Long-Term Changes in the Extractability and Availability of Lead, Cadmium, and Nickel in Soils Under Wastewater Irrigation

The objective of this study was to assess the long-term changes in the behavior of extractable and bioavailable lead (Pb), cadmium (Cd) and nickel (Ni) of soils in the Mexican Valley of Mezquital under wastewater irrigation. Soil samples (0–10 cm depth) were collected from sites with different years of irrigation management for the analysis of heavy metals. Plant tissue was also collected to determine their Pb, Cd and Ni concentrations. Statistical analyses were carried out to compare HNO3, EDTA and DTPA soil tests and to relate amounts of heavy metals extracted from the soils to their levels in plant tissue. Results showed that metal concentrations, in general, tended to increase in soil with increasing number of years of irrigation. Correlation coefficients were poor between HNO3–Ni and EDTA–Ni or DTPA–Ni of the soils, while the correlation between HNO3–Cd and EDTA–Cd was only moderate and significant between HNO3–Cd and DTPA–Cd. Significant correlation was found between lead extracted from the soil by the chelates (EDTA/DTPA) and HNO3. Leaf Pb and Cd concentrations of wheat from the soils explained 66 and 78% of the variability of their respective levels in wheat seed. Although Ni concentrations in leaf and seed of wheat were poorly related, content of this metal and that of Cd were higher in the latter plant organ in most sites, suggesting the absence of a physiological barrier in the transfer of metal from plant vegetative tissue to storage organ and reflecting a matter of concern about the consumption of this grain by animal or human population.