METAL MOVEMENT IN SLUDGE-TREATED SOILS AFTER SIX YEARS OF SLUDGE ADDITION

Abstract

Movement of the metals Ni, Co, Fe, Mn, Cr, and Hg in Dublin loam soil was studied after 6 yr of sludge addition. Sludges from two sources were added annually at rates ranging from 0 to 225 metric tons per hectare (t/ha) in increments of 45 t. Increasing rates of sludge addition resulted in almost no change in the concentration of metals in the surface soil, with the exception of Cr and Hg, where the metal concentrations in the added sludge were markedly greater than those found in the untreated soil. In these cases metal increased linearly with rate of sludge addition. Surface concentrations of HNO3-extracted Mn were less than those obtained at lower depths. The low values appeared to be due to the failure of HNO3 to extract all the Mn from sludge-treated soils. Only Fe of all elements increased significantly in the profile with time. No element moved more than 5 cm below the zone of sludge incorporation in either the highly acid (pH 4.5) Oakland or the near-neutral (pH 7.3) Pacheco sludge-treated soils. Relative metal availability followed the same order for both sludges, i.e., Mn > Co = Ni ≫ Fe > Cr. Percentage of available metal in Oakland sludge-treated plots was nearly double that in Pacheco sludge-treated plots. Percentage availability of Fe and Cr was extremely low.