Illinois Water Supply Planning


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Mercury in Illinois FishBack
Mercury Deposition Network Sites and Mercury Fish Advisories
The Illinois Fish Contaminant Monitoring Program advises people to limit the number of predator fish they eat because of methylmercury in fish tissue. Predator fish (bass, walleye, muskellunge, northern pike, etc.) eat other fish or aquatic animals. The advisory applies to predator fish from any surface water body in the state and especially targets women of childbearing age, pregnant and nursing women, and children younger than 15 years old. Recent studies have shown that methylmercury adversely affects the developing nervous system of children and fetuses and could result in lower IQ, abnormal muscle tone, and slowed motor function (Source: Illinois Department of Public Health Web site at

Small amounts of mercury occur naturally in the environment. Mercury also comes from burning coal or incinerating medical and municipal waste, and from some industries. According to a U.S. Environmental Protection Agency (USEPA) emissions inventory, these human activities are the largest source of airborne mercury in Illinois. The USEPA, along with state environmental protection agencies, has determined that the atmospheric deposition of mercury is the single largest cause of surface water impairment in USEPA Region V, which includes Illinois. However, the report does not explain how mercury in precipitation becomes methyl mercury in fish.

Methylmercury is a toxic and very persistent form of mercury that forms in lake and river sediments. Rainfall and runoff provide a way for methylmercury to enter lakes and streams. It then bioaccumulates in fish tissue as small fish consume tiny organisms containing methylmercury, large fish consume small fish, and so on, until the largest predator fish contain methylmercury levels deemed harmful for human consumption. Eating contaminated fish is the most important source of human exposure to methylmercury.

The Illinois State Water Survey is home to the National Atmospheric Deposition Program, which operates the Mercury Deposition Network (MDN). That network measures the amount of mercury in precipitation (rain, snow, and ice pellets). Additional information is available on the MDN Web site (


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