Our Methodology

Data Source

All data on this site comes from the EPA Safe Drinking Water Information System (SDWIS), the federal database that tracks compliance of public water systems across the United States. SDWIS is maintained by the U.S. Environmental Protection Agency and records whether water systems are meeting the standards set under the Safe Drinking Water Act (SDWA).

We access SDWIS data through the EPA Envirofacts REST API, a publicly available interface that provides programmatic access to EPA environmental databases. This allows us to query violation records, system characteristics, and geographic information in a structured format.

We include active Community Water Systems (CWS) serving 100 or more people. Community water systems are defined as systems that supply water to the same population year-round — these are your municipal utilities, suburban water districts, and similar providers. We exclude transient non-community systems (such as water at rest stops or campgrounds) and non-transient non-community systems (such as schools or offices with their own wells), because these serve different populations under different regulatory expectations.

SDWIS tracks compliance with National Primary Drinking Water Regulations (NPDWRs), which are legally enforceable standards for contaminants that may pose a risk to public health. These include limits on microorganisms, disinfection byproducts, inorganic chemicals, organic chemicals, and radionuclides. The database records every violation a water system incurs, creating a longitudinal compliance history that spans decades.

By focusing on community water systems, we cover the majority of Americans who receive their drinking water from a municipal or regional utility. Private well users — roughly 23 million households — are not covered by SDWIS and are therefore not represented in this data.

Data Collection Process

Our data pipeline is an automated script that fetches and processes SDWIS records for all 50 states. The process runs in several stages:

• System query: For each state, we query the EPA Envirofacts API to retrieve all active community water systems serving 100 or more people. This returns basic system metadata including the system name, PWSID (Public Water System ID), population served, and the city or area served.
• Violation retrieval: For each water system, we retrieve all violation records on file. These records include the violation type, the contaminant involved, the violation begin and end dates, and whether the violation has been resolved.
• City-level aggregation: Violations are grouped by city using the "city served" field in SDWIS. When multiple water systems serve the same city, their violations are combined to produce a city-level total. Contaminants are extracted from violation records and deduplicated, so a contaminant is listed once per city regardless of how many violation instances it appears in.
• File generation: We produce a per-state JSON file for each state containing city-level detail (violation counts, health violation counts, contaminants detected, and water system information), as well as a master index file with state-level summaries used for national rankings and the homepage.

The pipeline is designed to be re-runnable — each execution overwrites the previous data files with fresh API results. The current dataset reflects a fetch performed on February 17, 2026.

Violation Classification

Not all drinking water violations are equal. SDWIS records violations across multiple categories, and the distinction matters significantly when assessing water quality. We group violations into two broad types:

Health-based violations are the most significant category. These occur when a water system either detected a regulated contaminant above its legal limit or failed to apply a required treatment process:

• Maximum Contaminant Level (MCL) violations: A specific contaminant was measured above the legally enforceable limit set by the EPA. Examples include lead detected above the 15 parts per billion action level, coliform bacteria detected in distributed water, nitrates above 10 mg/L (a concern for infants), or disinfection byproducts such as trihalomethanes above their MCL.
• Treatment Technique (TT) violations: The system failed to properly apply a required treatment process, even if contamination wasn't directly measured. For example, a surface water system that didn't maintain adequate disinfection or filtration receives a TT violation. These indicate a failure in the treatment barrier that protects water quality.

Monitoring and Reporting (M&R) violations are procedural in nature. They mean the system failed to collect required water samples, test for required contaminants on schedule, or submit results to the state on time. M&R violations don't necessarily mean the water was unsafe — they mean the testing that would confirm safety either didn't happen or wasn't documented properly.

The distinction matters in practice. A water system might have accumulated hundreds of violations over its history, but the vast majority could be monitoring violations — missed sample deadlines or late paperwork. A different system with only a handful of violations might have all of them be health-based MCL exceedances. The health-based violation count is therefore the more meaningful indicator of actual water quality risk.

Our safety badge system, city pages, and rankings all surface health-based violation counts separately so users can distinguish between the two types at a glance.

Safety Rating System

Each city on Clean Water Index displays a simplified safety badge based on its historical health-based violation count. The three tiers are:

• Green — "No Health Violations": The city's water systems have zero health-based violations on record in SDWIS. This is the best-case outcome in our dataset.
• Yellow — "Some Concerns": The city's water systems have between 1 and 3 health-based violations on record. This may indicate isolated past incidents, resolved issues, or ongoing low-level concerns.
• Red — "Multiple Health Violations": The city's water systems have 4 or more health-based violations on record. This warrants closer examination of the specific violations involved.

There are important caveats to understand about these ratings. First, the badge reflects the full historical record — it includes violations from any year on file, not just recent data. A red badge may reflect problems that were identified and corrected years or decades ago. Second, this system does not account for severity — a single severe MCL exceedance for a dangerous contaminant is weighted the same as a minor, brief exceedance. Third, it does not reflect whether corrective action was taken or whether the violation is still open. The badge is a simplified starting point for exploration, not a definitive safety certification.

Analysis and Rankings

Clean Water Index ranks states and cities along several dimensions to help identify where water quality concerns are most concentrated nationally and within each state.

State rankings are computed across three primary metrics: total violations (all violation types combined), health-based violations only, and violations per capita. Per-capita calculations use the total population served by community water systems in each state — not the census population — because not every resident is served by a CWS. This gives a more accurate picture of per-person exposure within the systems we track.

City rankings work the same way within each state. Cities are ranked by total violations, health violations, and violations per capita (based on the population served by water systems in that city). Cities with multiple water systems have their figures aggregated before ranking.

Top contaminants lists are compiled by counting unique violation instances per contaminant type across all systems in a state or nationally. A contaminant that generated violations across many different water systems ranks higher than one that generated many violations in a single system. This approach highlights contaminants with broad geographic presence rather than those concentrated in one location.

The Analysis section of the site provides additional views including the cities and states with the most total violations, the most health-based violations, and the most distinct contaminants detected. These cross-cutting views are designed to surface different facets of the national water quality picture beyond simple violation counts.

Update Frequency

Data is fetched from the EPA Envirofacts API on a periodic basis and the generated data files are updated accordingly. The current dataset was last refreshed on February 17, 2026.

EPA data itself may not reflect the most current real-world conditions. There is inherent reporting lag between when a violation occurs, when it is identified by the water system or state regulator, and when it is entered into SDWIS. For the most up-to-date information about a specific water system — including any violations issued after our last data fetch — contact your local utility directly or search the EPA ECHO database, which is updated more frequently.

Known Limitations

We believe in being transparent about what this data can and cannot tell you. The following limitations apply to all information on this site:

• Historical data: Violation counts include the full historical record from SDWIS, not just recent years. A city with a high violation count may have resolved its water quality issues years or decades ago. Conversely, a city with a low count may have been tested less frequently. We do not currently filter by date range or recency.
• Private wells excluded: SDWIS only covers regulated public water systems. Approximately 23 million US households — about 15% of the population — use private wells that are not tracked in this database. If you use a private well, this data does not apply to your situation.
• Not all contaminants: The EPA currently regulates approximately 90 contaminants under primary drinking water standards. Thousands of other substances may be present in water supplies but are not currently regulated or required to be tested. PFAS (per- and polyfluoroalkyl substances) are present in many water systems but have only recently been added to federal regulation; historical PFAS data in SDWIS is limited. Pharmaceuticals, microplastics, and many industrial chemicals are not currently covered by NPDWRs.
• Tap vs. treatment plant: Compliance monitoring data reflects conditions measured at treatment plant outputs or at specific distribution system sampling points — not necessarily at your specific tap. In-home plumbing can introduce additional contamination that wouldn't appear in utility-level compliance data. Lead from household plumbing is a well-documented example: a system may be in full compliance while individual homes with lead service lines or fixtures experience elevated lead levels.
• Small system gaps: Very small water systems have less frequent testing requirements under federal rules. This means there are fewer data points for smaller systems, and violations may be underrepresented relative to larger systems with more frequent monitoring obligations.
• Data lag: Beyond the lag between our fetch date and EPA data entry, there is additional lag between real-world conditions and when violations are identified and reported. Violations can take months or longer to work through the reporting chain before appearing in SDWIS.
• City-to-system matching: Our pipeline groups water systems by the "city served" field in SDWIS records. Some water systems serve areas that span multiple cities, use informal geographic names, or use naming conventions that differ from standard city names. This can cause minor inaccuracies in city-level aggregation — for example, a suburban system that serves part of a city and part of an unincorporated area may be attributed entirely to one city.
• Not a substitute for professional testing: This site provides a starting point for understanding the historical compliance record of water systems in your area. It is not a substitute for independent water testing by a certified laboratory. If you have specific concerns about your water quality, we recommend requesting your utility's most recent Consumer Confidence Report, testing your tap water independently, and consulting local health authorities.