Testing well water for bacteria: coliform, E. coli and what to do next

A bacteria test is cheap, quick and the first item on every annual testing list. This page explains what the test looks for, how to take a sample the lab will accept, what a positive result means and how to clear the well.

What does a well water bacteria test look for?

Labs usually test for two things. Total coliform bacteria are a group of microorganisms found in soil and surface water, and E. coli is a type of fecal coliform. North Carolina DHHS: Total Coliform Bacteria, E. coli & Private Wells The results are read together: [NC-COLIFORM source note]

Mississippi State University Extension explains the distinction: coliform bacteria are common and do not necessarily indicate that the water has contacted human or animal waste, but fecal coliform or E. coli definitely indicates contamination. Mississippi State University Extension: What to Do about Coliform Bacteria in Well Water CDC adds that a high total coliform count makes it likely that harmful germs are present too. CDC: Guidelines for Testing Well Water [MSU-COLIFORM source note] [CDC-TEST source note]

How often should you test for bacteria?

At least once a year. CDC and EPA both recommend testing a private well every year for total coliform bacteria, along with nitrates, total dissolved solids and pH, and using a state-certified lab. EPA: Protect Your Home's Water Test again after flooding, after any work on the well or plumbing, and whenever the water's taste, color or smell changes. The Minnesota Department of Health also recommends yearly coliform testing. Minnesota Department of Health: Bacterial Safety of Well Water [EPA-PROTECT source note] [MDH-BACTERIA source note]

How to take a bacteria sample

Get the sterile bottle and instructions from the lab, and follow them exactly; a contaminated or late sample has to be repeated. Montana's state lab instructions, for example, say to:

1. Fill the 100 mL bottle to the mark without rinsing it.
2. Leave the powder or pill that comes in the bottle inside it.
3. Keep the sample cool after collection, not in a hot vehicle.
4. Get it to the lab within the stated time.

Your lab's kit will also say which tap to use and how long to run the water first.

Hold times vary by lab. Montana's state lab says bacteria samples must reach the lab within 48 hours of collection and should be kept cool, not left in a hot vehicle. Montana Environmental Laboratory: Bacteria Testing for Private Well Sample Submission Form Other labs set shorter windows and only accept samples on certain weekdays, so check before you collect. [MT-LAB-BACT source note]

What it costs

Bacteria tests are among the cheapest lab tests. Published public lab fees include $10 at the Indiana Department of Health, $25 at Montana's state lab, $35 at the Wisconsin State Laboratory of Hygiene and $41 at Maryland's state lab. Indiana Department of Health Laboratories Wisconsin State Laboratory of Hygiene Maryland Department of Health Laboratories Private labs and technician visits cost more; see well water testing cost. [IN-LAB-WELL source note] [WSLH-PRICES source note] [MD-LAB-WELL source note]

What to do after a positive result

Texas A&M AgriLife Extension sets out the steps: Texas A&M AgriLife Extension: What to Do About Coliform Bacteria in Well Water [TAMU-COLIFORM source note]

1. Stop drinking the water untreated. Use bottled water, or boil water for drinking, cooking and brushing teeth. New Jersey's health department advises boiling for at least one minute. NJ Department of Health: Total Coliform & E. coli in Private Well Drinking Water The Minnesota Department of Health notes that boiling can concentrate nitrate or arsenic, so check those results too.
2. Retest to confirm, including for E. coli if the first test only covered total coliform.
3. Find and fix the source. Common entry points are a loose or damaged well cap, a cracked casing, surface water pooling at the wellhead, a shallow well, an unplugged old well nearby, or a nearby septic system or animal waste. Check your well against state septic setbacks.
4. Shock chlorinate the well, ideally done by a licensed well driller or pump installer.
5. Retest before drinking untreated water, and keep testing: Texas A&M suggests a follow-up in 6 to 12 months after a clean result, then at least yearly.

New Jersey's guidance adds that a total coliform positive with E. coli negative calls for a retest within the week and again six months later, and that shock chlorination fits a one-time event; recurring results need the source corrected, or continuous treatment such as UV.

How shock chlorination works

Connecticut's Department of Public Health publishes a standard procedure: dose the well to at least 50 ppm chlorine using unscented bleach, run it through every tap, let the chlorinated water stand in the well and plumbing for at least six hours (overnight is better), then flush it to waste outdoors away from plants. A bacteria sample cannot be collected until no chlorine residual is detected, and if the retest is positive, disinfection is repeated. Connecticut DPH Publication #27: Disinfection Procedure for Private Wells The amount of bleach depends on the volume of water in the well, which depends on its depth, water level and diameter, so use the agency's table or have a contractor do it. [CT-DISINFECT source note]

If bacteria return after a proper disinfection, the problem is structural or environmental, and more chlorine will not fix it. Ask a licensed contractor to inspect the cap, casing and grout seal, and see how to read well water test results and the well maintenance checklist.