What is the primary difference between grab samples and composite samples for surface water monitoring?

The primary difference between grab samples and composite samples for surface water monitoring is that a grab sample represents a "snapshot" of water quality at a specific point in time and location, whereas a composite sample represents the average water quality over a defined period or across multiple locations. A grab sample is a single discrete sample collected at one specific time and place. It reflects the water quality conditions at that precise moment. A composite sample, on the other hand, is created by combining multiple grab samples collected either at the same location over a period of time (time-weighted composite) or at different locations at the same time (flow-weighted composite). Composite samples provide a more representative measure of average water quality, reducing the influence of short-term fluctuations or localized variations. For example, if monitoring a stream discharge, a grab sample might capture a temporary spike in pollutant concentration, while a composite sample taken over 24 hours would provide a better indication of the average pollutant load discharged during that day. Grab samples are useful for detecting acute pollution events, while composite samples are better for assessing overall water quality and long-term trends.