METAL MOVEMENT IN SLUDGE-AMENDED SOILS

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Abstract
Sewage sludges were incorporated annually into the surface 20 cm of Dublin loam over a period of 8 yr. Sludge rates varied from 0 to 225 metric tons per hectare (t/ha) in increments of 45 t. Data presented cover a 9-yr period, which includes a final year in which no sludge was added to the soil. Metal concentrations found by analysis were higher in the surface soil only when the concentration of metal in the added sludge exceeded that found in the untreated soil. Metal availability, expressed as DTPA/HNO3 ratios, was greater in the acid Oakland-sludge-treated soils for Cd, Zn, Ni, Co, Fe, and Mn than in the neutral to alkaline Pacheco-sludge-treated soils. The increased soil acidity and the high percentage availability of metals in the Oakland plots did not result in an increased metal movement within the soil profile. Metals tended to remain in the zone of soil incorporation over the 9-yr period in spite of sludge additions amounting to 1800 t/ha for Pacheco and 1440 t/ha for Oakland sludge. The apparent movement of Zn and Cd 5 cm below the area of sludge incorporation may be an artifact resulting from inaccurate depth measurements due to the decrease in bulk densities. Rototilling of the field resulted in a lateral movement of soil and sludge particles downslope from the plot areas. This soil movement made it impossible to balance metals added in sludge with metal contents of the soil. No significant movement of metals occurred in the year following termination of sludge additions.

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