Methodology

Data Source

All water quality data on WaterByCounty comes from the EPA Safe Drinking Water Information System (SDWIS), accessed through the EPA ECHO API.

SDWIS is the federal database that tracks compliance by public water systems across the United States. It records violations of the Safe Drinking Water Act (SDWA), including health-based violations, monitoring and reporting violations, and treatment technique violations. Every public water system that serves at least 25 people or has at least 15 service connections is required to report to SDWIS.

WaterByCounty aggregates this system-level data to the county level, giving you a single view of drinking water compliance across all public water systems serving each county.

Water Quality Metrics

We track several key metrics for each county:

• Health-Based Violations — Violations of Maximum Contaminant Levels (MCLs) or Treatment Technique rules that pose a direct risk to public health. These are the most serious type of violation and carry the most weight in our scoring.
• Health Violation Rate — The number of health-based violations normalized by the number of water systems in the county, providing a per-system violation intensity measure.
• Water Quality Grade — A letter grade (A through F) derived from the water quality score, making it easy to compare counties at a glance.
• Water Quality Score — A 0-100 score calculated using percentile-rank methodology across all US counties, where 100 represents the best water quality.

How Water Quality Scores Are Calculated

Water quality scores use percentile-rank methodology. Every county is ranked against all 3,100+ US counties based on its violation history and compliance record. The score is then normalized to a 0-100 scale:

• A score of 90 means the county has better water quality than 90% of all US counties.
• A score of 50 means the county is at the national median.
• A score of 10 means 90% of counties have better water quality.

Lower violation counts and lower violation rates result in higher scores. Counties with zero health-based violations score highest.

Letter Grade Scale

Geographic Coverage

WaterByCounty covers all 3,100+ counties and county-equivalents in the United States, including parishes (Louisiana), boroughs (Alaska), and independent cities (Virginia). Coverage depends on EPA reporting — counties where all public water systems report to SDWIS have the most complete data.

Some counties may have limited data if their water systems serve fewer than 25 people or if systems have not yet reported for the most recent period. Counties with insufficient data receive a null score rather than an inaccurate one.

Data Freshness

Water quality data is pulled from EPA SDWIS/ECHO on a periodic basis. Violation records reflect the most recently available compliance data from EPA, which typically lags real-time by several months as water systems report and EPA processes the data.

Scores and grades are recalculated each time we ingest new data from EPA. The current dataset was last updated in early 2026. We aim to refresh data quarterly to capture new violations and resolved compliance actions.

AI-Generated Content Disclosure

WaterByCounty uses artificial intelligence to generate descriptive narratives and editorial content on county and state pages. These AI-generated texts are based on real EPA data and are reviewed for accuracy, but they are machine-written summaries — not expert analysis.

All data tables, scores, grades, and numeric values displayed on WaterByCounty come directly from EPA SDWIS data processed through our scoring pipeline. The AI layer adds context and readability but does not alter the underlying data.

If you notice an error in any AI-generated content, please contact us so we can correct it.

Limitations

• SDWIS tracks public water systems only. Private wells — which serve approximately 13% of US households — are not included. If your county has a high percentage of private well users, the score reflects only the public system portion.
• Violation counts are cumulative and may include violations that have since been resolved. A county with historical violations may have fully compliant systems today.
• Water quality is complex and multidimensional. Our score captures regulatory compliance but does not measure taste, hardness, or contaminants below MCL thresholds. A passing grade does not guarantee perfect water — it means the water systems met federal standards.
• County-level aggregation means that a single large system with violations can affect the score for an entire county, even if smaller systems are fully compliant.