When sampling for which substance should a composite sample never be used?

A composite sample is a method where multiple individual samples are collected over a specific period or location and then combined into one sample for analysis. This approach is often beneficial for assessing average concentrations of various substances that might fluctuate over time.

However, when it comes to sampling for bacteria, composite sampling is not appropriate. This is largely because bacteria can be highly variable; their concentrations can change drastically from one sample to the next, often in very short time frames. Additionally, the presence of bacteria can vary significantly depending on environmental conditions, time of day, and other factors. This variability means that a single composite sample would not accurately represent the true concentration of bacteria in the water, leading to potentially misleading results.

In contrast, for substances such as heavy metals, nutrients, or chlorine, composite sampling is more acceptable because these substances tend to be more stable and their concentrations can be averaged over time, providing a more reliable overall measure. Therefore, when it comes to ensuring the safety and quality of drinking water, particularly regarding microbial contamination, discrete sampling methods (collecting individual samples at specific times) are preferred.