Contaminant Guides

PFAS are a group of thousands of man-made chemicals that have been used in industrial and consumer products since the 1940s. They do not break down in the environment or the human body, earning the name 'forever chemicals.' In April 2024, the EPA set the first-ever federal limits for six PFAS compounds in drinking water.

EPA limit: 4 ppt

Lead enters drinking water primarily through corrosion of lead service lines and lead-containing plumbing fixtures — not typically from the water source itself. There is no safe level of lead exposure for children. The EPA's 2024 Lead and Copper Rule Improvements require utilities to replace all lead service lines within 10 years.

EPA limit: 15 ppb (action level)

Arsenic is a naturally occurring metalloid found in geological deposits across the western United States, New England, and the Midwest. It can also enter water through industrial processes. Long-term exposure is strongly linked to bladder, lung, and skin cancer. The EPA reduced the arsenic MCL from 50 ppb to 10 ppb in 2001, though some researchers advocate for an even lower limit.

EPA limit: 10 ppb

Chromium-6 (hexavalent chromium, or Cr-6) is a carcinogenic form of chromium that occurs both naturally from serpentinite rock weathering and from industrial contamination. It was brought to widespread public attention by the Hinkley, California case documented by Erin Brockovich. The EPA has no specific federal MCL for chromium-6 — only a combined 100 ppb limit for total chromium. California attempted to set a state-specific 10 ppb limit for Cr-6 before it was rescinded following industry litigation.

EPA limit: 100 ppb (total chromium)

Copper enters drinking water primarily from copper household plumbing and service lines — not from the water source itself. It is regulated under the same Lead and Copper Rule as lead. At low levels, copper is an essential nutrient. At elevated levels (above 1.3 mg/L), it causes gastrointestinal symptoms acutely and liver and kidney damage with chronic exposure. Blue-green staining on sinks and fixtures is the most common sign of elevated copper. Corrosive (low-pH, low-alkalinity) water dramatically accelerates copper leaching.

EPA limit: 1.3 mg/L (action level)

Cadmium is a toxic heavy metal that enters drinking water primarily from industrial contamination, mining operations, and corrosion of galvanized steel pipes — not typically from natural geological sources. The EPA MCL is 5 ppb. Cadmium is an IARC Group 1 human carcinogen and accumulates in kidney tissue over decades, causing irreversible kidney damage. It is one of the most persistent environmental contaminants because it remains toxic in the body for 10–30 years after exposure stops.

EPA limit: 5 ppb (0.005 mg/L)

Mercury is a toxic heavy metal that occurs naturally and from industrial sources. In drinking water, the concern is inorganic mercury from industrial contamination, mining, and geological sources — distinct from the methylmercury exposure that comes from eating fish. The EPA MCL for inorganic mercury is 2 ppb. Chronic exposure damages the kidneys and nervous system. Mercury violations in public water systems are rare; the greater concern is private wells near gold and mercury mining sites and industrial facilities.

EPA limit: 2 ppb (0.002 mg/L)

Barium is a naturally occurring alkaline earth metal found in sedimentary rock formations across the United States. It enters groundwater through natural geological weathering and from oil and gas drilling operations that produce barium-containing brine. The EPA MCL is 2 mg/L. At elevated levels, barium raises blood pressure and can cause cardiovascular and kidney effects. Barium violations in public water systems are uncommon but occur in parts of the Midwest and Southeast where sedimentary geology concentrates barium in deep aquifers.

EPA limit: 2 mg/L

Nitrates are colorless, odorless compounds that occur naturally in soil but reach dangerous levels in water primarily from agricultural fertilizer runoff and septic system leakage. They pose an acute, potentially fatal risk to infants under 6 months and are increasingly linked to cancer risk even at levels below the EPA's 10 mg/L limit. An estimated 62 million Americans drink water with nitrate levels above 3 mg/L.

EPA limit: 10 mg/L

Atrazine is one of the most widely used herbicides in the United States — applied to corn, sorghum, and sugarcane across the Midwest — and is the most commonly detected pesticide in U.S. groundwater and streams. The EPA MCL is 3 ppb. It was banned in the European Union in 2004 over environmental and health concerns. Atrazine is an endocrine disruptor linked to hormonal disruption, reproductive effects, and possible cancer risk at levels found in drinking water.

EPA limit: 3 ppb (0.003 mg/L)

Disinfection byproducts form when chlorine or other disinfectants react with naturally occurring organic matter in source water. The two main regulated groups are total trihalomethanes (TTHMs) and haloacetic acids (HAA5). They are an unavoidable tradeoff of water disinfection — the risk of not disinfecting far outweighs the risk of DBPs, but minimizing exposure is prudent.

EPA limit: 80 µg/L (TTHMs) / 60 µg/L (HAA5)

Hard water contains elevated levels of dissolved calcium and magnesium. It is not a health risk and is associated with some cardiovascular benefits, but it causes scale buildup in pipes and appliances, soap scum, and reduces the effectiveness of detergents. Approximately 85% of U.S. homes have hard water. It is an aesthetic and infrastructure issue, not a regulatory one.

EPA limit: No federal limit

Fluoride is added to most U.S. public water supplies at low levels to reduce tooth decay — a practice called water fluoridation. At the EPA's enforceable limit of 4 mg/L, fluoride causes skeletal fluorosis. At higher natural levels (above 2 mg/L), it causes dental fluorosis (tooth mottling) in children. The current recommended fluoridation level is 0.7 mg/L. Naturally elevated fluoride in groundwater affects well owners in the western U.S., parts of New Mexico, Nevada, and some Texas formations.

EPA limit: 4 mg/L (MCL) / 2 mg/L (Secondary MCL)

Iron and manganese are naturally occurring minerals found in groundwater across the United States — they are the most common aesthetic problems reported by private well owners. Iron causes red-brown staining and a metallic taste; manganese causes black-brown staining and at high levels poses a genuine neurological health concern, particularly for infants and children. Neither has a federal health-based MCL (only aesthetic standards), but manganese above 0.3 mg/L warrants health attention beyond aesthetics.

EPA limit: 0.3 mg/L iron / 0.05 mg/L manganese (aesthetic SMCLs)

Selenium is a naturally occurring mineral essential to human health in trace amounts but toxic at elevated levels — it has one of the narrowest margins between beneficial and harmful dose of any element. In drinking water, selenium exceeds the EPA's 50 ppb MCL primarily near coal mining operations, coal-fired power plant sites, and in naturally selenium-rich agricultural soils of the western United States. Chronic high exposure causes selenosis, characterized by hair loss, nail brittleness, and neurological effects.

EPA limit: 50 ppb (0.05 mg/L)

Chlorine and chloramine are intentionally added to public water supplies to kill bacteria, viruses, and other pathogens. At regulated levels they are safe to drink, but they cause the characteristic taste and odor that many people associate with tap water. The more important concern is disinfection byproducts (DBPs) — compounds formed when chlorine reacts with natural organic matter — which are regulated separately and linked to long-term cancer risk.

EPA limit: 4 mg/L (MRDL)

Microplastics are tiny plastic particles under 5 millimeters in size found in tap water, bottled water, food, and air. There is currently no federal drinking water limit for microplastics in the U.S. — the EPA is studying the issue but no MCL has been set. Detection is widespread: a 2018 study found microplastics in 94% of U.S. tap water samples tested. The health effects of ingested microplastics are an active area of research, with growing evidence of concern but no established dose-response thresholds yet.

EPA limit: No federal limit

Bacteria — including E. coli, coliform, and other pathogens — are the most critical water quality concern for private well owners and the primary reason drinking water is disinfected. Public water systems are required to maintain disinfectant residuals and test regularly for coliform. Private wells receive no regulatory monitoring. Any detection of E. coli in drinking water is a health emergency requiring immediate action.

EPA limit: Zero E. coli / < 1 coliform per 100 mL

Cryptosporidium and Giardia are microscopic parasitic protozoa that cause severe gastrointestinal illness and are uniquely dangerous because they are resistant to chlorine — the standard disinfectant used in public water systems. Cryptosporidium caused the largest waterborne disease outbreak in U.S. history when it contaminated Milwaukee's water supply in 1993, sickening 403,000 people and killing over 100. Both are a concern for private well owners and anyone drinking from unfiltered surface water sources.

EPA limit: Zero (treatment technique standard)

Radon is a naturally occurring radioactive gas that dissolves into groundwater from uranium-bearing rock and soil. It is primarily a concern for private well owners — public water systems remove most radon before distribution. The main health risk is not from drinking radon-contaminated water, but from breathing radon gas that off-gasses when the water is used for showering, dishwashing, or laundry. Radon in water contributes to indoor air radon levels, which cause approximately 21,000 lung cancer deaths in the U.S. each year.

EPA limit: No finalized MCL

Uranium is a naturally occurring radioactive heavy metal found in rock and soil across the western United States. It enters groundwater through natural geological weathering and from uranium mining legacy sites. The primary health concern from uranium in drinking water is not radiation but kidney toxicity — uranium acts as a chemical nephrotoxin at concentrations found in some wells. The EPA MCL is 30 µg/L (ppb). An estimated 2 million private well users drink water with uranium above this limit.

EPA limit: 30 µg/L (30 ppb)

Radium is a naturally occurring radioactive element found in rock formations across the central and eastern United States. It enters drinking water by dissolving from uranium- and thorium-bearing rock. Radium is a bone-seeking element — it accumulates in bone tissue, delivering ongoing radiation dose from within. The EPA MCL covers combined radium-226 and radium-228 at 5 picocuries per liter (pCi/L). Radium is primarily a concern for public water systems in the Midwest, Gulf Coast, and parts of New England drawing from deep confined aquifers.

EPA limit: 5 pCi/L (combined Ra-226 + Ra-228)

Perchlorate is a chemical compound used in rocket propellant, solid fuel, fireworks, and airbag initiators. It contaminates drinking water near military installations, aerospace facilities, and fireworks manufacturing sites — and occurs naturally in arid Western soils, particularly in Texas and the Southwest. The EPA finalized the first federal MCL for perchlorate in 2024 at 0.056 mg/L. Perchlorate's primary health concern is thyroid disruption — it blocks iodide uptake and impairs thyroid hormone production.

EPA limit: 0.056 mg/L (56 ppb)

Volatile organic compounds (VOCs) are a broad class of carbon-based industrial chemicals that evaporate easily and dissolve into groundwater. The most common in drinking water include benzene (gasoline), trichloroethylene (TCE), perchloroethylene (PCE), toluene, and xylenes — typically from leaking underground storage tanks, dry cleaners, industrial facilities, and Superfund sites. Several VOCs are known human carcinogens. They are the most common class of groundwater contaminants in the United States.

EPA limit: Varies by compound: benzene 5 ppb; TCE 5 ppb; PCE 5 ppb; vinyl chloride 2 ppb

Hydrogen sulfide (H₂S) is a dissolved gas that causes the characteristic 'rotten egg' smell in well water — the most commonly reported well water odor complaint in the United States. It occurs naturally from sulfur-reducing bacteria in the aquifer and from geochemical reactions in sulfur-rich rock. At concentrations found in drinking water, hydrogen sulfide is not a health hazard, but it corrodes metal plumbing and fixtures, discolors laundry, and makes water unpleasant to drink. It often co-occurs with iron and manganese.

EPA limit: No MCL; Secondary MCL (aesthetic) of 0.05 mg/L

pH measures how acidic or alkaline water is on a scale of 0–14. The EPA's secondary standard for drinking water is 6.5–8.5. pH itself is not a direct health contaminant, but it has significant indirect effects: low pH (acidic) water corrodes pipes and leaches lead and copper into drinking water, while high pH can impair chlorine disinfection effectiveness and cause scale buildup. Private well users benefit most from pH testing.

EPA limit: 6.5–8.5 (secondary standard)