Localised mobilisation of metals, as measured by diffusive gradients in thin-films, in soil historically treated with sewage sludge.

The two dimensional, small-scale, distribution of labile trace metals was investigated in a sandy loam soil historically treated with sewage sludge. After deployment of two DGT (diffusive gradients in thin-films) probes, their 1.8×15 cm windows were sliced into 5×5 mm and 5×8 mm rectangles to provide three vertical profiles with 5mm resolution. Both sets of profiles showed sharp maxima of Mn, Cd, Co, Cu, Ni and Zn, which varied systematically, both vertically and horizontally. The observed mobilisation of metals was highly localised, as each measurement corresponded to a volume of soil as little as 25-40 μL. Regression analysis indicated mobilisation of Cd, Co, Cu, Ni and Zn were strongly linked to mobilisation of Mn, while there was a strong linkage between Pb and Fe mobilisation. Localised reductive dissolution of Mn oxides or a localised lowering of pH were considered as possible mechanisms.