Bacteria (Coliform & E. coli)

If you only test for one thing, test for this — most well failures aren't dramatic geology, they're a leaky septic 200 feet uphill

EPA MCL

Total coliform: zero detected per 100 mL. E. coli: zero detected per 100 mL.

Health concern

Acute gastrointestinal illness; serious risk for infants, elderly, immunocompromised, pregnant women

Testing method

Membrane filter or Colilert MPN; $20-30; standard on every well panel; many counties offer free testing

If you have a private well and you're going to test for one thing this year, test for total coliform and E. coli. They are the most common positive result on private well tests, the cheapest test you can run, and the contaminant where the right answer is almost always immediately fixable. Most well "failures" you'll experience as a homeowner aren't dramatic geological problems. They're a leaky septic system 200 feet uphill, a fractured well casing, a wellhead that flooded in last week's rainstorm. The geology of where you live didn't change. Something at the surface did, and the test is how you find out.

What the test actually means

Total coliform is a broad family of bacteria that live in soil, vegetation, and the digestive tracts of warm-blooded animals. Most coliforms are not themselves dangerous. The reason they're the standard indicator is that they're everywhere surface contamination is — if your well water has coliforms, then by inference whatever else is in that contamination source could also be in your water, including pathogens that aren't routinely tested.

E. coli is a specific subset of coliforms that lives almost exclusively in the gut of warm-blooded animals. A positive E. coli on a well test means fecal contamination — human or animal — has reached your well. That is the moment to switch to bottled water for drinking and cooking and find the source.

The two-step test pattern at most labs:

• First: total coliform present / absent. If absent, you're done.
• If total coliform present: confirm with E. coli specifically.
• If E. coli present: that's a "boil water" or bottled-water situation while you fix the source.

How it gets in

Bacteria don't migrate up through deep aquifers from below. They get into wells through specific surface pathways:

• Cracked, corroded, or shallow well casing — the casing is supposed to seal the well from surface water down to the aquifer. Old steel casings rust through; PVC casings can crack or pull apart at joints; shallow casings don't extend below the surface contamination zone.
• Loose or damaged well cap — the cap on top of the well casing is often the failure point. Wildlife (mice, snakes, frogs) get under loose caps; insects nest there; rainwater pools and drips in.
• Septic system uphill from the well — standard setbacks of 50-100 feet are often inadequate in karst geology, in sandy soils, or where the water table is shallow. A failing or undersized septic system uphill of a well is the most common chronic source.
• Surface flooding of the wellhead — if your wellhead is below grade or the surrounding soil settled, heavy rain can flood the cap and push surface contamination directly down the casing.
• Cross-connections (rare, usually a plumbing error) — backflow from irrigation, livestock troughs, or other non-potable plumbing.
• Recent well work — drilling, deepening, or pump replacement temporarily exposes the well to surface bacteria. A test 2-4 weeks after well work is standard.

What the test tells you about action

The test result by itself doesn't tell you much; the combination of results across testing rounds does:

• One-time total coliform positive, no E. coli, after a heavy rain — likely a wellhead-level issue. Inspect the cap and surface drainage; shock-chlorinate; retest.
• Persistent total coliform across multiple tests — the well or its environment has a chronic problem. Inspect casing; consider continuous UV treatment; check uphill septic systems.
• E. coli positive, ever — fecal contamination is reaching the well. Switch to bottled water. Find and fix the source. Don't trust the well until two consecutive negatives several weeks apart.
• E. coli plus illness in the household — call your health department. Most counties will help investigate and offer free or subsidized retesting.

Treatment

Treatment depends on whether you're managing a one-time event or a chronic source:

• Shock chlorination — for one-time contamination events (post-flood, after well work, after wildlife intrusion). Add household bleach to the well, run all taps until you smell chlorine, let sit overnight, flush thoroughly. Detailed protocols are published by every state cooperative extension service.
• Continuous UV treatment — point-of-entry UV lamp inactivates bacteria in real time as the water enters the house. Best whole-house solution for ongoing low-level contamination. Cost: $700-1,500 installed; bulb replacement annually.
• Chlorination injection — continuous chlorine dosing with subsequent contact tank and carbon polishing. More aggressive than UV, used for systems with iron bacteria or where UV is impractical.
• Whole-house filtration with UV — combines particulate filtration with UV inactivation. The standard package for ongoing problems.

Treatment manages the symptom. You also need to find and fix the source. A well with a perpetually leaky cap will keep introducing bacteria; UV will keep killing them; you'll keep paying for bulb replacements; the underlying problem won't go away. Mechanical fix first, treatment second.

Aquifers where this is a concern