Water Bacteria Test

Who should get a water bacteria test

Testing well water for bacteria is the core of the yearly check: CDC recommends that well owners test at least once every year for total coliform bacteria, along with nitrates, total dissolved solids and pH. The Vermont Department of Health gives the same advice for wells and springs: test for total coliform every year.

What a coliform test tells you. Total coliform bacteria are an indicator. CDC explains that these bacteria generally won't make you sick on their own, but they get into water the same way as germs that do cause disease, for example from a sewage leak, and they are much easier to test for. EPA's drinking water table describes total coliforms as not a health threat in itself, but a sign that other potentially harmful bacteria may be present. So a positive coliform result means something has found a way into your water system.

E. coli is the more serious finding. E. coli comes from human or animal fecal waste. CDC says a positive test for fecal coliforms or E. coli likely means that feces, and the harmful germs in them, have gotten into your well water. When you order a bacteria test, check that it is both a coliform test and an E. coli water test: the report should cover total coliform and E. coli.

When to test outside the yearly schedule. CDC lists times when you should test your well water again:

• There are problems near your well, such as flooding, land disturbances, or nearby waste disposal sites.
• You replace or repair any part of your well system.
• You notice a change in your water's taste, color or smell.
• You or someone you live with becomes pregnant.
• A child starts living with you.

A change you can see or smell is a reason to test, not an answer. Our pages on a rotten-egg smell and brown well water explain what those signs of well water problems can and can't tell you.

Buying a home with a well. A bacteria result gives you a starting point before you rely on the water. If a lender or a home-sale contract asks for a well test, ask them which lab and which collection steps they will accept before you book anything. Our guides to the VA loan well water test and the FHA well water test explain how those checks work.

Public water customers. If your water comes from a public system, the utility tests for coliform bacteria under federal rules. The case for your own bacteria test is strongest on a private well, where nobody else tests the water.

To see bacteria alongside the other tests that fit your home, you can build a test plan in a few questions. Our well water testing guide covers the full yearly well check.

Certified lab or home kit

For bacteria, the choice that matters is whether your result comes from a state-certified laboratory. CDC tells well owners that their state or county health department should have a list of state-certified (licensed) laboratories. Our directory of certified water testing labs lists state-certified labs in the states we cover, and our guide to well water testing cost shows verified lab prices.

A certified lab follows a set method and timeline. Certified drinking water labs use approved bacteria methods, such as the ones in the Lab methods list at the end of this section. They also control the conditions that make a bacteria result meaningful: a sterile bottle, a preservative where the water is chlorinated, and a limit on how long the sample can travel. For example, the Penn State lab is accredited by the Pennsylvania Department of Environmental Protection for drinking water analysis, and its bacteria test is part of every package it sells.

A home kit is not a certified-lab result. You may see bacteria kits you read at home. A reading you make yourself is not a result from a certified lab, and you can't compare it with a lab report. If a home kit shows anything at all, confirm it with a certified lab. Our guide on how to test water quality compares a certified lab, a mail-in kit and test strips.

Timing is the catch with bacteria. Unlike most chemical tests, a bacteria sample can't sit on a shelf. There is a short window between collection and the start of the test:

• For public water systems, federal rules say the time from sample collection to the start of incubation may not exceed 30 hours^[4].
• Labs that test private wells set their own window and print it on their instructions. Penn State's lab, for example, must receive a bacteria sample within 30 hours^[5] after sampling, asks you to send the kit by overnight mail, and cannot accept bacteria samples on a Friday.

Always follow your lab's own window. That short window is also why we route bacteria tests to labs near you: a lab you can reach within its window is worth more than a cheaper one that your sample can't get to in time. When a lab states its own transit window, we use that.

When to choose a certified lab:

• You want a result for total coliform and E. coli that a health department will recognize.
• You are buying or selling a home with a well.
• You just disinfected your well and need to know whether it worked.
• A home kit or an earlier test showed bacteria.

Lab methods

• Colilert (Standard Methods 9223 B)
• Standard Total Coliform Membrane Filter Procedure (Standard Methods 9222 B, C)
• Presence-Absence (P-A) Coliform Test (Standard Methods 9221 D)
• Membrane Filtration using MI medium (EPA Method 1604)
• EC-MUG medium (Standard Methods 9221 F.1)

Source: ecfr.gov (source for lab methods) (opens in a new tab)

How to collect a bacteria sample

A bacteria sample is easy to spoil. Anything that touches the inside of the bottle can add bacteria that were never in your well. Read your lab's instructions before you start, since they are written for that lab's bottle and method.

What comes in the kit. Use only the sterile bottle the lab sends. Don't swap in a jar from home. The bottle may contain a small amount of preservative: when the water has residual chlorine, federal methods call for sodium thiosulfate to be added to the bottle before sterilization to neutralize it. Leave it in the bottle. Some labs send more than a bottle. Penn State's kit, for example, includes a cooler, a shipping box, sample bottles, sampling instructions and a submission form.

1. Plan the timing first. Check your lab's receiving days and window before you fill the bottle. Collect early in the week and close to a shipping or drop-off time so the sample reaches the lab inside its window.
2. Pick the right tap. Use a cold-water tap you drink from, usually the kitchen sink. If you have a treatment device, ask the lab whether to sample before it, after it, or both.
3. Prepare the faucet as your kit says. If the instructions ask you to remove the aerator, wipe or disinfect the faucet tip, do that before you collect.
4. Wash your hands. Mississippi State University Extension advises washing your hands and not touching the inside of the container or its cap.
5. Run the cold water. Let it run for the time your lab gives you so the sample represents water from the well.
6. Fill without rinsing. Open the bottle only when you are ready, hold the cap by its outside edge, and fill to the line your lab marks. Don't rinse the bottle and don't let anything touch the rim. Cap it tightly.
7. Label, pack and send. Write the date, time and tap on the label and the form. Pack the sample the way your lab asks, using any cooler or cold pack it provides, and send it as soon as you can.

Common mistakes to avoid

• Collecting on a day the lab can't receive the sample in time.
• Touching the inside of the cap or bottle, or setting the cap face down on the counter.
• Sampling from an outdoor spigot or a hose when you wanted to know about your drinking water.
• Sampling right after disinfecting your well, before the chlorine has cleared, unless your lab tells you to.

What your bacteria result means

Most bacteria tests report presence or absence: the lab tells you whether total coliform bacteria and E. coli were found in your sample. Here is how to read each part.

Total coliform: an indicator. Under EPA's Revised Total Coliform Rule, total coliforms are a useful indicator of other pathogens. For public water systems, total coliform results trigger a treatment technique: systems that exceed a specified frequency of total coliform occurrences must conduct a Level 1 or Level 2 assessment of their system. There is no federal maximum contaminant level for total coliform under the current rule.

E. coli: the federal limit. EPA sets a maximum contaminant level for E. coli to protect against fecal contamination, and its health goal (the MCLG) is 0 CFU/100 mL^[2]. EPA notes that disease-causing microbes in human or animal waste can cause diarrhea, cramps, nausea and headaches, and may pose a special health risk for infants, young children, and people with severely compromised immune systems. A water test can't diagnose anything. If you have health concerns, talk to a health professional and bring your lab report.

Putting your result in context

• No coliform and no E. coli detected. No treatment needed based on this result. Keep the report as a baseline and retest each year, or sooner after flooding, well repairs or a change in your water.
• Total coliform present, E. coli absent. Something has found a way into your water system. The Vermont Department of Health advises verifying the result with a second bacteria test and looking for sources of contamination around the well. After any repairs, it advises disinfecting the well by shock chlorination and retesting two to three days^[6] after the smell of chlorine is gone.
• E. coli present. Treat this as fecal contamination until a follow-up test shows otherwise. Vermont advises boiling all water used for drinking, cooking, brushing teeth, making ice and preparing baby formula for one minute^[6], until follow-up test results show the absence of bacteria. Mississippi State University Extension suggests bottled water for drinking and cooking in the meantime. Contact your local health department for help.

Base every step on a certified-lab result, and retest with a certified lab after any fix to confirm it worked. For the full fix, from finding the cause to shock chlorination, read our guide to coliform in well water.