Accumulation of Heavy Metals in Sewage Sludge‐Treated Soils

Abstract The accumulation of Cd, Cr, Cu, Ni, Pb, and Zn in sewage sludge‐treated soils was studied after 6 y of continued annual sludge application at a cropland disposal site. Soil samples were taken at 15‐cm depth increments from Greenfield sandy loam (coarse‐loamy, mixed, thermic, Typic Haploxeralf) and Domino loam (fine‐loamy, mixed, thermic Xerollic Calciothid) that received 0 (control), 22.5, 45, and 90 Mg ha −1 y −1 of composted sludges or 0 (control), 3.75, 7.5, and 15 cm y −1 of liquid sudges since 1976. Samples were also obtained from areas where liquid sludge treatment was terminated after 1978. Heavy metals in soils were extracted by 4 M HNO 3 and analyzed by atomic absorption spectroscopy. Over 90% of the deposited heavy metals were found in the 0‐ to 15‐cm soil depth where sludges were incorporated; no statistically significant increase in heavy metal contents of the soil was detected below the surface 30 cm of the soil profile. Although crop absorption of heavy metals has increased with the sludge application rates, the total uptake amounted to < 1% of the metal introduced through sludge application. Through a material balance calculation, it was found that recovered heavy metals were low relative to amounts added due to incomplete extraction of metals from soils by 4 M HNO 3 and changes in soil bulk density.