Small-scale distribution of airborne heavy metals and polycyclic aromatic hydrocarbons in a contaminated Slovak soil toposequence

In this study soil "pools" (horizons, aggregate and particle size separates) were identified in which airborne heavy metals and polycyclic aromatic hydrocarbons (PAHs) accumulate. Along a deposition gradient caused by an Al smelter in Central Slovakia we sampled organic layers and A horizons of a soil toposequence under beech (Fagus sylvatica L.) consisting of three Dystrochrepts. We analyzed Cd, Cu, Ni, Zn and 20 PAH in the organic layers, in bulk soil, aggregate surface and core fractions, and PAH only in particle size separates of the A horizons. The concentrations of Cd, Cu, Ni, Zn, and PAHs in the organic layers were markedly higher than those in the mineral soil (up to 1.3 mg kg(-1) Cd, 53 mg kg(-1) Cu, 32 mg kg(-1) Ni, 148 mg kg(-1) Zn, 80 mg kg(-1) PAHs). As the concentrations increased while approaching the Al smelter, this pointed to the smelter as the source. The heavy metal concentrations generally were lower in the aggregate surface than in the core fractions. This probably was caused by preferential acidification of the aggregate surfaces resulting in preferential weathering and leaching and by a higher plant uptake from the surfaces than from the cores of aggregates. The depletion decreased as distance to the Al smelter decreased. Cu and Zn were even enriched at the aggregate surfaces of the site nearest to the smelter. Thus, the depletion of lithogenic heavy metals was compensated by the preferential sorption of deposited heavy metals. Like the heavy metals, PAHs were more enriched at aggregate surfaces nearer to the smelter. In addition, the percentage of total PAHs bound to the sand fraction increased as distance to the smelter decreased. Thus, airborne pollutants are sorbed preferentially to aggregate surfaces and to the sand fraction. This may affect the bioavailability of the pollutants.