My well water tested positive for coliform bacteria — what should I do?
You filled the sample bottle from the kitchen tap, mailed it to the lab, and three days later the report came back with a word you did not want to see: coliform detected. The well has been fine for years, no one is sick, but now every glass of water feels questionable. This is one of the most common — and fixable — problems private well owners face. The key is to treat the positive result as a warning sign, not a panic button, and follow a clear sequence before deciding on permanent treatment.
What a positive coliform test actually means
Coliform bacteria are a broad family of organisms found in soil, plants, and the digestive tracts of warm-blooded animals. A positive total coliform test means bacteria got into your water somewhere between the aquifer and the faucet. It is an indicator: it does not automatically mean fecal contamination, but it does mean the system is no longer sterile.
The result that changes the math is fecal coliform or E. coli. EPA’s Maximum Contaminant Level Goal for E. coli in drinking water is zero, and any confirmed detection should be treated as an acute health risk, especially for infants, elderly household members, or anyone immunocompromised. If your lab report separates total coliform from E. coli, focus on the E. coli line first.
Step-by-step response plan
1. Stop drinking the water if E. coli is present
Use bottled or boiled water for drinking, brushing teeth, and washing produce until the problem is resolved. Boil at a rolling boil for at least one minute — three minutes if you are above 6,500 feet elevation. For total coliform without E. coli, many health departments still recommend avoiding consumption until disinfection is complete.
2. Retest before you spend money
One positive sample can be caused by a dirty faucet screen, recent plumbing work, or a lab handling error. Collect a second sample from a clean, disinfected tap after running the water for several minutes, and send it to a state-certified laboratory. A standard total coliform/E. coli test costs $25 to $150 depending on your state and turnaround time.
3. Inspect the well for entry points
Walk the system with a flashlight. Look for a cracked or missing well cap, an unsealed casing joint, a well pit that floods, or a casing that does not extend at least 12 inches above finished grade. Also check the pressure tank, water softener, and any storage tanks for stagnant water or biofilm. Surface water intrusion is the most common cause of intermittent coliform problems after heavy rain.
4. Shock chlorinate the well and plumbing
Shock chlorination is the standard first treatment for a one-time coliform event. The goal is to raise the chlorine concentration throughout the well, drop pipe, pressure tank, and household plumbing to roughly 200 mg/L (about 200 ppm). Unscented household bleach is roughly 5–6% sodium hypochlorite; use a dosage chart based on well diameter and depth, or hire a water treatment professional for $200 to $500.
After introducing chlorine, circulate it by running each tap until you smell bleach, then let the system sit for at least six to eight hours — overnight is better. Flush the entire system the next day, preferably to a location where chlorine will not harm landscaping or a septic drain field, until the free chlorine residual reads below 4 mg/L. Wait about seven to ten days, then retest.
Long-term treatment options if coliform keeps coming back
If shock chlorination works and the next quarterly test is clean, you may not need permanent equipment. If coliform returns — especially after storms or snowmelt — the well is being recontaminated and a continuous disinfection system is the practical fix.
| Technology | How it works | Typical installed cost | Standard to look for | Best suited for |
|---|---|---|---|---|
| UV disinfection | 254 nm ultraviolet lamp inactivates bacteria as water flows past | $350 – $1,500 | NSF/ANSI 55 Class A | Clear water with low iron, sediment, and hardness |
| Chlorine injection | Pump adds measured chlorine dose before contact tank | $800 – $2,500 | NSF/ANSI 42 / 60 components | High iron, manganese, or sulfur bacteria; long distribution lines |
| Reverse osmosis at point of use | Membrane removes bacteria at a single tap | $250 – $800 | NSF/ANSI 58 with bacteria reduction claim | Drinking/cooking water only; not whole-house protection |
UV is the most common whole-house choice for coliform because it adds no chemicals and requires only an annual lamp change. It is not a filter — suspended particles can shield bacteria from the light — so pre-filtration down to at least 5 microns is usually required. Chlorine injection handles more complex water chemistry but needs routine chemical refills and careful residual management. A point-of-use reverse osmosis system protects drinking water at one tap but does nothing for showers or washing machines.
Common mistakes
- Treating only the well and not the plumbing. Bacteria can live in the pressure tank, water heater, and softener resin. If you chlorinate the well but skip the house lines, the problem comes back within days.
- Testing too soon after shock chlorination. Residual chlorine can suppress bacteria temporarily and give a false negative. Wait at least seven days before collecting a post-treatment sample.
- Installing UV without pre-filtration. Murky or rusty water blocks UV light. Test for hardness, iron, and turbidity before sizing the unit.
- Ignoring the well cap and casing. No disinfection system can keep up with a cracked cap or a casing flooded by surface runoff. Fix the source before or alongside treatment.
- Testing once and forgetting about it. EPA recommends private wells be tested for total coliform at least once per year, and more often after flooding, construction, or a change in taste, odor, or color.
FAQ
Is total coliform in well water an emergency?
Total coliform is a warning, not an automatic emergency. E. coli or fecal coliform is more serious and should trigger a boil-water response, especially for vulnerable household members. Either way, stop using the water for consumption until you disinfect and retest.
Can I just boil my well water long-term instead of treating it?
Boiling kills bacteria, but it is not a practical whole-house solution and does not address how the bacteria are entering the system. Use boiling as a temporary measure while you shock chlorinate or install continuous disinfection.
How much does it cost to fix coliform in well water?
A DIY shock chlorination costs $40 to $120 for bleach, test kits, and a chlorine test kit. Professional shock chlorination runs $200 to $500. A whole-house UV system with pre-filtration typically costs $350 to $1,500 installed, while a chlorine injection system runs $800 to $2,500. Annual maintenance for UV is usually $80 to $150 for a replacement lamp and sleeve cleaning.
Will a standard sediment filter remove coliform bacteria?
No. A sediment filter may physically trap some bacteria, but it is not a reliable barrier and can become a breeding ground. For documented bacterial contamination, use NSF/ANSI 55 Class A UV, chlorination, or a certified microbiological purifier.
ClearTap content is for informational purposes only and does not replace advice from a licensed plumber, state-certified water testing laboratory, or public health authority. If your well water tests positive for E. coli or anyone in your household shows symptoms of gastrointestinal illness, contact your local health department and a qualified water treatment professional.