Mercury Rising?

The liquid metal sometimes called quicksilver can have detrimental health effects if present in drinking water above levels set by the U.S. EPA.
Mercury Rising?
Facilities such as this cement smelting plant emit mercury and thallium into the atmosphere that come to the ground when it rains.

Florida is beginning a campaign to monitor mercury because of its accumulation in the state’s surface waters. A community close to a cement manufacturing plant asked Florida Gateway College for information about the impact of mercury on drinking water.

We explained that the U.S. EPA regulates mercury in drinking water to protect public health. Mercury can cause health problems if present in amounts greater than 2 parts per billion. The health effects of mercury contamination mainly include kidney damage.

Multiple sources

Large amounts of mercury are released naturally from the earth’s crust, and other sources include combustion of fossil fuels, metal smelters, cement manufacturing, municipal landfills, sewage, metal refining, and chloralkali plants.

Mercury is a liquid metal found in natural deposits along with other ores. Electrical products such as dry-cell batteries, fluorescent light bulbs, switches, and control equipment account for 50 percent of the mercury used. From 1987 to 1993, according to the EPA’s Toxic Chemical Release Inventory, mercury releases to land and water totaled nearly 68,000 pounds. These releases were primarily from chemical and allied industries.

The largest releases occurred in Tennessee and Louisiana, and the largest direct releases to water occurred in West Virginia and Alabama. Mercury is unique among metals in that it can evaporate when released to water or soil. In addition, microbes can convert inorganic forms of mercury to organic forms that can bio-accumulate in aquatic life.

Inorganic mercury is found in batteries and is used in the chemical industry. It is produced from elemental mercury through a process of oxidation. Inorganic mercury is the most common form present in drinking water but it is not considered seriously harmful to human health at levels in drinking water below the maximum contaminant level (MCL).

Organic mercury, primarily methyl mercury, is produced from inorganic mercury-specific bacteria in surface waters. Organic mercury is the form that poses a significant health threat. Methyl mercury is ingested by fish and bio-accumulates in their tissues, which people then may eat. Large predatory fish can contain as much as 100,000 times more methyl mercury than the surrounding water.

Methyl mercury (rarely present in drinking water) damages the central nervous system and can also cause birth defects. Both inorganic and organic mercury are considered more detrimental to children because both are more easily absorbed into their systems.

The mercury cycle

Mercury in the environment is constantly cycled and recycled through a biogeochemical cycle that has six major steps:

  • Degassing of mercury from rock, soils, and surface waters, or emissions from volcanoes and human activities
  • Movement in gaseous form through the atmosphere
  • Deposition on land and surface waters
  • Conversion of the element into insoluble mercury sulfide
  • Precipitation or bioconversion to more volatile or soluble forms like methyl mercury
  • Re-entry to the atmosphere or bioaccumulation in food chains

Mercury cycles in the environment through natural processes and human activities. The primary anthropogenic sources are fossil fuel combustion and smelting. These sources release elemental mercury vapor to the atmosphere.

In the atmosphere, mercury vapor can circulate for up to a year and becomes widely dispersed. The elemental vapor can undergo photochemical oxidation to become inorganic mercury, which can combine with water vapor and travel back to earth as rain.

The mercury water is deposited in soils and water bodies. In the soil, it accumulates until a physical event causes it to be released again. In water, inorganic mercury can be converted to insoluble mercury sulfide, which settles out into the sediment, or it can be converted by bacteria that process sulfate into methyl mercury.

The conversion of inorganic mercury into methyl mercury is important, because methyl mercury is much more toxic than inorganic mercury. The methyl mercury-processing bacteria can be consumed by the next higher organism in the food chain, or the bacteria can release methyl mercury into the water, where it can adsorb to plankton, which in turn can be consumed by the next higher organism in the food chain, and so on. Alternatively, elemental mercury and methyl mercury can vaporize, re-enter the atmosphere and cycle through the environment.

Protecting the public

The Maximum Contaminant Level Goal (MCLG) for mercury is 0.002 mg/L (2 ppb).  The EPA has set this level based on the best available science to prevent health problems.  The EPA has also set an enforceable regulation for mercury at 0.002 mg/L or 2 ppb.  MCLs are set as close to the health goals as possible, considering cost, benefits, and the ability of public water systems to detect and remove contaminants using suitable treatment technologies. 

The Safe Drinking Water Act requires the EPA to periodically review the national primary drinking water regulation for each contaminant and revise the regulation. The EPA recently reviewed mercury as part of the six year review and determined that the 2 ppb MCLG and the 2 ppb MCL for mercury are still protective.

A federal law called the Emergency Planning and Community Right to Know Act requires facilities in certain industries that manufacture, process, or use significant amounts of toxic chemicals to report their releases annually.

When routine monitoring indicates that mercury levels are above the MCL, the water supplier must take steps to reduce the mercury to below the MCL of 2 ppb. Water suppliers must notify their customers as soon as practical, but no later than 30 days after the system learns of the violation.

Additional actions, such as providing alternative drinking water supplies, may be required to prevent serious risks to public health. We explained to the local community that treatment methods proven effective for removing mercury to below 2 ppb include coagulation and filtration, granular activated carbon, lime softening, and reverse osmosis.

If contaminant levels are found to be consistently above the MCL, the water supplier must take steps to reduce the amount of mercury so that it is consistently below that level.

Elemental mercury is typically released from industrial processes; agriculture; household, commercial and medical products; sewage discharges; and sediment. We suggested the community intensify its monitoring for mercury in the water supply and surrounding natural waters in response to the presence of the manufacturing facility.

ABOUT THE AUTHOR

John Rowe, Ph.D., is a professor of Water Resources at Florida Gateway College in Lake City, Fla.



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