Coliform Bacteria in Well Water: What It Means and How to Fix It
July 5, 2026 · 6 min read · By Deep Dive Plumbing Maryland
Important: If your well has tested positive for coliform bacteria, do not drink the water until the well has been treated and retested. Use bottled water for drinking and cooking in the meantime.
You had your well water tested — maybe because you are buying or selling a home, or because someone in the family got sick — and the results came back positive for coliform bacteria. Now what?
Coliform bacteria in well water is one of the most common well water problems in Maryland. It does not mean your water is definitely making you sick right now, but it does mean your well has been compromised and needs immediate attention. Here is what you need to know.
What Causes Coliform Bacteria in a Well?
Surface water intrusion
Rainwater or floodwater entering the well through a damaged well cap, cracked casing, or improper grout seal.
Nearby septic system
A failing septic system or one that is too close to the well can leach bacteria into the groundwater.
Animal waste
Agricultural runoff, pet waste, or wildlife near the wellhead can introduce bacteria during heavy rain events.
Well construction issues
An old or improperly constructed well without a sanitary well cap is vulnerable to contamination year-round.
Flooding
Any time floodwater reaches the wellhead, the well should be tested and likely shocked with chlorine.
Total Coliform vs. E. coli: What Is the Difference?
A standard bacteria test checks for two things: total coliform and E. coli. Understanding the difference matters for how you respond.
Total Coliform Positive
Indicates bacteria are present in the well. The source may be environmental (soil bacteria) or fecal. Requires investigation and treatment, but not necessarily an immediate health emergency.
E. coli Positive
Confirms fecal contamination — human or animal waste has entered the water. This is a direct health risk. Do not drink the water. Treat immediately and identify the contamination source.
How to Treat Coliform Bacteria in a Well
1. Shock chlorination
A licensed plumber introduces a high concentration of chlorine (household bleach) into the well, circulates it through the system, and lets it sit for 12–24 hours. This kills bacteria throughout the well and distribution system.
2. Flush and retest
After the contact time, the system is flushed until the chlorine smell is gone. A follow-up bacteria test confirms the treatment was successful. Wait at least 5–7 days after flushing before retesting.
3. Identify and correct the source
If bacteria return after chlorination, the entry point must be found and fixed. Common fixes: new sanitary well cap, casing repair, grouting, or relocating a nearby septic component.
4. Install ongoing protection
A UV water treatment system provides continuous disinfection, killing bacteria, viruses, and other pathogens every time water passes through. It is the most reliable long-term protection for wells in vulnerable areas.
Frequently Asked Questions
What is coliform bacteria in well water?
Coliform bacteria are a group of bacteria used as indicators of water quality. Total coliform includes many types of bacteria found in soil and water. E. coli (a subset of coliform) specifically indicates fecal contamination — meaning human or animal waste has entered the water supply. Total coliform in a well does not always mean fecal contamination, but it does mean the well is vulnerable and should be investigated.
Is coliform bacteria in well water dangerous?
Total coliform bacteria are not always directly harmful, but their presence indicates the well may be contaminated with pathogens that are dangerous. E. coli specifically can cause serious illness — nausea, vomiting, diarrhea, and in vulnerable populations (children, elderly, immunocompromised), severe complications. Do not drink water that tests positive for coliform until the source is identified and the well is treated.
How do I know if my well has coliform bacteria?
You cannot tell by looking, smelling, or tasting the water. Coliform bacteria are odorless and colorless. The only way to know is to test. Maryland requires well water testing at the time of sale, but private well owners should test annually for bacteria. Test kits are available, but a certified lab test is more reliable.
How do you treat coliform bacteria in a well?
Shock chlorination is the standard treatment — introducing a high concentration of chlorine (bleach) into the well to kill bacteria, then flushing the system. This is effective for a one-time contamination event. If bacteria return after chlorination, the source of contamination must be identified and corrected (well cap, casing, nearby septic system). A UV water treatment system provides ongoing protection against bacteria.
How much does well chlorination cost in Maryland?
Professional well shock chlorination in Maryland includes the service call, chlorine, and follow-up testing. If the well requires structural repairs (new cap, casing repair, grouting), costs increase. A UV treatment system for ongoing protection is also available — contact us for a free estimate.
How often should I test my well for bacteria?
Maryland recommends testing private wells annually for bacteria (total coliform and E. coli). Also test after any flooding event, after well repairs, if you notice changes in water taste or odor, or if anyone in the household gets unexplained gastrointestinal illness. Annual testing is the only way to catch contamination before it causes illness.
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