Concentrations and total mass storage of fine sediment, potentially toxic elements (PTEs) and phosphorus in the channel bed of an urban river: a multi-year study

Purpose Given the rapid growth of people living in urban and peri-urban areas, this study examined the downstream trends in concentrations and determined the total storage of potentially toxic elements (PTEs) and phosphorus (P) associated with fine-grained sediment (0.45 to 63 µm) stored in the channel bed of an urban river. Materials and methods Samples were collected from seven sites along McMillan Creek (watershed area = 55 km 2 ) in the city of Prince George, British Columbia, Canada, between 2010 and 2016 using a resuspension approach. Grabs samples were also collected from two key tributaries and a large culvert. Results and discussion Some elements (e.g., As, Mn and P) decreased in a downstream direction due to inputs from agricultural sources in the headwaters, while others (e.g., Cr, Cu, Pb and Zn) increased downstream reflecting inputs from urban sources in the lower part of the watershed. Several PTEs increased significantly due to a large culvert which supplied road-deposited sediment (RDS) and other urban materials. In some cases (e.g., As, Mn, Zn) PTE concentrations exceeded Canadian sediment quality guidelines (SQGs) for the protection of aquatic organisms. While concentrations were similar for most years, they were elevated in 2016 which may reflect higher rainfall prior to sample collection. The average total storage of fine-grained sediment in the channel bed of McMillan Creek was 155 t (4198 g m −2 ), while for PTEs this ranged from 0.01 kg (0.0002 g m −2 ) for Hg to 1130 kg (30.7 g m −2 ) for Mn. Conclusions The channel bed of an urban river stored large amounts of fine sediment, PTEs and P, with concentrations that exceeded SQGs for some elements. Given the expected increase of people living in urban and peri-urban areas, more studies are required to assess the impacts of this growth on the amount and quality of fine-grained sediment stored in urban rivers.