Mercury is released to surface waters from rocks and soils and from industrial activities and wastewater treatment facilities. An indirect but significant source is the deposition of mercury emissions from power plants in rain to water surfaces and to soils. When mercury enters water, biological processes transform it to methylmercury, which is highly toxic and builds up in fish and animals that eat fish. For this reason, EPA sets a maximum contaminant level for mercury in drinking water and works with the states to issue consumption advisories. These advisories may be directed at the general population or at sensitive groups such as pregnant women, nursing mothers, and their children.
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