Recovery of biosolids-applied heavy metals sixteen years after application

Little is known of the long-term fate of biosolids-applied heavy metals in soil. The objective of this study was to quantify percent recovery of biosolids-applied heavy metals in a well-drained soil. Three annual applications of biosolids resulted in cumulative biosolids loadings of 0, 60, 120, and 180 Mg ha(-1). Cumulative metal loadings for the 180 Mg ha(-1) biosolids rate were 25, 141, 127, 43, 173, and 348 kg ha(-1) for Cd, Cr, Cu, Ni, Pb, and Zn, respectively. Soils were sampled to a depth of 0.9 m. Soil organic carbon (OC) was measured. Soil was extracted with 1 M HNO3 and metals were determined in the supernatants. Soil OC concentrations were linearly correlated to biosolids loadings and did not change significantly during the 16 yr after biosolids applications. Concentrations of extractable Cd, Cr, Cu, Ni, Pb, and Zn in biosolids-treated soils were much greater than the control to a depth of 0.30 m (P < 0.01), slightly greater at 0.30 to 0.45 m (P < 0.05) and same as the control below 0.45 m. For the 180 Mg ha(-1) biosolids loading, percent recoveries for Cd, Cr, Cu, Ni, Pb, and Zn were 112, 59, 119, 114, 102, and 97%, respectively. Low recovery of Cr was probably due to incomplete extraction from the soil. These results show that complete recovery of biosolids-applied heavy metals is possible when plot size is large enough to prevent crossmixing of treated soils during tillage operations and when sediment losses are minimal.