What’s in your drinking water?
The federal government tested 10,313 public water systems for PFAS “forever chemicals.” Enter your ZIP code to see what they found in yours.
What the national picture looks like
Every figure below is counted directly from EPA’s published results — not modelled, not estimated.
How this works
- Where the numbers come from
- The Fifth Unregulated Contaminant Monitoring Rule (UCMR 5) required public water systems to test for 29 PFAS compounds and lithium between 2023 and 2025. EPA published the final results. We joined those results to the Safe Drinking Water Information System for system names, service areas and population, and to EPA’s own PWSID‑to‑ZIP crosswalk.
- What the limits mean
- Only five compounds have enforceable federal limits: PFOA and PFOS at 4.0 parts per trillion, and PFHxS, PFNA and HFPO‑DA at 10.0. The rest were measured but are not federally regulated, so we show the value and say so plainly rather than implying a threshold that does not exist.
- A result above a limit is not a violation
- These are individual sample measurements. Compliance is judged on a running annual average at each entry point, and is not required until April 2029 — EPA has proposed extending that to 2031 for systems that qualify. The agency has also proposed rescinding the limits for PFHxS, PFNA and HFPO‑DA; both proposals are pending, so the 2024 rule still stands as shown here.
- What “nothing detected” does and does not mean
- It means none of the 30 monitored substances were found above the laboratory reporting limit in the samples taken. It is not a statement that the water is free of everything, and it does not cover lead, nitrate, arsenic, bacteria or disinfection by‑products, which are tested under separate rules.
- If your ZIP isn’t here
- UCMR 5 required testing of systems serving more than 3,300 people, plus a sample of smaller ones. Around 40,000 small rural systems were not included, and private wells are never federally tested — roughly 43 million Americans rely on them and are responsible for their own testing.
- Reading a result
- Values are shown in parts per trillion
ppt, the unit EPA uses for PFAS. One part per trillion is roughly one drop in twenty Olympic swimming pools — small numbers here still matter, which is why the federal limit is 4.
Drinking water, briefly Where it comes from, how it is treated, who regulates it
What tap water is
Water delivered to a building through pipes for drinking, cooking and washing, as distinct from bottled water or water drawn directly from a well. In the United States it is supplied by a public water system, a legal category covering any operation that pipes water to at least 25 people or 15 service connections. There are 142,891 active public water systems in the country, of which 49,378 are community systems — the ones that supply homes year-round. Between them they serve about 331 million people.
Where it comes from
Roughly two sources. Surface water is drawn from rivers, lakes and reservoirs and tends to supply larger cities; it is more exposed to run-off, industrial discharge and seasonal change. Groundwater is pumped from aquifers through wells and supplies most smaller systems; it is naturally filtered by rock and soil, which removes many pathogens but also dissolves minerals and, in some regions, arsenic or radon. Some systems blend both, and a few coastal utilities now desalinate seawater.
How it is treated
Conventional treatment runs in stages. Coagulation adds a chemical that makes suspended particles clump together; flocculation gently stirs them into larger flocs; sedimentation lets those settle out; filtration passes what remains through sand, gravel or membranes; and disinfection — usually chlorine or chloramine, sometimes ozone or ultraviolet light — kills the organisms that survived. Many systems also adjust pH to stop the water corroding the pipes it travels through, which is the control that failed in Flint, Michigan.
How it reaches you
Treated water leaves the plant at an entry point, which is where compliance samples are taken, then travels through mains, storage tanks and finally a service line into the building. This last stretch matters: it is private property, it is not covered by the utility's sampling, and in housing built before 1986 it may be lead. A system with a spotless record can still deliver lead into one specific kitchen, which is why lead is regulated at the tap rather than at the plant.
Who regulates it
The Safe Drinking Water Act of 1974 gives the Environmental Protection Agency authority to set enforceable national limits for contaminants in public water systems. In practice almost every state holds primacy, meaning it administers and enforces those rules itself, so sampling intensity and enforcement vary between states. Around 90 contaminants are regulated. Private wells fall outside the Act entirely and are the owner's sole responsibility.
Hard and soft water
Hardness is the dissolved calcium and magnesium a supply picks up from rock, and it is mostly a nuisance rather than a health matter. Hard water leaves scale in kettles and on fixtures and makes soap lather poorly; soft water does neither but can taste flat and is slightly more corrosive to plumbing. Groundwater is generally harder than surface water. Softeners exchange calcium and magnesium for sodium, which is worth knowing if you are restricting dietary sodium.
Fluoridation
Adding fluoride to drinking water to reduce tooth decay began in Grand Rapids, Michigan in 1945 and became widespread across the United States. It remains a decision made locally rather than nationally, so whether your supply is fluoridated depends on your utility. EPA sets a maximum limit for naturally occurring fluoride for health reasons, which is a separate matter from the lower level used in deliberate fluoridation.
A short history
Piped water is ancient; safe piped water is recent. The turning point was disinfection: Jersey City, New Jersey began continuously chlorinating its supply in 1908, and waterborne typhoid and cholera collapsed across American cities within a generation. Filtration and chlorination together are among the largest public health gains of the twentieth century. The modern regulatory era began with the Safe Drinking Water Act in 1974, and the questions have shifted since — from pathogens that act in days to industrial compounds that act over decades.