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T he enrichment or spiking of sewage sludge withThe enrichment or spiking of sewage sludge with heavy metals for heavy metals for experimental purposes is a fairly experimental purposes is a fairly widely used, although sometimes widely used technique (for example, Davis and Carltoncontroversial, technique. A study was undertaken, using a sequential Smith, 1981; Street et al., 1977; Coppola et al., 1988; fractionation scheme, to assess the degree of incorporation of Cu, Ni, Forge et al., 1993; Obbard et al., 1993). Spiking may be and Zn into sewage sludge samples spiked with these metals and undertaken to (i) increase the rate of buildup of metals as those metals already present in the sludge. This is important because there is evidence that the bioavailability of metals added to soil as metal salts may differ

Cadmium (Cd) is a nonessential element that occurs at above-background concentrations in many New Zealand (NZ) soils. Most of this Cd is due to the historical application of single superphosphate that was made from Nauru phosphate rock containing between 400 and 600 mg Cd kg P. Pasture Cd uptake exacerbates the entry of Cd into animal products. We sought to determine the critical environmental factors affecting Cd uptake in NZ pastures and to calculate the likely Cd intake of sheep and cattle. We tested 69 pastures throughout NZ for a range of variables, including Cd. Soil Cd and pasture Cd were positively correlated with soil P and soil concentrations of other elements found in phosphate fertilizers. We found that no single environmental variable adequately predicted pasture Cd uptake. Nevertheless, pseudo-total soil Cd and Cd extracted using a 0.05 mol L Ca(NO) solution were positively correlated with pasture Cd. Although soil pH, soil Fe, and soil Cd provided an excellent predictor of the Ca(NO)-extractable soil Cd fraction, regression models explained just 38% of the variation of the Cd concentration in pasture grasses. Incorporating the effect of pasture species composition is a crucial next step in improving these models. A calculation of the likely exposure to Cd of sheep and cattle revealed that no pastures tested resulted in sheep and cattle ingesting Cd at a rate that would result in breaching muscle-tissue food standards. For offal products, which the NZ meat industry does not sell for human consumption, food safety standards exceedence was calculated in a few cases.

The main objective of this study was to compare the effectiveness of different methods (heavy metals in pore water (PW), diffusive gradients in thin films (DGT), diethylene triamine pentaacetic acid (DTPA) extraction, and total heavy metals (THM) in soil) for the assessment of heavy metal bioavailability from soils having various properties and heavy metal contents. The effect of soil heavy metal pollution on shoot yield and sulfatase enzyme activity was also studied. Wheat (Triticum aestivum) was grown in different soils from Spain (n=10) and New Zealand (n=20) in a constant environment room for 25 days.

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.

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.