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

The key difference between a first draw sample and a grab sample lies in the nature of what each sample represents and how they are collected.

A first draw sample is specifically designed to capture the water that has been sitting in the plumbing system, particularly after a period of stagnation. This means that it reflects the quality of water that is present in the pipes, potentially including contaminants that may leach from the plumbing materials or from biofilms or sediments that may have accumulated when the water has not been in continuous motion. It is vital for assessing potential risks related to lead or other contaminants that may leach into the water when the system is not actively in use.

On the other hand, a grab sample does not specifically account for water stagnation. It is typically collected at one point in time and can reflect the water quality as it flows from the tap, capturing a representative sample of water at that moment. However, this sample may not represent the potential contaminants present in standing water within the plumbing system.

Thus, the correct answer emphasizes the significance of the first draw sample in representing water that has been standing in the plumbing, which is critical for assessing certain water quality parameters that could pose health risks.