HEAVY-METAL DISTRIBUTION BETWEEN SOIL AGGREGATE CORE AND SURFACE FRACTIONS ALONG GRADIENTS OF DEPOSITION FROM THE ATMOSPHERE

This study tests the hypothesis that heavy metals deposited from the a tmosphere are bound preferentially to soil aggregate surfaces mainly i n forms of higher solubility than in aggregate cores. Soil aggregates (2-20 mm in diameter) were sampled along two gradients of heavy metal deposition from the atmosphere. Near Dornach/Switzerland (DOR) heavy m etal deposition originates from a brass foundry, and at the second sit e near Ziar nad Hronom/Slovakia (ZNH) from the coal-based power statio n of an aluminum smelter. The soils at the DOR-sites have a neutral to weakly acid soil reaction (pH 6-7), those at the ZNH-sites have an ac id soil reaction (pH 3-4). At DOR the 0-20 cm layer and at ZNH the org anic layer, A and B horizons of three soils with decreasing deposition al burden, were sampled. Aggregates were divided mechanically into cor e and surface fractions. We sequentially extracted seven fractions of Cd, Cu, Zn (DOR and ZNH) and Ni (ZNH) from both aggregate fractions. A t DOR the higher the depositional burden, the more pronounced is the e nrichment of heavy metals in the aggregate surface compared with the c ore fractions. At the acid ZNH-sites preferential leaching of heavy me tals from the aggregate surfaces dominates, resulting in depletions of Cd > Ni > Zn > Cu in the aggregate surface compared with the core fra ctions according to the different element mobilities. The preferential leaching is, however, compensated in part by preferential sorption of deposited heavy metals at aggregate surfaces. Thus, the higher the de positional burden, the less pronounced is the depletion. Generally, at both locations greater proportions of the total metal concentrations were found in the bioavailable metal fractions in the aggregate surfac e than in the core fractions. This indicates that deposited metals are mainly bound in forms of higher solubility. From these results implic ations for the availability of deposited heavy metals to plants and fo r leaching may arise. (C) 1998 Elsevier Science B.V.