MassDEP outlines 24‑hour Tier 1 public‑notice steps after lead action‑level exceedance

A Massachusetts Department of Environmental Protection Drinking Water Program staff member said the 24‑hour Tier 1 public‑notice requirement is triggered “as soon as the system learns that they have exceeded the lead ninetieth percentile,” and that public water systems must distribute the notice within 24 hours.

The guidance explains how a system determines a “90th percentile” lead result, when the requirement applies, what must appear in the notice and how systems must certify distribution. The instruction applies to Massachusetts community and nontransient noncommunity public water systems regulated under the Lead and Copper Rule revisions (LCRR).

Why it matters: the LCRR became the compliance rule on Oct. 16, 2024, and MassDEP says the Tier 1 24‑hour public‑notice requirement remains active despite temporary pauses to most other LCRR provisions. The notice is intended to alert consumers rapidly when laboratory results indicate the system’s 90th‑percentile lead concentration exceeds the action level.

MassDEP’s explanation of the 90th‑percentile calculation: after a system collects the required number of samples and receives certified laboratory results, it organizes results from lowest to highest and multiplies the number of samples by 0.9 to locate the 90th‑percentile sample. If that sample result is above the action level, the…