How do I know if my well water has bacteria?

You can't tell by looking, smelling, or tasting it. Bacterial contamination leaves no visible sign. The only way to know is a lab test. A standard bacteria test costs $15-40 at a state-certified lab and takes less than 10 minutes to collect a sample.

What happens if you drink well water with E. coli?

Symptoms typically appear within 1-3 days and may include nausea, vomiting, diarrhea, and stomach cramps. Most healthy adults recover on their own. But E. coli O157:H7 can cause hemolytic uremic syndrome (HUS), a condition that may lead to kidney failure. Infants, elderly people, pregnant women, and immunocompromised individuals face the highest risk of serious illness.

How do I disinfect a well with bacteria?

Shock chlorination is the standard procedure. Pour unscented household bleach (5.25% sodium hypochlorite) into the well at a rate of about 1 quart per 100 gallons of water in the well. Run water through every tap briefly to circulate the chlorine, then let it sit for 12-24 hours. Flush the system completely afterward. Retest 1-2 weeks later before resuming normal use. The EPA provides a full guide at epa.gov/privatewells.

What is the difference between coliform and E. coli in water?

Total coliform is a broad group of bacteria used as an indicator. Many coliform bacteria are harmless, but their presence in water signals that a contamination pathway exists. E. coli is a specific type of coliform that indicates fecal contamination (human or animal waste getting into the water). E. coli is the more serious finding. Any confirmed E. coli requires immediate action.

Does a UV water filter kill bacteria in well water?

Yes. A UV disinfection system rated NSF/ANSI 55 Class A kills 99.99%+ of bacteria, viruses, and protozoa without adding chemicals. UV systems treat microorganisms only. They don't remove metals, nitrates, or chemical contaminants. They also require pre-filtration (sediment and carbon filters) to work correctly, since cloudy or turbid water blocks UV light.

Bacteria in Well Water: Testing, Risks, and Treatment

Health disclaimer: This page provides general information about bacterial contamination in private wells. It is not medical advice. If you or a family member shows symptoms of waterborne illness, contact a healthcare provider.

Nobody tests your private well water but you.

Municipal water is regulated. Utilities test it constantly and report results publicly. Private well water gets no such oversight. The EPA has no authority over private wells. Your state may have recommendations, but in most cases there are no requirements and no inspections. It’s on you.

About 15% of Americans (roughly 43 million people) rely on private wells for drinking water, according to the U.S. Geological Survey. The EPA recommends testing those wells for bacteria at least once a year, and after any event that could affect water quality: flooding, new plumbing work, a change in taste or odor.

If you or someone in your home develops sudden severe diarrhea, vomiting, or signs of dehydration after drinking well water, contact a doctor immediately. This applies especially to infants, elderly people, and immunocompromised individuals. Bacterial contamination can cause serious illness.

The Two Tests That Matter

A standard bacteria test checks for two things.

Total coliform is the broad indicator. It measures a large group of bacteria found in soil, vegetation, and the digestive tracts of animals and humans. Many coliform bacteria won’t make you sick. But their presence in a well tells you something is wrong. A pathway exists for outside material to enter your water supply. That could be a cracked casing, a failed seal, or surface water finding its way in. Total coliform is a red flag, not a diagnosis.

E. coli is the specific concern. E. coli is a type of coliform that lives in the intestinal tracts of warm-blooded animals. Finding it in well water means fecal contamination is entering the system. There is no safe level of E. coli in drinking water. Any confirmed positive result requires immediate action.

Labs typically test for both at the same time. The test costs $15-40 at a state-certified lab. Your state health department can point you to certified labs in your area.

How Bacteria Get Into Wells

Flooding is the most common acute cause. After Hurricane Helene in 2024, testing of private wells in the storm’s path found E. coli or coliform contamination in approximately 40% of sampled wells. Floodwater carries surface runoff, sewage, and animal waste directly into wells that aren’t properly sealed.

Other common pathways:

Cracked or deteriorating well casing that allows surface water to seep in
Improper grouting around the casing when the well was drilled
Well caps that don’t seal properly, allowing insects, rodents, or rainwater in
Proximity to a septic system, especially an aging or failing one
Livestock operations or agricultural land upgradient from the well
Wildlife getting into the well

Older wells are at higher risk. A well drilled 30 or 40 years ago may have casing or grouting that has degraded over time.

Who Faces the Highest Risk

Healthy adults often tolerate low-level bacterial exposure without severe symptoms. That doesn’t make it safe, but it does mean the stakes vary by household.

Infants, elderly people, pregnant women, and immunocompromised individuals face the most serious risk. Their immune systems are either undeveloped or compromised, which means the same exposure that causes mild symptoms in an adult can cause severe illness in a child or an elderly person.

E. coli O157:H7 is the strain that gets the most attention in outbreak coverage. In some cases, infection with this strain can progress to hemolytic uremic syndrome (HUS). HUS may cause kidney failure and can be life-threatening, particularly in children under five. The CDC tracks HUS cases as a marker for E. coli O157:H7 exposure.

Symptoms of waterborne bacterial illness generally appear within 1-3 days: nausea, vomiting, diarrhea, stomach cramps, and sometimes fever. Mild cases resolve on their own. Severe or prolonged symptoms need medical attention.

Testing: What to Do and When

A certified lab test is the only reliable option for bacteria testing. At-home bacteria test strips exist, but they’re not sensitive or specific enough to trust for safety decisions.

To collect a sample correctly: run the tap for 1-2 minutes first, then fill the sterile sample container your lab provides. Don’t touch the inside of the lid or the container’s mouth. Follow the lab’s instructions exactly. Bacteria testing requires the sample to reach the lab within a specific window, usually 24-30 hours, so plan accordingly.

The EPA recommends testing:

Once a year as a baseline
After any flooding event
After repairs to the well or plumbing
If you notice changes in taste, odor, or appearance
If a household member gets unexplained gastrointestinal illness

Check the EPA’s private wells page at epa.gov/privatewells for state-specific guidance.

For a full breakdown of well testing options, see Best Mail-In Water Tests.

If You Find Bacteria: What to Do Right Now

Stop drinking the water. Don’t use it for infant formula, cooking, or brushing teeth. Switch to bottled water immediately.

Then shock chlorinate the well. This is the standard first-response procedure for bacterial contamination.

The process:

Calculate the approximate volume of water in your well (your driller’s log will have the casing diameter and depth).
Add unscented household bleach (5.25% sodium hypochlorite) at a rate of about 1 quart per 100 gallons of water in the well.
Run water through every faucet, inside and outside, briefly to circulate the bleach through the pipes.
Let the chlorinated water sit for 12-24 hours.
Flush the system by running outdoor hoses until you can no longer smell chlorine. Avoid flushing into your septic system in large volumes.
Wait 1-2 weeks, then retest. The water must test clear before you resume normal use.

The EPA’s full shock chlorination guide is at epa.gov/privatewells.

Boil water as a temporary measure. If you have confirmed bacteria and you’re waiting on treatment or retesting, boiling kills bacteria. Bring water to a full rolling boil for at least 1 minute. At elevations above 6,500 feet, boil for 3 minutes. Let it cool before using. Boiling is a stopgap. It doesn’t fix the source of contamination.

Long-Term Treatment Options

Shock chlorination treats an existing problem but doesn’t prevent future contamination. If your well is at ongoing risk because of your location, soil type, or proximity to agricultural land, you’ll want a treatment system in place.

UV disinfection is the most common long-term solution for bacterial protection in private wells. UV systems kill bacteria, viruses, and protozoa by exposing water to ultraviolet light as it passes through a chamber. A system rated NSF/ANSI 55 Class A achieves 99.99%+ inactivation of microorganisms. Class B is supplemental only. Make sure any system you buy is Class A.

UV has real limitations. It treats microorganisms only. If your well also has iron, nitrates, arsenic, or other chemical contaminants, UV does nothing for those. You’ll need separate filtration for chemical issues.

UV systems also require pre-filtration to work correctly. Turbid or cloudy water blocks UV light and reduces its effectiveness. Most UV installations include a sediment filter and a carbon block filter upstream of the UV chamber.

For treatment options and system reviews, see the Treatment hub.

Continuous chlorination is the alternative approach. A chlorine injection pump doses the water before a contact tank, which gives the chlorine time to disinfect. A carbon filter downstream removes residual chlorine taste before the water reaches your taps. It’s effective but requires more maintenance than UV: monitoring chlorine levels, maintaining the injection pump, and changing carbon media.

Both options address ongoing bacterial risk. UV is simpler to maintain for most homeowners.

The Direct Recommendation

Test your well for bacteria once a year. A single test costs $15-40 and takes about 10 minutes to collect. That’s the lowest-effort, highest-value thing you can do for your household’s water safety.

If you’re in a flood-prone area, test every spring before the rainy season. And test again after any significant flooding event, regardless of when you last tested.

If you’ve never tested your well and you’ve been using it for years, start now. See Well Water Testing Guide for step-by-step instructions on what to test and how to collect samples properly.

Note: A dedicated page on testing your well after a flood is coming. Check back for specific post-flood testing protocols.

Sources:

U.S. EPA, Private Drinking Water Wells: epa.gov/privatewells
U.S. EPA, Potential Well Water Contaminants and Their Impacts: epa.gov/privatewells/potential-well-water-contaminants-and-their-impacts
U.S. Geological Survey, Private Wells and Domestic Water Use
NSF International, NSF/ANSI 55: Ultraviolet Microbiological Water Treatment Systems

Health disclaimer: WaterAnswer.com provides general information only. If you suspect bacterial contamination has made someone ill, contact a healthcare provider. This content does not constitute medical advice.