Reference

What's in your tap water?

The contaminants the EPA regulates in U.S. drinking water — what they are, the health effects, the legal limit, and how many community water systems are dealing with each one. Built from federal Safe Drinking Water Act records.

Across 48,769 community water systems, 14,233 have had at least one health-based violation since 2016.

Source: EPA Safe Drinking Water Information System (SDWIS) · 2026 Q1

Inorganic

Lead

Lead leaches from old pipes and fixtures, not the source water — which is why it varies house to house and why service-line replacement is the real fix.

Action level 0.015 mg/L (15 ppb)5,736 systems

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Arsenic

A naturally occurring carcinogen in many groundwater systems; the fix is treatment, since you can't avoid the geology.

MCL 0.010 mg/L (10 ppb)714 systems

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Nitrate

An agricultural contaminant that's acutely dangerous to infants; common across farm country and rural groundwater.

MCL 10 mg/L (as nitrogen)636 systems

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Copper

A plumbing-corrosion contaminant regulated alongside lead — usually a sign of corrosive water chemistry.

Action level 1.3 mg/L2 systems

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Fluoride

Beneficial at low added levels for teeth, but a violation where natural groundwater concentrations run high.

MCL 4.0 mg/L (enforceable)102 systems

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PFAS

Newly regulated 'forever chemicals' — a fast-emerging issue as EPA's first-ever PFAS limits take effect and monitoring expands nationwide.

PFOA & PFOS 4.0 ppt (ng/L)

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Disinfection byproducts

The trade-off of disinfection itself: chlorine kills pathogens but forms cancer-linked byproducts, making DBPs one of the most widespread health-based violations.

TTHM 0.080 mg/L · HAA5 0.060 mg/L4,670 systems

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Microbial

Bacteria & coliform

Coliform is the early-warning indicator that something could let pathogens into the water — the trigger behind most boil-water notices.

E. coli: any positive is a violation2,462 systems

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Radionuclide

Radionuclides

Naturally radioactive elements that seep from rock into groundwater; a geology-driven, region-specific cancer risk.

Radium 5 pCi/L · Gross alpha 15 pCi/L · Uranium 30 µg/L791 systems

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