Long-Term Bioavailability and Movement of Cd, Cr, Cu, Pb, Ni and Zn in a Soil Treated with Metal-Contaminated Sewage Sludges.

Long-term studies can help to understand how mobility and bioavailability of trace metals, accumulated in soil, will change over time. Sewage sludges from different wastewater treatment plants were applied annually from 1977 to 1986 to a Metea loamy sand soil (loamy, mixed, mesic Typic Hapludalfs) at 0, 240 (Tmt.1), 690 (Tmt.3), and 870 (Tmt.2) Mg ha^–1 and replicated four times in a randomized complete block design. The sewage sludge supplied varying quantities of metals that ranged from 0 kg ha^-1 (at 0 Mg ha^-1 sludge) to 44, 480, 1870, 2100, 3000 and 11,300 kg ha^-1 of Cd, Pb, Cu, Ni, Cr and Zn, respectively (the highest metal loading for one of the three sludge treatments). Total recoverable metals of soil samples taken in 2007 from surface depth (0-10 cm) are consistent with earlier samples and with the metal loadings applied in 1977-1986. Concentrations of Cd, Cr, Cu, Pb, Ni and Zn in diagnostic leaf samples of plants grown between 1985 and 2007 also reflect the differences in metal loadings between the three sludge treatments and suggest that some of the added metals are phytoavailable for a long time period (>20 yr). Concentrations of Cd, Cr, Cu, Pb, Ni and Zn in the 0 – 15 cm and 15-30 cm depths samples of 1990, 1999, and 2007 are consistent with metal loadings applied in 1977-1986. Soil samples taken in 1990, 1999, and 2007 from lower depths (> 30 cm) show no treatment effects and no increased metal concentrations compared to subsurface soils of control plots. This study suggests minimal or no vertical movements of sewage sludge-borne Cd, Cr, Cu, Pb, Ni and Zn but bioavailability of the metals in the tillage depth (0-30 cm) can persist for a long time period (> 20 yr).