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Common Drinking Water Contaminants and Their Sources

At a glance
The EPA regulates more than 90 drinking water contaminants under the National Primary Drinking Water Regulations (NPDWR). Each has a Maximum Contaminant Level (MCL, legally enforceable) and a Maximum Contaminant Level Goal (MCLG, the health-based goal, often zero). Contaminants fall into six categories: microorganisms, disinfectants, disinfection byproducts, inorganic chemicals, organic chemicals, and radionuclides. This hub links to deep-dive pages on the contaminants most relevant to residential treatment.

The MCL framework

EPA's Safe Drinking Water Act framework distinguishes between the legally enforceable Maximum Contaminant Level (MCL) and the health-based Maximum Contaminant Level Goal (MCLG). For most acute toxicants and known carcinogens, the MCLG is zero (no exposure is considered safe), while the MCL is set at the lowest level that water systems can feasibly achieve given current treatment technology and analytical detection capabilities.

Lead is a clear example. The MCLG for lead is zero. The Action Level (the regulatory threshold for required corrective action under the Lead and Copper Rule) is 15 ppb at the 90th percentile of household samples. There is no MCL for lead in the conventional sense, because lead is treated as a corrosion problem on the consumer side rather than a source-water problem on the utility side. This is why a clean CCR can coexist with lead in your tap water.

The six EPA contaminant categories

NPDWR organises drinking water contaminants into six categories. Each has different treatment implications.

  • Microorganisms: total coliform, E. coli, Cryptosporidium, Giardia, Legionella, viruses. Treatment: UV-C, chlorination, filtration.
  • Disinfectants: chlorine, chloramine, chlorine dioxide. Regulated under MRDL (Maximum Residual Disinfectant Level). Treatment: activated carbon.
  • Disinfection byproducts: trihalomethanes (THMs), haloacetic acids (HAA5), bromate, chlorite. Treatment: NSF/ANSI 53 activated carbon.
  • Inorganic chemicals: lead, arsenic, copper, cadmium, mercury, nitrate, fluoride, nitrate. Treatment varies by contaminant.
  • Organic chemicals: VOCs (benzene, vinyl chloride), pesticides (atrazine, glyphosate), PFAS. Treatment: GAC, RO.
  • Radionuclides: radium, uranium, gross alpha. Treatment: RO, ion exchange, activated alumina.

Deep dive pages

Lead

EPA Action Level: 15 ppb. MCLG: zero. The 2024 LCRI mandates lead service line replacement within 10 years.

NSF/ANSI 53 carbon block (POU); RO; lead service line replacement

PFAS (PFOA, PFOS)

EPA 2024 final rule sets 4 ng/L MCL for PFOA and PFOS individually. Compliance deadline 2029.

GAC (NSF P473); RO; activated carbon for whole-house, RO at POU

Chlorine, chloramine, and DBPs

Mandated municipal disinfectants. EPA MRDL: 4.0 mg/L chlorine. DBPs regulated under Stage 2 D/DBP Rule.

Activated carbon (NSF/ANSI 42 with chloramine claim); catalytic carbon for chloramine

Arsenic, iron, and manganese

Common well water minerals. EPA arsenic MCL: 10 ppb. Iron is aesthetic at low levels but significant in plumbing.

RO for arsenic V; oxidation plus filtration for iron and manganese

Coliform, E. coli, and nitrates

Total coliform indicates contamination pathway. E. coli indicates fecal contamination. Nitrate MCL: 10 mg/L.

UV disinfection for microbes; RO for nitrate; anion exchange for nitrate

How to identify your contaminants

You cannot pick a treatment without knowing what you are treating. Two paths apply, depending on your water source.

Municipal customers: Read your annual Consumer Confidence Report (CCR). It lists every contaminant detected by your utility, the highest level recorded, the EPA MCL, and any reporting violations. Detected contaminants below the MCL are still detected; some homeowners install treatment for contaminants well below the MCL because the MCLG is lower or zero. The water testing page walks through CCR interpretation.

Private well owners: CDC recommends annual testing for total coliform bacteria, nitrate, total dissolved solids (TDS), and pH. Add regional tests as appropriate: arsenic and radon in much of the western U.S. and the Northeast, fluoride and uranium in some aquifers, pesticides downhill from agricultural land. Use a state-certified laboratory; home test kits are useful as a flag but not as a treatment-system specification basis.

Common questions

What is the difference between MCL and MCLG?
MCL (Maximum Contaminant Level) is the legally enforceable concentration limit for a contaminant in drinking water. MCLG (Maximum Contaminant Level Goal) is the level below which there is no known or expected health risk, with an adequate margin of safety. For carcinogens and acute toxicants, MCLG is often zero, while the MCL is set at the lowest feasibly achievable level given treatment technology. Detected concentrations between MCLG and MCL meet the law but indicate a contaminant present below the goal.
Are EPA MCLs the same as the WHO drinking water standards?
No. The U.S. EPA sets enforceable MCLs under the Safe Drinking Water Act based on a domestic regulatory analysis combining health risk, treatment feasibility, and cost. The WHO publishes Drinking Water Quality Guidelines, which are recommendations rather than enforceable standards, and which sometimes differ in the values chosen for arsenic, lead, fluoride, and other contaminants. EPA standards govern U.S. utility compliance.
Does my CCR show every contaminant in my water?
No. CCRs report contaminants detected during routine compliance monitoring under the MCLs the utility is required to monitor. Many emerging contaminants (PFAS until 2029, certain pharmaceuticals, microplastics) may not yet be on the regulated list and may not appear in the CCR even if present. Some utilities voluntarily report additional unregulated contaminants under EPA{`'`}s UCMR (Unregulated Contaminant Monitoring Rule) results.
What contaminants are most common in well water?
Coliform bacteria, nitrate (especially in agricultural areas), iron (very common nationally), manganese, hardness (calcium and magnesium), arsenic (regional, especially the western U.S. and Northeast), and radon (regional, especially Northeast and Rocky Mountain states). Less commonly: fluoride, uranium, pesticides, and VOCs from industrial groundwater contamination. Annual testing through a state-certified laboratory is the only reliable way to know.
Which contaminants does the CDC recommend testing for in private wells?
The CDC recommends annual testing for total coliform bacteria, nitrate, total dissolved solids, and pH at minimum. Additional tests are recommended based on regional risk factors: arsenic and radon in many western and northeastern states, fluoride in some aquifers, pesticides where the well is downhill from agricultural use, and metals where local geology suggests elevated risk. State health departments publish region-specific testing guidance.

Sources

Last reviewed: April 2026

Updated 2026-04-27