Short definition
Coliform bacteria are an indicator group, not a specific pathogen. Their presence in drinking water means surface contamination — soil, runoff, sewage, or animal waste — found its way into the supply. Total coliform positive means a pathway exists; E. coli positive means active fecal contamination, and the water is unsafe to drink without treatment.
What it is
Coliform is a broad family of rod-shaped bacteria that live in soil, surface water, and the intestines of warm-blooded animals. In a well-sealed groundwater supply, coliform should be absent. When a lab finds it, that’s a signal that the system has a contamination pathway — usually a cracked well cap, missing sanitary seal, septic-field too close, or surface runoff entering through a damaged casing.
Lab reports separate the result into layers:
- Total coliform — the broad indicator group, includes environmental species from soil and vegetation. Positive means something is getting in.
- Fecal coliform — a subgroup that grows at higher temperatures (44.5°C); presence correlates with sewage or animal waste.
- E. coli — a single species; presence is unambiguous fecal contamination.
A typical home result reads “Total Coliform: positive/negative; E. coli: positive/negative.” Total coliform positive with E. coli negative usually means a surface entry pathway. Total coliform with E. coli positive means an active fecal pathway, and the well is unsafe to drink without treatment.
Why it matters to a homeowner
For Washington homeowners on private wells — common across Mason, Jefferson, Kitsap, Whatcom, eastern WA, and rural parts of Snohomish, King, and Pierce — coliform is the number-one routine test. WA Department of Health recommends annual coliform + nitrate testing on every private well, and lenders typically require a fresh test at home sale.
A positive doesn’t always mean someone is getting sick. Most surface-entry events show as positive total coliform with negative E. coli, and shock chlorination plus a follow-up test resolves the problem. But during the window between “test came back positive” and “follow-up confirms negative,” vulnerable people in the household — infants under 6 months, immunocompromised individuals, elderly — should drink bottled water.
When you’ll encounter this term
- Annual maintenance test on a rural well
- Selling rural property: closing-table coliform test required by lender or buyer
- After extreme weather (atmospheric river flooding, heavy spring melt), retest for surface contamination
- New construction or after well repair: shock chlorination + 48-hour wait + follow-up coliform test before drinking
- A purchased filter advertises “removes bacteria” — verify NSF/ANSI 53 cyst rating and UV component before trusting it
What a positive test means in practice
| Result | Likely cause | First response |
|---|---|---|
| Total coliform: negative; E. coli: negative | Normal | Routine — retest next year |
| Total coliform: positive; E. coli: negative | Surface entry pathway (cracked cap, missing seal, recent flood) | Shock chlorinate; retest in 1–2 weeks |
| Total coliform: positive; E. coli: positive | Active fecal contamination (septic intrusion, animal waste) | Stop drinking; bottled water; shock chlorinate; investigate root cause; retest |
Common variants and disambiguation
- Total coliform — broader indicator group, includes environmental species.
- Fecal coliform — subgroup growing at 44.5°C; correlates with sewage.
- E. coli — single species; unambiguous fecal contamination indicator.
- Coliform vs. cryptosporidium / giardia. Coliform is bacteria (chlorine-killable). Cryptosporidium is a protozoan oocyst (chlorine-resistant). Different test, different treatment.
- Coliform vs. nitrates. Both can come from septic or agricultural sources, but they’re tested separately and treated differently. Nitrates need RO or anion exchange; coliform needs disinfection.
Common failure modes (causes of positive coliform)
- Cracked well cap or missing sanitary seal — surface water entry; the most common cause of total-coliform-positive / E. coli-negative results.
- Submerged pump pulling from low water table during summer — drawing from a less-protected zone.
- Old shallow dug well — often inherently positive; candidates for replacement with a modern drilled well.
- Septic field too close to well (separation distance violation) — direct contamination route. WAC requires typically a 100-foot minimum.
- First-time test on new construction before final disinfection — needs shock chlorination per WAC 246-290-451.
Cost data
- Mail-in lab coliform test (single parameter): $25–$60.
- Coliform + nitrate combined panel: $40–$80.
- Full panel (coliform + nitrate + arsenic + heavy metals): $150–$400.
- Shock chlorination (DIY): $5–$15 in unscented household bleach + $30–$60 follow-up coliform test.
- Pro shock chlorination + follow-up: $300–$800.
- Whole-house UV treatment (continuous protection): $400–$1,200 hardware + install.
Washington note
WA Department of Health recommends annual coliform + nitrate testing for all private wells. Group A public water systems (15+ connections) follow WAC 246-290 with regulated routine monitoring; Group B systems (2–14 connections or seasonal) follow WAC 246-291. Private single-family wells aren’t regulated, but mortgage lenders typically require a fresh coliform test at closing, and FHA, VA, and USDA loans have specific water-test requirements.
After a positive coliform on a private well, WAC 246-290-451 provides the framework for shock chlorination — the standard one-time disinfection procedure most WA private well owners can do themselves. Follow with a confirmatory coliform test 1–2 weeks after the chlorine has fully flushed.
For a recurring positive (multiple events in a season), the well needs root-cause investigation: cap inspection, seal check, septic-separation review. That’s well-driller territory.
FAQ
How often should I test my well for coliform?
WA Department of Health recommends annual testing for total coliform and nitrates. A full panel including arsenic, heavy metals, and other parameters can be on a 3–5 year cycle. After any well repair, flooding event, septic system work, or unexpected change in taste/smell, test sooner.
Is a coliform-positive well dangerous to drink from?
It depends on the result. Total coliform positive with E. coli negative usually means a surface-entry pathway and is treatable with shock chlorination and follow-up testing. Vulnerable people in the household (infants under 6 months, immunocompromised, elderly) should drink bottled water until the follow-up test is clean. E. coli positive means active fecal contamination — stop drinking from the well immediately.
What does shock chlorination cost on a private well?
DIY with unscented household bleach: $5–$15 in materials plus a $30–$60 follow-up test, typically $40–$80 total. Pro disinfection through a well-service contractor: $300–$800 including the test. The pro service includes proper calculation, full circulation, and well-cap inspection.