The EPA’s Safe Drinking Water Act regulates over 90 contaminants in public water supplies. Ninety. Most people can name maybe three.
That gap matters. Municipal treatment handles many of them, but not all — and the ones that slip through are often the ones you’d most want to know about. Lead. PFAS. Arsenic. Nitrates. None of these have taste, smell, or color at the concentrations where they cause harm.
Here’s what the EPA recognizes, what’s actually showing up in US water supplies right now, and which treatment technologies address each category.
The EPA’s Four Categories of Drinking Water Contaminants
The EPA groups drinking water contaminants into four official categories under the Safe Drinking Water Act (SDWA), 42 U.S.C. § 300f et seq.
Source: U.S. EPA, “Types of Drinking Water Contaminants,” epa.gov/ccl/types-drinking-water-contaminants
1. Microbial Contaminants (Biological)
What they are: Living organisms — bacteria, viruses, protozoa, and helminths (parasitic worms).
Why they’re dangerous: Even at very low concentrations, pathogens can cause acute gastrointestinal illness, severe infections, and in vulnerable populations (elderly, immunocompromised, infants), life-threatening disease.
Key examples and EPA MCLs:
| Contaminant | EPA MCL | Health Effect |
|---|---|---|
| Total Coliform bacteria | Zero (MCLG) | Indicator of fecal contamination; not directly harmful but signals potential pathogen presence |
| E. coli / Fecal coliform | Zero (MCLG) | Gastrointestinal illness; source of dangerous strains like O157:H7 |
| Cryptosporidium | Treatment technique (99%+ removal required) | Cryptosporidiosis — severe diarrhea, fatal in immunocompromised |
| Giardia lamblia | Treatment technique (99.9% removal) | Giardiasis — prolonged diarrheal illness |
| Legionella | Treatment technique | Legionnaires’ disease — severe pneumonia |
| Turbidity | 1 NTU (monthly avg) | Indicator of filtration effectiveness; high turbidity can harbor pathogens |
Municipal treatment (coagulation + filtration + disinfection) handles most microbial contaminants. The failure points: distribution system contamination after treatment, private wells (not regulated by the EPA SDWA), and boil-water events during infrastructure failures.
What removes microbial contaminants:
– UV disinfection — highly effective against Cryptosporidium and Giardia (chlorine resistant); AMPAC offers home UV systems
– Reverse osmosis — physical barrier removes protozoa and most bacteria; viruses require RO + UV combination
– Ultrafiltration — removes protozoa and bacteria physically
2. Chemical Contaminants
This is the largest and most complex category. Chemical contaminants include naturally occurring minerals, synthetic compounds, agricultural chemicals, and disinfection byproducts.
Heavy Metals
| Contaminant | EPA MCL (mg/L) | Primary Source | Health Effect |
|---|---|---|---|
| Lead | Action Level: 0.015 | Lead service lines, solder, plumbing fixtures | Neurological damage, developmental delays in children; no safe level |
| Arsenic | 0.010 | Natural geological deposits, mining runoff | Increased cancer risk (skin, bladder, lung) |
| Mercury (inorganic) | 0.002 | Industrial discharge, natural deposits | Kidney damage |
| Cadmium | 0.005 | Industrial discharge, galvanized pipes | Kidney damage |
| Chromium (total) | 0.1 | Natural deposits, industrial discharge | Allergic dermatitis; Cr(VI) associated with cancer risk |
Lead deserves special mention. There is no safe blood lead level in children. The EPA’s Lead and Copper Rule Improvements (2024) requires utilities to inventory and replace lead service lines within 10 years. But that only covers the utility-side pipe — the in-home plumbing, fixtures, and solder are the owner’s responsibility. Over 9 million homes in the US still have lead service lines according to EPA data.
Source: U.S. EPA, Lead and Copper Rule Improvements, 40 CFR Parts 141 and 142 (2024).
PFAS (Per- and Polyfluoroalkyl Substances)
PFAS are the most significant emerging contaminant category in US drinking water. Known as “forever chemicals” because they don’t break down in the environment or the human body.
2024 EPA final MCLs for PFAS (effective 2024, compliance by 2029):
| Compound | MCL |
|---|---|
| PFOA | 4 parts per trillion (ppt) |
| PFOS | 4 ppt |
| PFNA, PFHxS, HFPO-DA (“GenX”) | 10 ppt individual |
| PFAS mixture | Hazard index-based |
This is the first federal drinking water standard for PFAS. Estimated 66–94 million Americans served by water systems with PFAS levels exceeding the new MCLs.
Sources: Military bases (AFFF firefighting foam), industrial manufacturing, airports, wastewater treatment plant discharges.
What removes PFAS: Reverse osmosis is the most effective residential treatment — 94%+ reduction. Activated carbon (especially granular activated carbon/GAC) provides significant reduction but less complete than RO. Ion exchange resins (PFAS-specific) are emerging as a residential option.
Source: U.S. EPA, PFAS National Primary Drinking Water Regulation, 40 CFR Parts 141 and 142 (April 2024).
Nitrates
MCL: 10 mg/L as nitrogen (EPA SDWA)
Primary source: Agricultural fertilizer runoff. Nitrate contamination in the US is concentrated in the Midwest Corn Belt, the Central Valley of California, and areas of intensive livestock operations. Over 15 million Americans rely on private wells, which are not regulated under the SDWA — many in agricultural areas have nitrate levels well above the MCL.
Nitrate is a particular concern for infants under 6 months: it causes methemoglobinemia (“blue baby syndrome”) by reducing the blood’s ability to carry oxygen.
What removes nitrates: Reverse osmosis (85–95% reduction), ion exchange, and distillation. Carbon filters do not remove nitrates.
Pesticides and Herbicides
Over 70 synthetic organic chemicals (SOCs) from agricultural and industrial sources are regulated under the SDWA. Atrazine (a herbicide) is the most commonly detected pesticide in US drinking water, particularly in Midwestern states. EPA MCL: 0.003 mg/L.
What removes pesticides: Activated carbon (high effectiveness), reverse osmosis, nanofiltration.
Disinfection Byproducts (DBPs)
These form when chlorine or chloramine disinfectants react with natural organic matter in source water. The main regulated classes:
- Total Trihalomethanes (TTHMs): MCL 0.080 mg/L — associated with increased cancer risk and adverse reproductive outcomes with long-term exposure
- Haloacetic Acids (HAA5s): MCL 0.060 mg/L — similar concerns
DBPs are a product of treatment, not the source water. Higher natural organic matter (NOM) in source water = more DBP formation potential.
What removes DBPs: Activated carbon (effective), RO (effective), aeration.
Arsenic
Particularly prevalent in the American Southwest, New England, and parts of the Midwest due to natural geological deposits. The EPA MCL of 0.010 mg/L (10 ppb) has been controversial — environmental groups argue it should be lower based on cancer risk data. Long-term exposure above the MCL is associated with skin, bladder, and lung cancer.
Source: WHO, “Arsenic in Drinking-Water,” WHO/SDE/WSH/03.04/75/Rev/1.
3. Physical Contaminants
What they are: Substances that affect the appearance, color, taste, or odor of water — but also include particles that can carry microbial contaminants.
Key examples:
- Turbidity — cloudiness from suspended particles. Regulated as an indicator of filtration performance (MCL 1 NTU monthly, 5 NTU single sample). High turbidity can harbor pathogens and interfere with disinfection effectiveness.
- Sediment / TSS — sand, silt, rust from pipes. Not directly regulated at the MCL level but affects taste, appearance, and can damage appliances.
- Iron and Manganese — regulated as secondary MCLs (aesthetic standards): iron at 0.3 mg/L, manganese at 0.05 mg/L. At higher concentrations both cause brown/black staining, metallic taste, and promote bacterial growth.
- Hardness — calcium and magnesium ions (not a health concern, but causes scale buildup). No MCL, but relevant for equipment protection and soap efficiency.
What addresses physical contaminants: Sediment filtration, multimedia filtration, water softeners for hardness, iron reduction filters.
4. Radiological Contaminants
What they are: Chemical elements with unstable atoms that emit ionizing radiation. They can enter water from natural geological sources or from industrial and nuclear activity.
| Contaminant | EPA MCL | Primary Source |
|---|---|---|
| Radium (combined 226/228) | 5 pCi/L | Natural geological deposits |
| Radon | Proposed MCL: 300 pCi/L (not yet finalized) | Natural geological, dissolved in groundwater |
| Uranium | 0.030 mg/L | Natural geological, mining |
| Beta/photon emitters | 4 mrem/y | Nuclear industry effluents |
Radon in drinking water is primarily a concern for private wells in granite-rich geology (New England, Appalachians). The main exposure route is actually inhalation when radon volatilizes from hot water during showering — not ingestion.
What removes radiological contaminants: Reverse osmosis (effective for radium, uranium), ion exchange, aeration (effective for radon removal from water), activated carbon.
Which Treatment Technology Addresses Which Contaminants?
| Contaminant | RO | Carbon Filter | UV | Water Softener | Sediment Filter |
|---|---|---|---|---|---|
| Lead | ✓ High | ✓ Moderate (with lead-rated filter) | ✗ | ✗ | ✗ |
| PFAS | ✓ High (94%+) | ✓ Moderate (GAC) | ✗ | ✗ | ✗ |
| Nitrates | ✓ High | ✗ | ✗ | ✗ | ✗ |
| Bacteria | ✓ | ✗ | ✓ High | ✗ | ✗ |
| Cryptosporidium/Giardia | ✓ | ✗ | ✓ High | ✗ | ✗ |
| Arsenic | ✓ High | ✗ | ✗ | ✗ | ✗ |
| DBPs (TTHMs, HAAs) | ✓ High | ✓ High | ✗ | ✗ | ✗ |
| Hardness | ✓ (reduces) | ✗ | ✗ | ✓ High | ✗ |
| Sediment | ✓ (with pre-filter) | ✓ (with sediment pre-filter) | ✗ | ✗ | ✓ High |
| Iron/Manganese | ✓ | ✗ | ✗ | ✓ (partial) | ✗ |
| Radium/Uranium | ✓ High | ✗ | ✗ | ✗ | ✗ |
The practical takeaway: Reverse osmosis is the only single residential technology that addresses all four contaminant categories effectively. For microbial protection, pairing RO with UV provides the most complete coverage — RO removes protozoa and bacteria physically, and UV inactivates viruses that may pass through.
AMPAC offers under-sink RO systems, whole-house RO systems, and home UV systems for comprehensive residential water treatment.
If you’re on well water, start with a water test before choosing a system. Most county health departments offer free or low-cost well water testing. At minimum test for: total coliform, nitrates, pH, hardness, iron, and arsenic. Add a full metals panel and PFAS screen if you’re near agricultural land or industrial sites.
Contact our water treatment engineers →
FAQ
Q: What are the four types of drinking water contaminants?
A: The EPA classifies drinking water contaminants as: (1) microbial/biological (bacteria, viruses, protozoa), (2) chemical (heavy metals, PFAS, pesticides, nitrates, disinfection byproducts), (3) physical (sediment, turbidity, color), and (4) radiological (radium, uranium, radon).
Q: What is the most common drinking water contaminant in the US?
A: Lead contamination affects the most people through aging infrastructure. PFAS contamination is the fastest-growing concern, with the EPA’s 2024 final MCL estimating 66–94 million Americans served by systems with PFAS above new limits. Nitrates are the most common chemical contaminant in rural well water.
Q: Does a carbon filter remove all drinking water contaminants?
A: No. Carbon filters are effective for chlorine, chloramines, DBPs, some VOCs, and certain pesticides. They do not effectively remove nitrates, most heavy metals (except lead, with specifically rated filters), PFAS (GAC provides partial removal), bacteria, viruses, protozoa, or radiological contaminants.
Q: What does reverse osmosis remove from drinking water?
A: RO removes 94–99% of most dissolved contaminants including lead, arsenic, nitrates, PFAS, fluoride, heavy metals, radium, and uranium. It also physically removes bacteria and protozoa. RO does not inactivate viruses (pairing with UV addresses this).
Q: Should I get my water tested before buying a filter?
A: Yes — especially if you’re on a private well. Municipal water customers can request the annual Consumer Confidence Report (CCR) from their utility, which lists all regulated contaminants detected. For well owners, independent testing or county health department testing is the only way to know what’s actually in your water.
References
- U.S. EPA, “Types of Drinking Water Contaminants” — epa.gov/ccl/types-drinking-water-contaminants
- U.S. EPA, National Primary Drinking Water Regulations, 40 CFR Part 141 — epa.gov/dwstandardsregulations
- U.S. EPA, PFAS National Primary Drinking Water Regulation (April 2024) — epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas
- U.S. EPA, Lead and Copper Rule Improvements, 40 CFR Parts 141 and 142 (October 2024)
- WHO, “Arsenic in Drinking-Water,” Background Document for Development of WHO Guidelines for Drinking-Water Quality, 2011.
- National Toxicology Program, “Health Effects of Exposure to PFAS,” NTP Technical Report, 2023.
Updated May 2026. Original post: June 2022.
AMPAC USA engineers custom water purification systems for commercial, industrial, and emergency applications — from 500 GPD to multi-million GPD. Trusted by municipalities, military, and industry worldwide.
