A moving legal target
Legal limits are written by agencies, and agencies answer to administrations. Health goals are set on what the human body can safely take, and they don't change with an election.
- April 2024 EPA sets the first national legally enforceable limits for six PFAS in drinking water, including 4.0 parts per trillion each for PFOA and PFOS.[1]
- May 2025 EPA announces it will keep the PFOA and PFOS limits and intends to rescind and reconsider the limits for PFHxS, PFNA, GenX and a PFAS mixture.[2]
- May 2026 EPA proposes removing the limits for PFHxS, PFNA, GenX and the PFAS mixture, and proposes letting systems request two more years, to 2031, to meet the PFOA and PFOS limits. Neither proposal is final.[3, 4]
- Now The existing limits remain in place while the rule is challenged in federal court. The court heard oral argument on September 18, 2026; a decision is pending.[5]
That's why AquaScore™ never depends on the law alone. We publish both numbers and the gap between them, so you always know where your water stands, no matter who's in office.
Your water can be legal and still not clean.
Two numbers for every regulated contaminant
For most contaminants it regulates, the U.S. Environmental Protection Agency sets two different numbers — and they mean different things.
The first is the Maximum Contaminant Level Goal (MCLG): "the maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur, allowing an adequate margin of safety." EPA is clear that MCLGs are non-enforceable public health goals.[6]
The second is the Maximum Contaminant Level (MCL): the maximum level allowed in water delivered to anyone served by a public water system. This is the enforceable number. By law, EPA sets it as close to the health goal as feasible — and "taking cost into consideration," EPA determines what is feasible.[6] For some contaminants, like lead, EPA uses an enforceable treatment technique with an "action level" instead of an MCL.[6, 7]
That's the whole idea in one sentence: the health goal is about your body; the legal limit is about your body and what treatment costs. The distance between them is what we call The AquaGap™.
Three gaps worth knowing
Lead. EPA's health goal for lead is zero, because there is no known safe level of exposure. The current action level is 15 parts per billion (ppb). Under the Lead and Copper Rule Improvements, it drops to 10 ppb starting November 1, 2027.[7, 8, 9]
Arsenic. The health goal is zero. The enforceable limit is 0.010 milligrams per liter — 10 ppb — in effect since 2006.[7]
PFOA and PFOS. For these two "forever chemicals," EPA's 2024 rule set the health goal at zero and the enforceable limit at 4.0 parts per trillion (ppt) each.[1] In May 2026, EPA proposed letting water systems request two more years — to 2031 — to comply; that proposal is not final.[4]
What The AquaGap™ shows you
A water system can meet every legal limit and still deliver water with contaminants above their health goals. That isn't a violation — it's how the rules are designed. But it's information you deserve to see.
The AquaGap™ puts the two numbers side by side for the water at your address: what the law allows, what the health goal is, and where your water falls between them. We report data and limits — not verdicts.
Modeled vs. measured
Every AquaScore™ is labeled by where its numbers come from. Measured means lab or field results for that location. Modeled means our AI foundation models estimated the value from surrounding data — public records, nearby results and system information — because no local measurement exists yet. That's one reason Guardians of H2O™ matter: every tap-level sample turns a modeled number into a measured one.
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Sources
- U.S. EPA. Per- and Polyfluoroalkyl Substances (PFAS): Final PFAS National Primary Drinking Water Regulation.
- U.S. EPA. EPA Announces It Will Keep Maximum Contaminant Levels for PFOA, PFOS (May 14, 2025).
- U.S. EPA. Proposed PFAS Rescission Rule.
- U.S. EPA. Proposed PFOA and PFOS Compliance Extension Rule.
- CourtListener. American Water Works Association v. EPA, No. 24-1188 (D.C. Cir.) — docket.
- U.S. EPA. How EPA Regulates Drinking Water Contaminants.
- U.S. EPA. National Primary Drinking Water Regulations.
- U.S. EPA. Basic Information about Lead in Drinking Water.
- U.S. EPA. LCRI Frequently Asked Questions for States and Public Water Systems (Oct. 2024).