3.3.2 Total and available heavy metals
The content of total heavy metals showed the presence of Cr2O3 in all samples (native soils, tailing and spoil), and SnO in all native soils and some layers of tailing profiles (Table 3). The total concentration of Cr2O3 toxic heavy metal was considerably higher in tailing (sand fraction) (204-286 mg kg-1) than the native soils (149-190 mg kg-1), indicating that Cr was occluded in the host-mineral structure. On the other hand, the SnO heavy metal was much higher in the native soil and spoil than in tailings. In addition, the SnO was higher in a native soil deriving from sandstone than the soil from granite.
Total concentrations of other heavy metals such as Pb, Cd, Ni and Hg were not detected in native soils and tailings, except for the TBB3 spoil profile, where Pb was present in the topsoil. The non-heavy metal (trace element) present in high amount was Cl and its concentration was much higher in native soils (1270-1380 mg kg-1) than tailing (370-970 mg kg-1).
The available heavy metals as measured by 0.05 M CaCl2showed the presence of Pb, Cr and Hg in all native soils and tailings, and few samples contained As (Table 4) but their concentration was very low. Although total concentrations of Pb and Hg were not detected in native soils and tailings, they present in small amounts in available forms. The available Pb of soils deriving from sandstone was similar in native and tailing conditions (< 0.30 mg kg-1), which in turn was lower than Pb in native soils and tailings deriving from granite materials (< 1.20 mg kg-1). The higher available Pb in tailings than native soil deriving from granite, indicating that the Pb was inherited from host rocks. This was supported by the total PbO in the granitic rocks was 48 mg kg-1 (Table 3).
For Sn available form, it was not detected in both native soils and tailings (Table 4), although its total concentration as measured by XRF was high in native soils (203-418 mg kg-1) (Table 3). This further indicates that the Sn-bearing mineral (cassiterite) was resistant to chemical weathering and Sn heavy metal should not raise concern for health risk in the post- tin mining areas. Other heavy metals such as Cd, Ni, and Co were not detected in both total and available forms of native soils and tailings.