Redistribution of Sludge‐Borne Cadmium, Copper, and Zinc in a Cultivated Plot

AbstractThe vertical and horizontal redistribution of sludge‐borne Cd, Cu, and Zn was studied in and around a 6.1 by 7.3 m plot on Plano silt loam (Typic Argiudoll) that had received 112 Mg−1 of sewage sludge in the fall of 1978. This plot had been moldboard‐plowed and disked each year thereafter. Calculated loadings of trace metals to the cropped plot from the sludge application were: Cd, 25.6 kg ha−1; Cu, 149 kg ha−1; and Zn, 538 kg ha−1. Two hundred surface soil samples (0–20 cm) were taken from areas within the plot and up to 3 m on each side of the plot and 39 subsoil samples (20–30, 30–40, and 40–60 cm) were taken within the plot in the spring of 1989. Estimates of total trace‐metal concentrations in the soil samples were obtained by determining metal concentrations in a 4 M HNO3 extract (70°C) by atomic absorption spectrometry. The determined distributions of sludge‐borne Cd, Cu, and Zn, respectively, were: 66.7, 67.5, and 68.0% in the 0‐ to 20‐cm plow layer within the original pot area; 15.5, 18.1, and 21.0% dragged outside the plot area; and 15.2, 12.4, and 11.6% moved into the 20 to 30‐cm layer within the plot area. No significant enrichment with trace metals from the sludge application was found below 30 cm. Of the metals found outside the boundaries of the original plot, about 56% moved into adjacent areas in the direction of tillage and about 44% into areas perpendicular to the direction of tillage. A mass balance calculation using an assumed soil bulk density of 1.3 Mg m−3 showed an average recovery of 98.7% of the added metals.