Coliform Bacteria Testing for Wells: Complete Guide

Coliform testing is the most important biological test for private well water. Unlike chemical tests that measure contaminants in parts per million, coliform testing tells you whether your water is microbiologically safe—whether harmful bacteria could be present. Understanding what these tests mean, how to collect proper samples, and what to do with results is essential for every well owner.

Understanding Coliform Bacteria

What Are Coliforms?

Coliform bacteria are a large group of organisms found naturally in:

• Soil and vegetation
• Surface water
• Intestinal tracts of warm-blooded animals

Most coliform bacteria are harmless. They're used as "indicator organisms"—their presence suggests that surface water or waste may have entered your well, potentially bringing pathogens with them.

Types of Coliform Tests

Total Coliform vs. E. coli

Total Coliform Positive:

• Could be environmental contamination
• May indicate well integrity issues
• Doesn't necessarily mean fecal contamination
• Common after heavy rain or flooding
• Often resolved by shock chlorination

E. coli Positive:

• Confirms fecal contamination
• Serious health concern
• Stop drinking water immediately
• Requires investigation of contamination source
• May indicate septic, animal, or sewage intrusion

When to Test

Annual Testing

All private well owners should test annually, typically in spring after snowmelt/rain when contamination risk is highest.

Test Immediately After:

• Flooding in your area
• Any well work (pump replacement, repairs)
• Breaking the well seal for any reason
• Unexplained illness in the household
• Changes in water taste, smell, or appearance
• New nearby septic system or animal operation

Test More Frequently If:

• Infants or young children in home
• Immunocompromised family members
• Previous positive coliform results
• Shallow well (<50 feet)
• Known septic issues nearby

Collecting a Valid Sample

Proper sample collection is critical. Contamination from improper technique causes false positives and unnecessary worry or treatment.

Get the Right Container

• Use sterile container from testing lab
• Bottles contain preservative—don't rinse
• Don't touch inside of bottle or cap

Sample Collection Steps

1. Choose the right tap: Use a faucet that draws directly from the well (before treatment equipment if possible). Avoid faucets with aerators, sprayers, or swivel heads—these harbor bacteria.
2. Remove aerator: If unavoidable, remove and clean the aerator before sampling.
3. Clean the tap: Wipe the faucet opening with a bleach solution (1 tablespoon bleach per quart water) and let dry.
4. Flush the tap: Run cold water for 2-3 minutes to clear standing water.
5. Fill the container: Carefully remove cap without touching inside. Fill to the line (not to the top—leave air space). Replace cap immediately.
6. Label and chill: Record date, time, and location. Keep sample cool (not frozen) and deliver to lab within 24 hours (6 hours is ideal).

Common Mistakes

• Sampling from outdoor hose bibs (contaminated by backflow)
• Not flushing long enough
• Touching bottle rim or inside
• Delay in getting sample to lab
• Sample getting too warm or freezing

Understanding Results

Presence/Absence Testing

Most coliform tests report as "Present" or "Absent" (positive or negative). Some labs report colony counts (CFU per 100 ml).

Result Interpretations

If Your Well Tests Positive

Total Coliform Positive (E. coli Negative)

This is common and usually not dangerous, but should be addressed:

1. Consider sources: Recent work? Heavy rain? New construction nearby?
2. Shock chlorinate: See our well shocking guide
3. Wait 7-10 days: Allow chlorine to dissipate
4. Retest: Collect new sample using proper technique
5. If still positive: Inspect well for integrity issues

E. coli Positive

This requires immediate action:

1. Stop drinking the water immediately
2. Use bottled water for drinking and cooking
3. Notify household members
4. Call a well professional to inspect for contamination sources
5. Shock chlorinate the well thoroughly
6. Retest after treatment
7. If persistent: Consider continuous disinfection (UV or chlorination)

Possible Contamination Sources

• Failing septic system nearby
• Animal waste runoff
• Cracked or corroded well casing
• Improper well cap or seal
• Surface water infiltration
• Cross-connection with contaminated source

Shock Chlorination

The standard treatment for coliform contamination:

Basic Process

1. Calculate chlorine needed based on well depth and casing diameter
2. Add household bleach (unscented) or well chlorination tablets
3. Recirculate to distribute chlorine
4. Let sit 12-24 hours
5. Flush until chlorine smell is gone
6. Wait 7-10 days, then retest

See our detailed shock chlorination guide for complete instructions.

Professional vs. DIY

• DIY works for simple total coliform positives
• Call a professional for E. coli, persistent positives, or unknown well details

Prevention

Well Construction and Maintenance

• Maintain proper well cap (vented, vermin-proof)
• Ensure well casing extends 12+ inches above ground
• Keep surface drainage away from wellhead
• Maintain 50+ feet from septic components
• Inspect annually for cracks, damage, settling

Continuous Disinfection (If Needed)

For wells with recurring contamination:

• UV disinfection: Kills bacteria with ultraviolet light; no chemicals
• Chlorine injection: Continuous low-level chlorination
• Ozone treatment: Powerful disinfection; more complex

We use Hach and LaMotte professional water testing equipment for field analysis, with comprehensive lab testing through certified California laboratories.

Frequently Asked Questions