SEGH Articles

An historical reconstruction of atmospheric heavy metals deposition from a peat bog record on the North Shore of the St. Lawrence Estuary, Quebec

01 October 2013
Peat bogs were used to reconstruct the history of atmospheric heavy metal deposition along the St. Lawrence Valley. Results from one of the study sites were presented at the 29th SEGH held in July 2013, Toulouse.

Steve Pratte is currently a Ph.D. student at the Department of Earth and Atmospheric Sciences of Université du Québec à Montréal (UQAM, Canada) and the National Polytechnical Institute of Toulouse (INPT, France). The research presented at the 29th SEGH Conference in Toulouse won the Hemphill prize for best poster presentation in July.  The research was carried out during his Master’s degree in Earth and Planetary Sciences at McGill University in Montreal, under the supervision of Dr. Alfonso Mucci and Dr. Michelle Garneau.

Human activities, especially since the Industrial Revolution, have left a legacy of trace metal contamination that is potentially harmful for natural ecosystems and human health (e.g. As, Cd, Pb) and affected their geochemical cycles. Atmospheric metal pollution is recorded in different environmental archives such as lake and marine sediments, snow and ice and peat bogs. Among these archives, peat bogs have proven to be effective in reconstructing the history of atmospheric metal deposition throughout Europe, but few studies have been carried out in North America or in Quebec. Being an important natural wind corridor, oriented from south-west to north-east, the St. Lawrence Valley is affected by long-range transport of contaminants.

The present study focuses on the reconstruction of the history of atmospheric As, Cd, Ni, Pb and Zn deposition in surface cores (<100 cm) from three peat bogs along the St. Lawrence Valley (Fig.1). Core chronologies were established using 210Pb for the upper horizons and 14C dating for the deeper sections. Metal accumulation rates were computed from measured concentrations and core chronologies. Stable lead isotopes (204, 206, 207 and 208) were also analysed to distinguish natural and anthropogenic sources of Pb. Arsenic, cadmium, lead and stable lead isotopes results from one of the study sites (Baie bog) were presented at the 29th SEGH conference.

Metal accumulation rates (AR) and concentrations start increasing from the beginning to mid-19th century and increase more sharply from early 20th century. At the same time, Pb isotopic values diminish from 1850 AD probably from deposition of coal burning particle, and stabilise from the 1920’s likely due to contributions from leaded gasolines. Lead accumulations rates peak in 1951 AD, which is earlier than other studies undertaken in the region. Maximum Pb AR (24 mg m-2 yr-1) are in good agreement with other studies, while As and Cd AR are much lower than accumulation rates obtained in the southwestern part of the St. Lawrence Valley. This is likely explainable by the more remote location of the site which allow more particles to settle before reaching the site. This is also reflected in lead isotope values which fall closer to Canadian aerosols values, the site further away from the US Mid-west, receives proportionally more contributions from Canadian leaded gasolines. A sharp decrease in metal accumulation rates and concentrations from the mid-60’s and increase in Pb isotopic ratios from the mid-1970’s is observed, which reflect the phasing out of leaded gasoline and the implementation of other mitigation policies (i.e. Clean Air Act). However, values are still an order of magnitude higher than pre-industrial values and other less radiogenic sources of Pb must be invoked (likely coal consumption and smelting activities) to explain the recent decrease in isotopic values.

Study site locations

In short, the Baie bog recorded the main trends in industrial activities since the Industrial Revolution. The site receives more pollution from Canadian than US sources in reason of its greater distance from the main industrial and urban sources. Mitigation policies (phasing-out of leaded gasoline, Clean Air Act) have been effective in reducing metal emissions and deposition in the environment. Nevertheless, other sources than leaded gasolines are still contributing to Pb and other metal emissions.

Link to an article in Atmospheric Environment arising from this study.

Steve Pratte

Department of Earth and Atmospheric Sciences of Université du Québec à Montréal (UQAM, Canada) and the National Polytechnical Institute of Toulouse (INPT, France).

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