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Arsenic makes up part of the Earth’s crust and is commonly found in groundwater. In 2001, the Environmental Protection Agency lowered the drinking-water standard from 50 parts per billion of arsenic to 10 parts per billion. The agency had initially proposed a limit of five parts per billion but faced criticism that it would be too costly for water companies to hit that target.

Arsenic is known to cause a variety of cancers as well as being linked to heart disease, strokes and diabetes. Recent research has found an association between arsenic below 10 parts per billion and IQ deficits in children.

This map is based on arsenic readings from 45,000 wells collected by the United States Geological Survey throughout the country going back four decades. In addition, the states of Texas and Minnesota provided data gathered on arsenic in private wells. In several other states, few readings were available.

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Jared Bennett is a deputy editor and reporter covering criminal justice and economic justice for the...