Scoring Methodology
A detailed explanation of how WaterQ evaluates and scores the water quality of US public water systems.
Overview
Each public water system receives a score from 0 to 100, where 100 represents the best possible water quality (no detected contaminants, no violations). The score is calculated by starting at 100 and applying deductions based on contaminant test results and violation history. For a quick, plain-language walkthrough before the technical details, see how to read a water quality report.
Scoring Formula
The final score is computed using the following formula:
Step 1: Base Points
For each contaminant detected in the water system, base deduction points are calculated by comparing the measured concentration to the EPA Maximum Contaminant Level (MCL):
| Concentration vs MCL | Base Points |
|---|---|
| < 50% of MCL | 1–3 points |
| 50–80% of MCL | 3–6 points |
| 80–100% of MCL | 6–10 points |
| > 100% of MCL (Violation) | 10–20 points |
Step 2: Contaminant Weight
Not all contaminants pose equal risk. Each contaminant is assigned a weight factor based on its potential health impact:
| Risk Category | Weight | Examples |
|---|---|---|
| High Risk | 2.0x | Lead, Arsenic, PFAS, Radionuclides |
| Medium Risk | 1.5x | Nitrate, Copper, Total Trihalomethanes |
| Standard | 1.0x | Fluoride, Chlorine, Barium |
Step 3: Time Decay
More recent test results carry greater weight. A time decay factor ensures that older data has diminishing impact on the current score:
| Data Age | Decay Factor |
|---|---|
| < 1 year | 1.0 (full weight) |
| 1–2 years | 0.8 |
| 2–3 years | 0.6 |
| 3–4 years | 0.4 |
| 4–5 years | 0.2 |
Violation Penalties
In addition to contaminant-based deductions, regulatory violations carry additional penalties:
| Violation Type | Penalty | Status Modifier |
|---|---|---|
| Critical | 10–15 points | Unresolved: ×1.5 |
| Major | 5–10 points | Unresolved: ×1.3 |
| Minor | 1–5 points | Unresolved: ×1.1 |
Resolved violations still count but at a reduced rate. The time decay factor also applies to violation penalties.
Aggregated Scores
City, county, and state scores are calculated as population-weighted averages of the individual water system scores within each geographic area. This means larger water systems that serve more people have a proportionally greater influence on the aggregated score. To see these results in context, visit the live national rankings page.
Data Reliability
WaterQ scores are based on publicly reported data from the EPA SDWIS database. While we strive for accuracy, there are inherent limitations:
- Testing frequency varies by contaminant and water system size
- Not all contaminants are required to be tested (e.g., some emerging contaminants like PFAS)
- EPA data may have a reporting lag of weeks to months
- Private wells and very small systems are not covered by federal monitoring
- Water quality at the tap may differ from what is measured at the treatment plant due to aging infrastructure
Limitations
WaterQ scores provide a useful summary but cannot capture every aspect of water quality. Our scoring system does not account for:
- • Unregulated contaminants not currently monitored by the EPA
- • Aesthetic qualities (taste, odor, color) that may not pose health risks
- • Distribution system conditions between the treatment plant and your tap
- • Individual health sensitivities (e.g., immunocompromised individuals)
This scoring methodology is designed for informational purposes only and should not replace professional water quality testing or medical advice.
Frequently Asked Questions
How is the WaterQ score calculated?
WaterQ scores range from 0 to 100, starting at 100 (best) and applying deductions based on contaminant detections, violation severity, and how recently violations occurred.
What do the letter grades mean?
Grades A (90-100) are excellent, B (70-89) good, C (50-69) fair, D (30-49) poor, and F (0-29) critical. Higher scores mean fewer contaminants and violations.
How often is WaterQ data updated?
WaterQ refreshes data regularly based on EPA SDWIS releases, typically within weeks of new federal monitoring and compliance reports being published.
Are private wells included in WaterQ scores?
No, WaterQ covers only public water systems regulated by the EPA. Private wells are not subject to federal monitoring and are not included in our database.
Why do some water systems lack scores?
Some systems may have incomplete testing data or recent ownership changes. We show available data while working to incorporate new information as it becomes available.