Factors affecting the three-dimensional distribution of exogenous zinc in a sandy Luvisol subjected to intensive irrigation with raw wastewaters.

Massive volumes of raw wastewater have been spread on sandy Luvisols in the Paris region since 1899, causing major soil contamination with metallic trace elements (MTE). To identify the factors influencing the vertical and horizontal redistribution of MTE at the plot scale, a study was carried out of the three-dimensional distribution of zinc. The background and exogenous fractions of zinc in the contaminated soil were discriminated using correlations between zinc and iron defined from unpolluted soils of the area. At the surface, the spatial distribution of zinc is determined by the geometry of the irrigation system and the terrain topography. The highest concentrations are observed near the irrigation outlets and in areas of very slight slope. The exogenous zinc has migrated down to the base of the studied solums (1 m depth), and its subsurface distribution depends upon the physico-chemical properties of the horizons. In the E horizon, the trapping of exogenous zinc is governed by the number of available reactive sites, whereas in the Bt horizon it is a function of the vertical and lateral behaviour of the drainage. Moreover, lateral transfer of exogenous zinc occurs in the C horizon. In the case of soil contaminated with raw wastewater, the organization of water flow on the surface and in the subsurface is a determining factor in the redistribution of zinc within a plot.