Fissure and mineral weathering impacts on heavy metal distribution in sludge-amended soil

Abstract

The purpose of this study was to follow the distribution and migration of the metallic trace elements (MTE) zinc (Zn), lead (Pb) and cadmium (Cd) in a sludge-amended soil, both at the metric scale of the bulk soil horizons and at the micrometric scale of mineral weathering microsites. In the soil scale approach, the MTE contents determined by ICP-AES and ICP-MS analyses in amended and control soil samples were compared through enrichment factor calculation to assess the extent to which spread MTE may have moved throughout the soil profile. In the mineral scale approach, the MTE were analysed on thin sections in specific weathering microsystems including (1) rock-forming minerals (amphiboles, biotites, plagioclases) and their specific weathering clay minerals; (2) weathering clayey plasma, which obliterates the original rock structure with newly-formed clay minerals; and (3) the fissural network with its clay minerals. The purpose of this mineralogical approach, using X-ray diffraction (XRD) for mineral identification and electron probe microanalyses (EPMA) for MTE analyses, was to determine where and at which concentrations spread MTE can concentrate within the soil and weathered rock. The chemical analyses of MTE in the bulk samples reveal strong Cd and Pb accumulation at the surface of the amended soil due to anthropic contamination. Cadmium undergoes a vertical migration in deeper soil horizons, whereas Zn and Pb do not show significative transfer within the amended soil. Accurate MTE analyses in weathering microsites indicate that, except in plagioclase microsites, (1) Zn and Cd accumulate in clay minerals from surface horizons and migrate downwards through the fissural system, and (2) Pb does not show any significant mobility throughout the amended soil. The MTE migration evidenced through the fissural system gives rise to two main environmental problems. Zn and Cd have the potential to move several meters deep along fissures in the soil profiles and may represent potential contaminants for unconfined aquifer. Secondly, because the plant root system grows preferentially along soil fissural pattern, it may adsorb MTE.