Arsenic in Drinking Water
Arsenic occurs naturally in rock, and New Hampshire's bedrock wells often contain it. EPA's legal limit (MCL) is 0.010 mg/L (10 ppb). Public systems must test and treat to meet it; private wells are not regulated, so the owner must test.
Arsenic violations on system pages show the measured value next to the 0.010 mg/L limit.
Detection vs violation
- Detection: a lab found a substance. Many substances are detected at levels below the legal limit.
- MCL (legal limit): the highest level EPA allows. Going over it, usually as an average over time, is a violation.
- MCLG: a health goal with no enforcement. It can be zero (for lead it is zero).
- Action level (lead and copper): a trigger for extra steps such as corrosion control and public education. Exceeding it is not a violation by itself.
- Monitoring or reporting violation: a test or report was late or missing. It does not by itself mean a contaminant was too high.