Grab vs. Composite Sampling
Choosing the right method matters as much as running the test correctly once the sample arrives.
Grab samples capture a single point in time and are required for parameters that change quickly or degrade during compositing, such as pH, dissolved oxygen, temperature, and residual chlorine.
Composite samples, collected incrementally over a period (commonly 24 hours) proportional to flow, better represent average conditions for parameters like BOD, TSS, and most permit-reported values.
Flow-proportional compositing (more sample volume collected during high-flow periods) gives a more representative result than simple time-proportional compositing when flow varies significantly over the sampling period.
Using a grab sample where a permit requires a composite, or vice versa, is a documentation and compliance error independent of whether the underlying water quality result would have differed.
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