What is a key difference between a first draw sample and a grab sample?

A first draw sample is specifically intended to capture the very first portion of water that comes from a tap after a period of stagnation, typically several hours, in order to assess the concentration of contaminants that may leach from pipes or plumbing fixtures into the water supply. This method is particularly important when evaluating potential lead levels or other heavy metals that may accumulate when water sits stagnant in the plumbing.

The notion that the sample is taken after a period of stagnation is critical—usually, a duration of six hours is recommended. This allows for sufficient time for any contaminants that may have leached into the water from the pipes to accumulate, ensuring that the sample accurately reflects the potential exposure levels for consumers.

In contrast, grab samples are typically taken without any specific stagnation period and do not necessarily follow the same protocol as first draw samples. They can be collected anytime and are meant to provide a snapshot of the water quality at that moment, which may not account for contaminants that have settled over time. Hence, the key difference in the sampling approach highlights the distinct purposes each method serves in water quality analysis.