Methodology

How we build each city report

Every number on this site comes from a public dataset. Here is exactly which ones, how we connect them, and where they can be wrong.

Cities and utilities

We cover the 200 largest US cities by the US Census Bureau 2024 population estimates and 2024 Gazetteer. For each city we find the community water system that serves it in EPA's Safe Drinking Water Information System, using the cities each system reports serving, the system's name, and the population it serves. Where the automatic match was wrong or missing, we fixed it by hand. 198 city pages meet our bar for a confident utility match and enough data to be useful; the rest stay hidden from search engines until we can verify them.

Large cities are often served by more than one utility, and suburbs by different ones entirely. Your water bill names your utility; if it differs from the one on your city's page, that utility's annual Consumer Confidence Report is the authoritative source for your home.

Chlorine or chloramine

We first read each utility's treatment records in EPA's database. Utilities that buy treated water from a wholesaler inherit the wholesaler's disinfectant, so we check the seller's records for those. Treatment records can lag real-world changes, so we then checked 181 of the largest cities against the utility's own water quality report or website, and those answers override EPA's records. Each page says which kind of source it relies on.

Lead and copper

We show the most recent 90th-percentile result each utility reported under the Lead and Copper Rule: the level that 90% of sampled homes were at or below. EPA's lead action level is 15 parts per billion (ppb) today and drops to 10 ppb in November 2027 under the Lead and Copper Rule Improvements. FDA's limit for bottled water is 5 ppb. EPA's health goal for lead is zero. It describes the system, not your tap.

PFAS

PFAS results come from the EPA Fifth Unregulated Contaminant Monitoring Rule (UCMR 5), sampling 2023-2025 (results released through August 2026). We show the highest single result and the average of detections for six compounds. In April 2024 EPA set enforceable limits of 4 parts per trillion (ppt) for PFOA and PFOS, and 10 ppt for PFHxS, PFNA and HFPO-DA (GenX). In May 2025 EPA said it intends to keep the PFOA and PFOS limits, extend the compliance deadline to 2031, and rescind the other three. We compare results to the 2024 limits and label which ones may change.

Violations

We count health-based violations (maximum contaminant level and treatment technique violations) that began on or after January 1, 2016, from EPA's violation records. Monitoring and reporting violations are not included.

Hardness

Hardness data: TapWaterData Water Hardness Dataset (tapwaterdata.com), licensed CC BY 4.0. Values come from utility-reported data where available, otherwise from area estimates built on EPA and USGS Water Quality Portal samples. We convert to grains per gallon at 17.12 mg/L per grain and use the US Geological Survey hardness scale.

Radon

EPA radon zones describe counties, not houses. Zone 1 counties have a predicted average indoor level above 4 picocuries per liter (pCi/L), Zone 2 between 2 and 4, Zone 3 below 2. Homes with high levels are found in every zone, which is why EPA recommends that every home be tested, and fixed at 4 pCi/L or above. We place each city in the county at its geographic center using the US Census geocoder; cities spanning several counties can have more than one zone.

Recommendations and money

Product suggestions follow fixed rules from the findings (for example, chloramine leads to chloramine-rated filters, hard water to a softener or scale inhibitor), not from commissions. Some links are affiliate links, and installer quote requests may be shared with local companies who pay for introductions. See our disclosure.

Corrections

If something on your city's page is wrong, email the details and a source, and we will fix it.