The most common well water contaminants in the USA

Latest update: Fall 2021

Whether you’re researching the best well water filtration system for your specific well, or are simply curious about what may be present in your drinking water, here are the most common water contaminants in private well water across the US.

Many wells face similar risks to water taste and purity—from iron staining and sulfur odors to bacterial contamination. Expanding agricultural land and frequent flooding are also increasing threats to groundwater safety.

In this article, we’ll describe every common well water contaminant, including:

  • Contaminant severity,
  • Effects on water quality,
  • Potential health consequences, and
  • How to identify and filter the contaminant.

Choose a contaminant category to find out more…

Why are these things in my well water?

Water spends long periods of time underground before it reaches a well. During that time, it’s filtered through many layers of rock, which is why many wells produce super clean drinking water.

Depending on the type of rock that water filters through, however, metals and minerals can dissolve into the water supply on its journey to your well. If wells are drilled to a sufficient depth, chemicals from soil layers may also get carried along for the ride.

Some wells are also vulnerable to direct contamination. Depending on well-depth and local features of the environment, well water can be polluted by chemicals and bacteria from farms, solvents from heavy industry, and even waste products from the local neighborhood.

How are well water contaminants managed?

Because they’re drilled on private property, the responsibility for managing a well falls to homeowners. This means that well users need to regularly test their water for contaminants and employ an appropriate water filter system.

Public water supplies are protected under the EPA’s Safe Water Act, which requires local bodies to filter water for over 90 of the most common contaminants.

Most groundwater is exposed to far fewer potential contaminants than surface water, but protecting well water supplies from common risks to health is still an essential task.

How can I protect my home and family?

Once you’ve tested your water to find out what kinds of contaminants may be present, water can be filtered using several different types of filtering technology.

From basic sediment filters for undissolved particles, to modern UV purifiers that use ultraviolet light to kill bacteria, there’s a filter device that works for every possible well water contaminant.

For more on water filtration, read our articles on the latest whole house filtration systems for well water. These systems contain multiple filter stages, and are designed to reduce levels of the most common water pollutants with minimal maintenance:

Chemical contaminants

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Chemicals from industrial processes and farming can seep into the soil or wash in shallow wells. With no taste or smell, many are undetectable and have unknown health risks.

Using granular activated carbon is an effective way to filter most common chemical contaminants.


Nitrates are an extremely common substance in the soil and water networks across the country. They enter the environment from manmade processes, such as when fertilizers and manure escape farmland and seep into the ground, and from natural processes, such as when bacteria in the soil interact with naturally occurring nitrogen.

Wells that are subject to heavy rainfall or flooding may have particularly high levels of nitrates, as nitrate-rich water can saturate the surrounding earth and make its way down to aquifers.

Researchers estimate that agricultural land in California and the midwest is sprayed with millions of pounds of nitrate-based fertilizers every year. While the consumption of some nitrates is inevitable and not thought to be a serious health risk, drinking water with high nitrate levels for many years has been associated with circulation issues, which can lead to heart conditions and symptoms such as dizziness and fatigue.

Contaminant severity: Moderate when consumed over long periods.

Effects on water quality: Does not affect water taste or appearance.

Potential health consequences: High consumption may cause nervous system and circulation disorders.

How to identify nitrates: Water testing.

How to filter nitrates: Reverse osmosis or anion exchange water filters.


PFAS is a relatively new form of chemical pollution that comes from both urban and industrial causes. Also termed “forever chemicals”, these substances are used in the production of non-stick, or waterproof objects and materials, such as Teflon pans, raincoats, fire extinguishers, and even pizza boxes.

While PFAS are useful in manufacturing due to their ability to form a protective barrier, that same feature means they rarely break down in the environments, hence the “forever” nickname. Researchers are only now conducting widespread testing to determine just how far PFAS have spread, and have found them in water, soil, and our bodies.

Because PFAS chemicals have only been around for several decades, the long-term health consequences of PFAS consumption are largely unknown. However, several correlations have been identified between high consumption and fertility and hormone disorders.

PFAS contamination is less of an issue for deeply drilled wells than shallow wells or those in areas where industrial or military activity occurs.

Contaminant severity: Moderate.

Effects on water quality: Does not affect water taste or appearance.

Potential health consequences: High consumption is correlated with fertility, developmental, and hormone disorders.

How to identify PFAS: Water testing.

How to filter PFAS: Granular activated carbon or reverse osmosis water filters.

Pesticides & Herbicides

Since the organic food revolution, the general public has become far more aware of the environmental dangers of chemical pesticides, herbicides, and fertizliers. However, many don’t realize that those same dangers apply to our drinking water. When these chemicals are sprayed onto crop fields, they can leach into the soil, spread into nearby rivers, and even evaporate into the air.

As a result, low levels of pesticides can be carried into shallow wells, especially in rural properties surrounded by farmland. The presence of pesticides or fertilizers in the surrounding area can also disrupt local plant life, leading to large areas of stagnant water that harbor bacteria, which may also contaminate wells.

Contaminant severity: Low.

Effects on water quality: Does not affect water taste or appearance.

Potential health consequences: High consumption is correlated with immunosuppression and hormone disorders.

How to identify Pesticides & Herbicides: Water testing.

How to filter Pesticides & Herbicides: Granular activated carbon water filters.

Volatile organic compounds

Another very common set of chemicals, Volatile organic compounds (often shortened to VOCs) are used in the production of paints, varnishes, cleaning products, and brake fluid. This means that they are constantly in contact with the environment, and have continual opportunities to escape into the ecosystem and find their way into the water supply.

Like PFAS, VOCs are long-lasting and accumulate in wells where there’s lots of nearby industrial or urban activity.

Contaminant severity: Moderate when consumed over long periods.

Effects on water quality: May worsen the taste of water.

Potential health consequences: High consumption is correlated with cancer, circulatory system damage, and nervous system damage.

How to identify VOCs: Water testing, water taste, water odor.

How to filter VOCs: Granular activated carbon water filters.

Metals & Minerals

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The levels of metals and minerals found in well water are rarely unsafe, but they do have a significant effect on water quality.

Many well owners struggle with brown stains, rotten-egg odors, and water hardness, thanks to the presence of calcium, iron, and sulfur.


Unlike the chemicals above, most of the substances in this category dissolve into well water from natural sources—primarily the soil and rock that surrounds groundwater aquifers.

Aluminum is found in certain types of sedimentary rock that are more likely to be found in warm or tropical environments. There are no reported health consequences of consuming small traces of aluminum, however, prolonged consumption is connected to kidney disorders and anemia.

Contaminant severity: Low.

Effects on water quality: Does not affect water taste or appearance.

Potential health consequences: High consumption is correlated with kidney issues and anemia.

How to identify Aluminum: Water test.

How to filter Aluminum: Ion exchange or reverse osmosis water filters.


Another metal that commonly occurs in bedrock is arsenic. As a heavy metal, it’s considered toxic and is associated with digestive issues and cancer when consumed in high amounts over long periods. If your well is situated near any intensive industrial areas where smelting or refining occurs, it’s important that you filter your well water for heavy metal contaminants.

Contaminant severity: Moderate.

Effects on water quality: Does not affect water taste or appearance.

Potential health consequences: Digestive issues (arsenic poisoning), cancers.

How to identify Arsenic: Water testing.

How to filter Arsenic: Ion exchange or reverse osmosis water filters.

Calcium & Magnesium

Also known as hard minerals, calcium and magnesium are found to differing levels in well water all across the US. Water with high levels of hard minerals tends to be cloudy and may have a more pleasing taste. Dissolved calcium and magnesium can form scale deposits on plates, glasses, and pipes, especially when water is heated.

Unlike the majority of well water contaminants, research suggests that drinking calcium and magnesium-rich water is actually beneficial for us, with surveys showing correlations between hard drinking water and elevated cardiovascular health.

Contaminant severity: None.

Effects on water quality: Can cause scale, worsen water appearance, and improve water taste.

Potential health consequences: Beneficial to cardiovascular health.

How to identify Calcium & Magnesium: Water taste, water feel.

How to filter Calcium & Magnesium: Water softener.


Iron is the most commonly complained about contaminant in well water, due to the way it produces discoloration, nasty tastes, and even staining of sinks and laundry at high levels. Iron is a prevalent natural substance in soil and rock, which is why it’s so common in groundwater.

Iron exists in several forms in drinking water. It occurs as undissovled particles (rust), that can be filtered using a sediment filter. It can also be dissolved into drinking water (ferrous iron), in which case it cannot be detected by taste or smell. Iron deposits, scale, or scum is produced by iron bacteria, which can thrive in iron-rich waters.

Contaminant severity: None.

Effects on water quality: Can worsen water taste, smell, and appearance.

Potential health consequences: High consumption over long periods can affect heart, liver, and kidney function.

How to identify Iron: Water appearance, water taste.

How to filter Iron: Chlorination, KDF filtration, or sediment water filters, depending on the type of iron contamination.


Manganese is often grouped alongside iron, calcium, and magnesium as another common mineral contaminant that affects water appearance and can cause staining and scale. If well water leaves black rings in sinks and toilet bowls, or black stains on towels, the likely cause is high levels of manganese.

Levels of naturally occurring manganese are unlikely to be high enough to present any health risks, but if manganase enters groundwater from mining or industrial activities such as glass production, it is known to cause neurological disorders.

Contaminant severity: Low.

Effects on water quality: Can cause scale, staining, and worsen water appearance.

Potential health consequences: High consumption is associated with neurological issues, especially in children.

How to identify Manganese: Water appearance, appliance staining.

How to filter Manganese: Water softener.


While you might think that a radioactive compound like radon is rarely found in the environment, it’s actually relatively common. Produced when uranium breaks down, radon is found in the earth and water supply.

It’s also present the air, where if trapped, it can concentrate to dangerous levels. Thousands of deaths per year are attirbuted high levels of radon, particularly when people spend large amounts of time in subterranean basements. 

If your well water comes from an aquifer surrounded by granite rock or sand, it’s more likely to contain high levels of radon, and should be tested and filtered.

Contaminant severity: High.

Effects on water quality: No effect on water appearance or taste.

Potential health consequences: High exposure to radon is associated with several cancers.

How to identify Radon: Water testing and analysis of local rock and soil type.

How to filter Radon: Filtering water through granular activated carbon.


Most people can easily identify the presence of sulfur in well water by its distinctive, rotten-egg odor. Beyond being unpleasant to drink, sulfur smells can also linger on clothes and towels if they’re washed in contaminated water.

While there’s no serious health risk to drinking sulfur-contaminated water, those unused to consuming high levels of sulfides may experience stomach upset. Poor-smelling water can also be concerning to guests and those unfamiliar with well water.

Contaminant severity: None.

Effects on water quality: Can worsen water taste and water odor.

Potential health consequences: Stomach upset in people unaccustomed to consuming sulfides.

How to identify Sulfur: Water odor.

How to filter Sulfur: KDF water fitlers or chlorination.


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Whether microorganisms enter wells from nearby sources of pollution, or from natural events like flooding, the presence of bacteria and protozoa in drinking water significantly increases the risk of illness.

Disinfection methods such as chlorination or UV purification reliably rid water of biological contaminants.

Algae (cyanotoxins)

What’s commonly known as blue-green algae is actually the cause of several serious problems across the US. These toxin-producing microorganisms have flourished in recent years due to fetrilizer pollution and warming water temperatures, resulting in “algal blooms” that have swamped many once-pristine bodies of water.

When an algal bloom event occurs in water systems near private wells, stagnant water can wash into drinking water supplies, carrying cyanotoxins, the harmful byproduct of algal growth. High levels of cyanotoxins in water can cause skin rashes and irritation, as well as breathing difficulties in some people.

Contaminant severity: Moderate.

Effects on water quality: Can worsen water taste.

Potential health consequences: high consumption can irritate skin and cause breathing difficulties.

How to identify cyanotoxins: Water test, water taste, water smell.

How to filter cyanotoxins: Granular activated carbon or reverse osmosis water filters.

Coliform bacteria

Often separating into two categories: total coliform bacteria and E. Coli bacteria, these microorganisms are found in the digestive systems of humans and animals. While not necessarily harmful in themselves, the presence of either type of coliform bacteria in a water supply suggests that water has been in contact with fecal matter or animal remains.

Common reasons for coliform bacteria contamination include nearby stagnant water that has seeped into a well, leaks in septic tanks, a large amount of nearby livestock or farming activity, or nearby sewage spills. The presence of these bacteria may mean that harmful, disease-causing pathogens are present.

Contaminant severity: Indicative of high-risk pathogens.

Effects on water quality: No effect on water taste, water odor, or water appearance..

Potential health consequences: High levels of coliform bacteria suggest the presence of pathogens that can cause gastrointestinal distress and disease.

How to identify coliform bacteria: Water testing.

How to filter coliform bacteria: Chlorination or UV purification.

Iron Bacteria

Another type of iron contamination, iron bacteria are small microorganisms that process iron as a form of energy, and create unpleasant orange or green-tinged scum or “bio-film” as a byproduct. While not harmful in themselves, when iron bacteria begin to proliferate inside a well, they can make the environment more hospitable for other forms of bacteria, as well as increasing levels of oxidization throughout plumbing systems.

Iron bacteria can also create orange or black stains in sinks, toilet bowls, and laundry. The accumulated film can block pipes and filter screens, resulting in flow restrictions that can in turn trigger existing corrosion and scale to release from piping.

Contaminant severity: Low.

Effects on water quality: Can worsen water appearance, water taste, and cause staining.

Potential health consequences: Not known to be harmful.

How to filter iron bacteria: Shock chlorination.

Giardia lamblia

Giardia is a type of waterborne parasite that’s found across the United States and can cause severe stomach upset. According to the EPA, no pathogen is more widely occurring than Giardia in public water supplies, with multiple outbreaks reported every year.

In well water, giardia contamination can occur simply due to its presence in the surrounding soil, as well as from contact with animal feces and matter, especially from mice, rats, beaver, and deer. The higher the source water temperature, the higher the risk of giardia. Because it’s contained inside a small cyst, giardia can also be resistant to disinfection and may require multiple filtering stages.

Contaminant severity: High.

Effects on water quality: No effect on water taste, water odor, or water appearance.

Potential health consequences: Gastrointestinal illness.

How to filter giardia: Granular activated carbon water filters, chlorination, or UV purification.


Cryptosporidium is another microscopic parasite that’s found in the digestive systems of infected humans and animals. Its presence in water signifies that fecal matter has contaminated the supply at some point, and that water should be disinfected to avoid the risk of diarrheal disease.

As one of the most common waterborne parasites in the US, wells can easily become infected with Cryptosporidium if they’re in rural environments, near livestock farms, or in areas with lots of human activity.

Contaminant severity: High.

Effects on water quality: No effect on water taste, odor, or appearance.

Potential health consequences: Diarrheal disease.

How to filter cryptosporidium: Chlorination or UV purification.


Often synonymous with food poisoning incidents, norovirus is actually a category of viruses known to cause gastrointestinal upset and irritation. If well water comes into contact with human fecal matter through sewage overflows, stormwater flooding, or other septic tank issues, then it’s important to test for noroviruses and disinfect the well with chlorine. (Multiple rounds of chlorination are often required).

Contaminant severity: High.

Effects on water quality: No change to water appearance or taste.

Potential health consequences: Severe gastrointestinal illness.

How to filter norovirus: Repeated cycles of chlorination and water testing.


The quality of well water can vary dramatically across the country. Even in the same neighborhood, drilling on different properties or at different depths can turn up vastly different types of groundwater.

Most well water is safe to drink with minimal filtering but is likely to contain aesthetic contaminants such as iron and sulfur. These substances can affect the way that water looks, tastes, and smells, making it offputting to drink.

Wells drilled in environments that contain sources of pollution such as sewage, industrial chemicals, or agricultural land need to be filtered for chemical and microbiological contamination.

Many harmful materials like volatile organic chemicals or E. Coli are undetectable by the senses, meaning there’s no way to identify them without testing and filtering well water.