How the numbers are computed

Every figure comes from two public EPA files, run through a published script. If the data cannot support a number, the page says so rather than guessing.

PFAS results (UCMR 5)

• Non-detects count as zero. For each sampling point, a "full set" is two results for groundwater sources or four for surface water.
• Each sampling point's results are averaged; a page shows the highest average at any point in the system.
• PFOA and PFOS count as above the 4 ppt limit at 4.05 ppt or more; PFHxS, PFNA and GenX count as above their 10 ppt level at 15 ppt or more; the hazard index counts at 1.5 or more with at least two components detected. These are EPA's own rounding rules (UCMR 5 Data Summary, Table 4).
• Validation: run on every public water system type, this method reproduces EPA's published counts of systems above each level: PFOS 875, PFOA 779, PFHxS 66, PFNA 9, GenX 8, hazard index 80, any 1,136.
• UCMR 5 results are not compliance determinations. Compliance uses a running annual average from the utility's regular monitoring.
• UCMR 5 covered all systems serving 3,300+ people and only a sample of smaller ones, so "not tested" never means "clean".

Violations and lead (SDWIS)

• Health-based violations: records flagged health-based (limit exceedance, residual-disinfectant exceedance or treatment-technique), with a noncompliance period beginning on or after 2021-10-26. Monitoring and reporting violations are excluded.
• Lead and copper: the latest 90th-percentile value in the EPA file. Values whose sampling period ended before 2021 are not shown on pages.

Coverage

49,123 active community water systems serving at least one person are in the file. 8,990 serve 3,300+ people and have a complete PFAS set; 1,053 of those averaged above a 2024 federal level at one or more sampling points. Population totals are not de-duplicated across systems that buy water from each other.

Known limits

• SDWIS data are reported by states and can lag by a quarter or more.
• City and county lists are as reported to EPA and are incomplete for some systems.
• System-wide data cannot describe your tap, your pipes or your well.

EPA UCMR 5 data · EPA ECHO SDWA download documentation · Download our computed tables