Characterization of heavy metals in contaminated volcanic soils of the Solofrana river valley (southern Italy)

International audienceA number of volcanic agricultural soils from the Solofrana river valley (southwestern Italy), irrigated for a long time with contaminated river water or subjected to overflowing, were collected and examined for fundamental soil parameters and total content and distribution of Fe, Cr, Cu, Mn, Ni, Pb and Zn. Micromorphological properties, the effect of main soil characteristics on the distribution of heavy metals in the various forms, and metal uptake or effects on vegetables were also investigated. Chromium and Cu were the only metal contaminants, occurring in soils in broad ranges of concentrations (Cr 62-335 and Cu 70-565 mg kg-1) and in the surface horizons always above the regulatory levels (Cr 150, Cu 120 mg kg-1), as established by the Italian Ministry of the Environment for soils of public, private and residential areas. Chromium and Cu, but also Ni, Pb and Zn, were concentrated in silt (20-2 micro m) and clay (50%). Mn was uniformly distributed among all the extracted fractions. For all metals the soluble and exchangeable forms made a small contribution to the total. Significant amounts of Cr and Cu were recovered in the acid ammonium oxalate extraction, suggesting association of metals with short-range-order aluminosilicates and organo-mineral complexes. The amounts of metals extracted by oxalate were found to be approximately equal to two thirds of the sequentially removed non-residual amounts. The results of DTPA extraction confirmed the low bioavailability of heavy metals in soil. The metal concentration in dwarf beans and lettuces growing on one contaminated soil did not exceed the maximum concentration recommended by the European Union. However, the enzyme activities in the bean roots indicated the induction of anti-oxidative defense mechanisms due to metallic stress. Optical microscopy (OM) showed occurrence of clay and silt coatings along elongated pores in the surface and subsurface soil horizons, suggesting a risk of metal-rich sediment transfer along the soil pore network during water movement