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Biomonitoring of the air quality by the accumulation in the area of Saclay: metal elements traces in lichens
Biomonitoring of the air quality by the accumulation in the area of Saclay: metal elements traces in lichens

Saclay station

After having tested the behavior of Evernia prunastri according to the conditions of exposure and durations varying from 3 to 6 months, we undertook a new series of exposures of transplants over one year. We retained lichen Xanthoria parietina. Indeed this foliaceous lichen has capacities of accumulation 2 to 3 times higher than those of E.prunastri. In addition it adapts more easily to the zones very strongly polluted.
The transplants come from a dune zone located at some km of Boulogne Sur Mer, moved away from any source of pollution by industries. The lichens are fixed on small branches of elder tree, which facilitates transport and the transplant.
We retained three situations of exposure:
1)Driving position with exposure to the rain,
2)Sheltered driving position,
3)Sheltered horizontal position,
4)X.parietina, present in a natural way in glazing bar which borders the enclosure of transplantation taken at the same time as the transplants.
The exposure lasted a little more than one year with four taking away every fifteen days.
 
Biomonitoring of the air quality by the accumulation in the area of Saclay: metal elements traces in lichens

Fig 1 : Variation of lithophiles elements contents during the exposure time

The samples are dried then cleaned under the binocular microscope, crushed with the agate mortar.
50 mg samples are weighed exactly, then, digested in Teflon bakers on sand bath, by reagents HNO3/HF/H2O2 ultra pure.
The solutions obtained are analyzed by ICP MS (Series X7-Thermo electron) as well as a sample of lichen reference (IAEA 336) treated in the same way.
18 elements were retained Mg, Al, Ti, V, mn, Fe, Co, Cu, Zn, Rb, Sr, Zr, Cd, Sb, Pb, Ag, Th, U. To establish the initial content (base line, before exposure) these elements were proportioned in 20 taking away of lichens of Boulogne, each taking away corresponding to an individual. In this way, the homogeneity of the population collected was tested.
The preliminary results confirm part of the conclusions advanced in preceding publications. Namely that pollution is not a continuous process but which it operates by jerks and which the cumulative effect is not easy to see even over such a long period.
From a methodological point of view, we also confirm that the sheltered situation and the horizontal position offer the best chances of accumulation.
In detail, the elements considered of origin lithophile, like Al accompanied by Fe and Mg show during the analyzed period a considerable reduction recorded in the four described situations.
 
Biomonitoring of the air quality by the accumulation in the area of Saclay: metal elements traces in lichens

Fig2 : Evolution of the pollutants content during the exposure time

Vanadium, element considered as characteristic of the hydrocarbon combustion and coal (refineries, incinerators), do not show any significant variation and in the four types of taking away, with rather low values close relations of 2 ppm remains.
The lead generally associated the road traffic, in spite of some fluctuations increases gradually and can quadruple compared to its initial content. In spite of that it remains in values not exceeding 10 ppm, therefore close to those measured to start from E.prunastri. Copper follows an evolution similar to that of Pb, but the absence of correlation between these two elements excludes that they can have a common source.
During the time considered and on the basis of data exploited up to now, there was a stationary “state of pollution” even a tendency to improvement being given the least mobilization of dust, more especially as the sensor used (X.parietina ) is two to three times more sensitive than E. prunastri.

 

 

Last update : 09/28 2007 (905)

 

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