Trace metal sediment loading in the Mill Creek: A spatial and temporal analysis of vehicular pollutants in suburban waterways

Vehicular wash-off introduces toxic heavy metals into riverine environments, but the delivery patterns of such contaminants are normally difficult to observe. This study examines the spatial and temporal heterogeneity in metal loading of riverbed sediments at two river fords in the Mill Creek Watershed located in suburban Philadelphia, Pennsylvania. Riverbed sediment samples were collected over the course of the 2015 driving season (April to December) upstream and downstream of the two fords, which greatly differ in traffic density, and analyzed for their trace metal concentrations (As, Cd, Cu, Ni, Pb, Zn) and percent particulate organic carbon (%POC). Overall, metal concentrations were in the upper range of literature values for road-derived sediments. Comparing the sites, the more heavily trafficked ford had significantly higher concentrations, confirming the role of traffic density on metal accumulation. Highly enriched concentrations of Cu and Zn coupled with diagnostic metal:Cu ratios points to the strong influence of brake lining dust. While an increase in metal concentration was observed over the driving season, the highly variable spatial and temporal signal suggests other factors such as local geomorphological and hydrological conditions also play a role in metal accumulation and dispersal. Metal concentrations in excess of levels known to impact aquatic ecosystems, coupled with strong correlations between metal concentrations and the %POC content of the sediments, suggest vehicular-derived metals may play a role in the historically observed decrease in macroinvertebrate diversity in the watershed. These findings add to a growing body of literature highlighting the potential for vehicular and road sourced metals to persist and potentially impact riverine environments. (C) 2017 Elsevier Ltd. All rights reserved.