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Impact radioécologique des retombées de poussières sahariennes Episode majeur du 21/02/2004 dans le sud de la France

Rapport DEI/SESURE n°2005-04

Summary

Lithometeors, Sirocco or more commonly “red mud” are all in fact related to a single phenomenon which affects France every year: the wind transport and deposit of desert particles from the Sahara.

On the 21st of February 2004, the southern part of France is swept by a weather event of wind transport of Saharan particles. The recordings of atmospheric dust contamination and the deposit of dust, which results from it, make an episode of exceptional width. In a few hours, the thickness of the deposit exceeds 1 mm (up to 4 mm in Corsica) with a maximum density of surface charge of 50 g.m-2 (50 tons per km2). The loads of the PM10[1] type particles in the air, recorded by associations of monitoring of the quality of the air, indicate concentrations multiplied to the maximum by 10 and an influence on the ground of the plume ranging between 300 000 and 350 000 km2. To the end, 2 million tons are deposited on a portion of the territory located at the south of a line from Nantes to Besançon.
This event also had a significant radio-ecological impact, leading to significant 137Cs, (239+240)Pu, 241Am, activity levels of 38 Bq.kg-1 sec, 1 Bq.kg-1 sec and 0,46 Bq.kg-1 sec, respectively. Quality of air monitoring organisations recorded 10-fold increases in the concentration of charged PM10 type particles within the cloud; ground coverage stretched over a 300 000 km2 surface area. Across this whole area, the artificial radioactivity deposits are estimated to 37.1010 Bq. In term of flow of deposit, this episode represents, with him only, i.e. in a few hours, a 137Cs deposition equivalent to that recorded on average in a cumulated time of one year. Data from this study show that these weather-climatic episodes generate today, environmental samples which on average, present the highest levels and flux of artificial radioactivities, more than those in the sediments of the Rhone river deposited by flood events, for example.

Changes in artificial radionuclide activity levels in the atmospheric area close to ground level are routinely attributed to resuspension of formerly deposited aerosols. In the particular case of the Saharan dust deposits, apart from the resuspension mechanisms that caused the Saharan soil particles to be taken up, other mechanisms may have intervened during transports up until deposit on French soil. Such mechanisms, which have already been identified for other atmospheric compounds or pollutants, give a lead in attempting to understand the enhanced relative or absolute radioactivity of the mineral particles. Two hypotheses have been put forward to explain this enhanced radioactivity: either a process rather like a ‘’horizontal leaching’’ of compounds or pollutants present in the atmosphere during transport, or the early fall of the heavier and thus less radioactive mineral particles, giving rise to a relative increase in specific activity.
One cannot overlook the contribution these sporadic phenomena make to atmospheric deposits on an annual basis. Further studies are needed to evaluate the weight of these phenomena on a multiannual scale in the reduction of activity levels in the atmospheric compartment by dry or wet process and at short time scale the deposition of radioactivity with dust from remote regions.

Taking into account current metrological performance devices at the IRSN (Institute for Radiological Protection and Nuclear Safety), regular monitoring of these events allow opportunity to follow the changes of plutonium isotopes in the atmosphere.


[1] Particulate matter with aerodynamic diameter less or equal to 10 microns (breathable particles)


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