What Chemicals and Contaminants Can Be Found in Drinking Water

Drinking water can contain naturally occurring substances, chemicals from human activity, byproducts of water treatment, and biological contaminants. Examples include arsenic, nitrate, pesticides, PFAS, lead, bacteria, and viruses. The types and amounts vary by water source and location.

Finding a substance in water does not, by itself, show that it poses a health risk. The concentration, duration of exposure, and the substance’s health effects all matter. The CDC describes contaminants from natural sources, human activity, and water treatment. The WHO assesses drinking-water safety by comparing concentrations with health-based guideline values.

Common types of drinking-water contaminants

These categories suggest possible sources, but they do not identify what is in a particular home’s water. A private well near farmland may need testing for nitrate and pesticides. A household with older plumbing may need to check for lead, even if the water leaving the treatment plant meets applicable standards.

Naturally occurring substances

Rock and soil can release elements and gases into groundwater. Arsenic, uranium, and radium can occur in some aquifers, while radon may enter water from underground sources. Their presence varies by local geology, so a result from one area does not predict what is in water elsewhere.

Chemicals from agriculture and industry

Nitrate can enter water from fertilizer, animal waste, or other sources. Pesticides may reach groundwater or surface water through their use and movement in the environment. Industrial solvents can enter water through spills or leaks. PFAS are a large group of manufactured chemicals associated with uses that include some firefighting foams and products made to resist stains or water.

A possible source nearby does not prove that a contaminant is in someone’s drinking water. Testing is needed to check the water itself.

Disinfection byproducts

Water systems may disinfect water to control disease-causing microbes. Some disinfectants can react with organic matter in the water and form byproducts, including trihalomethanes and haloacetic acids. Levels depend on factors such as the source water and treatment process. These substances differ from contaminants that enter through a spill or leak: they can form during treatment.

Biological contaminants

Bacteria, viruses, and parasites can contaminate water when sewage, animal waste, or other sources enter a water supply. Problems with a private well, water system, or treatment process can also create risks. Microbial contamination can cause illness, so public water systems use treatment and monitoring to control it. A water test for chemicals does not necessarily test for microbes, and vice versa.

Lead and household plumbing

Lead can enter drinking water as it moves through lead-containing service lines, solder, or plumbing components. The amount can vary between buildings and even between taps in one building. A test of source water alone may not show what a household receives after water passes through its plumbing. If lead is a concern, testing at the tap is more relevant.

One example: nitrate in a private well

A household with a private well near farmland might ask whether nitrate could be present. Fertilizer and animal waste are among its possible sources, but location alone cannot confirm contamination. The practical step is to have the well water tested for nitrate. If a result is above a health-based or local limit, the appropriate response depends on the result and local guidance. A health department or qualified water professional can help interpret it and discuss treatment or another water source.

This example shows why one general “water quality” test may not answer every question. Tests need to include the contaminants that fit the water source, local conditions, and the concern being investigated.

Detection, standards, and risk

Laboratories can detect some substances at low concentrations. A detection does not automatically mean water is unsafe. Results need to be compared with the relevant health guidance or regulatory limit and interpreted in light of the test method and exposure.

In the United States, the EPA sets requirements for public water systems under the Safe Drinking Water Act. Public systems test for regulated contaminants and provide customers with water-quality information. Meeting current requirements does not mean water contains no substances; it means the system is operating within applicable standards.

Private wells are not covered by the same federal monitoring requirements for public water systems. Well owners are responsible for arranging tests, with the appropriate schedule and list of contaminants depending on local conditions and advice from public-health authorities.

What to do if you are concerned

First, identify whether your water comes from a public system or a private well. Customers of public systems can review the annual consumer confidence report or contact their water provider with questions about reported results. The report does not assess contaminants that may enter water inside a home’s plumbing, so a separate tap test may be needed for concerns such as lead.

For a private well, contact a local health department or water-testing laboratory about tests suited to the area and well. A single test cannot check for every possible contaminant. If results show a problem, ask a qualified professional or local authority to explain the options. Treatment depends on the contaminant and the water system, and a filter that reduces one substance may not reduce another.

Key Takeaways

• Drinking water may contain natural substances, agricultural or industrial chemicals, treatment byproducts, biological contaminants, or contaminants from household plumbing.
• The types and amounts vary by location and water source. Testing is needed to confirm what is present.
• A detected contaminant does not automatically mean the water is unsafe. Compare the result with relevant health guidance or regulatory standards.
• Public water systems and private wells have different monitoring responsibilities. Well owners should arrange testing suited to local conditions.
• Lead can enter water through plumbing, so a tap test may be needed even when source water has been tested.