What does a positive coliform test mean for well water?

A total coliform positive means bacteria have found a way into your well, usually through surface water infiltration, a cracked well casing, or contaminated groundwater. It doesn't automatically confirm fecal contamination, but it requires action. Follow up immediately with an E. coli test if your original panel didn't include one. E. coli presence confirms fecal contamination, which is a more serious finding.

Is it safe to shower with coliform bacteria in well water?

Generally yes for healthy adults. Skin contact with coliform-contaminated water is low risk for intact skin. The concern is ingestion, primarily drinking and cooking with contaminated water. Avoid drinking, cooking, brushing teeth, or using ice from the contaminated supply until testing confirms it is clear. Infants, pregnant people, the elderly, and anyone with a compromised immune system should avoid any contact until the water is confirmed safe.

How long does it take to fix a coliform problem?

Shock chlorination takes one day to perform and requires a 24-hour contact time for the chlorine to be effective. After chlorination, flush the system completely. Wait at least 24 hours before collecting a retest sample, and ideally wait several days. Lab results take 1 to 3 days. From positive test to confirmed clean result, plan for 1 to 2 weeks.

Does UV treatment remove bacteria from well water?

UV light damages bacterial DNA and prevents reproduction. NSF 55 Class A certified UV systems achieve a 6-log (99.9999%) reduction of bacteria and viruses and are designed as a complete disinfection solution for private wells. UV doesn't remove the bacteria from the water physically, it inactivates them so they can't reproduce or cause illness. A UV system must be paired with a sediment pre-filter to work correctly.

Can coliform come back after shock chlorination?

Yes. Shock chlorination is a one-time treatment that kills bacteria present at the time of treatment. It doesn't address the source of contamination. If surface water is entering through a cracked casing, if nearby land use is contributing to groundwater contamination, or if iron bacteria are established in the well, coliform can return. Always retest after treatment. If it comes back, install a UV disinfection system and investigate the source of infiltration.

Positive Coliform Test in Well Water: What to Do Next

Stop drinking your well water until it tests clear. A positive bacteria test requires immediate action. Use bottled water for drinking, cooking, and brushing teeth until you complete treatment and confirm clean results.

Your lab report says total coliform positive, or worse, E. coli detected. Here’s exactly what that means and what to do about it, step by step.

Total Coliform vs. E. Coli: They’re Not the Same Problem

Your test may have flagged one or both.

Total coliform positive (E. coli negative) means bacteria are present in your well, but not necessarily fecal bacteria. Total coliform bacteria occur naturally in soil and plants. Their presence in well water indicates that surface water is finding a path into your well, through a cracked casing, improperly sealed wellhead, or pressure changes after heavy rain. It requires action, but it’s a lower-urgency finding than E. coli.

E. coli positive means fecal contamination. Human or animal waste has reached your water supply. This is a more serious finding. Don’t drink the water. Don’t cook with it. Don’t use it to mix infant formula.

If your original test didn’t include an E. coli test alongside total coliform, order one now.

Immediate Steps

Switch to bottled water for all drinking, cooking, and brushing teeth. Use it until you get clean test results.
Tell everyone in the household, particularly if there are infants, pregnant people, elderly residents, or anyone with a compromised immune system.
Note any recent events that could have introduced contamination: flooding, heavy rain, nearby construction, recent well work, dead animals near the wellhead.

Step 1: Shock Chlorination

Shock chlorination is the standard first response to a positive bacteria test. It disinfects the well casing, pump, and distribution pipes by introducing a strong chlorine solution that kills bacteria on contact.

The full procedure is on the shock chlorination how-to page. The short version:

Calculate the amount of unscented household bleach (5.25% sodium hypochlorite) based on your well’s casing diameter and water depth
Pour the chlorine solution into the well casing
Circulate the water to distribute chlorine through the full system
Wait 12 to 24 hours for contact time
Flush the system thoroughly until chlorine smell is gone
Wait 48 to 72 hours before collecting a retest sample

Do not retest immediately after flushing. Chlorine residual can give a false-negative result. Wait until chlorine is fully flushed before sampling.

Step 2: Get a Retest

Submit a fresh water sample to the same certified lab, or another NELAP-accredited lab, following the same collection procedure as your original test.

Results take 1 to 3 days. A clean retest after shock chlorination is a good sign, but it doesn’t guarantee the problem won’t recur.

If Coliform Comes Back

A second positive result after shock chlorination means the contamination source is ongoing, not a one-time event. At this point, two things need to happen:

Find the physical source. Have a licensed well contractor inspect the casing for cracks, the wellhead seal for gaps, and the surrounding area for potential contamination pathways. Surface runoff from nearby animal areas, septic systems within 100 feet, and flooded fields are common sources. Without fixing the entry point, treatment alone won’t solve a recurring problem.

Install a UV disinfection system. A UV system treats every gallon of water leaving your well before it reaches your household plumbing. It’s the standard long-term solution for private wells with bacterial contamination.

UV Disinfection: What to Know Before You Buy

Not all UV systems are equivalent. The certification matters.

NSF/ANSI 55 Class A systems are validated for 6-log reduction of bacteria and viruses. This is the standard required for wells with confirmed bacterial contamination. Class A systems are rated at 40 mJ/cm2 UV dose or higher.

NSF/ANSI 55 Class B systems provide supplemental treatment only. They’re rated for 16 mJ/cm2 UV dose and are not appropriate as the sole disinfection method for a contaminated well.

For a well with confirmed coliform or E. coli, you need Class A. Check the NSF certification before purchasing.

A UV system also requires a sediment pre-filter upstream. Particles in the water absorb UV light and protect bacteria from the dose. A 5-micron or 1-micron sediment filter installed before the UV chamber is standard practice.

For product recommendations, see best UV water purifiers, which compares Class A systems at different flow rates.

What UV Doesn’t Do

UV inactivates bacteria and viruses. It doesn’t remove chemicals, nitrates, arsenic, heavy metals, or any dissolved contaminants. If your water test also showed elevated nitrates, arsenic, or other chemical contaminants alongside bacteria, you need separate treatment for those.

A UV system combined with a whole-house carbon filter handles bacteria, chlorine, and some organics. For nitrates or arsenic, add a point-of-use RO system at the kitchen tap.

For more on bacteria in well water, including sources and risk factors: bacteria in drinking water.

WaterAnswer.com provides general information only. This page is not a substitute for water quality testing by a certified laboratory or guidance from a licensed well contractor or water treatment professional.

Sources: EPA Private Wells | CDC Private Wells | NSF/ANSI 55