Total coliform, E. coli, and microbial contamination in drinking water

Why we test for coliform bacteria

Total coliforms are a broad family of bacteria found in soil, vegetation, and the intestines of warm-blooded animals. Most species do not themselves cause disease. They are tested in drinking water as an indicator: their presence means the water has come into contact with an environmental source that could also harbor disease-causing organisms. The most consequential of those is fecal contamination.

The hierarchy:

• Total coliform: presence indicates a general breach (treatment failure, distribution-system contamination, sampling-tap contamination, or simply a positive sample-handling artifact). Triggers investigation and follow-up sampling.
• E. coli: presence indicates fecal contamination specifically. Treated as an acute, immediate-action violation. Utilities must issue a Tier 1 public notification (usually a boil-water advisory) within 24 hours.

What the rules require

Under the Revised Total Coliform Rule (RTCR, effective 2016):

• Utilities sample for total coliform on a schedule based on population served, ranging from monthly to annually.
• Any positive triggers same-day repeat sampling at the same and adjacent locations, plus an investigation of the cause.
• An E. coli positive triggers a Tier 1 public notification within 24 hours.

The Ground Water Rule (2006) and Surface Water Treatment Rule (1989, updated multiple times) layer additional requirements specific to source-water type. Surface-water utilities must filter and disinfect; the rule that drove this was the 1993 Milwaukee Cryptosporidium outbreak.

Pathogens of public-health concern

What coliform monitoring is designed to catch indirectly:

• Cryptosporidium: chlorine-resistant; the 1993 Milwaukee outbreak sickened ~400,000 people and reshaped U.S. drinking-water rules. Removed by adequate filtration.
• Giardia: similar story to Crypto, also chlorine-resistant.
• Legionella: more often a plumbing-system biofilm issue than source-water contamination. Regulated separately under the Revised Total Coliform Rule’s distribution-system maintenance requirements.
• Pathogenic E. coli strains: O157:H7 and others. These are the fecal-contamination scenarios coliform monitoring catches.
• Viruses: norovirus, hepatitis A, rotavirus. Disinfection (free chlorine, UV, ozone) is the primary control.

Boil-water advisories

When your utility issues one:

• Bring water to a rolling boil for 1 minute (3 minutes at altitudes above 6,500 feet). CDC’s standard guidance.
• Boiled water is safe for drinking, cooking, brushing teeth, and washing produce.
• Cold tap water (or treated bottled water) is fine for hand-washing.
• Continue boiling until the utility lifts the advisory and confirms two consecutive clean samples.

Private wells are not covered

Private wells are not regulated under SDWA microbial rules. CDC and most state health departments recommend annual coliform and E. coli testing for any household well. Treatment for contaminated private wells:

• Disinfection: chlorination or UV
• Source-water protection: proper well casing, distance from septic systems, surface drainage

If a private well tests positive for E. coli, do not drink it without treatment.

Sources

• US EPA, “Revised Total Coliform Rule”: https://www.epa.gov/dwreginfo/revised-total-coliform-rule-and-total-coliform-rule
• CDC, “Making Water Safe in an Emergency”: https://www.cdc.gov/healthywater/emergency/drinking/making-water-safe.html
• US EPA, “Drinking Water Regulations and Contaminants”: https://www.epa.gov/dwreginfo/drinking-water-regulations-and-contaminants
• CDC, “Drinking Water from Private Wells”: https://www.cdc.gov/healthywater/drinking/private/wells/