Highlights Flow-paced composite sampling (FPCS) is widely used to measure pollutant loads, especially in stormwater. We used high frequency nitrate, DOC, and TSS concentration data to simulate and assess FPCS strategies. Upgrades to minimize uncertainties on event loads include (1) avoiding pumping aliquots before the first flow interval, (2) sampling 90% of an event, and (3) filling the composite bottle to at least 30% of full capacity. ABSTRACT. Flow-paced composite sampling (FPCS) is widely used in stormwater quality monitoring for its practicality, cost-efficiency, and ability to reflect event concentrations and loads. Existing guidelines help configure FPCS protocols that minimize measurement uncertainty, and prior studies have evaluated its accuracy. However, these evaluations have been typically based on discrete sampling or modeled pollutographs, which lack the temporal resolution of hydraulic measurements and can fail to capture actual pollutant dynamics during storm events. This loss of resolution is largely driven by limitations of the automated sampling equipment’s capacity. In this study, high-frequency (4-minute) water quality data from UV-Vis spectrophotometers were used to simulate thousands of FPCS scenarios and quantify uncertainty introduced by different sampling configurations. The simulations and literature review identified three primary sources of uncertainty: (1) percentage of event flow volume captured, (2) number of samples composited per event, and (3) timing of the first sample. Results show that to achieve ±10% accuracy with 90% confidence, of 100 samples composited in a jar, a minimum of 9 samples for nitrate-N, 13 for DOC, and 20 for TSS would be required. For a stricter ±5% accuracy target, 19 samples for nitrate-N, 22 for DOC, and ~29 for TSS would be needed. Sampling less than 85%–90% of the event flow volume resulted in systematic under- or overestimation depending on pollutant dynamics. Inclusion of a first flush sample taken prior to completion of the first flow interval, which therefore is not a flow-paced sample, introduced very large errors (100%) and should be avoided especially for multi-peak events. This study provides an upgraded framework for configuring autosamplers based on expected runoff and capacity constraints, and introduces a post-event diagnostic tool to assess data quality, improving the reliability of FPCS-based stormwater pollutant monitoring. Keywords: Event mean concentration, Flow paced composite sampling, Stormwater, Uncertainties, Updated guidelines.
Moin et al. (Thu,) studied this question.