What types of methods are used to collect surface water samples?

The correct choice highlights two fundamental sampling techniques used in surface water quality assessment: grab samples and composite samples.

Grab sampling involves collecting a single, instantaneous sample at a specific point in time and location. This approach is useful for capturing the variability in water quality at that moment, as it can reflect contaminants or changes in conditions that might occur unexpectedly. For instance, during rain events, a grab sample can reveal spikes in pollution levels as stormwater runoff enters the water body.

Composite sampling, on the other hand, entails collecting multiple samples over time or from different locations and then mixing them to create a single sample representative of the overall conditions. This method is particularly effective for understanding average water quality over a specified period, thus smoothing out temporal variations that may be present in grab samples. Composite samples are advantageous for tracking changes in water quality due to gradual pollutant inputs.

Together, these sampling methods provide a comprehensive overview of surface water health, helping scientists and regulators monitor contamination, assess ecological impacts, and inform water management decisions.