Where total coliform comes from
- surface water infiltration through shallow or poorly cased wells
- nearby septic systems or leach fields
- agricultural runoff and manure application
- wildlife fecal contamination of wellhead area
- failed or cracked well casing or grouting
- flooding events contaminating wellhead
Health considerations
- indicator of fecal contamination pathway — Total coliform bacteria are not themselves harmful at typical concentrations, but their presence in a well signals a pathway for fecal pathogens. EPA MCL: no more than 5% of monthly samples may be total coliform positive (public systems). Private well standard: should be absent (0 CFU/100 mL).
- gastrointestinal illness (if fecal pathogens present) — Total coliform-positive wells carry elevated risk of harboring Giardia, Cryptosporidium, norovirus, or pathogenic E. coli. A positive total coliform result should trigger follow-up E. coli testing and well inspection.
EPA MCLG = 0 for total coliform (public water systems). For private wells, no regulatory standard exists, but the public health guidance is absence of total coliform. A positive result should be followed immediately by E. coli testing — the presence of total coliform without E. coli still warrants investigation and likely well disinfection.
Who is most at risk
- infants and young children
- elderly adults
- immunocompromised individuals
- pregnant women
- households within 100 feet of a septic system or livestock operation
Testing
EPA Method 9222B (membrane filtration), IDEXX Colilert presence/absence or MPN method; results reported as CFU/100 mL or present/absent; certified lab required; sample must be collected in sterile container with sodium thiosulfate preservative
Find a certified lab and learn how to collect a sample
Treatment
- UV ultraviolet disinfection (whole-house, NSF/ANSI 55 certified)
- continuous chlorination (chemical feed pump + contact tank)
- shock chlorination (one-time well treatment for episodic contamination)
- well casing inspection and repair (address root cause)
- reverse osmosis (point-of-use for drinking water only)