Which statement about grab sampling and composite sampling best describes their use?

• A. Grab sampling collects multiple samples over time to estimate average conditions.
• B. Grab sampling detects long-term trends; composite sampling detects spikes.
• C. Composite sampling is never used in regulatory programs.
• D. The best approach depends on the objective; grabs detect short-term spikes, composites assess average conditions over a period or area.

Answer: (D)

The main idea is choosing a sampling approach based on whether you need a momentary snapshot or an average over time or area. A grab sample captures the condition at a single moment, which makes it best for identifying short-term spikes or episodic events and for getting quick results after a suspected incident. A composite sample combines multiple grabs over a period or across a area to produce an average value, which smooths out variability and reflects longer-term or area-wide conditions. In regulatory programs, composites are often used to assess typical exposure over a monitoring period, while grabs are useful for detecting peaks. So the best approach depends on the objective: grab samples for short-term spikes, composite samples for averages over time or space. The other statements don’t fit because they misstate the roles or applicability of grabs and composites.