In what situation should grab samples be collected?

Grab samples are collected in situations where the pollutant in the sample degrades over time. This is particularly important for substances that may change chemically or biologically, making it essential to capture a snapshot of the pollutant at a specific moment. Grab sampling allows for the immediate collection of a sample that accurately reflects the concentration and characteristics of the pollutant before any potential changes occur.

In contrast, collecting grab samples during scenarios where there is turbulence or when the sample has been frozen may not provide reliable information about the actual conditions of the discharge. Turbulence can cause mixing that leads to sample variability, while freezing can alter the state of certain pollutants, possibly affecting their concentration and behavior in water. Therefore, the context of pollutant degradation underscores the necessity of timely and direct sampling to get the most representative analysis.