Coliform in Well Water: What to Do After a Positive Test

First: E. coli, or total coliforms only?

Total coliforms live in soil, on plants and in feces; finding them means surface water may be reaching your well^1,2. E. coli comes from human and animal feces¹. Penn State puts it plainly: a positive E. coli result is much more serious than coliforms alone³.

Your result · what agencies say to do now

Result	What it means	What to do now
E. coli present (1 or more)	Human or animal waste is reaching the well2	Stop drinking it untreated: boil or use another safe source until tests show it is safe1. Contact your public health unit as soon as possible2
Total coliforms only	The water “might not be safe to drink”; more information is needed1	Agencies differ (see below). Retest as soon as possible1
Total coliforms, Ontario lab, more than 5	Significant bacterial contamination2	Stop using the well water, contact public health, resample2
Total coliforms, Ontario lab, 5 or less, no E. coli	No significant contamination2	Keep testing regularly2

Where agencies disagree: total coliforms only. Health Canada says you can continue to use your water while you wait for further results¹. Minnesota says to use boiled, bottled or other safe water for drinking and cooking until you address the issue⁴, and Alberta says not to drink it without boiling it first⁵. Our take: boil until the retests come back clean, especially if a baby, an older adult, someone pregnant or someone with a weak immune system lives in the house. These are the groups Minnesota lists as more at risk⁴.

Water you can use in the meantime

• Boil it for 1 minute at a rolling boil^6,7. At high elevation, boil longer: British Columbia says 2 minutes above 2,000 m (6,500 ft)⁸, the US EPA 3 minutes above 5,000 ft⁹.
• Not if nitrate or arsenic were also found. Boiling can concentrate them; use bottled water⁴.
• Babies and children. While E. coli is present, Québec advises boiled water for ice, baby drinks and food, washing raw food and brushing teeth. Adults can shower while avoiding swallowing the water; give babies and children sponge baths⁷.

If someone in the house has stomach illness that may come from the well, Health Canada says to talk to your doctor and local health unit¹.

The steps, in order

1. Retest. Health Canada says to test a second sample as soon as possible. If it is clean, test a third to confirm. If the second or third confirms contamination, shock disinfect the well and fix the cause¹. Penn State suggests asking the lab for a counted result rather than present or absent³.
2. Find how the bacteria get in. Use the checklist below. Don’t skip this step: shock chlorination kills the bacteria but does not close the path they used.
3. Fix it, then shock chlorinate. Our shock chlorination guide has the doses and steps.
4. Retest after shocking. Health Canada considers the treatment probably successful after 2 safe tests in a row over 1 to 3 weeks, with one more test 3 to 4 months later⁶.
5. If it comes back, add continuous disinfection. Health Canada suggests treatment when the problem is recurrent or you can’t identify or remove the source⁶.

Finding the cause

Health Canada’s yearly well check¹⁰:

• the well cap is securely in place and watertight, with no cracks;
• the vent is screened and not blocked;
• the ground slopes away from the well, with no pooling or settling around the casing;
• a trimmed grassy area of at least 3 metres (10 ft) around the well, with nothing else in it;
• no gaps around the casing, and no leaky connections between the well and the house.

Alberta adds the simplest cause of all: a sample that was not collected properly⁵. Higher-risk wells, according to Minnesota, include dug wells with leaking walls, wells with corroded casing, wells at the bottom of old frost pits, and wells too close to a septic system or feedlot⁴.

How close is too close? Rules for new wells set minimum distances to septic systems:

Minimum distance between a new well and a septic system

Where	Septic tank	Leaching bed / drain field
Ontario	15 m (50 ft)	15 m with watertight casing at least 6 m deep; otherwise 30 m (100 ft)11
British Columbia	15 m from a holding tank	30 m from a sewerage system12
Minnesota	50 ft	50 ft, doubled for a well with less than 50 ft of watertight casing13

If it comes back: continuous disinfection

The two usual options are continuous chlorination and UV light. Penn State notes that UV adds no chemical but leaves no residual: it kills bacteria only inside the unit³.

For a well with bacteria, the UV unit must be certified to NSF/ANSI 55 Class A. Class B units deliver a lower dose and are only for water that is already safe; Ohio’s health department notes that such units can clear the total coliform indicator without being enough for many other pathogens¹⁴. Penn State also advises against UV when total coliforms exceed 1,000 or fecal coliforms 100 per 100 mL³.

UV units certified NSF/ANSI 55 Class A in our catalogue

Model	Class A flow	Maker
LUMINOR BlackComb (LB6-15XA)	7.9 gpm	LUMINOR, Guelph, ON
VIQUA PRO10	10 gpm	VIQUA, Guelph, ON
UV Pure Hallett 500PN	16.5 gpm	UV Pure, Toronto, ON

UV needs clear water to work, so most wells need a filter and sometimes a softener ahead of it. Our UV systems guide covers sizing, pretreatment limits and upkeep.