The call comes from the lab, or the letter arrives in the mail, and the result says: positive for total coliform. Your well water has bacteria.
The immediate reaction is usually panic. Bacteria in the water. That sounds serious. That sounds like Flint, like a boil water notice, like something that could make your family sick.
Sometimes it is serious. Often it isn't — or at least, it isn't in the way you're imagining. Understanding what the result actually means is the difference between an appropriate response and an expensive overreaction, or worse, an inappropriate dismissal of a real problem.
What Are the Three Types of Coliform Bacteria in Well Water?
Total coliform is a broad category of bacteria that includes hundreds of species found in soil, vegetation, and the digestive tracts of warm-blooded animals. Most total coliform bacteria are not harmful to humans. Their presence in well water is significant not because they themselves are dangerous, but because they indicate that a pathway exists for contamination — that surface water or soil is somehow reaching your well.
Fecal coliform is a subset of total coliform that is found specifically in the intestines and feces of warm-blooded animals. A positive fecal coliform result means recent fecal contamination — from wildlife, livestock, a failing septic system, or surface runoff from agricultural land. This is a more serious finding than total coliform alone.
E. coli is a specific type of fecal coliform. Most E. coli strains are harmless, but certain strains — particularly E. coli O157:H7 — can cause severe illness, as the CDC warns. A positive E. coli result in well water is a serious finding that requires immediate action: stop drinking the water until the source is identified and the well is treated.
What Causes Coliform in Connecticut Wells
Connecticut's geology and land use patterns create several common pathways for coliform contamination:
Septic system proximity: Connecticut has a high density of private wells and septic systems in close proximity. The state requires a minimum separation distance between a well and a septic system, but older properties may not meet current standards, and even compliant systems can fail or become saturated during heavy rain.
Surface water intrusion: Wells with damaged or improperly sealed casings can allow surface water to enter directly. This is particularly common after heavy rain events — the same storms that cause flooding also push surface water into vulnerable wells.
Agricultural runoff: In farming areas — the Connecticut River Valley, the agricultural towns of Tolland and Windham counties — runoff from fields with animal waste can reach well water, particularly in shallow wells. The Connecticut River Valley has a well-documented nitrate problem from farming that often travels alongside bacterial contamination.
What Should You Do After a Positive Coliform Result?
First: determine which type of coliform was found. Total coliform alone is a different situation from fecal coliform or E. coli.
Second: shock chlorinate the well. This is the standard first response to a positive coliform result — introducing a high concentration of chlorine into the well to kill bacteria, then flushing the system. Your water testing lab or a licensed well contractor can walk you through the process.
Third: retest after shock chlorination. If the result is still positive, the contamination source needs to be identified and addressed.
Fourth: consider a UV disinfection system. For wells with recurring coliform issues, a UV disinfection system installed at the point of entry — certified to NSF/ANSI Standard 55 — provides continuous protection against bacteria and viruses without adding chemicals to the water.
A positive coliform result is not a reason to panic. It is a reason to act. And if your well has bacteria issues, it may have other problems too — the standard mortgage water test doesn't cover most of them.
Worried about bacteria in your well? Enter your ZIP code at CheckYourTap.com to see what's in your tap water — free, in 30 seconds.
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Frequently Asked Questions
Is total coliform in well water dangerous?
Not necessarily. Total coliform bacteria are mostly harmless — they're found in soil and vegetation. Their presence indicates a pathway exists for contamination to reach your well, not that the water is making you sick. Fecal coliform is more serious (indicates recent fecal contamination), and E. coli is a serious finding requiring immediate action — stop drinking the water until the source is identified and the well is treated.
How do you fix bacteria in a Connecticut private well?
The standard first response is shock chlorination — introducing a high concentration of chlorine into the well to kill bacteria, then flushing the system. Retest after shock chlorination. If the result is still positive, the contamination source (damaged casing, septic proximity, surface water intrusion) needs to be identified and addressed. For wells with recurring coliform issues, a UV disinfection system at the point of entry provides continuous protection without adding chemicals.
What causes coliform bacteria in Connecticut wells?
The three most common causes in Connecticut are: (1) septic system proximity — the state has a high density of wells and septic systems close together, and older properties may not meet current separation distances; (2) surface water intrusion through damaged or improperly sealed well casings, especially after heavy rain; and (3) agricultural runoff in farming areas like the Connecticut River Valley, where animal waste reaches shallow wells.
Sources: CT DPH Private Well Testing Guidelines; EPA Coliform Bacteria in Drinking Water; NSF International Well Water Testing Guide; CT DPH Well Contractor Licensing Program.