a significant source, then approaches for reducing groundwater fluxes (especially deep groundwater >9 m, which has very high levels of sulfate) could include reducing any unnecessary groundwater usage, and repairing leaky canal bottoms.

There is probably little that can be done to reduce sulfur sources from soil oxidation in the EAA and elsewhere in the Everglades' watershed. This soil sulfur represents sulfur that was applied for agricultural purposes (new and legacy) and natural sulfur in the soil. Agricultural practices in the EAA are dependent on a dry and oxic surface layer of soil. Microbially mediated oxidation transforms most of the sulfur in this layer to sulfate, which is readily washed into canals during rain events. Maintaining reducing conditions in surface soils by waterlogging would prevent oxidation of reduced sulfur species, and effectively sequester sulfur in the soil, but most crops cannot be grown under such conditions. Unless water tolerant sugarcane can be developed, oxidation of EAA soil will likely continue. Rotation of rice with sugarcane, however, does help the situation by reducing oxidation during the period of rice cultivation.

In addition to reducing anthropogenic uses of sulfur, it is likely that active mitigation strategies will be needed to reduce sulfur loads to levels that will significantly benefit the Everglades ecosystem. A number of different mitigation approaches have been briefly outlined in the preceding sections of this report. Strategies range from passive approaches such as the use of minerals that absorb sulfate, or anaerobic microbial processes to sequester sulfur, to more active (and expensive) approaches such as nanofiltration or ion exchange. The relative merits of the various mitigation strategies in terms of effectiveness in removing sulfate and cost (initial and ongoing costs) are summarized in Table 2.

The current configuration of existing macrophyte-dominated STAs appears to have limited capacity to sequester sulfate as reduced sulfur species in soils. This is probably due to three factors: (1) the slow rate of diffusion of sulfate into sediments where MSR and sulfur sequestration occurs, (2) inefficiencies in the microbial consortia that supports MSR , especially in regard to cellulose decomposition, and (3) limitations in the availability of iron for the sequestration of reduced sulfur species as iron mono- and disulfides. Periphyton-dominated STAs (PASTAs) may be more efficient at removing sulfate through formation of extensive anoxic floc layers at the sediment/water interface. These floc layers may enhance sulfate diffusion to sites of MSR. Also, the algal material is more biodegradable than vascular plant-derived organic matter from macrophytes and may stimulate production of organic substrate supporting MSR. The efficiency of PASTAs in removing sulfate, however, is unclear at this time. Studies of sulfate removal efficiencies in macrophyte-dominated STAs versus PASTAs would provide useful information.

The creation of zones in the STAs or PASTAs that resemble PRBs might also be effective in sulfate removal. Trenches at the ends of STAs and PASTAs composed of organic-rich material and zero-valent iron may increase MSR and sulfur sequestration. One approach for evaluating this could involve initial laboratory testing with Everglades' canal water, and follow-up field tests in an up scaled PRB. Sequestration of sulfate from canal water could also be achieved using a bioreactor approach, using either addition of barium to precipitate sulfate as BaSO₄ under oxidizing conditions, or using a reducing bioreactor to facilitate MSR and sequestration of sulfide. Bioreactor approaches, while effective, would likely be extremely costly to construct, operate, and maintain. They also present the problem of disposal of the precipitated BaSO₄ or metal sulfides.

Active mitigation using nanofiltration or ion exchange are extremely effective at removing sulfate from water both at the laboratory scale, and in desalination plants for production of drinking water. Up scaling to the volumes involved in treatment of water from Everglades' canals, however, may be problematic. In addition, issues of biofouling of membranes in nanofiltration or clogging of ion exchange resins would have to be overcome. These approaches may be most effective in controlling sulfate outputs from feeder canals coming from individual farms. Individual farms are locations where sulfate loads may be highest, and water volumes lowest, allowing the most efficient application of the nanofiltration or ion exchange approaches.

The problem of sulfate contamination of the Everglades ecosystem is many decades old, and it will likely take some time to fully address this issue. Observations from this report suggest some possible initial sulfur mitigation strategies: (1) further studies to determine the sources of sulfate would constrain which approaches to reduce these sources would be most effective, (2) explore ways to eliminate or reduce these sources of sulfur, especially through reductions in anthropogenic uses of sulfur, (3) examine biological removal processes, improvements to existing STAs, and PASTAs as sulfate-removal wetlands, (4) examine possible use of PRBs (possibly in combination with STAs or PASTAs) to reduce sulfate loads, and (5) examine the economics and practicality of nanofiltration and ion exchange at the individual farm level for active mitigation. The first three strategies involve the use of existing scientific expertise and infrastructure. If the economics of active mitigation using nanofiltration or ion exchange look reasonable, initial pilot studies could be undertaken.

The success of sulfur mitigation strategies do not depend on attaining pre-development levels of sulfate in the ecosystem. Although this would be desirable, current conditions in the sulfur source areas likely preclude attaining levels of < 1 mg/L sulfate in large areas of the Everglades. Nevertheless, studies in the Everglades have demonstrated that any significant reduction in current sulfate loads to the ecosystem will have beneficial results, especially with regard to levels of MeHg, and that the response of the ecosystem to reduced sulfur loads is likely to be rapid.

Table 2. Relative effectiveness and costs among different active and passive sulfate mitigation strategies for the Everglades (L = Low, M = Moderate, H = High).