The Drinking Water Contaminants Reference Guide: Sources, EPA Standards & Testing
Drinking water can contain substances from natural geology, source-water pollution, treatment processes, distribution systems, and household plumbing. The contaminants present—and their concentrations—vary by location, water source, season, treatment method, and the materials between the utility and your tap.
Public water systems in the United States must monitor for regulated contaminants and meet federal and state requirements. That oversight is important, but a utility-wide result does not always describe conditions at every faucet. Lead and copper, for example, can enter water from service lines and building plumbing after treatment. Private wells generally fall outside federal public water regulations and place more responsibility on the owner.
This reference guide explains major contaminant groups, how they reach drinking water, how EPA addresses them, and which tests can provide useful answers. It is designed as a starting point; linked contaminant guides provide deeper coverage.
How to Use This Guide
Start with the quick-reference table, then read the sections relevant to your source and plumbing. Each profile answers six practical questions:
- What is the contaminant or contaminant group?
- Where does it come from?
- How can it reach drinking water?
- How does EPA regulate or evaluate it?
- Who may want to pay particular attention?
- How can it be tested?
Drinking Water Contaminants: Quick Reference
| Contaminant | Common source or pathway | Federal drinking-water status | Useful first check |
|---|---|---|---|
| Lead | Lead service lines, solder, fixtures, and plumbing corrosion | Treatment technique and action-level framework | Tap-specific certified laboratory test |
| Fluoride | Natural deposits or controlled addition by a utility | Primary MCL; secondary standard also applies | CCR or fluoride-specific test |
| TTHMs and HAA5 | Reactions between disinfectants and natural organic matter | Regulated groups with MCLs | CCR or laboratory DBP panel |
| Arsenic | Natural mineral deposits; some industrial and mining sources | Regulated with an MCL | Certified laboratory test, especially for wells |
| PFAS | Industrial releases, firefighting foam, landfills, and products | Federal rule is evolving; PFOA and PFOS limits remain central | EPA-method laboratory test |
| Chlorine/chloramine | Intentionally used by utilities as disinfectants | Maximum Residual Disinfectant Levels | Utility information or residual-disinfectant test |
| Nitrate/nitrite | Fertilizer, manure, septic systems, and natural processes | Regulated with separate MCLs | Certified laboratory test |
| Chromium-6 | Natural deposits and industrial activity | Federal MCL applies to total chromium, not chromium-6 alone | Speciated chromium laboratory test |
| Copper | Corrosion of copper plumbing and fixtures | Treatment technique and action-level framework | Tap-specific first-draw test |
| VOCs | Fuel, solvents, industrial releases, spills, and waste sites | Many individual VOCs have MCLs | Laboratory VOC panel |
| Microplastics | Breakdown of plastics, fibers, source water, and distribution | No federal MCL; under study and candidate-list review | Specialized analysis; methods remain developing |
| Radium | Naturally radioactive rock and aquifer materials | Combined radium-226/228 MCL | Certified radiological laboratory |
| Pharmaceuticals | Wastewater, improper disposal, and human or veterinary use | No group-wide federal MCL; under study | Specialized targeted analysis |
| Other metals | Geology, mining, industry, and plumbing | Varies by metal | Laboratory metals panel selected for local risks |
| Hardness minerals | Dissolved calcium and magnesium from geology | Not a federal health-based contaminant standard | Hardness strip, utility data, or laboratory test |
Regulatory status is summarized for orientation and is not a substitute for current EPA, state, tribal, or local requirements.
From Water Concern to Filter Choice
Testing tells you what is actually present. Filter selection should then be based on the exact contaminant or water-quality issue you want to address—not on a generic “removes everything” claim.
Understanding EPA Drinking Water Standards
Under the Safe Drinking Water Act, EPA establishes national requirements for public water systems. States and tribes with primary enforcement authority may implement the federal rules and can adopt additional or more stringent requirements.
Terms that are easy to confuse
- Maximum Contaminant Level Goal (MCLG): a non-enforceable public-health goal set at a level where no known or expected adverse effect is anticipated, with a margin of safety.
- Maximum Contaminant Level (MCL): an enforceable limit for a contaminant in water delivered by public systems. EPA considers health information as well as the feasibility of analysis and treatment.
- Treatment technique: an enforceable process or performance requirement used when regulating a contaminant through a numeric MCL is not practical. Lead and copper are prominent examples.
- Action level: a sampling threshold that triggers specified system actions. It is not simply interchangeable with an MCL or a universal boundary between “safe” and “unsafe.”
- Maximum Residual Disinfectant Level (MRDL): the enforceable limit for a disinfectant residual in drinking water. This term applies to substances such as chlorine and chloramine.
- Secondary standard: a generally non-enforceable federal guideline addressing aesthetic or operational qualities such as taste, odor, staining, or scale.
- Unregulated contaminant: a substance without a federal drinking-water limit. It may still be monitored, studied, regulated by a state, or included on EPA's Contaminant Candidate List.
Public Water Systems vs. Private Wells
Public water systems
Public systems monitor under federal and state schedules, report violations, and provide annual CCRs to customers of community water systems. A CCR is the best first look at the source, detected regulated contaminants, and compliance results—but it may not reveal a plumbing-related issue at one home.
Private wells
EPA's public-water rules generally do not regulate an individual private well. Owners should follow state or local testing guidance and select tests based on local geology, nearby land use, flooding, septic systems, agriculture, and previous results.
Lead
What it is and where it comes from
Lead is a toxic metal. In drinking water, its most common pathway is corrosion of lead service lines, galvanized lines that have captured lead, lead-containing solder, and some older fixtures. Water can meet requirements when it leaves a treatment plant yet acquire lead as it moves through the distribution system or building plumbing.
How EPA addresses it
Lead is regulated through the Lead and Copper Rule, a treatment-technique framework centered on corrosion control, tap sampling, public communication, and service-line requirements. Because implementation dates and action-level provisions have changed, consult EPA and your utility for the rule currently applicable to your system.
Who should pay attention and how to test
Households with known or possible lead service lines, pre-1986 plumbing, recent plumbing work, or prior elevated results have a clear reason to investigate. Infants, children, and pregnant people are especially important populations in lead guidance. A certified laboratory test from the specific tap is more informative than a utility-wide average; follow the laboratory's sampling instructions, as water stagnation and sample sequence can affect the result.
Fluoride
What it is and where it comes from
Fluoride is a naturally occurring mineral. Concentrations vary with geology, and some community water systems adjust fluoride to a level intended to help prevent tooth decay. Other systems do not fluoridate.
How EPA addresses it
EPA has a health-based primary MCL for fluoride and a lower secondary standard associated with cosmetic dental effects. The U.S. Public Health Service recommendation for community water fluoridation is a separate public-health recommendation, not the EPA MCL.
Who should pay attention and how to test
Private-well users in areas with naturally elevated fluoride and households seeking to know their exact concentration may want a specific test. Public-system customers can first check the CCR and the utility's fluoridation information; certified laboratories can measure fluoride directly.
Read the complete guide to fluoride in drinking water → · View Berkey PF-2® Elements
Disinfection Byproducts: TTHMs and HAA5
What they are and where they come from
Disinfection byproducts (DBPs) form when disinfectants react with naturally occurring organic matter and other constituents in source water. Two regulated groups are total trihalomethanes (TTHMs) and five haloacetic acids (HAA5). Their presence reflects a treatment tradeoff: utilities must control harmful organisms while also limiting byproduct formation.
How EPA addresses them
EPA's Disinfectants and Disinfection Byproducts Rules establish MCLs, monitoring, and related treatment requirements. Compliance is based on specified system sampling and averaging procedures, so one result should be interpreted in that context.
Who should pay attention and how to test
Customers of chlorinated or chloraminated systems can review their CCR for TTHM and HAA5 results. Levels can vary within a distribution system and seasonally. A certified laboratory can analyze a tap sample, but correct containers, preservation, and handling are essential.
Arsenic
What it is and where it comes from
Arsenic is an element found naturally in some rocks, sediments, and groundwater. Mining, smelting, wood preservatives, and other past or present activities can also contribute. In affected aquifers, groundwater can dissolve arsenic from mineral deposits without any change in taste, smell, or appearance.
How EPA addresses it
EPA regulates arsenic in public drinking water with an MCL. Utilities monitor and treat as required under the Arsenic Rule.
Who should pay attention and how to test
Private-well owners in arsenic-prone regions should follow local testing recommendations. A certified laboratory test is necessary; standard taste, odor, hardness, or basic home-strip tests cannot rule arsenic out. If treatment performance must be evaluated, use paired before-and-after samples and follow the laboratory's instructions.
PFAS
What they are and where they come from
Per- and polyfluoroalkyl substances (PFAS) are a large family of persistent synthetic chemicals. Potential sources include industrial releases, certain firefighting foams, landfills, wastewater, and products manufactured with PFAS. Contamination may reach surface water or groundwater and travel beyond the original source.
How EPA addresses them
Federal PFAS regulation is actively evolving. EPA's 2024 rule established MCLs for six PFAS or PFAS mixtures. In May 2026, EPA proposed retaining the PFOA and PFOS limits while extending implementation flexibility and separately proposed rescinding parts of the rule for PFHxS, PFNA, HFPO-DA (GenX), and the Hazard Index mixture. Readers should verify the final status on EPA's current PFAS drinking-water page rather than relying on an older summary.
Who should pay attention and how to test
Communities near known industrial uses, firefighting training areas, airports, military installations, landfills, or documented releases may have particular reason to review local data. PFAS testing requires a laboratory using an appropriate method and careful sampling practices designed to minimize contamination. A broad “water test” does not automatically include PFAS.
Read the complete guide to PFAS in drinking water → · Review Phoenix PFAS test data
Chlorine
What it is and where it comes from
Chlorine is intentionally used by many utilities to inactivate harmful organisms and maintain a protective residual in the distribution system. The residual at a home can vary with distance, water age, temperature, demand, and utility operations.
How EPA addresses it
EPA regulates chlorine as a disinfectant using an MRDL rather than an ordinary contaminant MCL. Utilities must also manage DBPs that can form when chlorine reacts with source-water constituents.
Who should pay attention and how to test
People investigating a noticeable disinfectant taste or odor, aquarium owners, and users of equipment or processes sensitive to chlorine may need source-specific guidance. Free-chlorine test kits can provide a screening measurement when used correctly; utility data or laboratory results offer more context.
Read the complete guide to chlorine in drinking water → · Review Phoenix chlorine test and certification information
Chloramine
What it is and where it comes from
Chloramines form when chlorine and ammonia are combined. Some utilities use monochloramine as a longer-lasting distribution-system disinfectant. A utility may use it continuously or change disinfectants during maintenance periods.
How EPA addresses it
Chloramine is regulated with an MRDL. EPA also regulates relevant DBPs and requires public systems to meet operational and monitoring requirements.
Who should pay attention and how to test
Dialysis providers and patients must follow medical facility protocols; aquarium owners also need to prepare the water appropriately. Consumers can ask the utility which disinfectant it uses. Testing should measure total chlorine when chloramine is present, because a free-chlorine result alone can be misleading.
Read the complete guide to chloramine in drinking water → · Review Phoenix chloramine test data
Nitrate and Nitrite
What they are and where they come from
Nitrate and nitrite are nitrogen compounds. Drinking-water sources can be affected by fertilizer, manure, septic systems, wastewater, decaying organic matter, and natural soil processes. Shallow wells and agricultural areas can be more vulnerable, especially after major runoff or flooding events.
How EPA addresses them
EPA has separate MCLs for nitrate and nitrite in public water systems. Results must be reported in the units specified by the laboratory and regulation; “as nitrogen” and “as nitrate” are not interchangeable without conversion.
Who should pay attention and how to test
Infants are the central sensitive population in nitrate guidance. Private-well households, particularly those with infants or pregnant people, should follow health department testing advice. Use a certified laboratory for decisions; strips can screen but vary in accuracy and may report different units.
Chromium-6
What it is and where it comes from
Chromium-6, or hexavalent chromium, is one chemical form of chromium. It can occur naturally and can also be associated with metal finishing, pigment production, wood preservation, and other industrial activities.
How EPA addresses it
EPA's federal MCL applies to total chromium, which includes chromium-3 and chromium-6; there is not a separate federal MCL for chromium-6 alone. Some states may impose different requirements. See EPA's chromium drinking-water information for the current federal position.
Who should pay attention and how to test
Areas with relevant natural geology, industrial sites, or documented releases may warrant closer review. A total-chromium result does not state how much is chromium-6. Ask a certified laboratory for a method that distinguishes chromium species and follow its preservation and holding-time instructions.
Copper
What it is and where it comes from
Copper is an essential nutrient, but excessive concentrations in drinking water can be a concern. It usually enters household water through corrosion of copper pipes, fittings, and fixtures rather than from the treatment plant. Blue-green staining, a metallic taste, or pinhole leaks can signal corrosion but cannot quantify the extent of exposure.
How EPA addresses it
Like lead, copper is managed under the Lead and Copper Rule's treatment-technique and action-level framework, not a conventional system-wide MCL.
Who should pay attention and how to test
New plumbing, corrosive water, prolonged stagnation, and recurring blue-green stains warrant investigation. A tap-specific laboratory test following first-draw or other prescribed instructions is appropriate; pH and alkalinity may help diagnose corrosion but do not replace copper analysis.
Volatile Organic Compounds (VOCs)
What they are and where they come from
VOCs are carbon-based chemicals that readily evaporate. The category includes solvents, fuel components, degreasers, and industrial chemicals such as benzene, tetrachloroethylene, and trichloroethylene. Spills, leaking tanks, waste sites, industrial releases, and contaminated soil can affect groundwater or surface water.
How EPA addresses them
EPA regulates numerous individual VOCs with contaminant-specific MCLs; “VOCs” are not governed by one group-wide number. The applicable limit and health information depend on the specific compound.
Who should pay attention and how to test
Properties near fuel releases, dry cleaners, industrial facilities, or known groundwater plumes may need location-specific testing. VOC samples require special vials and bubble-free collection; use a certified laboratory's VOC panel and collection kit.
Read the complete guide to VOCs in drinking water → · Review Phoenix VOC test data
Microplastics
What they are and where they come from
Microplastics are small plastic particles and fibers produced intentionally at small sizes or created as larger plastics break down. They have been detected in source waters and drinking water, but findings are difficult to compare because sampling, size cutoffs, identification methods, and contamination controls differ.
How EPA addresses them
There is no federal drinking-water MCL for microplastics. In 2026, EPA included microplastics as a priority group on the draft Contaminant Candidate List 6. Candidate-list placement supports evaluation and does not, by itself, create an enforceable limit.
Who should pay attention and how to test
Consumers may be interested because of the widespread use and environmental persistence of plastics. However, routine consumer testing is not yet standardized like lead or nitrate testing. Specialized laboratories may use spectroscopy or microscopy, and results must be interpreted in light of the particle-size range and method used.
Read the complete guide to microplastics in drinking water →
Radium
What it is and where it comes from
Radium is a naturally radioactive element formed in the uranium and thorium decay series. Groundwater can acquire radium as it interacts with certain rocks and aquifer materials; occurrence is strongly influenced by regional geology and water chemistry.
How EPA addresses it
EPA's Radionuclides Rule includes an MCL for combined radium-226 and radium-228, along with standards for other radionuclide measures.
Who should pay attention and how to test
Private wells in areas known to have radionuclides may need testing based on state geological or health department advice. A specialized certified radiological laboratory is required. Gross-alpha screening and isotope-specific analyses answer different questions, so select the panel with expert guidance.
Pharmaceuticals and Personal-Care Compounds
What they are and where they come from
This broad category includes residues of prescription medicines, over-the-counter drugs, veterinary medicines, and personal-care ingredients. Pathways include human and animal excretion, wastewater discharge, septic systems, manufacturing releases, and improper disposal.
How EPA addresses them
There is no single federal MCL for pharmaceuticals as a group. EPA included pharmaceuticals as a priority group in draft CCL 6 in 2026. Inclusion is a research and regulatory-evaluation step, not proof that every listed compound occurs at a concerning level in a given supply.
Who should pay attention and how to test
Testing is generally specialized and target-specific because “pharmaceuticals” includes many compounds at very low concentrations. A laboratory must know which analytes to measure. To reduce source pollution, use official medication take-back programs rather than flushing unused medicines unless disposal instructions specifically direct otherwise.
Other Metals and Inorganic Contaminants
“Heavy metals” is an imprecise umbrella term. Each element has different sources, chemistry, standards, and testing needs. A location-specific panel is usually more useful than ordering every available analyte.
| Substance | Common reason it appears | How to interpret it |
|---|---|---|
| Iron | Natural geology, well components, or corroding iron pipes | Often an aesthetic or operational issue involving color, sediment, taste, or staining; investigate source and form |
| Manganese | Natural groundwater conditions and mineral deposits | Can cause black staining and taste issues; use current health and state guidance, particularly for infants |
| Mercury | Natural deposits, industrial pollution, and atmospheric deposition | Regulated with an MCL; use certified laboratory analysis |
| Cadmium | Industrial releases, metal operations, waste, or pipe corrosion | Regulated with an MCL; do not infer from a general hardness test |
| Barium | Natural deposits and some industrial activities | Regulated with an MCL; occurrence is geology-dependent |
| Uranium | Natural rock and soil | Regulated with an MCL; radiological and chemical considerations differ |
Iron guide · Manganese guide · Review Phoenix element-specific heavy-metal test information
Hard Water Minerals
Hardness is primarily a measure of dissolved calcium and magnesium. Water acquires these minerals as it moves through limestone, dolomite, and other mineral-bearing formations. Hardness is generally treated as a water-quality and operational characteristic rather than as a federally regulated health-based contaminant.
Hard water may contribute to scale buildup, spots on dishes, reduced soap lather, and increased maintenance needs. A softening process addresses hardness, while many drinking-water filters are not designed to reduce dissolved hardness minerals.
| USGS classification | Hardness as CaCO3 |
|---|---|
| Soft | 0–60 mg/L |
| Moderately hard | 61–120 mg/L |
| Hard | 121–180 mg/L |
| Very hard | More than 180 mg/L |
Learn what causes hard water and how treatment differs from filtration →
How to Read a Consumer Confidence Report
Community water systems provide annual CCRs describing the water source, detected regulated contaminants, compliance results, violations, and required educational information. Find yours on the utility website, through customer service, or with EPA's CCR resources.
- Confirm the system and reporting year. A neighboring utility may use a different source or treatment process.
- Identify the source. Surface water, groundwater, and purchased water can have different vulnerabilities.
- Read the units. One ppm is roughly one mg/L in water; one ppb is roughly one µg/L. One ppm equals 1,000 ppb.
- Compare the detected range and reported compliance value with the correct benchmark. Do not compare an individual result with an annual average or action level as though they were the same measure.
- Check violations and explanatory notes. A violation may concern a contaminant result, monitoring, reporting, or a treatment requirement.
- Look for plumbing-specific limits. A CCR cannot fully characterize lead or copper at every residence.
Common report and laboratory terms
| Term | Meaning |
|---|---|
| ND | Not detected above the method or reporting limit; it does not necessarily mean absolute zero. |
| Reporting limit | The lowest concentration the laboratory reports with its stated performance. |
| Running annual average | An average of results over a specified rolling period, often used for compliance. |
| 90th percentile | A ranked statistic used in lead and copper compliance; it is not an average. |
| TT | Treatment technique requirement. |
| AL | Action level that triggers specified requirements. |
How to Build a Sensible Water-Testing Plan
- Define the question. Are you checking a private well, a plumbing problem, a known local release, an odor, or treatment performance?
- Review existing information. Read the CCR, utility notices, state well guidance, and local environmental records.
- Select analytes by risk. Well construction, geology, agriculture, septic systems, industry, plumbing age, floods, and prior results should shape the panel.
- Choose an appropriate laboratory. Use a state-certified or otherwise appropriately accredited laboratory for decisions involving regulated contaminants or health concerns.
- Follow sampling instructions exactly. Container, tap location, stagnation time, flushing, preservation, temperature, and delivery time can change the validity of results.
- Interpret before acting. Confirm units, detection limits, benchmarks, and whether a repeat or confirmatory sample is appropriate.
For a public-water plumbing concern, testing at the tap may be more informative than testing at the service entry. For a private well, sampling before and after any treatment can help separate source-water conditions from treatment performance.
Frequently Asked Questions
Does EPA regulate every substance found in drinking water?
No. EPA regulates contaminants that meet the Safe Drinking Water Act criteria through MCLs, treatment techniques, or other requirements. Other substances may be monitored or evaluated through programs such as the Unregulated Contaminant Monitoring Rule and the Contaminant Candidate List. States can also establish their own requirements.
Does a contaminant detection mean the water violates a standard?
Not necessarily. A laboratory may detect a substance below an applicable limit, and some detected substances have no federal MCL. Compliance may also depend on averages, sampling locations, or treatment requirements. Compare the correct value, unit, and period with the correct benchmark.
Can water look, taste, and smell normal while containing contaminants?
Yes. Lead, arsenic, nitrate, PFAS, and many other substances may have no reliable taste, odor, or visible sign at concentrations of interest. Sensory changes can justify investigation, but normal senses cannot replace testing.
Should private-well owners test their water?
Yes, according to a schedule and panel informed by state or local guidance. Annual testing typically includes basic indicators, while geology, land use, flooding, nearby septic systems, household changes, and prior results may warrant additional tests.
How often should tap water be tested?
There is no single schedule for every household. Public-system customers can review the annual CCR and test when there is a plumbing-specific concern, an advisory, a change in taste or appearance, or a known local issue. Private-well owners should follow state and local schedules and retest after relevant repairs, flooding, or changes in conditions.
Can contaminant concentrations change during the year?
Yes. Rainfall, drought, runoff, source blending, temperature, treatment changes, water demand, and distribution-system conditions can affect results. One sample is a snapshot of a particular place and time.
Is a broad home test kit enough?
It can be useful for screening certain characteristics, but no single consumer kit reliably covers every contaminant. Many substances require certified laboratory methods, special containers, preservation, or very low detection limits.
Where can I find reliable local water information?
Start with your water utility, CCR, state drinking-water agency, local health department, and a certified laboratory. Private-well owners can also consult state geological surveys and well programs. EPA's drinking-water pages provide national regulatory context.
How do I choose a water filter after I know which contaminants are present?
Match the filter to the specific contaminant or water-quality concern and review performance information for the exact filter element and configuration you plan to use. For Berkey® systems, use the Phoenix Gravity test-results page for contaminant-specific laboratory information and the replacement-filter guide for compatibility and current filter options.
Ready to Compare Filtration Options?
Once you know what is in your water, the next step is to review the performance information for the exact filter element you are considering. TheBerkey.com provides current Phoenix Gravity laboratory reports, system comparisons, and replacement-filter guidance to help you evaluate the available Berkey® options.
A Better Starting Point for Water-Quality Decisions
Drinking-water questions are best answered with a source-to-tap approach. Start with the water source and local report; consider distribution and household plumbing; identify credible local risk factors; and test using a method suited to the contaminant.
Regulation provides an essential foundation for public water systems, but it is not a substitute for tap-specific investigation when plumbing or a private well is involved. By distinguishing an MCL from an action level, a disinfectant limit from a contaminant limit, and a detection from a violation, consumers can make more informed decisions without treating every result as either harmless or alarming.
Last reviewed: August 2026. Regulatory information can change; verify current requirements with EPA and the authority responsible for your water system.