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U.S. Department of the Interior
U.S. Geological Survey
OFR 2007-1374

Sulfur Contamination in the Florida Everglades: Initial Examination of Mitigation Strategies

By William H. Orem, U.S. Geological Survey, 956 National Center, Reston, VA 20192 USA, 703-648-6273, borem@usgs.gov

photograph showing aerial view of Stormwater Treatment Areas in the Northern Everglades


> Summary
Review of Sulfur Contamination
Reducing Sulfate Sources
Bio. Mitigation
Natural Minerals Mitigation
Chem. Treatment Mitigation
Active Removal Mitigation
Tables and Figures

  • Sulfate in excess of 1 mg/L contaminates about 60% of the freshwater Everglades (background sulfate levels in the Everglades are estimated to be < 1 mg/L).
  • Excess sulfate originates from EAA canal discharge. Isotopic data is consistent with sulfur used in agriculture (current applications and legacy in soil) as a primary source of the excess sulfate. Deep groundwater could also contribute, however, currently available data does not support groundwater as a major source of sulfate to the Everglades.
  • Sulfate entering the Everglades stimulates microbial sulfate reduction (MSR), production of sulfide, and methylmercury production. A unique combination of conditions in the Everglades, including high mercury deposition, sulfate contamination, and favorable environmental conditions (extensive wetland area, wet/dry cycles, high dissolved organic carbon) result in high levels of methylmercury production and bioaccumulation.
  • Buildup of toxic sulfide in Everglades - soils from sulfate stimulation of MSR makes soils more reducing , impacts macrophyte growth, and may impact other flora and fauna. Greenhouse experiments show that growth of sawgrass is adversely affected by sulfide toxicity at sulfide levels above 9 ppm. Levels as high as 13-15 ppm have been observed in heavily sulfur impacted parts of the northern Everglades where sawgrass has been replaced by natural invasion by cattail.
  • Sulfate loading can stimulate phosphate and ammonium release from wetland soils via a process referred to as internal eutrophication (Lamers et al., 1998). Mesocosm studies in the Everglades have demonstrated that sulfate loading at levels equivalent to those observed at sulfur-contaminated sites in the northern Everglades enhanced remobilization of ammonium, sulfate, and dissolved organic matter from soils to porewater and surface water.
  • Current restoration plans to deliver more water to the Everglades will likely increase overall sulfur loads to the ecosystem, impacting areas that currently do not have elevated levels of sulfur. Delivery of sulfate contaminated water to areas like Everglades National Park, ARM Loxahatchee National Wildlife Refuge, and Big Cypress National Preserve through the canal system and may serve to exacerbate the harmful effects of sulfate on the ecosystem (Gilmour et al., 2007 a, b). Sheet flow over expansive marsh areas that reduces sulfate loading is preferable.
  • Dry/rewet cycles have been shown to temporarily increase surface water sulfate concentrations (due to oxidation of reduced sulfur in soil), stimulating MSR and methylmercury production. Although dry/rewet cycles are a natural phenomenon in the Everglades, current water management practices and present conditions of sulfur-contaminated soils and high atmospheric mercury deposition make these cycles more damaging by exacerbating methylmercury production and bioaccumulation. Minimizing dry/rewet cycles would help limit methylmercury production in the Everglades.
  • Surface water stored in underground aquifers (aquifer storage and recovery) may acquire significant additional sulfate through interaction with connate seawater or dissolution of gypsum in the underground reservoirs, and costs versus benefits of using this approach in water management need to be considered.
  • Monitoring data suggests that the ecosystem response to declines (or increases) in sulfate loading is rapid. A decline in sulfate concentrations in surface water in the central Everglades during the late 1990s (probably due to changes in water discharge management) resulted in a rapid decline in methylmercury production and bioaccumulation here within 3-7 years.
  • Because of the serious impacts of sulfate on the Everglades, and the rapid response of the ecosystem to reductions in sulfate loading, a comprehensive Everglades restoration strategy could include reduction of sulfur loads as a goal. Mitigation of sulfate contamination in the ecosystem could be multifaceted, and might incorporate reductions in the many uses of sulfur in agriculture, reduction of groundwater sources (if important), investigation of methods for passive sequestration of sulfate as solid-phase reduced sulfide, reengineering of existing stormwater treatment areas (STAs) for better sulfate sequestration, and consideration of active mitigation of sulfate in runoff water (nanofiltration, ion exchange) at the individual farm level.
  • Existing macrophyte-dominated STAs remove limited amounts of sulfate from surface water, possibly due to slow rates of diffusion of sulfate into soil where sequestration occurs, limited availability of iron for metal sulfide precipitation, and limitations on substrate production for microbial sulfate reduction. Periphyton-dominated STAs (PASTAs) may provide more extensive floc to fuel microbial sulfate reduction and sequestration of sulfur.
  • Engineering permeable reactive barriers (zero-valent iron/organic substrate combined) into the inflow and outflows of STAs may enhance their effectiveness for sequestering sulfate.
  • Active mitigation strategies such as nanofiltration and ion exchange can be highly effective in removing contaminants like sulfate from water, but are expensive and subject to biofouling. Testing the use of active mitigation technologies at the individual farm level would provide cost/benefit information on this technology.
  • The reduction of sulfate concentrations in the Everglades from current levels (60 mg/L at some sites) to levels approaching background (< 1 mg/L) would be a desirable goal, but is unlikely to be achieved as long as current agricultural practices persist in the source area and flow path of water that feeds the Everglades. It is clear that any reduction in sulfate loads entering the Everglades will benefit the ecosystem's overall health. A multifaceted approach employing reduction in anthropogenic source loads of sulfur, and passive and active mitigation will help achieve lower overall sulfate levels in the Everglades, and resulting benefits.

Abbreviations >

U.S. Department of the Interior
Dirk Kempthorne, Secretary

U.S. Geological Survey
Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia 20192

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Suggested citation:
Orem, W., 2007, Sulfur contamination in the Florida Everglades: Initial examination of mitigation strategies: U.S. Geological Survey Open-File Report 2007-1374.

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Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted material contained within this report.

Related information:

SOFIA Project: Linking Land, Air and Water Management in the Southern Everglades and Coastal Zone to Water Quality and Ecosystem Restoration: Task 2, Sulfur and Nutrient Contamination, Biogeochemical Cycling, and Effects

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