A study released by the US Geological Survey found that arsenic exceeded federal safety standards for public drinking water at 13 percent of the sites tested -- nearly double the national rate. More than 10 micrograms of arsenic per liter is considered potentially harmful.

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• Less than 5 micrograms per liter
• 5 to less than 10 micrograms per liter
• Greater than or equal to 10 micrograms per liter(public supply well)
• Greater than or equal to 10 micrograms per liter(domestic well)

NOTE: Data come from Crystalline-Rock Aquifers in New England,New Jersey, and New York, 1995-2007.

SOURCE: US Geological Survey report

Patrick Garvin, Daigo Fujiwara/Globe Staff