EPA Water Contaminant Limits Explained
"Meets EPA standards" sounds reassuring until you understand how those standards are set, how many contaminants have no standard at all, and how far behind the science the regulations have fallen.
MCLGs vs MCLs
The EPA sets two numbers for each regulated contaminant. The Maximum Contaminant Level Goal (MCLG) is the level at which no known or anticipated health effects occur, with a margin of safety. It is a health-only target with no consideration of cost or feasibility. For carcinogens like lead and arsenic, the MCLG is zero.
The Maximum Contaminant Level (MCL) is the enforceable standard. It is set as close to the MCLG as "feasible" -- meaning the EPA weighs treatment costs, available technology, and the economic burden on water systems. The MCL for arsenic is 10 ppb. The MCLG is 0. The gap between those numbers is the compromise between health and economics.
Key distinction: the MCL is a legal limit, not a safety threshold. Water at 9.9 ppb arsenic is "compliant" but not necessarily safe. It simply falls below the level at which the EPA decided enforcement was worth the cost.
How Limits Are Set
The Safe Drinking Water Act requires the EPA to identify contaminants that may have adverse health effects, occur in public water systems, and where regulation presents a meaningful opportunity for health risk reduction. The process:
- Health assessment: EPA reviews toxicology data and sets a Reference Dose (RfD) -- the estimated daily exposure that's likely to be without appreciable risk over a lifetime. This becomes the MCLG.
- Feasibility analysis: EPA evaluates best available treatment technology (BAT) and the cost to water systems. Small systems get different treatment than large ones.
- Cost-benefit analysis: Required since 1996 SDWA amendments. If the cost of treatment exceeds the monetized health benefits, the EPA can set the MCL higher than what health science alone would suggest.
- Public comment and rulemaking: Years of regulatory process. The arsenic MCL took over a decade to move from 50 ppb to 10 ppb (finalized 2001, enforced 2006).
Contaminants Without Federal Limits
The EPA regulates approximately 90 contaminants. There are thousands of chemicals in commerce. Notable gaps:
Chromium-6 (hexavalent chromium)
The EPA regulates "total chromium" at 100 ppb but does not distinguish between chromium-3 (essential nutrient) and chromium-6 (carcinogen). California set a state MCL of 10 ppb for chromium-6, then withdrew it. An EWG analysis found chromium-6 in the tap water of 218 million Americans.
Microplastics
No federal standard exists. No standardized testing methodology is approved. Microplastics have been found in 94% of US tap water samples (Orb Media, 2017) and in bottled water at even higher rates. The health effects of chronic ingestion are still being studied.
PFAS
The EPA finalized enforceable limits for 6 PFAS compounds in April 2024: PFOA and PFOS at 4 ppt each, PFHxS/PFNA/HFPO-DA at 10 ppt each, and a hazard index for mixtures. Before 2024, there was only a non-enforceable health advisory. Water systems have until 2029 to comply. The remaining 12,000+ PFAS compounds remain unregulated.
Pharmaceuticals
Trace levels of antibiotics, hormones, antidepressants, and other pharmaceuticals are routinely detected in drinking water. No federal limits exist. Conventional water treatment was not designed to remove them.
Secondary Standards
The EPA also sets Secondary Maximum Contaminant Levels (SMCLs) for substances that affect taste, odor, color, and cosmetic effects (skin/hair staining). These include iron (0.3 mg/L), manganese (0.05 mg/L), chloride (250 mg/L), sulfate (250 mg/L), and total dissolved solids (500 mg/L).
SMCLs are non-enforceable guidelines. A utility can exceed every secondary standard and remain in full compliance. The water may taste terrible, stain laundry, and smell like rotten eggs -- but it's legally fine.
Why Limits Lag Science
The regulatory pipeline is structurally slow. New contaminants must go through the Contaminant Candidate List (CCL), the Unregulated Contaminant Monitoring Rule (UCMR) for data collection, regulatory determination, and then the full rulemaking process. From identification to enforceable standard takes 10-20 years minimum.
The EPA has not added a new contaminant to its enforceable list since 1996 (PFAS in 2024 was the first in 28 years). During that time, thousands of new chemicals entered commerce and peer-reviewed research identified health effects at concentrations below existing limits.
The system is reactive by design. This is why independent testing and transparency matter. Check actual contaminant levels in your water using our water lookup tool.