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Devenir du Zn, Pb et Cd issus de retombées atmosphériques dans les sols, à différentes échelles d'étude. -Influence de l'usage des sols sur la distribution et la mobilité des métaux-

Abstract : The impacts of land-use on the fate of airborne metal pollutants in soils are still relatively unknown. Such impacts were examined in this work by a detailed study on the behaviour of 3 major metal pollutants (Zn, Pb, Cd) displaying a different mobility in soils. This work was performed both for surface horizons of a small region affected by industrial airborne metal fallout, and for a paired study site including two agricultural soils under long-term distinct different land use. The first soil was under permanent pasture (PP), the second under conventional arable land (CA). Both soils were located at comparable distance form the former emission source and developed under comparable pedogenetic conditions. The aim of this work was to i) quantify the proportions of endogenous and exogenous metals in soils, their distribution patterns in the soil horizons and in pedological compartments (microfabrics, aggregates, constituents) and ii) to identify the main mechanisms of metal migration and incorporation in soils. For this, detailed analyses of total concentrations of org. C, Fe, Zn, Pb, La and Cd were carried out, as well as macro- and micromorphological study, kinetic EDTA extractions, and Pb-isotopic composition studies. Ranking of soil parameters, characteristic for land use, influencing distributions and mobility of metals in soils was also aimed. Our results revealed that 40 years after cessation of metallurgical activity, the actual distribution patterns of total metal contents in surface horizons did not corroborate the distribution pattern generally predicted by simple logarithmic modelling of atmospheric deposition as a function of the distance to the emission source and dominating wind directions. The Zn, Pb, and Cd contents in the upper 30 cm of soils under different land use differed notably according to soil pH, redox potential, soil texture or biological activity. For instance, in surface horizons of acid forest soils, a loss of about 80% of deposited Zn was found, whereas about 60% of Zn was shown to have migrated out of surface A horizon, ascribed to earthworm bioturbation in permanent grassland soils, compared to "only" 40% for Ap horizons of arable land. The ratio Zn/Pb and Cd/Pb were revealed as useful indicators to distinguish typical 'metal-fate vs land use' areas. At the scale of the soil profile and soil horizons, a century of distinct different agricultural practices led to clear different phenoforms. In particular, different biological activity observed for the PP and CA soils modified biochemical cycling of metals, leading to different incorporation and mobility of metal pollutants. The two soils displayed clear different distribution patterns of exogenous metal stocks, with a marked accumulation in the Ap horizon of the CA soil (53% of Zn, 92% of Pb and Cd) whereas exogenous metals were more deeply incorporated in grassland soils (only 40% of Zn , 73% of Cd and 82% of Pb stocks at a depth <26 cm). Metal incorporation is significant until 80-cm depth in the PP soil. At the scale of soil fractions, in the PP soil, three types of soil fractions were shown to contribute to metal dynamics due to the high biological activity (aggregates of 50-100 μm and 2-20μm, and the <2-μm fraction). In the CA soil, only the 2-20μm and <2μm fractions were found predominantly implicated in metal retention and extractability. The analysis of Pb-isotopic composition performed both on soil samples and EDTA extracts, was a powerful tool for providing detailed insight on the origin and the incorporation dynamics of anthropogenic lead in soils. It gave evidence for incorporation of anthropogenic Pb until 1-m depth in the PP soil and to about 60 cm in the CA soil. This isotopic approach was found perfectly complementary to current chemical analytical approaches used for studying metal fate in soils, by validating data on estimations of exogenous metal contents. It highlighted the necessity for careful considering pedological parameters in order to guarantee representativity of metal polluted soil samples in studies and to authorize a more pertinent assessment of environmental risks based only on total metal concentrations.
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Submitted on : Tuesday, April 3, 2007 - 8:00:00 AM
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Christelle Fernandez-Cornudet. Devenir du Zn, Pb et Cd issus de retombées atmosphériques dans les sols, à différentes échelles d'étude. -Influence de l'usage des sols sur la distribution et la mobilité des métaux-. Sciences of the Universe [physics]. INAPG (AgroParisTech), 2006. English. ⟨NNT : 2006INAP0012⟩. ⟨pastel-00002321⟩



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