Abstract

The extent of metal (Cd, Cr, Cu, Ni, Pb, Zn) leaching from soils treated with metal-spiked sewage sludge was examined using large undisturbed soil lysimeters of 5 different New Zealand soils (2 pasture and 3 forest soils). Metal leaching from the soils was monitored continuously over a period of 3 years following a single application of sewage sludge. Metal speciation in leachates was estimated using GEOCHEM-PC. Application of the sludge resulted in decreased pH and increased concentrations and total amounts of Cd, Ni, and Zn in drainage leachates from some of the soils, but had little effect on, or decreased concentrations of, Cr, Cu, and Pb. In the case of Cu, and to a lesser extent Pb, the reduced leaching of these metals in the forest soils was considered related to the lack of plant cover and reduced pH and soluble organic C in leachates from the sludge-treated lysimeters. The peaking of Ni and Zn concentrations well before 1 pore volume of water had passed through the lysimeters indicated that macropore flow was an important process facilitating the leaching of these metals. The extent of metal leaching varied greatly between soils and was dependent on a number of soil properties and other factors, including plant cover (pasture v. forest soils). However, even under the extreme conditions of the study, very small proportions (~1% or less) of the applied metals were leached over the 3-year period. In some cases, Ni and Zn concentrations in drainage leachates exceeded drinking water and/or environmental standards. However, with further attenuation and dilution in the vadose zone, in the short-term at least, the leaching of these metals would seem unlikely to pose a major environmental threat.