In the Campine region zinc and lead were being refined from the end of the 19th century until mid 1970, which resulted in a widespread soil metal pollution. Due to the sandy soil type in the region, risks of leaching and spreading of the metals is increased. During the past years several polluted agricultural soils in the region have been afforested. Trees seem very well suited to reduce the metal mobility and thus the dispersion of the metals in the ecosystem, due to their extensive root systems and high transpiration capacity. But on the other hand, tree growth might enhance metal leaching, because of soil acidification and production of dissolved organic matter. The aim of this study is to investigate the effects of 6 different tree species on Cd and Zn mobility after 10 years of tree growth on agricultural, sandy soil near a former Zn refinery. The study focuses on the processes that have the most relevant influence on metal mobility. Those are, on the one hand, decomposition of litter and roots and the resulting modifications of soil properties (biogeochemical), and evapotranspiration on the other hand (biophysical). The final output of the study will be a model that predicts leaching of Cd and Zn to the groundwater as a function of tree species.