Methodology
How we score water, in plain words.
Every score comes with its band, a factual finding, its basis (measured, modeled or mixed), its confidence and its date.
The five bands
A band describes what was measured or modeled against the strictest health benchmark, never whether water is safe. ELEVATED and HIGH are the AquaGap™ zone: legal, not clean.
| TRACE | All scored contaminants below a small fraction of their health benchmarks. |
|---|---|
| DETECTED | Measurable amounts, still at or below the health benchmarks. |
| ELEVATED | At least one contaminant above its health benchmark. |
| HIGH | At least one contaminant 10 times or more above its health benchmark. |
| SEVERE | At least one contaminant above its enforceable legal limit. |
How the score is calculated
Each reading is divided by its health benchmark. That ratio is weighted by the contaminant's severity, and the weighted ratios are added into one health hazard. The hazard is converted to the 0 to 100 scale so that a combined hazard of 30 scores 50. Any single contaminant at 10 times its health benchmark or more sets the score to 0, and any detection of E. coli does too. The same readings are rated a second time against legal limits; the distance between the two results is the AquaGap™.
Readings older than their freshness window are listed as old record and left out of the score and the band. A regulated contaminant a utility's report doesn't list counts as not detected, because federal rules require detected contaminants to be reported (40 CFR 141.153). Non-detects are not counted at half the detection limit when the lab's reporting limit is above the health benchmark.
Published thresholds
Method v2.0.0 reference table. TRACE means every scored contaminant is at or below one tenth of its health benchmark; HIGH starts at 10 times. Health benchmarks come from public-health agencies, chiefly California's public health goals (OEHHA) and EPA's health goals. An independent science panel is forming to review every value, and changes are logged below.
| Contaminant | Legal limit | Health benchmark | Freshness window |
|---|---|---|---|
| Arsenic | 10 ppb EPA MCL | 0.004 ppb CA OEHHA PHG, 2004 | 3 years |
| Nitrate (as N) | 10 ppm EPA MCL | 10 ppm CA OEHHA PHG, 2018 | 180 days |
| Nitrite (as N) | 1 ppm EPA MCL | 1 ppm CA OEHHA PHG, 2018 | 180 days |
| Lead | 15 ppb EPA action level (MCLG is 0) | 0.2 ppb CA OEHHA PHG, 2009 | 3 years |
| Copper | 1.3 ppm EPA action level | 0.3 ppm CA OEHHA PHG, 2008 | 3 years |
| Chromium-6 (hexavalent) | None no federal MCL; total chromium MCL is 100 ug/L | 0.02 ppb CA OEHHA PHG, 2011 | 3 years |
| Atrazine | 3 ppb EPA MCL | 0.15 ppb CA OEHHA PHG, 1999 | 3 years |
| Total trihalomethanes (TTHM) | 80 ppb EPA MCL | Not yet set Scored on the legal track until a health benchmark is set | 1 years |
| Haloacetic acids (HAA5) | 60 ppb EPA MCL | Not yet set Scored on the legal track until a health benchmark is set | 1 years |
| PFOA | 4 ppt EPA MCL | 0.007 ppt CA OEHHA PHG, 2024 | 3 years |
| PFOS | 4 ppt EPA MCL | 1 ppt CA OEHHA PHG, 2024 | 3 years |
| PFHxS | 10 ppt EPA MCL (rescission proposed May 2026) | Not yet set Scored on the legal track until a health benchmark is set | 3 years |
| PFNA | 10 ppt EPA MCL (rescission proposed May 2026) | Not yet set Scored on the legal track until a health benchmark is set | 3 years |
| GenX (HFPO-DA) | 10 ppt EPA MCL (rescission proposed May 2026) | Not yet set Scored on the legal track until a health benchmark is set | 3 years |
| Benzene | 5 ppb EPA MCL | 0.15 ppb CA OEHHA PHG, 2001 | 3 years |
| Radium-226 + Radium-228 | 5 pCi/L EPA MCL | Not yet set Scored on the legal track until a health benchmark is set | 3 years |
| Uranium | 30 ppb EPA MCL | Not yet set Scored on the legal track until a health benchmark is set | 3 years |
| Manganese | None no federal MCL; secondary standard 50 ug/L | Not yet set Scored on the legal track until a health benchmark is set | 3 years |
| Barium | 2 ppm EPA MCL | 2 ppm CA OEHHA PHG, 2003 | 3 years |
| Selenium | 0.05 ppm EPA MCL | 0.03 ppm CA OEHHA PHG, 2010 | 3 years |
| Fluoride | 4 ppm EPA MCL (secondary standard 2.0) | 1 ppm CA OEHHA PHG, 1997 | 3 years |
| E. coli | None EPA treatment technique; MCLG 0 | Not yet set Scored on the legal track until a health benchmark is set | 90 days |
The fixed line on every readout
AquaScore describes what was measured or modeled against health benchmarks. It is not a determination that water is safe or unsafe to drink.
The five actions
Watch · Assess the AquaGap™ · Test · Equip · Report
Changelog
| Oct 15, 2026 | Method v2.0.0: score scale calibrated (a combined hazard of 30 scores 50), stale readings left out of the score, unlisted regulated contaminants count as not detected, and the threshold table published. First public score: City of Atlanta, 62 · ELEVATED, within all federal legal limits. |
|---|---|
| Oct 12, 2026 | Band codes retired in public. Bands are words only, shown with the score (for example, 72 · ELEVATED). |
| Oct 9, 2026 | Five bands defined by facts in the method, not by score ranges: TRACE, DETECTED, ELEVATED, HIGH, SEVERE. |