About the data

Three public sources, joined city by city. Every page says which kind of number it shows.

Water hardness

Hardness figures come from the TapWaterData US Water Hardness dataset (tapwaterdata.com), published under CC BY 4.0. The version we use was generated on July 6, 2026. Values are total hardness in mg/L as calcium carbonate; grains per gallon is the same number divided by 17.1.

The four classes are the ones the US Geological Survey uses: soft up to 60 mg/L, moderately hard 61 to 120, hard 121 to 180, very hard above 180.

How sure each number is

The dataset sorts every city into one of three tiers, and so do we.

• Utility report (3,144 city pages). The hardness a water utility published in its own water quality data or Consumer Confidence Report.
• Calculated from utility data (199). TapWaterData computed hardness from the calcium and magnesium a utility reports, with the standard formula.
• County estimate (423). The median of field samples from the EPA and USGS Water Quality Portal across the whole county. These samples come mostly from wells and streams, not from treated tap water, so a city that softens its water or draws from a distant reservoir can be far off. Pages with a county estimate label it next to the number.

For 673 cities, different reports for the same system disagree. We show the dataset's main value with the full range next to it. Where the range spans two or more hardness classes, we leave the city out.

Which cities get a page

Cities and towns with at least 20,000 residents get a page whatever the tier. Smaller places down to 5,000 residents get one only when the number comes from their utility. Population comes from the Census Bureau, not from the dataset, because the dataset sometimes lists the size of the whole water system instead of the town. That leaves 3,766 city pages.

We also skip a city when the dataset links it to a utility in another state, or to a system that serves far fewer people than live in the city. Both are signs that the match went wrong.

Two more filters catch numbers we do not trust: county estimates built from fewer than 20 samples, and values under 3 mg/L, which look like a unit mix-up in the source. Those cities get no page and stay out of state averages.

EPA violation records

Violations come from EPA's Enforcement and Compliance History Online (ECHO), the Safe Drinking Water Act download for 2026Q3. We count violations whose compliance period started in 2016 or later, one per violation ID.

Health-based violations are the ones that matter for what comes out of the tap: a contaminant or disinfectant above its legal limit, or a missed treatment step. Monitoring and reporting violations mean a required test or public notice was late or missing; they say something about how the system is run, not about the water itself. "Open now" counts violations EPA lists as unaddressed.

Matching cities to water systems

A city can be served by one system, several, or a system named after somewhere else. We match in this order:

• Confirmed (1,449 pages). The hardness source names a system ID, and EPA lists that system as serving the city, or the system carries the city's name.
• City's own system (1,006). EPA lists an active community system that serves the city or is named after it, and serves at least a quarter of its residents. If the hardness figure came from a different system, the page says so.
• Source only (1,199). The hardness source names a system, but EPA does not list the city in its service area. Many of these are correct, such as county utilities serving unincorporated places, but some are not. The page tells you to check the name on your water bill.
• No match (112). No EPA record shown.

In a hand check of random samples, confirmed and own-system matches were right in nearly every case; source-only matches were right roughly seven times in ten.

Limits

Hardness varies across a city and with the seasons, depending on which plant or well feeds your street. If you need an exact number, a test strip from a hardware store or your utility's annual report will tell you. Nothing on this site is medical advice.

Questions or corrections: [email protected].