For THM samples, where should they be collected in a distribution system?

Collecting THM (trihalomethanes) samples from 25% at extreme ends of the distribution system and 75% from representative locations is essential for accurate monitoring. This method provides a comprehensive understanding of the water quality throughout the entire distribution network.

Sampling from extreme ends helps identify potential variations in water quality that may occur due to factors such as stagnation or different flow rates, which can lead to higher concentrations of THMs. Meanwhile, collecting the majority of samples from representative locations ensures that the data reflects the overall condition of the water supply, accounting for various conditions that might be present throughout the system.

This balanced approach ensures that water quality assessments provide a true representation of THM levels in the distribution system, which is critical for compliance with health regulations and for protecting public health. Other methods, such as sampling solely from treatment facilities, may not capture the potential changes and issues that can occur along the distribution system.