Oil and Grease Measurement in Highway Runoff—Sampling Time and Event Mean Concentrations

Abstract

An event mean concentration (EMC), usually collected with an automatic, flow-weighted composite sampler, is often used to characterize stormwater pollutants. Automatic samplers are not recommended for collecting oil and grease (O&G) samples due to possible biases associated with interactions with tubing and pumps. To measure the EMC without sampler interferences, a series of grab samples (often over ten samples) must be collected along with the flow measurement to compute the EMC. This paper examines 22 O&G pollutographs from small, impervious highway sites, to determine when a single O&G grab sample most closely approximates a flow-weighted composite sample. Samples collected within the first hour of a storm event overestimated the O&G EMC by 20 mg∕L or more, while samples collected toward the end of the event underestimated the EMC. The best time to collect a single grab sample ranged from 1 to 6 h after the beginning of runoff, and was related to site or storm-specific factors. Results obtained from th...