DISTRIBUTION AND SEQUENTIAL EXTRACTION OF SOME HEAVY-METALS FROM SOILS IRRIGATED WITH WASTE-WATER FROM MEXICO-CITY

A sequential extraction procedure was used to fractionate Cu, Cd, Pb a nd Zn in 4 soil profiles into the designated forms of water soluble exchangeable, organically bound, carbonate and Mn oxides bound. Soil p rofiles were obtained from the Rural Development District 063, State o f Hidalgo, which have been irrigated with wastewater coming out of the basin of Mexico. The total heavy metal contents range as follows: Cu, 8.9 to 86.5 mg kg(-1) Cd, 0.86 to 5.07 mg kg(-1). Pb, 18.1 to 131.7 m g kg(-1); and Zn, 101 to 235.5 mg kg(-1). The highest concentrations o f total heavy metals were found in the surface layers at all soil prof iles. Sequential chemical fractionation indicated that the four metals were predominantly associated with the organic fraction at most soil samples. The contents in all fractions of the four metals showed a dec rease with depth which has been explained by the variations in the org anic matter and CaCO3 contents in the different layers of soils. These soil properties were also the most important variables in the biologi cal availability of the metals in these soils.