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Why One Big Water Utility Can Move a Whole State's Ranking
A state's drinking-water rank weights each utility by the people it serves. See the arithmetic with a worked example, and how to read a low rank.
A state near the bottom of a drinking-water violations ranking usually isn't there because most of its taps have problems; it's there because of how many people the systems with violations serve. Here is the arithmetic behind that, so you can read the ranking correctly.
Each system gets a penalty, capped at 100
The Worst Water Violations by State tool starts with every active community water system in a state. EPA defines a community water system as "a public water system that supplies water to the same population year-round" (US EPA — Information about Public Water Systems), and EPA counts "over 148,000 public water systems in the United States" in all, so community systems are one slice of that total.
Each system's violations on EPA's record are turned into a penalty using four weights, set in this site's code (lib/zip-score.js):
| Violation on the system's record | Penalty points each |
|---|---|
| Open health-based violation | 25 |
| Resolved health-based violation | 8 |
| Open monitoring or reporting violation | 4 |
| Resolved monitoring or reporting violation | 1 |
A single system's penalty is capped at 100. The code comment explains why: the cap keeps one failing system from zeroing out an otherwise sound area. For what the two violation types mean, see health-based vs. monitoring violations.
Then every system is weighted by the people it serves
The state's penalty is not a simple average of its systems. Each system's penalty is multiplied by its share of the state's total population served, and the shares are added up. The state's water score is 100 minus that penalty. The site's code says the point of the weighting is that "a 25-connection system does not outvote a city utility."
Two made-up states show the effect
These numbers are invented to show the arithmetic. They are not EPA data.
- State A: one city utility serves 90,000 people and has one open health-based violation (penalty 25). Ten small systems serve 1,000 people each with no violations (penalty 0). Total served: 100,000. The city utility's share is 90%, so the state penalty is 25 × 0.9 = 22.5, and the state water score is 100 − 22.5 = 77.5.
- State B: the same ten small systems and the same city utility, but now the city utility is clean and one small system, serving 1,000 people, has four open health-based violations. Four × 25 is 100, which is the cap. That system's share is 1%, so the state penalty is 100 × 0.01 = 1, and the state water score is 100 − 1 = 99.
A violation at a utility that serves most of the state's residents moves the state's rank far more than a worse record at a system serving a few hundred people. That is deliberate: the ranking asks how many people are served by systems with violations on file, not how many systems have them.
What this means when you read the list
- A low rank is not a share of utilities with problems. The tool's own FAQ says the same: the pooled score "is not a share of utilities with a clean record."
- A state with a few very large systems can swing a long way on one system's record.
- The ranking summarizes records on file with EPA. It is not a measurement of what comes out of a tap. For one system's own record, use Drinking Water by Utility.
- Open the state on the ranking page to see which counties sit behind its position, since the state figure alone hides that spread.
Why a clean record still isn't a guarantee is covered in why "no violations" doesn't mean perfectly clean water.
Try it
Open Worst Water Violations by State, pick a state near the bottom, and expand it to see which counties carry the weight.
Informational only, not professional advice. This is a summary of public compliance records at the utility and county level. It is not a health assessment or a water test, and it says nothing certain about the water at any one address.
Last reviewed: October 2026. Written by Eric, AirWaterScore founder.
Sources
Frequently asked questions
Does a low state rank mean most of its water systems have violations?
No. The state score weights each system by the people it serves, so it reflects how many people are served by systems with violations on file, not the share of systems with violations.
Why is one system's penalty capped at 100?
The site's code caps it so that one failing system cannot zero out an otherwise sound area.
Does the ranking measure the water coming out of my tap?
No. It summarizes violation records on file with EPA for community water systems, and it cannot speak for the water at one address.
Where can I see which counties are behind a state's rank?
Expand the state on the Worst Water Violations by State page, which lists the counties behind its position.
This guide is for informational purposes only. It explains how to read public EPA records and what this site's tools compute from them — it is not a health assessment, a water test, or a substitute for medical or professional advice about a specific address or a specific person's health.
Last reviewed: October 2026 · Against primary sources cited in the body.