The number of monthly distribution system chlorine residual samples required is based on:

The correct answer relates to population, which is a critical factor in determining the number of monthly distribution system chlorine residual samples required. In drinking water systems, the sampling frequency is designed to ensure water quality and safety for consumers. As the population served by the water system increases, the potential for variations in water quality also increases, necessitating more frequent sampling to monitor chlorine residual levels effectively.

The regulations that govern water quality often stipulate that larger systems, which typically serve a greater number of people, need to conduct more frequent monitoring to ensure that there is adequate disinfection throughout the distribution system. This is because maintaining proper chlorine residual levels is essential for preventing microbial contamination and ensuring safe drinking water.

Other factors like the water withdrawal permit limit or system size might influence operational practices or compliance but do not directly determine the sampling frequency as explicitly as population does. Additionally, while different states may have varying regulations regarding sampling and monitoring requirements, the fundamental guideline linking sampling frequency to population is a common regulatory framework adopted across many jurisdictions.