Leaching of macronutrients and metals from undisturbed soils treated with metal-spiked sewage sludge. 3. Distribution of residual metals

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Abstract
Sewage sludge is increasingly been applied to land in New Zealand in an attempt to recycle the valuable plant nutrients such material contains. However, there are concerns regarding the environmental fate of heavy metals contained in the sludge. This study examines the soil profile distributions of metals (Cd, Cr, Cu, Ni, Pb, Zn) in large undisturbed soil lysimeters treated with metal-spiked sewage sludge, following 3 years of drainage leachate monitoring. For 3 forest soils that had all received surface applications of sludge, analytical results demonstrated considerable transport of metals from the sludge into the underlying litter and mineral soil layers. Determination of individual metal/Cr ratios revealed considerable differential mobility between metals, with Cd, Ni, and Zn being most mobile, and Cu and Pb being only slightly more mobile than Cr. Total metal analysis was relatively insensitive in detecting metal movement down the soil profile compared with determination of extractable metals (EDTA or Ca(NO3)2 extraction). Maximum depths of metal penetration varied between metals, but were similar for all 3 forest soils, averaging 0.15, 0.20, 0.25, 0.35, and 0.45 m for Pb, Cu, Cd, Ni, and Zn, respectively. In contrast to the forest soils, when sludge was incorporated into the top 0.10 m of 2 pasture soils, little movement of metals could be detected below the layer of incorporation. However, even in these soils, the determination of metal/Cr ratios indicated that some differential movement of metals had taken place. For the forest soils, the results indicate considerable potential for the long-term and sustained leaching of metals by normal convective and diffusive transport processes. This would appear to be of less concern where sludge has been incorporated into the pasture soils. These results clearly have implications for selection of the most appropriate soils and conditions for sludge application to the land.