When should grab samples be collected?

Grab samples are a type of sampling method that collects a single, instantaneous sample of a substance, providing a snapshot of the characteristic of the water, effluent, or other material at a particular point in time. This method is particularly useful for specific regulatory compliance purposes.

When an effluent limit is defined as an instantaneous maximum, it means that the concentration of a particular pollutant should not exceed a certain level at any given time. Therefore, grab samples should be collected at the moments when the discharge is occurring, specifically during those instances where there is a concern that the concentration of a pollutant may spike above the regulatory limit. This allows for immediate assessment and compliance with the environmental standards set in place, aiming to minimize negative impacts on the surrounding ecosystem.

In contrast, while it may be relevant to consider the timing of samples during heterogeneous discharges or periods of pollutant increases, those scenarios do not specifically address the regulatory requirements tied to instantaneous maximum limits. Therefore, the focus on collecting grab samples refers directly to ensuring compliance with instantaneous maximum thresholds.