Arsenic in Drinking Water
Unlike most contaminants, arsenic is usually natural, dissolved out of rock into groundwater. It is tasteless and invisible, and the long-term health risks are serious, which makes testing the only way to know.
Arsenic stands apart from contaminants like lead or PFAS because its main source is nature, not industry. It is a naturally occurring element in the earth’s crust, and in many regions groundwater slowly dissolves it out of rock and sediment. The result is that arsenic is most often a problem in wells and in some communities that rely on groundwater, rather than in surface-water systems.
What arsenic is
Arsenic in water is colorless, odorless, and tasteless, so there is no way to detect it by sight or smell. It occurs in two main forms, and the inorganic forms found in groundwater are the more toxic. Mining, some historic pesticides, and industrial activity can add to natural levels in certain areas, but geology is the usual driver.
Where it shows up
Elevated arsenic is more common in parts of the West, the Upper Midwest, and the Northeast, but it can appear anywhere groundwater chemistry favors it. Public water systems test and treat for arsenic, so the highest exposures in the United States tend to be among private well owners, who have no such testing done for them. If you draw from a well, arsenic is one of the contaminants worth testing for specifically.
The health risks
Long-term exposure to arsenic, even at low levels, is linked to skin, bladder, and lung cancers, as well as skin changes, cardiovascular disease, and effects on development. The risks come from years of exposure rather than a single glass, which is part of what makes arsenic insidious: there are no immediate symptoms to warn you, and the harm accumulates quietly.
The EPA limit
The EPA sets the maximum contaminant level for arsenic at 10 parts per billion (0.010 milligrams per liter), a standard tightened from 50 ppb in 2001 and fully in effect since 2006. The health-based goal is zero, reflecting that arsenic is a known human carcinogen, while the enforceable limit balances health protection with what treatment can achieve. Public systems must meet 10 ppb; wells have no such requirement, so a well at 30 or 40 ppb can go unnoticed for years.
Testing and removal
Because arsenic is undetectable to the senses, a laboratory test is the only way to know your level, as we cover in how to test your water and private well water safety. If arsenic is present, effective home treatments include reverse osmosis, covered in our reverse osmosis guide, and specialized adsorptive media and anion exchange systems designed for arsenic. The right choice depends partly on which chemical form of arsenic is present, which a good test will identify, so test first and match the treatment to the result.