PurityMap is built on one principle: water quality information should be transparent, data driven, and understandable.

This page explains how data is collected, how contaminants are evaluated, how Purity Scores are calculated, and how filtration recommendations are made. All sources referenced are publicly available and verifiable.

How PurityMap Collects Water Data

PurityMap analyzes publicly available drinking water data from trusted federal, state, and municipal sources. Our reporting focuses on the most relevant information households need to understand local water quality and common exposure risks.

Primary data sources include:

U.S. Environmental Protection Agency (EPA) Safe Drinking Water Act resources and related drinking water datasets, including the Safe Drinking Water Information System (SDWIS), which reports contaminants, violations, and compliance data for public water systems. https://www.epa.gov/sdwa

Environmental Working Group (EWG) Tap Water Database, which aggregates utility testing and provides health based comparisons. https://www.ewg.org/tapwater/

Municipal Consumer Confidence Reports (CCRs), annual water quality reports published by local water utilities as required by law.

State environmental and public health agencies, when available.

Data is collected at the utility, city, or service area level depending on availability. When multiple sources exist, data is cross referenced to identify consistent patterns and highlight common issues most likely to affect households.

Local applications of this data are presented in our City Water Quality Reports.

How Contaminants Are Evaluated

Most official water reports focus on whether water meets legal compliance limits.

PurityMap also evaluates detected contaminant levels against health based guidelines to provide additional context for long term household exposure.

Evaluation criteria may include:

  • EPA Maximum Contaminant Levels (MCLs)
  • EPA health advisories and related guidance
  • Independent health benchmarks used by Environmental Working Group and peer reviewed research

This approach helps answer a common household question:

Is my water legally compliant but still suboptimal for long term health?

Contaminants commonly evaluated include PFAS, lead, chlorine and disinfection byproducts, arsenic, and hardness related minerals.

More detail is available in the Water Contaminant Encyclopedia.

What a Purity Score Means

Each city or ZIP level report includes a Purity Score shown as a letter grade from A to F.

Purity Scores reflect:

  • Presence of known contaminants
  • Consistency and frequency of detection
  • Comparison to health based benchmarks rather than legal limits alone

A lower Purity Score does not mean water is unsafe or illegal.

It indicates that detected levels exceed optimal health guidelines or that multiple contaminants may be present in meaningful amounts.

Purity Scores are designed to be:

  • Easy to understand
  • Comparable across locations
  • Independent of product recommendations

Limitations & Data Gaps

Public water quality data has limitations, and PurityMap is transparent about them.

Common limitations include:

  • Testing conducted at the utility level rather than individual homes
  • Multiple treatment plants serving a single city
  • Aging household plumbing not reflected in municipal testing
  • Reporting delays or incomplete datasets

PurityMap does not replace laboratory testing or professional water analysis.

Our reports are intended to provide a reliable starting point for understanding local water conditions, identifying potential concerns, and selecting practical next steps.

Product Recommendations Policy

PurityMap may earn commissions when users purchase filtration products through links on the site.

These commissions do not influence:

  • Purity Scores
  • Contaminant reporting
  • Health context
  • Product eligibility

All data analysis is completed before any product is considered.

Products are recommended only if they meet relevant certification standards for the contaminants detected in a given location.

If a product does not address the identified water issues, it is not recommended, regardless of commission potential.

For full transparency, see our Affiliate Disclosure.

Sources & Certifications

PurityMap references guidance and public information from the following organizations:

External links are provided for verification and further reading.

Learn More

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