Q. Under the Safe Drinking Water Act, the Environmental Protection Agency establishes quality standards for public water supply systems. The EPA sets two kinds of standards: “Primary” and “Secondary.” What’s the difference?
Primary Water Quality Standards
Primary Standards are health-based, and the EPA sets Primary Standards for contaminants that threaten public health. For each contaminant, a Primary Standard either specifies a treatment technique or sets a Maximum Contaminant Level (MCL) and leaves it up to the public water system to figure out a way to purify the water below that level [40 CFR 141].
Secondary Water Quality Standards
Secondary Standards are aesthetic, not health-based. The EPA sets Secondary Standards for contaminants that do not present a health hazard but may make water unpleasant to drink [40 CFR 143].
Safe Drinking Water Act (SDWA) Enforcement
While the U.S. EPA sets Water Quality Standards, the standards are enforced by both the U.S. EPA and at the State level.
Every public water system must consistently monitor and sample its water supply for contaminants. While the EPA does not require monitoring for secondary contaminants (considering them non-enforceable recommendations), the Agency does recommend monitoring for secondary contaminants on the same schedule as primary inorganic contaminants [40 CFR 143.4]. State regulators may enforce secondary standards and require water systems to monitor and sample for secondary contaminants.
Notices of Violation
When a public water system violates a Primary Standard, by exceeding the authorized maximum contaminant level, it must notify its customers. The notification must contain:
- Clear and readily understandable explanations of the violations,
- The potential health effects,
- What steps the water system is taking to correct the violation, and
- Precautions customers should take until the violation is corrected and the water is again safe (“boil water” notifications, etc.)
When a public water system violates a Secondary Standard, the EPA (with one exception) does not require a notification, but State regulators may require one. The one exception is “fluorine.” When a water system violates the Secondary Standard for fluorine without violating the primary standard, it must notify its customers [40 CFR 141, Subpart Q].