Bacteria (coliform & E. coli) in private well water
Coliform bacteria and E. coli reach private wells from septic systems, surface water, and a compromised wellhead. There is no national groundwater grid for it; the universal recommendation is to test every private well for coliform bacteria at least once a year.
Is it a health risk?
Bacteria are a health hazard: E. coli indicates fecal contamination and can cause serious gastrointestinal illness. Bacteria samples have a short (~30 hour) hold time, so a positive result means acting fast — disinfect and re-test.
What is Bacteria (coliform & E. coli)?
Most bacteria in groundwater are harmless. Total coliform bacteria are used as an INDICATOR that surface water (and possibly disease-causing organisms) may have reached the well, while E. coli specifically signals fecal contamination.
Health effects of Bacteria (coliform & E. coli)
Bacteria are a health hazard: E. coli and other pathogens cause gastrointestinal illness. A positive coliform result does not always mean a pathogen is present, but it means the well is vulnerable and should be acted on.
Symptoms & signs
- Diarrhea and stomach cramps
- Nausea and vomiting
- Headaches
Who is most at risk
- Infants and young children
- Elderly adults
- Pregnant women
- People with weakened immune systems
Bacterial illness is acute — a positive E. coli result calls for prompt action (boil-water precautions, disinfection, and re-testing).
How Bacteria (coliform & E. coli) gets into a well
Bacteria enter through septic systems, surface runoff from pastures, feedlots, and sewage, flooding, and a compromised wellhead — a cracked or missing well cap, failed grout/seal, or a damaged casing.
Where Bacteria (coliform & E. coli) is common
Any well can be affected. Risk rises after flooding, near septic systems and livestock, and with older or poorly sealed wells. The universal recommendation is to test every private well for coliform bacteria at least once a year.
How common is Bacteria (coliform & E. coli) in US private wells?
No national grid — observed context only
There is no national groundwater grid for Bacteria (coliform & E. coli) in private wells, so we do not show a national risk number for it. The only signal available is the observed sampling context from the EPA/USGS Water Quality Portal (64,484 reported samples across 1,097 counties in our current dataset), which is biased toward wells that were already sampled. The only way to know your level is to test your own well.
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How to remove Bacteria (coliform & E. coli): treatment options
Shock chlorination
Point-of-use or whole-houseDisinfects the well and plumbing as a first response to a positive result.
Fix the underlying problem (wellhead seal, septic) first, then re-test after disinfecting.
UV disinfection
Whole-house (point-of-entry)Continuously inactivates bacteria as water passes a UV lamp; treats the whole house.
Requires clear water (pre-filtration) and a working lamp; it does not remove chemicals.
Whole-house (point-of-entry) systems are a larger investment — typically into the thousands of dollars installed — and total cost varies widely with water chemistry, system type, and professional installation.
Continuous chlorination + filtration
Whole-house (point-of-entry)Injects chlorine for ongoing disinfection, followed by carbon to remove the residual.
Whole-house (point-of-entry) systems are a larger investment — typically into the thousands of dollars installed — and total cost varies widely with water chemistry, system type, and professional installation.
Costs are typical installed ranges that vary widely by system, water chemistry, region, and whether you install it yourself or hire a professional. Last reviewed 2026-06. Always confirm a device is certified (NSF/ANSI or WQA) for the specific contaminant.
Testing for Bacteria (coliform & E. coli)
- EPA standard (Revised Total Coliform Rule)
- Total coliform and E. coli: none detectableThe MCL goal is zero. Test every private well for coliform bacteria at least once a year.
- Can you taste, smell, or see it?
- None — bacteria are invisible, tasteless, and odorless.
- Collecting a sample
- Bacteria samples have a short (~30-hour) hold time — collect and deliver to the lab quickly. If positive, disinfect and re-test.
Sources
The facts on this page are drawn from primary public-health and government sources:
By TapWaterData Editorial
Bacteria (coliform & E. coli) in well water FAQ
Is Bacteria (coliform & E. coli) a health risk in private wells?
Bacteria are a health hazard: E. coli indicates fecal contamination and can cause serious gastrointestinal illness. Bacteria samples have a short (~30 hour) hold time, so a positive result means acting fast — disinfect and re-test.
How common is Bacteria (coliform & E. coli) in US private wells?
There is no national groundwater grid for Bacteria (coliform & E. coli) in private wells. We can only show the observed sampling context from the EPA/USGS Water Quality Portal (64,484 reported samples across 1,097 counties in our current dataset), which is biased toward already-sampled wells. The only way to know your level is to test your own well.
How do I find out if Bacteria (coliform & E. coli) is in my well?
Bacteria (coliform & E. coli) is not something you can see, taste, or smell your way to certainty about. Order a test that covers Bacteria (coliform & E. coli) from a state-certified drinking-water lab, collect the sample exactly as the kit instructs, and compare the result to the EPA limit. County-level area estimates describe a region as a whole and cannot stand in for testing your own well.
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