Which of the following statements about THM sampling is correct?

The correct statement regarding THM sampling is that a sample must be taken after the water has sat for a minimum duration. This is important because trihalomethanes (THMs) can form when chlorine reacts with organic matter in the water. Allowing the water to sit for a certain period helps ensure that any THM formation that may occur during storage and distribution is accurately reflected in the sample. This practice ensures that the sampling results provide a realistic picture of the THM levels in the water system, which is critical for compliance with water quality standards and for protecting public health.

In contrast, sampling at any time regardless of water age may not yield accurate results, as THM concentrations can vary based on how long the water has been in the distribution system and its exposure to environmental conditions. The notion that only one sample is necessary overlooks the complexity of water systems where variations in quality can occur throughout the network. Additionally, while immediate analysis of samples is ideal for some parameters, THM samples typically require a specific holding time and preservation methods rather than immediate analysis, as changes can occur during transport and waiting to be analyzed.