Distribution and Mobility of Zn, Pb and Cd in a Sewage Sludge-Amended Soil

Abstract

The distribution of the metallic trace elements (MTE) Zn, Pb and Cd in a sludge-amended soil and their partitioning in specific soil microystems are studied by comparing their contents in amended and control soils. This comparison is achieved at the metric scale of the bulk soil horizons and at the micrometric scale of the weathering microsites (weathering rock-forming minerals and their specific weathered products). The chemical analyses of the MTE in the bulk samples do not show any anthropic contamination of the amended soil with repeated sewage sludge spreading. The chemical analyses of the bulk < 2 μm clay fractions indicate the occurrence of local MTE concentrations and vertical migration in deeper soil horizons. Precise chemical analyses in the weathering microsites indicate that, as a general rule and whatever microsite is considered, Zn, Pb, and Cd accumulate in clay minerals from surface horizons where the sludge was spread. On the contrary, the vertical MTE migration is restricted to the connected macroporosity of the fissural system filled with clay minerals and does not affect the weathering clays of rock-forming minerals. Such MTE mobility through the fissural system gives rise to two main environmental problems: 1) Zn, Pb, and Cd have the potential to move several meters deep along fissures in the soil profiles and may represent potential contaminants for unconfined aquifer and 2) because plant root system grows preferentially along soil fissural pattern, it may adsorb migrating MTE.