The safety of public drinking water relies on routine testing for microbial contaminants. Testing focuses heavily on coliform bacteria, a broad group of microbes found naturally across the environment. While many strains are harmless, their presence in treated water signals a breakdown in the system’s ability to keep the water clean. A specific subgroup is found in the intestines of warm-blooded animals, making them a marker for fecal contamination. Understanding acceptable limits is fundamental to protecting public health.
The Zero-Tolerance Standard for Drinking Water
The acceptable level of coliform bacteria in public drinking water is defined by the Revised Total Coliform Rule (RTCR) in the United States. This rule establishes that the Maximum Contaminant Level Goal (MCLG) for Escherichia coli (E. coli) is zero. This zero-tolerance approach means no sample of drinking water, measured in a standard 100 milliliter (mL) sample, should contain this specific bacterium.
Detecting E. coli is an acute violation, immediately signaling a failure in the water treatment or distribution system and indicating fecal contamination. While E. coli is the most severe indicator, the regulation also monitors the broader category of Total Coliforms. For large public water systems collecting 40 or more samples monthly, the maximum contaminant level (MCL) for Total Coliforms is exceeded if more than 5.0 percent of samples test positive.
If a system collects fewer than 40 samples per month, the limit is exceeded if more than one sample tests positive for Total Coliforms. Total Coliforms alone are not a direct health threat, but their presence triggers a required assessment of the system’s integrity, such as checking for cracks or treatment deficiencies. However, any positive sample for E. coli or Fecal Coliforms requires immediate, mandatory action due to the risk of sewage-related pathogens.
Why Coliforms Are Used as Contamination Indicators
Coliform bacteria are used as indicator organisms because directly testing for every possible pathogen would be too complex, time-consuming, and expensive. These indicators are present in higher numbers than the pathogens they signal, and they are easier and quicker to detect. Their behavior in water treatment processes is similar to that of many harmful microbes, making them an effective surrogate.
Total Coliforms are a general indicator of the water system’s overall sanitary quality and the effectiveness of the treatment process. Their presence suggests conditions are favorable for bacterial growth or that surface contamination has entered the distribution system through a leak or breach. This group includes bacteria that naturally live in soil, on vegetation, and in surface water, meaning their source is not always fecal matter.
Fecal Coliforms are a specific subgroup that originate from the intestines of warm-blooded animals. E. coli is the most reliable indicator of recent fecal pollution. Detecting E. coli provides a clear warning that human or animal waste has entered the drinking water supply, and this waste contains other dangerous pathogens.
Health Risks and Public Water System Response
The health risk associated with coliform detection stems from the disease-causing pathogens whose presence is indicated by the coliforms, not the bacteria themselves. When E. coli is detected, it signals that the water is contaminated with feces, which can carry viruses, parasites, and bacteria like Giardia or Cryptosporidium. These organisms cause acute gastrointestinal illness, leading to symptoms such as diarrhea, vomiting, abdominal cramps, and fever.
Infants, the elderly, and individuals with compromised immune systems are at increased risk of severe illness or death from consuming contaminated water. The presence of E. coli necessitates an immediate, mandatory response from public water systems. This response begins with rapid public notification, often a “boil water advisory.”
A boil water advisory instructs consumers to boil all water vigorously for at least one minute before using it for drinking, cooking, brushing teeth, or washing dishes. Concurrently, the water system must begin corrective actions, including taking the contaminated source, such as a well, immediately offline. The system must conduct a thorough investigation to identify the source of contamination, checking for leaks in distribution lines or issues with the water source.
Remediation steps involve increasing the level of disinfectant, such as chlorine, in the water supply and flushing the water mains to remove the contamination. If contamination is traced to a structural issue, such as a damaged wellhead or a failing septic system, these issues must be repaired or eliminated. The system must continue testing until multiple follow-up samples confirm the water is free of E. coli and Total Coliforms before the boil water advisory can be lifted.