According to MCL standards, what percentage of samples can test positive for total coliform?

The correct choice reflects the regulatory standards for total coliform in drinking water as outlined by the Microbiological Contaminants Action Level (MCL) standards established by the U.S. Environmental Protection Agency (EPA). Under these standards, the allowable threshold for positive test results indicates that a specific percentage of samples is permissible if the distribution system has a high number of samples being tested over a month.

In the context of MCL standards, the requirement for total coliform bacteria is that no more than 5% of samples can test positive within a month if 40 samples are collected. This allows for some level of monitoring flexibility in larger water systems while ensuring that overall water quality remains safe.

This choice underscores the importance of consistent water quality monitoring and reflects the understanding that maintaining a zero percentage of positive samples, while ideal, may not be practical for all systems, especially those serving larger populations. Instead, it creates a threshold that aligns with routine operational variability while safeguarding public health.

The other options do not accurately reflect the accepted limits set forth by the MCL standards for total coliform, as they either set too low a threshold or are not aligned with the guidance provided for conventional water sampling protocols.