Well Water Contaminants: The Complete Guide (2026)

Is My Well Water Safe?

Clean-looking water is not proof of safe water. The contaminants that actually hurt people are usually invisible, tasteless, and odorless.

If your home runs on a private well, you are your own water utility. City water systems are tested constantly and held to enforceable federal standards. Private wells are different: the Safe Drinking Water Act does not cover the well that serves a single household, so no agency samples your water, flags a problem, or fixes it. Testing, treatment, and upkeep are entirely on you.

That matters because contamination is more common than most owners assume. The U.S. Geological Survey, which has sampled private wells across the country, found that roughly one in five contained at least one contaminant at a level of potential health concern. The specific risks depend on your geology, what happens on the land nearby, and how your well is built - but the headline is simple: do not assume your water is clean because it looks clean.

~1 in 5

private wells sampled by the USGS contained a contaminant above a health benchmark

Source: USGS

Contaminants fall into a few broad families: biological (bacteria), heavy metals and inorganics, nutrients like nitrate, radiological elements, nuisance or aesthetic issues you can taste and smell, and synthetic man-made chemicals. The directory below covers each one, with a link to a full guide. Some contaminants - uranium, or iron and manganese - show up in more than one family because they are both a health and a nuisance issue.

Why looks and smell are not enough

The most dangerous contaminants - coliform bacteria, nitrate, arsenic, lead, radon, and PFAS - give no warning to your senses at harmful levels. By the time water tastes or smells off, you are usually dealing with a nuisance problem, not the silent hazards. The only reliable answer is a lab test.

The Contaminant Directory

Every contaminant we cover, grouped by family. Tap any card for the full guide - what it is, where it comes from, how to test, and how to treat it.

Biological

Bacteria and microbes that signal surface water or sewage reaching your well.

Coliform & E. coli BacteriaThe standard indicator of contamination. A positive coliform test means surface water or worse has a path into your well.Read the guide ->

Heavy Metals & Inorganics

Naturally occurring or plumbing-derived metals that build up with long-term exposure.

ArsenicA naturally occurring carcinogen common in bedrock aquifers. Tasteless and odorless - only a lab test finds it.Read the guide ->LeadUsually leaches from older pipes, solder, and fittings - not the aquifer. No safe level for children.Read the guide ->UraniumA radioactive metal in granite-region groundwater that is both a kidney toxin and a radiation source.Read the guide ->Iron & ManganeseCommon metals that stain fixtures and laundry; high manganese is also a recognized health concern for infants.Read the guide ->

Nutrients

Fertilizer, septic, and manure runoff that concentrates in shallow wells.

NitratesFrom fertilizer, septic systems, and livestock. The classic farm-country contaminant and a serious risk to infants.Read the guide ->

Radiological

Radioactive elements from the rock the water passes through.

RadonA radioactive gas that escapes into the air when you shower or run the tap, raising lung-cancer risk.Read the guide ->Uranium & RadionuclidesUranium and its decay products add a radiation dose on top of their chemical toxicity. Test if you are in a granite region.Read the guide ->

Nuisance & Aesthetic

Problems you can taste, smell, or see - usually a quality issue, sometimes a clue to something worse.

Iron & ManganeseRusty stains, metallic taste, and clogged plumbing. Mostly a nuisance, but worth treating.Read the guide ->Hydrogen Sulfide (Rotten-Egg Smell)That sulfur smell from the tap. Often harmless sulfur bacteria, but it can mask other problems.Read the guide ->

Synthetic & Man-Made

Industrial and treatment-byproduct chemicals that do not belong in groundwater at all.

PFAS ("Forever Chemicals")Persistent industrial chemicals linked to serious health effects, now regulated by the EPA at parts-per-trillion levels.Read the guide ->VOCs & MTBESolvents and fuel additives from spills, leaking tanks, and industry that migrate into groundwater.Read the guide ->TrihalomethanesDisinfection byproducts formed when chlorine meets organic matter - relevant after shock-chlorinating a well.Read the guide ->

Some contaminants appear twice

Uranium is listed under both heavy metals and radiological because it is chemically toxic and radioactive. Iron and manganese show up under metals and under nuisance issues. That overlap is real - a single contaminant can be a health hazard and an aesthetic headache at once.

How to Know What Is in YOUR Water

The directory tells you what is possible. Two things tell you what is actually in your well: a test, and your local context.

Test it. A state-certified laboratory or your local health department is the definitive source. At a minimum, the CDC recommends an annual test for total coliform bacteria and nitrate, plus anything of local concern, and a fresh test after any flood, repair, or change in taste, odor, or color. Which panel makes sense depends on where you live and what is around you. Our well water testing guide walks through choosing a lab, building the right panel, and collecting a clean sample.

Know your context.Geology and land use drive risk. Bedrock and granite aquifers raise the odds of arsenic, uranium, and radon; farm country raises nitrate; older plumbing raises lead; nearby industry, landfills, or fuel tanks raise PFAS and VOCs. You can use DrillerDB's well map to see wells near you, their depths, and the aquifer you share - a useful clue to what your water might contain (though never a substitute for an actual test).

Test-kit tip

Mail-in test kits are convenient, but accuracy depends on following the sampling instructions exactly - use the supplied sterile bottle for bacteria, do not rinse it, and ship same-day so the sample does not sit. For results you can act on, choose a lab that is certified by your state for drinking water, and ask for a panel built for your area rather than a generic kit. Keep every report; year-over-year trends are as informative as any single test.

If a Test Comes Back High

A high result is a call to action, not a panic. Work the problem in order.

Confirm it. Re-test with a certified lab before you spend money. Sampling and lab errors happen, and a second test tells you whether the problem is real and how big it is.
Reduce exposure now. While you sort out treatment, switch to bottled or known-safe water for drinking and cooking for the affected contaminant. (Note that for radon and some VOCs, breathing the vapor during showers matters too, not just drinking.)
Find the source. Lead usually points to plumbing, not the aquifer; bacteria points to a compromised well cap, casing, or nearby septic; nitrate points to land use. Fixing the source sometimes fixes the water without any treatment system.
Match treatment to the contaminant. There is no universal filter. Choose a technology proven for what you found, sized to your water chemistry and usage - see the overview below and the individual guides.
Verify and maintain. Re-test after installing treatment to prove it works, then keep testing on schedule. Treatment systems need upkeep; an unmaintained filter can do nothing or make things worse.

One filter does not fix everything

The single most common mistake is buying a filter before knowing the contaminant. A carbon filter that removes a rotten-egg smell does nothing for arsenic or nitrate; reverse osmosis handles many dissolved contaminants but not gases like radon. Always select treatment from a real test result.

Treatment Overview: Which Method Handles What

A quick map of the main home treatment technologies and the contaminant classes each one is suited to. Use it to narrow options, then confirm sizing and selection against your test on the specific contaminant page.

Home well-water treatment technologies and what they address
Treatment method	Typically handles	Not the right tool for
Reverse osmosis (RO)	Arsenic, nitrate, uranium, lead, PFAS, many dissolved metals	Bacteria, gases (radon)
Ion / anion exchange	Nitrate, arsenic, uranium (anion); hardness, some metals (cation)	Bacteria, gases, most organics
Granular activated carbon (GAC)	VOCs, MTBE, radon (in water), PFAS, taste and odor	Nitrate, most heavy metals
Oxidation + filtration	Iron, manganese, hydrogen sulfide (rotten-egg smell)	Nitrate, arsenic (unless paired), PFAS
UV disinfection	Bacteria and other microbes (does not remove chemicals)	Arsenic, nitrate, metals, gases
Aeration	Radon, some VOCs, hydrogen sulfide (drives gases out of water)	Metals, nitrate, bacteria

General guidance only. Actual performance depends on water chemistry, concentration, and system design - confirm with a certified product (NSF-rated) and your test results.

For the specifics - target levels, certified product types, and the tradeoffs of point-of-use versus whole-house - follow the contaminant guide that matches your result: arsenic, nitrate, lead, uranium, radon, PFAS, VOCs and MTBE, trihalomethanes, coliform bacteria, iron and manganese, or the rotten-egg smell guide for hydrogen sulfide.

Treatment is one half of keeping a well healthy; routine care is the other. Our well water upkeep guide covers the maintenance and testing rhythm that keeps contaminants from sneaking up on you in the first place.

Frequently asked questions

The only way to know is to test. Well water can look, smell, and taste perfectly clean while carrying bacteria, nitrate, arsenic, or other contaminants you cannot detect with your senses. The CDC recommends testing your private well at least once a year for total coliform bacteria, nitrate, and anything of local concern, plus after any flood, repair, or noticeable change in taste, odor, or color.

No. The Safe Drinking Water Act sets enforceable standards for public water systems, but it does not cover private wells serving individual households. No agency tests your well or fixes a problem if one shows up - testing, treatment, and maintenance are entirely the well owner's responsibility.

It is common enough that you should not assume your well is clean. The USGS has reported that roughly 1 in 5 (about 23 percent) of sampled private wells contained at least one contaminant at a level of potential health concern. Local conditions vary widely, so your risk depends on your geology, land use nearby, and well construction.

At a minimum, test annually for total coliform bacteria and nitrate. Beyond that, test for what is likely in your area: arsenic and uranium in many bedrock and granite regions, lead if you have older plumbing, radon in radon-prone areas, and PFAS or VOCs if there is industry, a landfill, or a fuel source nearby. A state-certified lab or your local health department can recommend a panel for your location.

Yes. The most serious contaminants - bacteria, nitrate, arsenic, lead, radon, and PFAS - are invisible, tasteless, and odorless at dangerous levels. Clear, good-tasting water is not evidence of safety. A nuisance problem like a rotten-egg smell or rusty staining is a separate (and usually less dangerous) issue from these health hazards.

First, confirm the result with a second test from a certified lab, since sampling errors happen. If it is genuinely high, stop drinking the water for that contaminant (use bottled water for drinking and cooking in the meantime), identify the source, and choose treatment matched to the specific contaminant. A single filter does not handle everything - the right technology depends on what you found. The individual contaminant guides linked above walk through treatment for each one.

Not by itself. Most basic carbon pitcher and fridge filters are designed to improve taste and reduce chlorine, not to remove arsenic, nitrate, bacteria, or many other well contaminants. Effective treatment is matched to the contaminant - reverse osmosis or anion exchange for arsenic and nitrate, UV or chlorination for bacteria, and so on. Always size and select treatment from an actual test result, not a guess.

DrillerDB's well map lets you look up nearby wells, their depths, and local geology, which is a useful starting point for understanding what your aquifer might contain. It does not replace a water test, but it can tell you whether you share conditions - like a bedrock aquifer or a shallow sand-and-gravel aquifer - with wells known to have specific issues.

Keep reading

Sources & further reading

Protect Your Home's Water (Private Wells) — U.S. EPA (accessed June 2026)
Human Health Benchmarks and Private Well Water Quality — U.S. EPA (accessed June 2026)
Well Testing - Private Water Systems — CDC (accessed June 2026)
Drinking Water - Private Wells — CDC (accessed June 2026)
Quality of Water from Domestic (Private) Wells — USGS (accessed June 2026)
National Primary Drinking Water Regulations — U.S. EPA (accessed June 2026)