The 50 Largest US Water Systems, Graded (2026)

The 50 largest water systems in our data serve a combined 72.8 million people, and their grades are mostly average: 1 A, 9 B, 21 C, 10 D and 9 F. The only A goes to Memphis Light, Gas & Water (87). The lowest scores belong to Aqua Pennsylvania’s main system (43), San Jose Water (45) and Miami-Dade’s main system (46). Twenty-eight of the 50 had no EPA violation in any of the last 12 quarters, and 9 have a health-based violation in their EPA record.

Below is the full list with each system’s grade, score, water source and EPA record, followed by what the patterns mean.

How this list was built

We took TapWaterSafety’s dataset of 1,000 graded utilities (each serving more than 50,000 people) and sorted by population served. The 50th-largest system serves 659,500 people.

A few things to know before reading the table:

• Population figures are as reported for each system, so they can differ from a city’s census count. Some systems sell water to others, so the totals overlap. For example, the MWRA supplies wholesale water to communities that include Boston, and the Boston Water and Sewer Commission is listed separately.
• Grades run from A (85 to 100) to F (54 or below). The score weighs detected contaminants against EWG health guidelines, which are usually much stricter than legal limits (40%), health-based EPA violations (20%), high-risk contaminants such as PFAS, lead and arsenic (20%), monitoring and reporting violations (10%), and source water vulnerability (10%). Full details are on our methodology page.
• EPA records are from EPA ECHO compliance records, checked October 7, 2026; last 12 quarters. We count a violation as health-based if EPA lists it as a maximum contaminant level (MCL), maximum residual disinfectant level (MRDL) or treatment technique (TT) violation. Monitoring, reporting and notice violations are about testing schedules and paperwork, not contamination findings.

The 50 largest systems

“Quarters in violation” is the number of the last 12 quarters with at least one open or resolved violation. Click a system to see its full page, contaminant details and EPA report link.

What the numbers show

Grades cluster in the middle. The average score is 66.3 and the median is 68, both a C. That’s close to the national average for all 1,000 systems (68.3). Being large does not make a system better or worse on our scale.

Disinfection byproducts show up almost everywhere. For 48 of the 50 systems, the top concerns include haloacetic acids or total trihalomethanes above EWG’s health guidelines. These byproducts form when chlorine or chloramine reacts with natural organic matter, and they are common in treated surface water. Levels can be within EPA’s legal limits and still be above the much stricter health guidelines. Our explainer on disinfection byproducts covers what that means.

PFAS separates many of the low grades. Twenty-one of the 50 systems list “PFAS contamination” among their top concerns, including all nine F-graded systems. For the federal limits and timelines, see our EPA PFAS rule explainer.

Most are surface water systems. Forty-five of the 50 rely on surface water (6 of them buy it from another utility). The five groundwater systems (Miami-Dade, Suffolk County, Jacksonville’s JEA, Tucson and Memphis) average 70.6, versus 65.8 for the surface water systems.

Most large systems have clean or minor EPA records. Twenty-eight had no violation in any of the last 12 quarters. None of the 50 carries EPA’s serious violator label. Two, Boston Water and Sewer Commission and Portland Water Bureau, are listed as over the lead action level.

The nine health-based violations, explained

A health-based violation is more important than a missed test, but the details matter. Here is what EPA lists for each:

• New York City: an open treatment technique violation under the Long Term 2 Enhanced Surface Water Treatment Rule, since 2017. According to a 2019 Federal Register notice of a proposed consent decree, LT2 requires the city to cover its Hillview Reservoir in Yonkers to protect treated water from microbial recontamination. The decree schedule runs to 2049 for the second basin cover.
• Portland Water Bureau: an open LT2 treatment technique violation since December 2017. Portland lost a state variance that had exempted it from treating for Cryptosporidium after the parasite was detected in its Bull Run source in 2017, according to OPB. The compliance schedule requires treated water by September 30, 2027.
• City of Houston: a treatment technique violation under the surface water treatment rules from 2025, now listed as closed.
• City of Baltimore: an open MCL violation under the Stage 2 disinfection byproducts rule, since 2026.
• Metropolitano (Puerto Rico): an open treatment technique violation under the surface water treatment rules, since 2023.
• San Jose Water: an open Surface Water Treatment Rule treatment technique violation, since 2023.
• Aqua PA Main System: a Ground Water Rule treatment technique violation from 2024, now listed as closed.
• MCWA (Monroe County, NY): an open Lead and Copper Rule treatment technique violation, since 2025.
• Boston Water and Sewer Commission: an open Lead and Copper Rule treatment technique violation since 2020, and over the lead action level.

None of these labels, on its own, tells you what is in the water at your tap today. Check your utility’s notices and its Consumer Confidence Report for what was found and what is being done. If lead is your concern, our guide on how to find out if you have a lead pipe is a good next step.

Large systems vs. big cities

This list ranks systems, not cities. A city can be served by several systems, and a single system can serve many cities. For a cutoff of 250,000 people served, see our lists of the cleanest big-city tap water and the lowest-graded big-city tap water. For every state’s averages, see tap water grades by state.

What to do with this information

1. Find your own system on our search page. Your utility may not be on this list, and a large system’s grade doesn’t describe smaller neighbors.
2. Read the top concerns. They tell you which contaminant is pulling a grade down, which matters more than the letter.
3. Check whether a violation is health-based and whether it is still open, using the EPA report linked from each utility page.
4. If you choose a filter, match it to the contaminant and confirm the claim in the certifier’s listing (NSF, IAPMO, WQA, CSA or UL).

Note: utilities may have resolved issues since October 7, 2026. Check each utility’s EPA ECHO report or its latest Consumer Confidence Report for current status.

Sources

• TapWaterSafety.org: Methodology
• EPA ECHO: Safe Drinking Water Act Resources and FAQs
• EPA ECHO: Detailed Facility Reports (linked from each utility page)
• Federal Register: Notice of Lodging of Proposed Consent Decree, United States v. City of New York (March 26, 2019)
• OPB: Portland must filter Bull Run water by 2027 (December 2017)