Significance of Using Different Particulate Matter Indices on Representing Particulate-Bound Metals Load and BMP Performance during Stormwater Events

The common sampling method for PM-bound metals is by collection of an aliquot or sub-sample of an original sample. However, there are a growing number of studies suggesting that aliquot/sub-sampling methods, commonly used for measuring total suspended solids (TSS) can mis-represent PM compared to whole-sample analysis; as suspended sediment concentration (SSC). In this study, the hypothesis is tested that different particulate matter indices makes a significant impact on representing particulate bound metals load and subsequently BMP performance during stormwater events. This representation was examined for unit operations with TSS and SSC methods for PM-bound metals of Cd, Cu, Pb and Zn (as aqueous, Me_p [μg/L] and solid-phase, mp [μg/g]); and with event-based mass balance errors (MBE) conducted for a hydrodynamic separator (HS). For coarser hetero-disperse particle size distributions (PSDs), results indicate that sub-sampling of TSS analysis underestimated influent Mep loads, resulting in larger MBEs across the HS as compared to events examined with SSC. Mep results measured by TSS were strongly influenced by PSDs. As PM coarseness and hetero-dispersivity increased, whole-sample analysis provided better representation of Me_p, and BMP performance for Me_p load reduction.