Natural attenuation of toxic metal phytoavailability in 35-year-old sewage sludge-amended soil.

Toxic heavy metals persist in agricultural soils and ecosystem for many decades after their application as contaminants in sewage sludge and fertilizer products This study assessed the potential long-term risk of cadmium (Cd), lead (Pb), zinc (Zn), and copper (Cu) in land-applied sewage sludge to food crop contamination. A sewage sludge-amended soil (SAS) aged in the field more than 35 years was used in a greenhouse pot experiment with leafy vegetables (lettuce and amaranth) having strong Cd and Zn accumulation tendencies. Soil media with variable levels of available Cd, Zn, and Cu (measured using 0.01 M CaCl2 extraction) were prepared by diluting SAS with several levels of uncontaminated control soil. Despite long-term aging in the field, the sludge site soil still retains large reserves of heavy metals, residual organic matter, phosphorus, and other nutrients, but its characteristics appear to have stabilized over time. Nevertheless, lettuce and amaranth harvested from the sludge-treated soil had undesirable contents of Cd and Zn. The high plant uptake efficiency for Cd and Zn raises a concern regarding the quality and safety of leafy vegetables in particular, when these crops are grown on soils that have been amended heavily with sewage sludge products at any time in their past.