Water potability is defined by regulatory standards that establish the maximum concentration of each contaminant permitted in public drinking water. Understanding the distinction between enforceable limits, health goals, and what happens between your treatment plant and your tap puts water quality reports in proper context.
The highest concentration of a contaminant permitted in water delivered by a public water system, set by EPA as close to the MCLG as technically and economically feasible. Violation triggers mandatory public notification and remediation requirements.
The level at which no known or anticipated adverse health effects occur, with an adequate margin of safety. The MCLG for known carcinogens (including some PFAS compounds) is often zero — below any technically achievable concentration. The MCL is set above the MCLG when zero is not achievable.
Non-enforceable guidelines for contaminants that affect taste, odor, or appearance but are not primary health hazards at typical concentrations. Iron (0.3 mg/L), manganese (0.05 mg/L), and turbidity (4 NTU) have SMCLs that inform consumer experience without triggering regulatory action.
When EPA establishes that a contaminant poses a health risk but has not yet finalized an MCL, it may issue a health advisory — a non-enforceable recommended concentration. PFAS health advisories existed for years before the 2024 final MCL rule was promulgated.
| Contaminant | MCL | MCLG | Primary Health Concern | Source in Drinking Water |
|---|---|---|---|---|
| Lead | Action level: 15 ppb | Zero | Neurological development impairment; no safe level in children | Leaching from lead service lines, lead-tin solder, brass fixtures |
| Nitrate | 10 mg/L as N | 10 mg/L |
Methemoglobinemia (blue baby syndrome) in infants under 6 months | Agricultural runoff, fertilizers, septic systems |
| Arsenic | 10 ppb | Zero | Carcinogenic; bladder, lung, skin cancer with chronic exposure | Natural geological deposits; some industrial discharge |
| Total Coliform | 0% monthly sample positive rate (large systems) | Zero | Indicator of fecal contamination and potential pathogen presence | Treatment failure, distribution contamination |
| Cryptosporidium | 99% reduction required (Enhanced Surface Water Treatment) | Zero | Gastrointestinal illness; severe risk for immunocompromised populations | Surface water; animal fecal matter in watershed |
| PFOA & PFOS | 4 ppt (EPA final rule) | Zero | Cancer, immune system effects, developmental toxicity | Industrial discharge, firefighting foam, consumer products |
| Trihalomethanes (TTHMs) |
80 ppb | Zero | Long-term cancer risk with chronic exposure | Disinfection byproducts from chlorine reacting with organic matter |
Chemicals added to make particles clump (floc). Addresses turbidity, suspended solids, and some pathogens attached to particles.
Heavy floc settles out. Removes majority of suspended particulate matter before filtration. Reduces filter loading.
Remaining particles removed through sand, gravel, or membrane filters. Post-filter turbidity must meet 0.3 NTU in 95% of samples.
Chlorine, chloramine, or UV inactivates pathogens. Residual disinfectant maintained throughout distribution to protect against contamination en route to tap.
Test finished water for all regulated contaminants on EPA's monitoring schedule
Notify the public within 24 hours of any MCL violation posing acute health risk
Publish an annual Consumer Confidence Report with all detected levels and MCL comparisons
Implement treatment techniques appropriate to the source water type
Maintain corrosion control to minimize lead dissolution from distribution infrastructure
Water quality at your specific tap — SDWA applies to the utility's delivery point, not your faucet
Zero lead — the MCLG is zero but the action level is 15 ppb, and building plumbing is outside SDWA scope
Regulation of all contaminants — only those on EPA's regulated list; emerging contaminants may not yet be covered
Building-side plumbing quality — service lines, risers, branch lines, and roof tanks are owner and building responsibility
Building constructed before 1986. Children under 6 or pregnant person in the household. Formula preparation in a pre-1986 building. Recurring cold-tap discoloration. Metallic taste in first-draw water. Building has had recent plumbing work or a nearby main replacement.
First-draw lead at the primary drinking fixture. For a full picture: lead, copper, iron, manganese, pH, hardness, TDS. For well water: add nitrates, arsenic, total coliform, and common agricultural contaminants. Laboratory tests are definitive — consumer test strips are not reliable for health decisions.
New York: request a NY ELAP-certified lab list from NYSDOH. New Jersey: NJ DEP maintains the certified laboratory directory. Always specify "first-draw, no pre-flush" collection protocol for lead — this captures worst-case building plumbing exposure, which is the clinically relevant scenario.