SULFUR/MERCURY POLLUTION THREATENS HUMANS,
WILDLIFE AND THE EVERGLADES
John F.H. Purcell, Ph.D.
The issue of phosphorus contamination flowing from the Everglades Agricultural Area (EAA) into The Everglades has been a major focus of water quality concerns in recent years. But an equal and in some ways greater threat to the Everglades ecosystem, one which has received much less attention than phosphorus, is the discharge of sulfur compounds (mainly sulfates) from the EAA into the Everglades protected areas. Metallic mercury, normally harmless to the ecosystem, is present in most parts of Florida and indeed in many wetlands in the United States. Some is of local origin but most is delivered by rainfall from distant areas either in the United States or abroad. But the Everglades is unfortunate in its location just downstream from a very large source of sulfates produced by the large sugar farms in the EAA. Sulfates react with metallic mercury to form methyl mercury, a deadly neurotoxin, which causes injury to nerves and nerve tissues of wildlife and humans.
Whats so bad about methyl mercury?
Methyl mercury is a bioaccumulative toxin meaning that it enters the Everglades food chain at the lowest levels in microbes and algae and becomes more concentrated as smaller animals are eaten by larger ones and eventually large amounts of the toxin accumulate in the tissues of the top predators such as large freshwater fish, wading birds alligators and mammals like raccoons and Florida panthers. But the damage does not stop here. Unlike phosphorus, methyl mercury is a threat to human health. Humans who consume the flesh of top predators from the Everglades such as bass or alligator tails are at risk, particularly pregnant women, fetuses, and small children. In recognition of this danger, The Florida Department of Health has warned against eating fish caught in the Everglades and notices have been posted at methyl mercury hot spots (areas of high concentration) in The Everglades, warning fishermen not to consume their catch. Methyl mercury has been measured at dangerous levels in the tissues of top predators in several parts of the Everglades, including the National Park and other protected areas from Lake Okeechobee to Florida Bay.
The Science Behind the formation of methyl mercury-The Goldilocks effect.
Even by the 1990s, the connection between sulfur and methyl mercury was well understood by scientists and described in peer-reviewed research in universities as well as by the U.S Geological Survey (see suggested reading: Orem and Krabbenhoft 2004). Yet the claim is still heard from representatives of Big Sugar (and, amazingly, recently echoed by the State Department of Environmental Protection) that there is insufficient understanding of mercury cycling in the environment given as a justification for resisting restrictions on the dumping of sulfates into The Everglades. This view is demonstrably outdated and scientifically incorrect.
Methyl mercury has been ignored (and its connection to sulfur denied by the sugar interests) as a dangerous pollutant, while phosphorus nitrogen and pesticides flowing from the Everglades Agricultural Area are the target of lawsuits and some ameliorative action by courts and State and Federal Agencies. (See the article by Albert Slap in this newsletter). This odd contradiction may be due to the more complex relationship between cause and effect with respect to the role of sulfur in the formation of methyl mercury.
1. Sulfur does not become part of the toxic compound but simply triggers its formation. This makes its role less obvious.
2. Characteristic of most wetlands is an anoxic (oxygen free) environment in the sediments where anoxic bacteria metabolize the rotting vegetation. In the absence of sulfur (even if metallic mercury is present), this metabolism takes the form of methane gas production. However, the presence of sulfate sets the stage for the dominance of sulfur reducing bacteria, which use sulfur instead of oxygen as an electron acceptor during respiration. In the process, the bacteria produce an enzyme, which attaches a methyl group to metallic mercury already within the sediment. This reaction, creating methyl mercury, occurs within the bacteria, and is simply a by-product of the bacterial metabolism.
3. In wetlands like The Everglades, metabolism of sulfur reducing bacteria associated with methyl mercury production also converts sulfate to sulfide. Sulfide in high concentrations is toxic to plants but the buildup of sulfides also inhibits methyl mercury production.
4. Thus a high concentration of sulfates produces a high concentration of sulfides and methyl mercury production is curtailed. This balance between methyl mercury production in the presence of sulfates and its inhibition by the resulting sulfides has been called the Goldilocks effect. Harmless metallic mercury is converted to the neurotoxin, methyl mercury, when the concentration of sulfates is neither too low, (insufficient sulfates do not stimulate the activities of sulfate reducing bacteria) nor too high (large concentrations of sulfates lead to the creation of sulfides which inhibit the formation of methyl mercury), but just right. Because of the Goldilocks effect, methyl mercury hotspots usually do not occur immediately south of the Stormwater Treatment Areas (STAs) where concentrations of sulfates from the agricultural area are highest, but instead occur further south where they have been diluted to the just right range. As the dumping of sulfur into the protected areas has continued, some of these hot spots have developed in the Shark River area within the National Park itself.
How do we know the sulfur comes from the agricultural areas?
The sugar industry and its apologists, not surprisingly, question the claim that sulfates come mainly from agriculture in the EAA. Other significant sources, they say, are rainfall, deep groundwater that mixes with seawater (which contains large natural concentrations of sulfur) and so-called legacy sulfur which was applied to fields in previous years and flows out when old fields are flooded. The issue has been the subject of considerable research (see suggested reading: Aiken et. al. 2011), which left little doubt that while perhaps not the only source of sulfates, the EAA is a major one. There is little argument that thousands of metric tons of sulfur compounds are applied to sugar fields in the EAA every year. It can also be shown that tons of sulfate flows downhill into canals, through the Stormwater Treatment Areas (STAs) and into the protected areas of The Everglades every year. Friends of the Everglades has calculated, using
data from the South Florida Water Management District that over the past five years, nearly 300,000 metric tons of sulfate have flowed through the STAs into the Everglades. This figure does not include the difficult to calculate but certainly larger amounts that flow directly into canals without passing through the STAs and are discharged into protected areas further south. By comparing different isotopes of sulfur in water present in different possible sources, scientists have been able to show that there is virtually no sulfur entering via rainfall and that there is very little connection between Everglades water and deep groundwater (see suggested reading: Corrales, et. al. 2011). They conclude that there can be only two significant sources: sulfur recently applied to fields in the EAA or legacy sulfur from soil oxidation of reflooded fields. In both cases, the sources are within the EAA.
What is to be done?
The scientific literature for at least the past decade-and-a-half shows that attacking the problem by attempting to remove metallic mercury cannot succeed. While this may be a worthy goal in itself it is probably a hopeless one from the local perspective of reducing methyl mercury in The Everglades. First, local sources of mercury have already been reduced without any impact on levels of methyl mercury. Secondly, the main source of mercury is the burning of coal and other fossil fuels, delivered by rainfall either from other states or from as far afield as China and Brazil. Successful solutions will need to come from closer to home. Control of sulfur discharges represents the only practical solution since other contributing factors such as organic carbon and iron would be impossible to control in an environment like the Everglades. Increasing the size and number of STAs to remove pollutants as is being done to reduce phosphorus pollution is impractical due to the extremely high concentration of sulfur discharged through the STAs every year. The amount of sulfur that would have to be removed by plants in the STAs is a thousand times the amount of phosphorus. The reduction of water flows (which carry the sulfates) into the Everglades would be pointless since it destroys a major
purpose of the restoration project which is to provide greater water flows to the Everglades. If the large amounts of Sulfur compounds used in the agricultural areas to the north of the Everglades can be eliminated or greatly reduced, over time this can reduce the discharges. Suggestions to achieve this goal include the following:
• Introduction of Best Management Practices in the application of sulfur compounds backed up by State and Federal disciplinary action on groups of farmers and/or individuals who do not comply. This will require pressure, either political through the Florida legislature and/or judicial on the relevant agencies to motivate action.
• Demonstration by authoritative agricultural research bodies that the levels of current sulfur usage is unnecessary. There has been some early research, which suggests that this may be the case.
• Legal action to require the State government to enforce the polluter pays amendment, which already exists in the Florida Constitution.
Friends of the Everglades has, over the past year or so, undertaken initial research to quantify the threat of sulfur discharges into the Everglades. We depend on the support of our contributors to continue work to protect and restore the River of Grass.
1. Corrales, J. et. al. 2011. Sulfate threshold target to control methylmercury levels in wetland ecosystems, in The Science of the Total Environment 409(11) 2156-2162.
2. Orem, W.H. and Krabbenhoft D. 2004. Impacts of sulfate contamination on the Florida Everglades ecosystem. USGS Fact Sheet FS109-03 United States Geological Survey, Reston VA.
3. Aiken, George R. 2011. et. al. Mercury and sulfur monitoring research and environmental assessment in South Florida. South Florida Environmental Report Volume I Chapter 3B. 1-43.