Common Drinking Water Contaminants

How Drinking Water Limits Work

Under the federal Safe Drinking Water Act, “SDWA authorizes the United States Environmental Protection Agency (US EPA) to set national health-based standards for drinking water to protect against both naturally-occurring and man-made contaminants that may be found in drinking water.”⁠^[1] EPA’s National Primary Drinking Water Regulations “are legally enforceable primary standards and treatment techniques that apply to public water systems.”⁠^[2]

Two terms come up often:

• Maximum contaminant level goal (MCLG): “The level of a contaminant in drinking water below which there is no known or expected risk to health.” EPA notes that goals are “non-enforceable public health goals.”⁠^[2]
• Maximum contaminant level (MCL): “The highest level of a contaminant that is allowed in drinking water.” MCLs “are enforceable standards.”⁠^[2]

New York adds its own requirements. The New York State Department of Health says “Public water systems in New York must monitor for more than 100 different contaminants on a regular schedule, including PFOA, PFOS and 1,4-dioxane.”⁠^[3] Private wells are different: “SDWA does not regulate private wells which serve fewer than 25 individuals.”⁠^[1] Our private well guide explains what that means for homeowners.

EPA Limits at a Glance

Nitrate and Septic Nitrogen

EPA’s limit for nitrate in public drinking water is 10 mg/L, measured as nitrogen, and EPA lists its common sources as “Runoff from fertilizer use; leaking from septic tanks, sewage; erosion of natural deposits.”⁠^[2] EPA’s health information for nitrate focuses on infants below the age of six months.⁠^[2]

Septic systems are one source of nitrogen. Suffolk County, which runs a septic improvement program, explains:

“A conventional onsite septic system was never designed to remove nitrogen. The average residential septic system discharges approximately 40 pounds of nitrogen per year.”

The county adds that “Even inland, nitrogen from septic systems will eventually reach the groundwater and surface waters.”⁠^[4] For homes on a private well, EPA’s advice is to “Test your private well annually for total coliform bacteria, nitrates, total dissolved solids, and pH levels.”⁠^[5]

Lead

Lead in tap water usually comes from plumbing. EPA explains that “Lead can enter drinking water when plumbing materials that contain lead corrode, especially where the water has high acidity or low mineral content that corrodes pipes and fixtures. The most common sources of lead in drinking water are lead pipes, faucets, and fixtures.”⁠^[6] Where a lead service line connects a home to the water main, EPA says “these pipes are typically the most significant source of lead in the water.”⁠^[6]

The age of your home matters. According to the New York State Department of Health, “Homes built before 1986 are more likely to have lead pipes, fixtures and solder.” It also notes that “Even if you get your water from a private well, there may still be a concern about lead in your water if the plumbing in your home contains lead solder or lead materials.”⁠^[7]

Public water systems have their own duties. “The federal Lead and Copper Rule requires a public water system to test tap water from sites likely to have plumbing containing lead.”⁠^[7] In October 2024, EPA “issued a final rule requiring drinking water systems across the country to identify and replace lead pipes within 10 years.”⁠^[8]

For treatment at home, the CDC notes that “Reverse osmosis systems will remove some types of chemicals from water, including lead, copper, chromium, chloride, and sodium (salt).” Whatever you choose, the CDC advises: “Check your treatment system’s label to make sure the system removes the chemicals or germs you are concerned about.”⁠^[9]

Coliform Bacteria

EPA recommends testing private wells for total coliform bacteria every year.⁠^[5] EPA describes total coliforms as “Not a health threat in itself; it is used to indicate whether other potentially harmful bacteria may be present,” and explains that “Coliforms are naturally present in the environment; as well as feces; fecal coliforms and E. coli only come from human and animal fecal waste.”⁠^[2]

For private wells, the New York State Department of Health advises that “Private well owners should test their tap water at least once a year for bacteria and every three to five years for other contaminants.”⁠^[10] On treatment, the CDC explains that “Ultraviolet treatment systems use ultraviolet (UV) light to disinfect water or reduce the number of germs in water.”⁠^[9]

Arsenic

EPA’s limit for arsenic in public drinking water is 0.010 mg/L. EPA lists its common sources as “Erosion of natural deposits; runoff from orchards, runoff from glass and electronics production wastes.”⁠^[2]

Industrial Solvents

EPA sets limits of 0.005 mg/L for the solvents trichloroethylene (TCE) and tetrachloroethylene (PCE), and for benzene. It lists TCE’s common source as “Discharge from metal degreasing sites and other factories,” PCE’s as “Discharge from factories and dry cleaners,” and benzene’s as “Discharge from factories; leaching from gas storage tanks and landfills.”⁠^[2]

Another industrial chemical, 1,4-dioxane, is regulated in New York drinking water.⁠^[3] Our 1,4-dioxane guide covers it, including its federal status.

Disinfection Byproducts and Chlorine

Disinfecting water keeps germs in check, and it can leave byproducts that EPA also limits: 0.080 mg/L for total trihalomethanes and 0.060 mg/L for five haloacetic acids, each described by EPA as a “Byproduct of drinking water disinfection.”⁠^[2]

Chlorine itself has a maximum residual disinfectant level of 4.0 mg/L, and EPA describes it as a “Water additive used to control microbes.”⁠^[2]

Other Topics

• PFAS: the federal and New York limits and the 2026 updates are covered in our PFAS guide.
• 1,4-Dioxane: our 1,4-dioxane guide explains what it is and how New York regulates it.
• Iron and manganese: EPA covers them with secondary standards, which it describes as “non-mandatory water quality standards” for aesthetic considerations such as taste, color and odor.⁠^[11] Discolored water and hard water are covered in Water Safety & Quality.

How to Check Your Water

If your home is on public water, your supplier’s annual drinking water quality report is the place to start. EPA says “Reports must be sent by your water supplier each year by July 1,” and each report includes “The regulated contaminant found in local drinking water.”⁠^[12] If you have a private well, testing is up to you.

Testing also tells you whether you need treatment at all. As the CDC puts it, “If your tap water does not contain harmful chemicals or germs, you may not need a home water treatment system.”⁠^[9]