Our Data & Methodology
How we collect, verify, and present water quality data for 1000 US cities.
Data Sources
CheckMyTap compiles water quality data from four primary public sources. EPA Safe Drinking Water Information System (SDWIS) provides regulatory compliance data for every public water system in the United States, including violations, enforcement actions, and contaminant monitoring results. EPA Unregulated Contaminant Monitoring Rule 5 (UCMR5) is a federal monitoring program that collected PFAS occurrence data from public water systems between 2023 and 2025; we use UCMR5 to report PFAS levels for approximately 965 systems where this data is available. USGS Water Hardness Studies provide regional hardness data based on geological surveys and groundwater sampling. Municipal Consumer Confidence Reports (CCRs) are annual reports published by water utilities as required by the Safe Drinking Water Act, containing detailed contaminant testing results specific to each system. We cross-reference utility-reported contaminant data against both federal legal limits and independent health guidelines.
What We Measure
Hardness (PPM and GPG): Total hardness is measured as milligrams per liter of calcium carbonate equivalent (mg/L CaCO₃), also called parts per million (PPM). We convert to grains per gallon (GPG) using the standard factor of 17.1 PPM per GPG. Our hardness values represent the most recent annual average reported by the utility.
PFAS (ppt): Per- and polyfluoroalkyl substances measured in parts per trillion. We report PFAS levels sourced from EPA UCMR5 monitoring data where available, supplemented by utility-reported contaminant data. The EPA established maximum contaminant levels of 4 ppt for PFOA and PFOS individually in 2024.
Lead (ppb): Lead measured in parts per billion at the 90th percentile of household sampling, as reported under the EPA Lead and Copper Rule. Because lead enters water from plumbing, not the source, these values represent system-wide averages and individual homes may be higher or lower.
Chlorine (mg/L): Free or total chlorine/chloramine residual in milligrams per liter. We report the average distribution system residual.
TDS (PPM): Total dissolved solids in parts per million, an overall water purity indicator.
Nitrate (mg/L): Nitrate-nitrogen in milligrams per liter. The EPA MCL is 10 mg/L; health guidelines suggest concern above 5 mg/L, especially for infants.
How We Classify Hardness
We use a six-tier hardness classification based on USGS methodology and water treatment industry thresholds: Soft (0-16 PPM), Slightly Hard (17-59 PPM), Moderately Hard (60-119 PPM), Hard (120-179 PPM), Very Hard (180-249 PPM), Extremely Hard (250+ PPM). Our recommendation thresholds are: below 60 PPM (no treatment needed), 60-120 PPM (optional conditioning), above 120 PPM (softening recommended), above 180 PPM (softening strongly recommended).
Limitations
Water quality varies by neighborhood, season, and even individual plumbing. The values on CheckMyTap represent utility-reported averages and may not match your specific tap. Homes with lead service lines, older plumbing, or private wells may have significantly different contaminant levels. For exact data about your home, we recommend independent testing or requesting your utility's most recent CCR with neighborhood-level data.
Updates
We update our database when new CCRs are published (typically annually) and when the EPA updates regulatory standards. Last comprehensive update: 2026-03-26.
Contaminants We Track
We maintain detailed reference guides for the most common contaminants found in US drinking water, covering health effects, EPA regulations, testing methods, and treatment options.