Sampling Issues in Urban Runoff Monitoring Programs: Composite versus Grab

Abstract

Storm-water monitoring generally uses flow-weighted automatic composite samplers to collect a representative sample of an entire storm event. Automatic samplers are convenient but unfortunately they can be expensive, especially for temporary sampling needs or for short-term research projects. An alternative method is to use a series of grab samples. This paper examines the accuracy of event mean concentrations (EMCs) and mass first flush ratios calculated from a finite number of grab samples, and compares them to results from flow-weighted automatic samples. Both sampling techniques were evaluated using data collected from a three-year investigation of three highway sites. A large number of grab samples is needed to approach the accuracy and precision of flow weighted composite samples, and 30 grab samples per storm event generally estimated the EMCs within 20% average error. To detect a first flush, it is necessary to take even more grab samples or to adjust the timing of the sample collection toward the beginning of the storm. The superiority of automatic sampling for estimating EMCs for constituents compatible with automatic sampling is demonstrated.