Urban runoff and flocculated sediment quality in the South Saskatchewan River

Abstract

Urban runoff poses a threat to water quality. Some contaminants in urban runoff, such as heavy metals, are preferentially bound to fine-grained sediment. Fine-grained sediment in fluvial systems is transported as composite particles known as "floes". Floes are deposited as a thin layer of surficial fine-grained laminae (SFGL) during periods of low flow velocity and shear stress. SFGL deposits have been identified as a possible in-channel sink and source of fine-grained sediment and associated contaminants that may be useful in determining anthropogenic impacts on rivers. The objective of this study was to examine changes in the physical and chemical characteristics of SFGL in response to urban runoff, and to assess the applicability of SFGL to water and sediment quality studies. SFGL samples were collected from the South Saskatchewan River, upstream and downstream of the city of Saskatoon, before, during and after rainstorms. The sediment was analyzed for trace element composition, organic carbon content and particle size distribution to characterize the physical and chemical nature of SFGL. Fine-grained storm water outflow sediment was also collected to characterize urban runoff inputs to the river. The physical characteristics of SFGL varied with season and suspended sediment concentration. Variations in the trace element chemistry of SFGL were due to changes in the physical composition of sediment in terms of % silt and clay and the chemical characteristics of the sediment in terms of % TOC (total organic carbon), and were not directly attributed to changing hydrologic conditions such as discharge, rainfall and runoff. As, Cr, Cu and Ni concentrations in SFGL exceeded sediment quality guidelines. The downstream decrease in trace element concentrations of SFGL may be attributed to dilution by uncontaminated sediment, or the presence of a sediment sink upstream of the downstream sampling site. Fine-grained sediment carried by urban runoff and discharged to the river at storm water outflows was a source for Cu, Zn, Pb and Sn. Nevertheless, there may not have been a sufficient quantity of this contaminantenriched sediment to have an impact on sediment-associated trace element concentrations downstream of the city.