How drinking water can become contaminated

Sand filtration tank at water treatment plant2

The concept of ‘naturally pure’ water is a misnomer. In fact, if you wanted to produce straight H2O without any accompanying minerals, metals, chemicals, or other dissolved stuff, you’d need to use several different types of artificial filtration and distillation.

Even after extensive filtering, it’s possible that trace contaminants will still exist in water supplies. This is because water is a fundamental mode of transmission in the natural environment, supplying nutrients and materials to everything from the cells in our body to alpine forests. As a result, public water systems and treatment works aren’t looking to remove everything from your drinking water; instead, they target a specific set of contaminants—a minimum of 90 different substances as required by the EPA’s Safe Water Act.

Most water systems across the country are dealing with similar water sources and a familiar roster of pollutants. Here’s a quick description of the major types of groundwater and surface water sources in the US, before moving on to discuss the most common contaminants in our drinking water, and how they frequently enter the supply.

Where does water come from in the US?

Groundwater aquifers and wells (source:

Groundwater aquifers and wells (source:


Around one-third of the country regularly gets its tap water from a groundwater source. Groundwater counts as any underground store of water, which can filter through rock and soil for over a century before reemerging.

Aquifers are the major source of groundwater for drinking. These are areas of permeable rock or loose material that sit beneath the water table, acting as huge underground water containers—in some cases so big that, according to USGS, there may be more than a thousand times the amount of water in the world’s lakes and rivers held underground at any one point.

Aquifers are normally accessed via wells. Public wells in the US are regulated at the state and federal levels, but private wells are not monitored in this way. This leaves over 15 million American households depending on some form of private well water monitoring and filtration, where a whole-house water filter for well water is recommended.

How does groundwater become contaminated?

Groundwater can be extensively filtered by geological pressure and sediment, to the extent that it appears crystal clear. But that doesn’t mean contaminants haven’t found their way into the supply. There are three major sources of groundwater contamination in the US:

  • Minerals and metals that dissolve into water during its time spent underground

  • Nitrates and chemical runoff that seeps into the earth from large scale agriculture

  • Toxic spills or leaks near well-water access points

Groundwater wells and likely sources of contamination (source: USGS).

Groundwater wells and likely sources of contamination (source: USGS).

Surface water

Rain which never makes it into the ground forms the surface water network for our public systems. This includes America’s great river systems, natural lakes, and artificially damned reservoirs. According to the EPA, 68% of community water systems make use of surface water, many of these feeding cities and urban populations.

Surface water is normally accessed via a treatment utility plant. Utilities pass surface water through a multi-stage, agitation, sedimentation, and filtration process, which removes sediments and particulates, mimicking the process of groundwater filtration. After this, water is disinfected (normally with chlorine or ozone).

How does surface water become contaminated?

Surface water is generally more prone to contamination than groundwater, as it may not have passed through any form of natural filtration. There are three major sources of surface water contamination in the US:

Map of real-time surface water

Map of real-time surface water streamflow compared to historical streamflow for Washington state. Watch the live streamflow for your state via this USGS tool (Source: USGS).

  • Waterborne bacteria and pathogens such as blue-green algae

  • Chemical spills or runoff, which can dramatically worsen during high rainfall

  • Open or leaking sewers and other types of urban dumping

What are the most common contaminants in US tap water, and where do they come from?

Almost all water sources will contain some cocktail of the following categories of contaminants. Here, the world ‘contaminant’ doesn’t necessarily suggest ‘harmful’ or ‘unwanted.’ Instead, it simply refers to materials that have entered the water supply, and can be desirable or undesirable in specific amounts.


Many people pay a premium to enjoy water that’s been bottled for its specific mineral makeup. While it’s true that many types of high mineral content water have a distinctive, appealing taste, almost all water has at least some kind of mineral composition.

In the US, minerals most often found in the water supply include potassium, sodium, zinc, calcium, and magnesium. Research indicates that sodium and magnesium concentrations are generally higher in the west and north of the country, while midwest water has the most variable mineral content.

How do they enter the water supply?

Mostly, minerals reach our taps from groundwater, where they’re able to dissolve from aquifers into water. Generally speaking, the longer water spends underground—and the farther it travels—the higher the mineral content will be. This how some waters come to be considered ‘rare’ or more premium, given their scarcity and age.

Of course, groundwater needs to pass through a mineral-rich rock to collect these beneficial substances, meaning that some areas of the country have higher mineral-content water than others. For example, the cities of Oahu and San Jose boast tap water with mineral levels on par with many bottled water brands.

Is this about water hardness?

A map of approximate water hardness across the country, from the USGS (source: USGS).

A map of approximate water hardness across the country, from the USGS (source: USGS).

Areas rich in calcium carbonates, such as places with limestone or dolomite bedrock, will produce water with much higher concentrations of minerals—particularly calcium and magnesium. When concentrations reach a certain point, they produce notable changes to water texture and taste. This is known as hard water.

While many people prefer the taste of hard water, high mineral content can spell bad news for household chores. Hard water can leave mineral stains on dishes and bathroom appliances, while building up inside dishwashers, laundry machines, and coffee-makers, thereby reducing efficiency. What’s more, soft water is often considered preferable for hair washing and skincare.

Most of the US runs on hard to moderately hard water. Whereas many coastal and southern states get to enjoy the benefits of softer water, Texas, Arizona, Indiana, and Utah are home to some of the hardest water in the country—with many, if not most, residents taking steps to soften their home supply.

Given the nutritional benefit of minerals such as calcium and magnesium to the body, some people consider hard water to contribute to general public health. There’s currently no conclusive evidence of the health benefits of mineral water, given how tiny mineral levels in water are. But, associations between hard water regions and lower incidences of cardiovascular disease have been identified.

Metals and trace elements

When people talk about metals rather than minerals in drinking water, they’re generally referring to contamination from a manmade source. For example, lead—the most well-known metal contaminant—can occur naturally in small deposits in the earth. But when people bring up lead pollution, they’re not referring to groundwater aquifers. Instead, they’re highlighting the historic and dangerous use of the material in piping and fixtures (more on that below).

Likewise, arsenic, which is technically a metalloid rather than a metal, is found widely throughout the earth’s crust. However, major concerns over arsenic contamination often revolve around its use in insecticides and its production in industries such as mining and those burning fossil fuels.

Though small amounts of metals and other elements dissolved in groundwater are rarely a threat to health, metal pollution certainly is—and no metal is desirable in drinking water. Despite this, manganese, arsenic, iron, radium, uranium, copper, lead, and mercury are all found to some extent in America’s water.

How do they enter the water supply?

An EPA infographic on lead sources in residential buildings (source: EPA, Public Domain).

An EPA infographic on lead sources in residential buildings (source: EPA, Public Domain).

Beyond natural erosion and dissolution, metals and trace elements enter the water supply through pollution, mining, or the construction of the water network itself.

According to the EPA, industries release thousands of pounds of arsenic into the environment each year, which can settle on the ground, and eventually reach surface and groundwater sources. Private wells in rural and farming areas are at higher risk of direct contamination from fertilizers and insecticide runoff.

Lead and copper usually enter the water supply much closer to your faucet, when aging pipes and fittings are corroded. While the EPA banned the use of lead piping in 1986, the ban didn’t include a requirement to remove existing plumbing, meaning that many areas of the country still receive water through lead pipes.

To reduce lead corrosion in these areas, water suppliers often treat their product to lower its ph level, making it less acidic, and therefore less corrosive. Home filtering can also be effective against lead, as long as a filter has been properly certified by the NSF to handle the metal.

What exactly happened in Flint, Michigan,
and how worried should I be?

Concerns about lead in the country’s water supply have remained high since the 2014 disaster concerning drinking water in the town of Flint, Michigan. Issues with the town’s water began when the local utility attempted to switch water sources, meanwhile using the Flint River as an interim solution.

Due to a (presumed) lack of foresight and planning, insufficient treatment meant that bacterial contaminants including E.coli were quickly transmitted from the river. But worse, the different profile of the new water source sparked wide-scale corrosion of the town’s lead piping, leading to contamination levels off the charts (nearly double the classification for toxic waste), a federal state of emergency, and health issues that disproportionately affected Flint’s children, which persist today.

Unfortunately, the risk of lead contamination is by no means limited to the past, or to Flint. Whenever the ph level of water changes in a region containing outdated plumbing, pipes are susceptible to corrosion, leaking lead into any home that doesn’t filter at the point of use.


Perhaps lesser known than the above categories are the chemicals that can contaminate water sources and often make it all the way into your glass. Modern life is full of chemical-producing products and processes—from the act of pumping gas to spraying tough carpet stains with cleaning fluids, to the chemicals used to disinfect water itself.

Thousands of chemical additives are manufactured for hundreds of uses, and they escape into the natural environment every day. Before long, many of these find their way into surface waters.

How do they enter the water supply?

THMs are the most common class of chemical contaminants in water. They include a range of hard to spell compounds such as bromodichloromethane and trichloromethane (chloroform), and are usually created when disinfectants, primary chlorine, come into contact with decaying organic matter, such as dead leaves. This makes them more common in supplies sourced from surface water.

Volatile Organic Chemicals (VOCs) are created for use in cleaning solutions, solvents, and different types of pesticides and plastic. VOCs generally enter the water system either directly, through spills and improper disposal, or via evaporation into the air. Because they are evaporable, VOCs tend to register in higher amounts in groundwater areas, where they are less able to escape.

Fluoride is also deliberately added to the water supply in many areas of the country, as a supplement intended to boost teeth and bone health.

What’s all this talk about PFAS?

One class of contaminating chemicals that are quickly becoming a cause for concern is Per- and Polyfluorinated Substances (PFAS). These currently unregulated substances are used in the production of numerous home and industrial products, from non-stick cooking surfaces to waterproofing techniques.

PFAS are designed to be deliberately hard to break down, giving them the nickname ‘forever chemicals’, and allowing them to easily infiltrate the environment when improperly disposed of. Preliminary tests commissioned late last year by the EWG found the presence of PFAS in rainwater, as well as several major US populations, including Miami, Philadelphia, and New Orleans. These findings come in addition to an EPA study that found the chemicals in all of the samples that researchers tested.

Results of EWG testing of PFAS in tap water samples in 2019 (source: EWG.)

Results of EWG testing of PFAS in tap water samples in 2019 (source: EWG.)

Such research follows incidents involving PFAS around the country, such as the chemical spill in Connecticut’s Farmington River, and the Marinette firefighting foam facility in Wisconsin—where large amounts of PFAS were found to have entered the water supply.

As this article is being written, a number of state and nonprofit organizations are introducing debate and legislature over PFAS use in foams and other products. Such regulations are hoped to be implemented as soon as possible, as (in high enough doses) PFAS can cause liver damage, thyroid disease, high cholesterol, and hormone suppression.


The last category of common contaminants in the US water supply is living things. Microorganisms, in the form of bacteria, viruses, and protozoa, are commonly found in natural water sources, especially surface water in areas where the temperature and environmental factors make a good Petri dish for microscopic life.

Vermont, for example, is one of several states whose lakes have a growing issue with blue-green algae. Large blooms of the tiny plant can turn pristine water into a soupy, murky green, releasing toxins as they die and break down. Large algae growths, therefore, risk the health of boaters, swimmers, and anyone who doesn’t adequately filter drinking water downriver.

How do they enter the water supply?

No living organism should make it into tap water when treatment methods are functioning adequately. However, there are scenarios when outbreaks of bacteria and other microbes can and do occur.

E. Coli is the most prevalent type of coliform bacteria in the US and the tenth most common cause of waterborne disease outbreaks in public water systems. These bacteria often enter private wells when nearby septic tanks and farm animal waste overflow, and can reach the public system when weather events cause excess sewage overflows, or runoff from polluted stormwater.

As mentioned above, blue-green algae (Cyanobacteria) can reach home water systems when growth explodes and levels of cyanotoxins overwhelm treatment capacity. While public utilities in areas vulnerable to algae growth should have protocols in place, any point along the water system where light can enter pipes and fittings may form a breeding ground for cyanobacteria.

Protozoans (small cysts containing organisms like giardia and cryptosporidium) and viruses can occasionally find their way into public drinking water when infrastructure ages to the extent that leaks occur. This means that water may be exposed to fecal matter from infected humans or wildlife. If contaminated water is not adequality disinfected, organisms can then potentially appear in tap water.

Giardia lamblia, seen through a scanning electron micrograph (Source: Wikipedia, public domain.)

What is giardia?

According to the EPA, “Since 1971, Giardia has been the most commonly identified pathogen in waterborne outbreaks reported in the United States. More than 130 waterborne outbreaks have been reported in 27 states.”

Giardia is a gastrointestinal-causing parasite that’s been detected in many of the country’s most common wildlife: beaver, wading birds, mice, rats, and deer, to name a few. Because it occurs so widely across the country, it’s likely that the soil itself is contaminated with giardia in many areas, through organic decomposition and sewage disposal.

Many, if not most, cases of infection from parasites like giardia occur in people using off-grid water systems—either while moving through the backcountry or from private wells. But all surface waters, including those accessed by public water municipalities, likely contain giardia, especially as water temperatures rise in the summer.

Giardia is removed by common filtration methods, such as coagulation and granular filters, or by adding chemical disinfects such as chlorine (which actually deactivates rather than removes microorganisms). So, giardia contamination should not be an issue for most unless filtration methods are dysfunctional. The most common reason for the presence of giardia and cysts in tap supplies is stagnant water collecting at some point along home or local community plumbing—often in wells or pressure tanks.

The last category of common contaminants in the US water supply is living things. Microorganisms, in the form of bacteria, viruses, and protozoa, are commonly found in natural water sources, especially surface water in areas where the temperature and environmental factors make a good Petri dish for microscopic life.

Vermont, for example, is one of several states whose lakes have a growing issue with blue-green algae. Large blooms of the tiny plant can turn pristine water into a soupy, murky green, releasing toxins as they die and break down. Large algae growths, therefore, risk the health of boaters, swimmers, and anyone who doesn’t adequately filter drinking water downriver.


Does the US have a drinking water problem?

Water flows into stainless steel kitchen sink

Almost all drinking water across America falls within the legal requirements set out by the EPA in their Safe Water Act. In cases where unexpected levels of contamination are identified, there are also efficient state and federal protocols for announcing Do Not Drink or Boiling orders.

These safeguards oversee an extensive public water system drawing from some of the largest river networks and bodies of water in the world. As a result, it’s estimated that over 90% of the population supplied by community water systems receives a continuous stream of safe water.

However, there is a growing skepticism over the quality and availability of water produced by our country’s infrastructure. Due to climate change and slowly increasing demands, long-relied upon water sources are quickly degrading or drying up in many states. Add to this the fact that large, mostly rural sections of the country use private well-water sources, not regulated by the EPA—plus the emerging findings on PFAS—and you may start to wonder whether enough is being done to protect our water.

In fact, many organizations have claimed for decades that EPA legal levels are nowhere near stringent enough to produce a drinking water supply that can genuinely be called ‘safe’. The truth is that almost all of the minerals, metals, chemicals, and organisms in this article can appear to some extent in tap water samples that would sail through EPA guidelines.

All of which prompts the question of what individuals can do to protect their supply of drinking water at home. The answer is both simple and long-proven: home water filters.

Home filtration is highly effective as a final line of defense against contaminants—when they’re used after filtration by a functioning water treatment plant or process. As long as you choose the right filtration mechanism for contaminants in your region, a point of use or whole house water filter is more than capable of removing all of the pollutants listed above.

Read next: Comparing the different types of home water filter.