Downstream Distribution and Postdepositional Mobilization of Cadmium in Alluvial Soils

The geochemical signature in alluvial soils is a witness of human activities that took place in a river catchment. Sampling of alluvial soils at depth, in combination with information on sedimentological history and age of samples, may even allow to reconstruct the pollution history of the river basin. In the present study, data on alluvial soils contaminated by a major pollution source were analyzed, with special attention for these soils as an archive for information on the pollution history of a river/river catchment, and on the postdepositional downward migration of metal(loid)s in the alluvial soils. Besides the lateral variation of soil properties and metal(loid) concentrations in the alluvial soils, the vertical distribution of metal(loid)s in soil profiles, as well as the evolution of soil composition in relation to the distance from the river, was addressed. The postdepositional mobilization of Cd was evaluated in a fine-scale sampled alluvial soil core, by comparing data from 137Cs dating with data about the Cd emissions through time and by using leaching tests to calculate the downward migration of Cd. A substantial amount of Cd could leach from superficial to deeper soil layers. Therefore, the low-resolution (cm-scale) sampling of the alluvial soil was not reliable to reconstruct the pollution history of the river catchment, because the elevated chloride-concentrations in the river water increased the downward leaching of Cd through the formation of chloro-complexes. Moreover, the variability in flooding and sedimentation regimes along the river resulted in a heterogeneous composition of the alluvial soils, allowing very large differences in metal(loid) concentrations in places only a few meters apart.