Disinfection Byproducts (TTHM and HAA5)
Chlorine and other disinfectants kill germs but can react with natural matter to form byproducts. EPA limits total trihalomethanes (TTHM) to 0.080 mg/L and five haloacetic acids (HAA5) to 0.060 mg/L, each as a running annual average at each sampling location. Surface water systems tend to have more byproducts than well systems.
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.