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3. Reducing the Source(s) of Sulfate to the Everglades
3.1 Sulfur use in agriculture -Sulfur plays three principal roles in agriculture: (1) as a plant nutrient, (2) as a soil amendment for pH adjustment, and (3) use as a fungicide (Meyer, 1977). Sulfur is also present in a number of fertilizers where it serves the purpose of an additional plant nutrient and as a counter ion to the principal nutrient in the fertilizer (Table 1). Sulfur (as sulfuric acid) is also used in the production of phosphate fertilizer from phosphate rock, with phosphogypsum produced as a byproduct (Ober, 1999). Only a small amount of phosphogypsum is used as a soil amendment in agriculture because of its high radium content (derived from the phosphate rock). More than 700 million tons of phosphogypsum is stored in large piles in central Florida, where it poses a hazard to surface and groundwater (Johnson and Traub, 1996).
Sulfur is a basic nutrient requirement for plant life including agricultural plants such as sugarcane and vegetables grown in the EAA, citrus trees cultivated north of Lake Okeechobee, and grasses used by cattle north of the lake (Fig. 4). Indeed, sulfur is required in about the same amounts as phosphorus by plants (Beaton, 1966; Tabatabai, 1984). Plants utilize sulfur for the synthesis of essential amino acid and proteins, some vitamins and coenzymes, glycoside oils, disulfide linkages and sulfhydryl groups, and for enzyme activation (Coleman, 1966). The molar sulfur/nitrogen ratio (a measure of plant sulfur requirements) ranges from about 0.02 to 0.03, which is about the same as the molar S/N ratio in plant proteins that constitute about 80 per cent of the organic S and N present (Dijkshoorn and Van Wijk, 1967). For sugarcane, sulfur requirements in the plant vary with age from 0.36% (whole plant) and 0.24% (leaf blades) for early growth, to 0.10% (leaf blades) and 0.08% (leaf sheaths) in 70 day old plants (Fox, 1976). Sulfur for plant nutrition can be applied directly as elemental sulfur, sulfur-bentonite mixes, ammonium sulfate, potassium sulfate, or superphosphates. Decreases in atmospherically deposited sulfur from air pollution in recent years may increase the need for sulfur fertilization of crops in some locations (Donald et al., 1999).
As a soil amendment, agricultural sulfur is used to adjust soil pH (Boswell and Friesen, 1993). The pH of soil can affect the uptake of essential nutrients by sugarcane and other agricultural plants. For example, at a pH of 7.5 or higher, virtually all phosphorus will be tied up as calcium phosphate and unavailable for plant growth. Other nutrients (e.g. potassium and nitrogen) also become less available for supporting plant growth at higher pH. The optimum pH for uptake of most nutrients is about 6.5. To adjust pH to optimum values for nutrient uptake by plants, elemental sulfur (agricultural sulfur) is often added to the soil. In oxic soils (such as surface soils in the EAA) the elemental sulfur is oxidized to sulfuric acid (S + 3/2 O2 + H2O -> H2SO4), with the process usually catalyzed by aerobic bacteria (e.g. Thiobacillus sp.). The rate of acid release from elemental sulfur can be controlled by the size of the sulfur particles added. Agricultural sulfur has historically been added to EAA soils, and continues to be used as a soil amendment in the EAA. Gypsum may also be added to soil to increase the sulfur content and as a soil amendment. Sulfate derived from both elemental sulfur oxidation and from gypsum has been shown to be highly mobile in organic matter-rich soils (Rhue and Kamprath, 1973). Gypsum or agricultural sulfur applied to soil in the EAA may be readily leached into drainage canals as sulfate. In contrast, Sakadevan and others (1993) found sulfur applied as fertilizer (superphosphate) to grazing fields was stored in the soil mostly as organic sulfur, and released as sulfate primarily through mineralization of the soil organic matter rather than directly from the fertilizer.
Elemental sulfur is also among the oldest fungicides still in use. ulfur and copper-containing mixtures were the major fungicides used in agriculture until the advent of synthetic organic compounds (e.g. alkyldithiocarbamates, organotins, quinones, and phthalimides) in the 1940s. In the 1960s systemic materials with more specificity for individual fungal organisms were developed (Wheeler, 2002). Bacterial fungicides, bacteria that compete with and attack specific fungi, have recently been developed as a potential alternative to chemical fungicides (National Academy of Sciences, 2000). These bacterial fungicides are not widely used yet, but offer a potentially more environmentally friendly alternative to chemical fungicides in the future. Natural chemicals produced by bacteria, plants, and other organisms are another focus of study for environmentally safe fungicides. For example, a number of substances isolated from Bacillus bacteria have been shown to control some significant fungal diseases of corn, potatoes, and beans (National Academy of Science, 2000). These natural substances offer a wide variety of active chemical ingredients with new mechanisms of antifungal action, and they have low risks to the environment. Sulfur and sulfur-containing compounds, however, are still used extensively as broad-spectrum fungicides at rates of 100 tons/yr for vegetable growing areas and 583 tons/yr for citrus growing areas within the South Florida Water Management District management area (Miles and Pfeuffer, 1997). Copper sulfate is another sulfur-containing fungicide still widely used in citrus production (Michaud and Grant, 2003; McCoy et al., 2003). Methyl bromide (3064 tons/yr) and chloropicrin (374 tons/yr) are probably the most important fungicides used in the EAA, mostly on vegetable crops (Miles and Pfeuffer, 1997).
The amount of total sulfur used in various soil amendments, fertilizers, and fungicides in the EAA is unknown. Also unknown is the total sulfate entering canals as runoff from EAA fields. The EAA soils in general have pH values ranging from about 5 to 7.5, and possibly higher than 7.5 in some cultivated fields (Bottcher and Izuno, 1994). Ideal pH for phosphorus uptake from soil by crops is about 6, while many metal micronutrients are taken up most efficiently at pH <6 (Lucas, 1982). Thus, for most efficient uptake of phosphorus and metals, the pH of soil in the EAA are often reduced. This is typically accomplished by the addition of elemental sulfur (agricultural sulfur), as discussed earlier.
Schueneman and Sanchez (1994) indicate that elemental sulfur in the amount of 500 to 1,700 kg/ha-yr is needed to reduce the pH of soil by 0.2 to 0.7 units for vegetables in the EAA, and 560 kg/ha-yr for multiyear sugarcane production. In a later publication, Schueneman (2000) suggests that about 37 kg/ha-yr of sulfur (about 111 kg/ha-yr converted from sulfur to sulfate) is currently added to EAA soil, based on interviews with farmers in the EAA and estimates from fertilizer sales. This estimated total includes sulfur additions to EAA soil from agricultural sulfur and superphosphate, but does not include additions of other fertilizers (Table 1), nor does it include fungicides containing sulfur. Schueneman (2000) also incorrectly estimates contributions from Lake Okeechobee by a factor of 3 due to a calculation error. Schueneman (2000) also assumes none of the sulfate in Lake Okeechobee originates from sulfur use in the EAA, an unlikely assumption considering the high sulfate levels in the EAA rim canal adjacent to the lake. Lake Okeechobee also receives some sulfate contributions from sources north of the lake, but the source is unclear at this time (Zielinski et al., 2006). The release (as sulfate) of sulfur sequestered in soil in the EAA as a result of soil oxidation is the largest single contributor to sulfate in EAA canals in the estimate of sulfur sources by Schueneman (2000). The sulfate released following soil oxidation likely includes current and legacy sulfur used for agriculture within the EAA as well as some natural (background) sulfur. Soil sulfur levels in the EAA are considerably higher than those of the nearby Everglades, indicating anthropogenic contributions to soil total sulfur. Thus, the conclusion in Schueneman (2000) that most of the sulfur entering canals in the EAA is from "natural" sources is misleading. Evidence that the high sulfate levels in canal water within the EAA originates largely from agricultural sources of sulfur was discussed earlier (also, see Bates et al., 2002).
Understanding where sulfur in EAA canals originates is critical for developing strategies for eliminating or reducing the source(s). Mass balance studies of the total sulfur used yearly in agricultural lands and urban areas, and sulfate runoff from EAA lands would constrain these anthropogenic sources. Additional studies of the contribution (if any) of deep groundwater to sulfate loads in EAA canals would also provide important information to managers (see section 3.2). Based on the current data, it is reasonable to assume that current agricultural practices (fertilizers, soil amendments, fungicides) introduce some of the sulfate entering canals in the EAA. Any reduction in sulfate load is likely to benefit the ecosystem's health. Reductions in the amount of sulfur used in agriculture will be necessary to achieve any significant reductions in sulfate loads to the ecosystem. Reducing the use of sulfur in the EAA will only be accomplished by involving all stakeholders in determining ways to balance the sulfur needs of agriculture with minimizing sulfur loading to the Everglades. Fertilizer manufacturers could consider the use of chloride instead of sulfate as the counter ion in many fertilizers. Unfortunately, the sulfur and phosphorus contamination issues are in conflict because reducing phosphorus use encourages use of sulfur to make more phosphorus available to crops. Best management practices (BMPs) in the EAA will need to incorporate considerations for the use of both sulfur and phosphorus in agriculture, but will need to balance profitability for farmers with protection of the environment.
3.2 Sulfate in groundwater -Groundwater in the Everglades has variable sulfate contents. In general, groundwater < 9 m below the surface often has relatively low sulfate concentrations, generally < 10 mg/L, while groundwater deeper than 9 m may have sulfate concentrations ranging from 100s to 1,000s of mg/L, probably representing connate seawater (Sprinkle, 1989; Bates et al., 2001). These very high sulfate and total dissolved solids levels in deep groundwater show that deep groundwater could be a significant contributor to sulfate contamination of canals and Everglades' wetlands. The available dataset, however, does not support deep groundwater as a major source of sulfate to the ecosystem (Gilmour et al., 2007b). Further work is needed to examine the role (if any) of groundwater as a source of sulfate contamination to the ecosystem.
If groundwater does contribute to sulfate loads to the Everglades ecosystem, this most likely occurs from advection of groundwater through the fractured bottoms of canals or via direct pumping of groundwater for fire control or other purposes. Studies of porewater from soil profiles to bedrock throughout the Everglades provide no indications of significant groundwater flux (advection or diffusion) of sulfate through marsh soil (Orem, unpublished data). Reduction of any groundwater pumping released to the canals (directly or indirectly) could be considered. Sealing of canal bottoms (a potentially expensive undertaking) could solve problems of advection of high sulfate groundwater.
3.3 Water from aquifer storage and recovery -One part of the Comprehensive Everglades Restoration Plan (CERP) calls for storage of surface water in underground aquifers for later removal, so-called aquifer storage and recovery (ASR) (National Research Council, 2002). Water accumulated during wetter periods would be stored in underground aquifers for later removal during drier periods. The principal storage reservoir planned for use in south Florida under CERP is the Upper Floridan aquifer. Unfortunately, in south Florida this aquifer contains brackish to saline water that may affect the quality of the surface water collected and stored for later use (Reese, 2001). Sulfate concentrations in the Upper Floridan aquifer generally range from 100 - 1,000 mg/L (Reese, 2000; Reese and Memberg, 2000), which is similar to values observed in groundwater > 9 m deep as reported by Bates and others (2002). Mixing of the high sulfate groundwater in the aquifer with lower sulfate in the stored freshwater will increase sulfate levels in the recovered water (Fig. 5). In addition, surface freshwater stored in the Upper Floridan aquifer may acquire sulfate from dissolution of gypsum in the aquifer matrix (Reese, 2000; Wicks and Herman, 1996). Sulfate could be lost from water stored in an ASR system if MSR causes precipitation of sulfide minerals or other forms of sulfide sequestration.
Preliminary studies of ASR water quality in south Florida (Mirecki, 2004) indicate that sulfate concentrations in recovered water do not exceed the 250 mg/L standard for drinking water supplies (Code of Federal Regulations, 2002). The 250 mg/L level, however, is far in excess of sulfate concentrations in surface water of marshes and canals in the freshwater Everglades (Bates et al., 2002). This indicates that the surface water stored in the Upper Floridan aquifer has acquired a significant load of sulfate during short term storage. Discharge of ASR water into the ecosystem will therefore increase sulfate loading. The drinking water standard is based on human health effects (gastrointestinal issues) from drinking high sulfate water, and does not take into account the adverse impacts on the ecosystem of sulfide and MeHg production from MSR stimulated by the excess sulfate. It is important to consider the use of ASR water in terms of costs (lower water quality) and benefits (more water) to the ecosystem. Mitigation of sulfate in ASR water will likely be needed to avoid excessive sulfur contamination of the Everglades and resulting impacts, such as MeHg production and bioaccumulation, sulfide toxicity to biota, and enhanced N and P remobilization, that would be detrimental to the ecosystem.
3.4 Flow path -One of the major goals of Everglades' restoration is the movement of more water to areas to the south, especially Everglades National Park (ENP) (Perry, 2004; U.S. Army Corps of Engineers, 1996; U.S. Army Corps of Engineers and South Florida Water Management District, 1999). Ultimately, planners hope to achieve sheet flow across the current Water Conservation Areas and into ENP, simulating the flow of water in the ecosystem prior to development and water management beginning in the early 1900s (Clarke and Dalrymple, 2003).
From the standpoint of reduction of sulfate contamination across the ecosystem, sheet flow is probably the most desirable option. Slow sheet flow across a wetland will allow effective diffusion of sulfate into Everglades' soil (peat) where MSR and sequestration of the resulting sulfide can occur. Moving sulfate-contaminated canal water through re-engineered STAs or PASTAs, and then by slow sheet flow across the northern Water Conservation Areas may prove an effective treatment for protecting the more unimpacted parts of the Everglades further south. A less desirable option is the directed flow of sulfate-contaminated canal water via canals for discharge in currently uncontaminated parts of the Everglades. Although this may achieve short term goals of providing more water to pristine areas such as ENP, it would likely lead to extensive contamination of portions of ENP with sulfate, triggering MeHg production and bioaccumulation in fish and other wildlife (Fig. 6). There is already some evidence of canal water containing sulfate entering ENP down the L67 canal. Increases in MeHg levels of fish in ENP have been reported recently (Axelrad et al., 2007).
3.5. Control of dry/rewet cycles -Another water management issue of concern with respect to sulfate sources is control of dry/rewet cycles. The USGS and the Smithsonian Institution have jointly examined the impacts of dry rewet/cycles on the geochemistry of the Everglades, in both field and laboratory studies (Krabbenhoft and Fink, 2001; Gilmour et al., 2004). Results of these studies show that drought (or fire) followed by rewet causes: (1) oxidation of organic soils (peats), transforming reduced S in sediments (organic sulfur and metal sulfides) to sulfate, (2) remobilization of this sulfate into the water column following rewetting, and (3) stimulation of MSR and MeHg production from the remobilized sulfate (Fig. 7). The fire/drought model linking sulfur and MeHg production has important implications for management of the Everglades and STAs, especially STAs that are routinely dried by surface water draw down. Problems with STA 2 producing periodic large plumes of MeHg were shown to be linked to routine dry down and rewet cycles. Limiting dry/rewet cycles in STA2 resulted in much lower levels of MeHg production. This also applies to the greater Everglades, especially those areas with elevated sedimentary sulfur resulting from decades of elevated surface water sulfate loading. Although dry/rewet cycles are a natural phenomenon in the Everglades, present conditions of sulfur-contaminated sediments and high atmospheric mercury deposition exacerbate MeHg production and bioaccumulation in these dry/rewet cycles.
U.S. Department of the Interior, U.S. Geological Survey
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