Essai de la dendrochimie sur le Frêne de Pennsylvanie pour la datation d’événements de contamination en zone riveraine (Sud du Québec, Canada)

Dendrochemical methods are increasingly being used to reconstruct the contamination events that occurred in the past, in particular accidental leakage of pollutants into waterways or along railroads, on waste or fallow land, or in riparian forests. At times, contamination is localised whereas in other cases several sites are impacted. Noticeable amounts of pollutants are sometimes detected and have a corresponding effect on the soil, groundwater and immediate environment. In this respect, trees can act as “environmental archives” that retrace the events associated with the contamination of the sites. For the purposes of this study, dendrochemical analysis is used to attempt to reconstruct the contamination events along two rivers that have been subject to industrial and mining pollution. Sampling was performed on red ash (Fraxinus pennsylvanica Marsh.) growing on the forested banks, contaminated by hydrocarbons and heavy metals (Al, As, Cd, Cu, Ni, Pb and Zn). The dendrochemical data retraced several pollution peaks with high concentrations of metals detected in the growth rings of the trees. It is apparent on the dendrochemical graphs that some metals are on the decline, e.g. copper, whereas others are increasing, e.g. aluminium and zinc. Some of these peaks may be associated with the contamination of the floodplains by polluted water moving down the river whereas others are probably attributable to releases of air pollutants.