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Learn how we calculate water quality ratings and what your city's score means for your health and safety.
Environmental Data Analysis
This content has been reviewed by Dr. Sarah Chen, PhD, Environmental Science Advisor, to ensure accuracy and alignment with current environmental science standards.
Last reviewed: September 2026
Every graded city starts at 100 points and loses points only for findings that appear in EPA's own records for its public water system. Nothing is estimated from pipe age or modelled; the “Why this grade” list on each city page is the literal output of the scoring function. Full rules and data vintages are on the methodology page.
−10 for each MCL or treatment-technique violation in the last five years (SDWIS), capped at −40, plus a flat −20 if any health-based violation is still unresolved with EPA or the state.
−1 per missed sample or late report in the last five years. Paperwork, not measured contamination, which is why it is weighted lightly.
The system's latest Lead and Copper Rule 90th-percentile result: −25 at or above the 15 ppb action level, −10 at 10 ppb, −3 at 5 ppb. Results older than 2020 are shown but not scored.
−10 each for PFOA or PFOS at or above EPA's 4 ppt limit in UCMR5 monitoring (2023–2026); −2 for every other PFAS compound detected, capped at −6.
EPA's Six-Year Review samples: arsenic, nitrate, fluoride, chromium, uranium and radium lose −10 when more than 5% of samples exceeded the MCL, −3 for any exceedance, −2 when the average ran above 80% of the MCL. Disinfection byproducts (TTHM/HAA5) are scored on their average only, because the limit is a running annual average. Total historical penalty is capped at −20 because the data is 7–14 years old.
Excellent water quality. All contaminants well below legal limits. No recent violations. Safe to drink without filtration, though some may prefer it for taste.
Good water quality. Meets EPA safety standards. Some contaminants detected at low levels. Generally safe to drink. Filtration optional for extra protection.
Fair water quality. Meets minimum EPA standards but has contaminants worth monitoring. Consider water filtration, especially for vulnerable populations.
Below average water quality. Multiple contaminants at elevated levels or recent violations. Water filtration recommended for all household members.
Poor water quality. Contaminants exceed EPA limits or significant compliance issues. Certified water filtration strongly recommended. Consider bottled water for drinking and cooking.
While water quality scores provide a helpful overview, they don't tell the complete story:
Your water quality is good. Consider these optional steps:
Your water has some concerns. We recommend:
Your water quality needs attention. Take action:
A city can be "EPA compliant" (meeting minimum legal requirements) but still have contaminants at levels that reduce the score. We consider health guidelines that are more protective than EPA legal limits, total contaminant count, and historical compliance patterns.
Water quality scores are updated quarterly based on the latest EPA data from the Safe Drinking Water Information System (SDWIS). However, testing frequency varies by contaminant type and water system size.
Yes. Large cities may have multiple water systems or treatment plants. Additionally, local infrastructure (pipes, fixtures) significantly affects water quality at the tap. Our scores reflect the primary water system serving each city.
First, review the specific contaminants detected in your water. Then, choose a certified water filter that removes those specific contaminants. Contact your water utility for their latest Consumer Confidence Report and ask about improvement plans. Consider home water testing to verify quality at your tap.