WaterQ

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.

A
90–100
Excellent
B
70–89
Good
C
50–69
Fair
D
30–49
Poor
F
0–29
Critical

Scoring Formula

The final score is computed using the following formula:

Score = 100 - Σ Deductions
Where each deduction is calculated as:
Deduction = BasePoints × Weight × TimeDecay

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.

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.