Protecting Public Health and the Environment.

Mercury in Surface Water

Mercury is a naturally occurring metallic element found in trace amounts in air, water, and soil. Mercury is toxic to both aquatic life and humans, but its toxicity is primarily a human health concern.

Inorganic mercury occurs naturally due to its presence in rocks and soils, where it is slowly released through erosion and weathering into surface waters. Most of the mercury in surface waters remains inorganic, but in certain environments (low pH, low dissolved oxygen, and high organic matter, such as are found in the bottoms of lakes, marshes, and wetlands), some of it is converted to a much more toxic organic form—methylmercury. Airborne mercury from coal-fired power plants, mining operations, and other industrial sources can settle into soil and rivers, lakes, and oceans, where aquatic microbes convert it to methylmercury through a biochemical reaction. Elemental mercury released to the atmosphere can circulate around the world. The mercury people are exposed to comes not just from locally caught fish, but also, and for some primarily, from fish and shellfish caught elsewhere and sold in the market. Mercury pollution is truly a global problem.

Methylmercury tends to accumulate in the tissue of fish as they feed on other aquatic organisms. As larger fish eat smaller ones, concentrations of the pollutant increase in the bigger fish, a process known as bioaccumulation. Thus, mercury enters the food chain and becomes concentrated and is of particular interest from a human health standpoint. By far the most common route of mercury exposure in humans is eating fish and shellfish contaminated by methylmercury, according to the federal Centers for Disease Control.

Fish Consumption Advisories

To help protect public health, the Idaho Department of Health and Welfare through the Idaho Fish Consumption Advisory Program issues fish consumption advisories when fish in Idaho water bodies are found to have methylmercury levels above what is considered safe. The need for fish consumption advisories for some Idaho water bodies indicates that mercury pollution is a factor in Idaho, as do the results of water quality studies that show some water bodies in Idaho are impaired and fail to meet water quality standards for mercury.

Fish consumption advisories don't mean you should stop eating fish from affected water bodies, just that you should be aware of the risks and may need to limit your consumption to be safe. Fish are a good source of protein and low in saturated fat. Simply limit fish consumption to amounts specified in the advisory.

What DEQ is Doing to Address Mercury in Idaho Surface Waters

  • Mercury Monitoring in Surface Waters: DEQ investigated mercury in Idaho's major rivers in 2006 and 2008 and in lakes and reservoirs during 2007. Fish were collected at a number of randomly selected sites across Idaho. Fish tissue samples from these sites were analyzed for mercury and results compared to Idaho’s mercury fish tissue criterion. Random sampling allows extrapolation of the results to water bodies not monitored and estimation of the rate at which water bodies throughout Idaho meet or exceed the fish tissue criterion.

    From 2004 through 2008, through a statewide cooperative with the US Geological Survey (USGS), additional fish tissue samples were collected at 30 gaging sites in Idaho. Some sites were sampled twice during that period. These fish samples were analyzed for mercury and results reported DEQ.

    DEQ is developing a database for water and fish monitoring data. This database will hold the mercury data DEQ is collecting along with data collected by the USGS. This database will facilitate analysis of statewide mercury conditions in various water body types and locations around Idaho. DEQ intends to make the data in this database available to the public, along with a fish calculator that will enable people to enter their weight and determine consumption advice based on specific mercury fish tissue data.
  • Mercury Deposition Network: DEQ previously operated three wet deposition monitors located in Idaho. These sites were part of the Mercury Deposition Network (MDN). The MDN is a monitoring network comprised of more than 100 sites in North America. Its purpose is to develop information on spatial and seasonal trends in mercury deposited via precipitation to surface waters, forested watersheds, and other sensitive areas. The information will be used to help determine how mercury emissions impact human health. Link to more information on the Mercury Deposition Network.

    The first MDN site in Idaho was at Craters of the Moon. It began operation in late 2006. DEQ operated two additional wet deposition monitors, one at Deer Flats in Nampa and the second in McCall. Data collection in McCall began in late 2007, and data collection began in Nampa in early 2008. The purpose of these two monitors was to gauge mercury inputs to Brownlee Reservoir and to compare inputs obtained in a dry climate (Nampa) versus an area with more precipitation (McCall). Operation of all monitors was discontinued in 2010 due to lack of funding. In 2013, DEQ completed an analysis of the wet deposition data collected in Idaho and surrounding states.

  • Total Maximum Daily Load Development and Idaho's Mercury Fish Criterion: In April 2005, a new fish tissue criterion for methylmercury was developed as part of DEQ's water quality standards. It was approved by EPA in September 2005. The mercury fish tissue criterion is a benchmark for evaluating whether recreational use of water (i.e., fish consumption) is impaired by mercury. Impairment triggers a need by DEQ to develop and implement water quality improvement plans known as total maximum daily loads (TMDLs). The goal of mercury TMDLs is to lower mercury in water bodies such that fish tissue tests below the 0.3 milligrams methylmercury per kilogram of fresh weight fish level established in the water quality standards as safe for human health.

  • Suction Dredge Mining Activities: Mercury can be found in Idaho’s environment from historic gold mining practices. To enhance gold recovery from hydraulic mining, hundreds of pounds of liquid mercury were added to riffles and troughs in a typical sluice. The high density of mercury allowed gold and gold-mercury amalgam to sink, while sand and gravel passed over the mercury and through the sluice. However, large volumes of turbulent water would flow through the sluice causing many of the finer gold and mercury particles to wash through and out of the sluice. Much of this mercury is still present in Idaho water bodies today, and suction dredge miners frequently encounter and collect mercury. To encourage safe collection, transport, and disposal of elemental mercury, DEQ has developed Best Management Practices for Mercury Collection from Suction Dredging Activities (April 2018).

  • Air Quality Permitting: DEQ is the state agency responsible for administering the federal Clean Air Act in Idaho. This responsibility includes issuing permits that limit the volume of hazardous air pollutants, including mercury, that facilities may emit and conducting inspections of these facilities to ensure compliance with federal and state air quality standards. In 2011, Idaho adopted a new rule that applies to facilities that are not subject to a federal national emission standard for hazardous air pollutants and have mercury air emissions that exceed certain levels. These facilities are required to implement best available control technology (BACT) for their mercury emissions.

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Staff Contacts

Water Quality Standards Lead
DEQ State Office
Water Quality Division
1410 N. Hilton
Boise, ID 83706
(208) 373-0502

DEQ Resources

Mercury Studies

More Information

Health Effects of Methylmercury

Related Pages

Mercury and Air Quality

Mercury Pollution Prevention