Use of inventories of natural and artificial radionuclides in soils to estimate total aerosols deposition and origin in French mountains

Titre du congrès :21ème RST (Réunion des Sciences de la Terre)
Ville du congrès :Dijon
Date du congrès :04/12/2006

  Transport and deposition of aerosols are essential factors in climatic and environmental changes. Deposition of aerosols carrying contaminants mainly governs soil acidification and pollution. Most of the acid, heavy metals and anthropogenic radionuclides are delivered to the ground by wet deposition. However other mechanisms like dry and occult depositions can be essential, especially in forested and/or mountainous areas.
We use inventories of 210Pb, a natural radionuclide and artificial radionuclides (137Cs and Pu) in undisturbed soils to quantify mechanisms influencing aerosol deposition in different French mountains. First, radionuclide and aerosol inventories are correlated with precipitations estimated by the Model AURELHY of Météo France: either annual rainfall (210Pb, 137Csnuclear tests) or precipitation of may 1986 (137CsChernobyl).
However, orographic clouds induce increased deposition on the mountain peaks. For example, high altitude sites show a >30% increase in 210Pb inventories compared to 210Pb-rainfall relationship established in the surrounding lowlands. Because artificial radionuclides and 210Pb have different past and present vertical distributions in the troposphere, we use 137Cs/210Pb and Pu/210Pb ratios to estimate occult deposition and aerosol origins. For example, 210Pb/239-240Pu ratios of the soil inventories are increasing with altitude (Fig.1). We will show that is due to larger deposition of surface-air aerosols in mountainous areas.
Occult deposition increases with altitude mainly due to cloud interception and the feeder-seeder effect scavenging aerosols from the surface air enriched in 210Pb.
Radionuclide inventories in undisturbed soils are therefore good estimators of total deposition of aerosols. Radionuclide ratios help to determine the origin of the aerosols. The combination of radionuclides inventories and ratios can therefore be used to assess the sensitivity of the different French mountains to local/global and discrete/continue atmospheric contaminations.