U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet FS 109–03

January 2004

Impacts of Sulfate Contamination on the 

Florida Everglades Ecosystem

Printed on recycled paper

Introduction

Water quality is a key issue facing the restoration of the Florida Everglades. For example, agriculturally derived phosphorus 

entering the Everglades has caused eutrophication of 6 to 10 

percent of the ecosystem, resulting in changes in the native plant 

communities. Phosphorus contamination remains an important 

issue facing Everglades restoration, but recent work by the U.S. 

Geological Survey (USGS) has pointed to other water-quality 

issues critical to the overall health of the Everglades ecosystem. 

Among the most important of these other water-quality 

issues in the Everglades is sulfate (SO4

2-) contamination. Sulfate 

contamination affects as much as one-third of the freshwater 

Everglades, and sulfate concentrations are 60 to 100 times 

historical levels in heavily affected areas. The effects of sulfate 

contamination on ecosystem health have not been fully studied, but links between sulfate contamination and high levels 

of methylmercury in the ecosystem have been documented. 

Also, sulfate contamination may be contributing to declines in 

native plants by altering chemical conditions in the sediments. 

Ongoing USGS research will further delineate these effects and 

explore possible options for mitigating sulfate contamination in 

this unique ecosystem.

Sulfate in the Everglades

Pristine areas of the freshwater Everglades have low concentrations of sulfate in surface water, typically 1 milligram per 

liter (mg/L) or 1 part per million (ppm) or less. Historical levels 

of sulfate in the freshwater Everglades were probably also 1 mg/

L or less. Today, about one-third of the ecosystem has sulfate 

concentrations higher than 1 mg/L, and some marsh areas have 

sulfate concentrations in excess of 60 mg/L (fig. 1). In general, 

sulfate concentrations decrease from north to south.

Sources of Sulfate

Figure 1 shows that the highest concentrations of sulfate 

in the freshwater Everglades are found in (1) canals in the 

Everglades Agricultural Area (EAA), (2) canals in the Everglades, and (3) marsh areas that receive direct discharge from 

these canals (especially in Water Conservation Area 2). Lake 

Okeechobee is the source of water for EAA canals and for much 

of the water entering the Everglades. The concentrations of 

sulfate in Lake Okeechobee, although elevated, are significantly 

lower than those in EAA canals. This information strongly 

indicates that canals within the EAA are the principal source of 

sulfate contamination in the Everglades. Much of the sulfate in 

Lake Okeechobee may also originate from the EAA, from the 

backpumping of canal water into the lake.

Sulfur isotopic composition can be used to trace the sources 

of sulfate contamination. This approach uses the ratio of different isotopes of sulfur (34S and 32S) in sulfate to trace its source. 

Figure 2 shows a plot of sulfate concentration versus sulfur 

isotope composition (δ34S) for water samples from canals and 

Everglades marshes. At low concentrations, the sulfur isotope 

composition of sulfate varies over a wide range, indicating that 

multiple sources of sulfate are present. As sulfate concentrations 

increase, the sulfur isotope composition begins to form a trend 

line, indicating that one source is becoming dominant. The trend 

line points to a source having a sulfur isotope composition of 

about 15–16 per mil, which is the sulfur isotope composition of 

agricultural sulfur used in the EAA as a soil amendment. These 

data, therefore, are consistent with agricultural sulfur as a major 

source of sulfate contamination in the Everglades.

Figure 1. Map of the Everglades area, superimposed on a satellite image 

of South Florida, showing the distribution of sulfate contamination. The 

checkered area is currently the area of highest methylmercury production in the Everglades. NWR, National Wildlife Refuge; WCA, Water Conservation Area. Heavy lines indicate canals; large dots indicate terminal 

discharge areas; arrows indicate discharge points and leakage areas.
Figure 3 shows the major sources contributing sulfate to a 

heavily sulfur contaminated site in the Everglades. Rainfall and 

shallow ground water have sulfate concentrations too low to 

account for the high levels of sulfate in the marsh water. Deep 

ground water has high enough concentrations of sulfate to be a 

significant source, but its sulfur isotope composition and high 

content of chloride differ significantly from those of the marsh 

water. Clearly, EAA canal water is the best match, in terms of 

sulfate concentration and sulfur isotope composition, as the 

major source of sulfate contamination to the ecosystem (fig. 4). 

Sulfate Reduction

Microbial sulfate reduction is an important biogeochemical process in the recycling of elements at the Earthʼs surface. 

Sulfate reduction takes place only under oxygen-free (anoxic) 

conditions, usually in sediments. Sulfate-reducing bacteria use 

sulfate instead of oxygen in their metabolism of organic matter; 

in the process, the bacteria change (reduce) sulfate to sulfide, 

a very reactive substance. Sulfide reacts with metals to form 

insoluble metal sulfides such as pyrite (fig. 5) and with organic 

matter to form organic sulfur compounds. Anyone who has ever 

visited a salt marsh may remember the “rotten egg” smell of the 

sulfide produced by sulfate reduction. 

Sulfate reduction is a key process in marine environments 

where sulfate is abundant, but it is usually limited in freshwater 

wetlands by the low levels of sulfate. Sulfate contamination 

from EAA canal discharge, however, has greatly increased the 

rates of sulfate reduction in a large area of the Everglades. Pristine areas of the Everglades have virtually undetectable levels of 

sulfide in sediment pore water, but sulfate-contaminated areas 

have the “rotten egg” odor of sulfide present at levels 10,000 

times background in the sediment (as measured with a sulfide 

electrode). The sulfur cycle for the Everglades is illustrated in 

figure 6. 

The Methylmercury-Sulfate Connection

Methylmercury (MeHg) is another major water-quality problem in the Everglades. MeHg is a neurotoxin that is bioaccumulated and found in high concentrations in fish and other wildlife 

in the Everglades. Fish consumption advisories have been 

posted throughout the Everglades by the Florida Department of 

Environmental Protection because of the danger that this neurotoxin poses to human health, especially to pregnant women. The 

MeHg problem is directly linked to sulfate contamination of the 

Everglades, because sulfate-reducing bacteria produce MeHg 

(fig. 6). Mercury enters the Everglades mainly in rainfall after 

atmospheric transport from human and natural sources (such as 

burning of fossil fuels, incineration of medical waste, volcanoes). The mercury raining on the Everglades is transformed to 

toxic MeHg by sulfate-reducing bacteria in sediments.

Sulfate entering the Everglades in canal discharge stimulates 

microbial sulfate reduction and MeHg production. There is a twist, 

however, in the MeHg-sulfate connection. Buildup of sulfide 

in sediment pore water actually inhibits MeHg production, 

Figure 2. Plot of sulfate concentration and sulfur isotope composition 

(δ34S), suggesting agricultural sulfur use within the Everglades Agricultural Area (EAA) as a major source of sulfate contamination to the 

Everglades. WCA, Water Conservation Area; mg/L, milligram per liter.

Figure 4. Canal and adjacent agricultural fields in the Everglades 

Agricultural Area. Photograph by William H. Orem.

Figure 3. Sulfate sources at a heavily contaminated site, showing discharge 

of Everglades Agricultural Area (EAA) canal water as the major source. 

Sulfate concentration is measured in milligrams per liter (mg/L); sulfur 

isotope composition is measured per mil.
probably by making mercury less available to sulfate-reducing 

bacteria for MeHg production. Thus, sulfate stimulates production of MeHg, but sulfide is an inhibitor. In the ecosystem, this 

relation plays out as follows (fig. 7):

(1) Heavily sulfate-contaminated areas have low to moderate 

MeHg production due to stimulation by sulfate, but inhibition 

by sulfide.

(2) Pristine areas have low MeHg production because low sulfate concentrations limit sulfate reduction.

(3) The areas of highest MeHg production occur where sulfate 

contamination is moderate (2–10 mg/L) and sulfide levels are 

low enough to avoid the inhibition of MeHg formation. The 

area of highest MeHg production in the Everglades is currently located near the center of Water Conservation Area 3 (the 

checkered area in fig. 1), where sulfate and sulfide concentrations are optimum for MeHg production.

Sulfate contamination of the Everglades and inputs of new mercury from rainfall appear to be the major factors influencing the 

intensity and distribution of MeHg production in the ecosystem. 

Figure 5. Pyrite framboids (FeS2) from sediments in Taylor Creek, near 

Lake Okeechobee. Bar scale equals 20 micrometers (μm).

Figure 6. The Everglades sulfur cycle. Terms that are italicized (for example, Diffusion) are processes; bold type indicates interfaces.
Other Effects of Sulfate Contamination

USGS studies have shown that areas of the Everglades heavily affected by sulfate contamination and resulting high sulfide 

levels in sediments have much lower sediment redox conditions than pristine sites. Lower redox conditions (a measure 

of sediment reducing capacity or available electron balance) 

can alter the natural chemical conditions in sediments, affect 

microbial activity, and decrease dissolved oxygen penetration 

into sediments. Studies in other freshwater wetlands suggest 

that high levels of sulfide (from sulfate contamination) may 

result in changes in the native plant communities. Numerous 

metals, some of which are important micronutrients for plants, 

react with high levels of sulfide to form insoluble chemical 

compounds that plants are unable to use. USGS scientists and 

academic collaborators are using environmental chambers (fig. 

8) to study the effects of high levels of sulfate and resulting sulfide contamination on wetland plants native to the Everglades. 

Preliminary observations indicate a need to mitigate sulfate 

contamination of the Everglades. 

Sulfate Contamination and Everglades Restoration

Restoration efforts aimed at improving water quality in the 

Everglades have focused primarily on reducing phosphorus 

contamination in the ecosystem. These efforts have included 

using (1) Best Management Practices (BMPs) to reduce the use 

of phosphorus fertilizer in the EAA and (2) Stormwater Treatment Areas, which are constructed wetlands designed to remove 

phosphorus from water before it is discharged to the Everglades. 

These approaches have greatly reduced the amount of phosphorus from the EAA discharged into the Everglades, although 

further progress is still needed.

Unfortunately, little has been done to reduce sulfate contamination of the ecosystem. Indeed, increasing the flow of water to 

the Everglades, which is a major goal of restoration, will likely 

increase the amount of sulfate entering the Everglades. An 

increase in the amount of sulfate entering the ecosystem could 

exacerbate the MeHg problem and other effects of sulfate on the 

ecosystem. Geochemical conceptual models also suggest that 

increasing amounts of sulfate entering the ecosystem could also 

move the zone of maximum MeHg production south from the 

central Everglades into Everglades National Park.

Implementation of BMPs for sulfur use in the EAA could 

significantly reduce the amount of sulfate entering the Everglades. BMPs for sulfur could include (1) reducing the amounts 

of agricultural sulfur used, (2) replacing agricultural sulfur 

with another soil amendment that would enhance phosphorus 

uptake by plants, and (3) using chloride instead of sulfate as the 

counter-ion in many fertilizers and other agricultural products. 

Unfortunately, Stormwater Treatment Areas appear to have 

little effect on reducing sulfate in agricultural discharge waters. 

Research on other approaches for mitigating sulfate inputs to 

the ecosystem is needed.

By William H. Orem

For further information on sulfur contamination in the 

Everglades, please contact

William H. Orem

U.S. Geological Survey

956 National Center

Reston, VA 20192

Telephone: 703–648–6273

E-mail : borem@usgs.gov

For further information on mercury contamination in 

the Everglades, please contact

David P. Krabbenhoft

U.S. Geological Survey

8505 Research Way

Middleton, WI 53562

Telephone: 608–821–3843

E-mail: dpkrabbe@usgs.gov 

Figure 7. The methylmercury-sulfate connection in the Everglades. Hg, 

mercury; P, phosphorus; MeHg, methylmercury; NWR, National Wildlife 

Refuge.

Figure 8. Experimental chambers in the Everglades used to study the 

effects of sulfate contamination. Photograph by William H. Orem.