Spatial and temporal patterns of metallic pollution in Québec City, Canada: Sources and hazard assessment from...

Québec City (QC, Canada) is an important urban center developed along the Saint-Charles River, at the confluence with the Saint-Lawrence River. Here, environmental issues related to pollution have been recently raised for sediments trapped upstream a dam built in the early 1970s. The major concern is about downstream transport of sediments and contaminants toward the Saint-Lawrence Estuary, a protected marine area of high socioeconomic value. This article deals with metallic contaminants in reservoir sediments collected along a longitudinal transect in the Saint-Charles River. The spatial and temporal patterns of metallic pollution have been assessed by the calculation of enrichment factors, geoaccumulation indexes, and metallic pollution index on 68 samples from a set of sediment cores and surface sediment samples. Severe to extreme pollutions are recorded with respect to silver (Ag), chromium (Cr), copper (Cu), mercury (Hg), and lead (Pb). Spatial analyses show contaminated samples are trapped in the downstream section of the river, where several point (industries, mall, harbor) and diffuse (dense urban habitat, road network) sources of pollution were evidenced using historical documents and multivariate statistics such as PCA/FA. A 50-yr sedimentary record indicates these metals were mainly delivered to the river system by the accumulation of fine-grained, organic-rich sediments during the 1970s and the 1980s. Since then, the commissioning of wastewater treatment plants in the city and environmental regulations likely played a key role to reduce the metallic yield in the Saint-Charles River. More recently, the river flow management within the reservoir favored the accumulation of much less contaminated sediments, burying the contamination. Yet, a significant environmental hazard remains if this sandy layer is removed by erosion, allowing for the remobilization and transport of contaminated sediments downstream toward the Saint-Lawrence River.