publications > open file report > OFR 2007-1374
U.S. Department of the Interior
U.S. Geological Survey
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, email@example.com
- 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parts of the northern Everglades where sawgrass has been replaced by natural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
- 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
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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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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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