Reconstructing contamination events on riverbanks in southern Québec using dendrochronology and dendrochemical methods

The riverbanks of the Saint-François and Massawippi Rivers are affected by hydrocarbons and heavy metals to varying degrees. The events associated with this riverbank contamination are not documented in the government reports or archives, which is why dendrochemical methods are useful for dating such pollution events. Samples of alluvial soils and trees were taken from the banks of the Saint-François River in Windsor and Richmond as well as the Massawippi River, where the level of hydrocarbon (C10–C50) contamination in the banks is low to moderate (300–700 mg/kg of dry soil). Core samples were taken from seven red ash (Fraxinus pennsylvanica Marsh.) and dendrochemical analyses were done on the tree rings. Dendrochemistry was successfully used to determine that there were heavy metals in the rings, and also to estimate the date of the contamination events by using tree-ring wood. Several contamination events were identified through the presence of lead and other contaminants in the xylem of the trees analyzed at the selected sites. This suggests that various contaminants were transported by the river on several occasions and deposited on floodplains during successive floods. Also, a marked increase of some heavy metals like aluminum and nickel in the tree specimens can be attributed to recent atmospheric pollution. The emissions of metal-bearing particles may have varied widely over the century due to the different types of industrial activities, technological changes and atmospheric emission regulations. All of these anthropogenic changes reflected in the rings of the trees that showed variations in the geochemical profiles. Though several major contamination events can be determined, it is still difficult to identify all of the potential sources of pollution, of which there are many in this vast catchment.