TRUST & TRANSPARENCY
Data sources & methodology
WaterCheckup pulls from the same public federal databases that public water systems report to. We merge them by ZIP code, translate the results into plain language, and match them to filter recommendations. We are not a laboratory and we do not sample your tap.
This page explains every data source we use, how the Water Quality Score is calculated, and what our reports do — and don't — tell you.
Our 6 data sources
SDWIS is the EPA's master registry of roughly 150,000+ community water systems in the United States (plus smaller non-community systems). It tracks every reported violation — health-based, monitoring, and reporting — along with enforcement actions, penalties, and compliance status. We query SDWIS live for every ZIP search so you always see the most current violation record on file. A "violation" means a public water system exceeded an EPA limit or failed to properly test and report. Most violations are monitoring failures (late or missing tests), but health-based violations for things like PFAS, nitrates, or disinfection byproducts are the ones that matter most.
Source: EPA ECHO / SDWIS public API
The UCMR is how EPA tests for contaminants that don't yet have federal limits. UCMR5 is the fifth round and is the most comprehensive PFAS dataset ever collected — public water systems were required to test for 29 PFAS compounds between 2023 and 2025 and report results to EPA. We ship a snapshot of the UCMR5 results database inside the app and match it to your water system by PWSID. This is how we show you specific PFAS compound levels (in parts per trillion) even before your public water system is required to include them in their annual water quality report.
Source: EPA UCMR5 public dataset
Lead doesn't come from source water — it leaches from aging lead service lines and in-home plumbing into your tap. The EPA's Lead & Copper Rule requires public water systems to collect tap samples from high-risk homes (typically those with older plumbing) and report the 90th percentile result. We surface these results alongside a flag if the action level of 15 ppb has been approached or exceeded. Keep in mind: sampling is done at a limited number of high-risk homes, not at every tap. Your home's actual lead exposure depends on its plumbing age and whether your service line is lead.
As of October 2024, all US public water systems are required by federal rule to publish an inventory of their lead service lines. We pull this data where available to flag whether your public water system has a significant number of known lead lines still in service. This is a public water system-level number — it tells you whether lead line risk exists in your system, not whether your specific address is affected. For address-level lookup tools, we link to city-specific resources where they exist (e.g. Chicago, NYC, Philadelphia).
Beyond violation records, ECHO tracks formal enforcement actions, inspection history, and penalty data. A public water system can have an open violation and still be working toward compliance — or it can have a history of repeat violations that suggest a systemic problem. We use enforcement history to contextualize the violation record and flag public water systems with patterns of non-compliance that a raw violation count might underrepresent.
The EPA's legal limits (MCLs) are set with economic and technical feasibility in mind, not pure health science. The Environmental Working Group publishes independent health guidelines for many contaminants that are often much stricter than EPA limits — for example, the EWG health guideline for PFOA is 0.1 ppt, compared to EPA's legal limit of 4 ppt. We use EWG health guidelines as a secondary layer to show when your water may be technically "within limits" but still above a stricter independent health benchmark. This context is clearly labeled as a health guideline, not a legal limit.
Source: EWG Tap Water Database
How the Water Quality Score is calculated
Every city and ZIP report includes a Water Safety Score from 0–88 and a letter grade (A–F). The score blends EPA compliance with real-world concerns — hardness, PFAS detections, and contaminants above independent health guidelines (similar in spirit to the EWG Tap Water Database, which does not publish letter grades). It is not a lab test result and it is not a medical assessment.
Starting point
No public municipal water supply scores above 88. Even clean supplies have chlorine, disinfection byproducts, and trace detections federal testing does not fully capture.
Deductions
Points are deducted for: open SDWIS violations; PFAS above EPA MCLs; any PFAS detection; very hard water (plumbing, taste, appliances); arsenic or radium above EWG-style health guidelines even when under EPA legal limits; and profile issues such as lead risk or disinfection byproducts. Meeting EPA limits alone does not earn an A — systems with hardness, PFAS, or metals flagged in the CCR are scored down accordingly.
Grade bands (0–88 scale)
Important limitations
- Whether your specific home's plumbing adds contaminants (especially lead). In-home plumbing can leach lead regardless of what your public water system reports.
- Whether your private well is contaminated — SDWIS only covers public water systems. Well users should test annually through a certified lab.
- Contaminants that aren't federally regulated and haven't been tested under UCMR. There are thousands of chemicals in use that EPA hasn't set limits for.
- Very recent changes — EPA violation data can lag 30–90 days behind real-world events. A boil water advisory issued this week may not yet be in SDWIS.
- Whether your bottled water is any safer — bottled water is regulated by FDA under different rules and isn't systematically tested for PFAS.
Data freshness
SDWIS violation data is queried live from the EPA ECHO API each time you search — it reflects whatever EPA has on file at that moment. UCMR5 PFAS data is a snapshot packaged into the app (the federal dataset was finalized in 2025 and updates infrequently). Lead pipe inventory data is pulled from federal reporting as of late 2024. Each report includes data freshness notes where we can surface them.
For the most current data, always cross-reference the official EPA links included in your report, and call your public water system directly if you have concerns about recent events.
Who we are
WaterCheckup is an independent water quality information service. We are not affiliated with the EPA, the EWG, or any public water system. We earn revenue through affiliate links to water filters — our recommendations are based on certifications and third-party test data, not paid placements. The affiliate relationship does not influence which data we surface or how we calculate scores.
Legal notice
WaterCheckup reports are provided for general information only. They are not legal advice, medical advice, engineering sign-off, or a statement of compliance by your public water system. EPA violation and monitoring data are reproduced from federal public records; the 0–88 score and A–F letter grades are calculated by WaterCheckup using our published methodology and may differ from how a utility describes its own compliance status.
A low grade does not mean every tap in a city is unsafe, and a high grade does not mean water is contaminant-free at your home. Plumbing, service lines, and neighborhood-level variation are not fully captured. Utilities and residents should rely on official Consumer Confidence Reports, EPA sources linked in each report, and certified laboratory testing for decisions that affect health or property.