IN-SITU MEASUREMENTS OF SOLUTION CONCENTRATIONS AND FLUXES OF TRACE-METALS IN SOILS USING DGT

Assessments of the availability of toxic or nutrient metals in soils n eed to consider solution speciation, mobility, and the kinetics of exc hange between solution and solid phase. The newly developed technique of diffusive gradients in thin films (DGT) accumulates metals in a wel l-defined geometry that allows quantification of their supply from the solid phase. DGT was used to measure potential maximum fluxes of avai lable metals in old arable and ex-woodland soils variously treated wit h sewage sludge with and without metal amendments. The flux increased with increasing sludge application. The results suggest that for Zn an d Cd there are two separate pools of metal associated with the solid p hase. In untreated soils or those with 100 m(3) ha(-1) yr(-1) sludge t reatments, Zn and Cd appear to have higher resupply rate constants fro m the solid phase than in soils with 300 m(3) ha(-1) yr(-1) sludge tre atments (assuming the ratio of labile solid-phase metal to soil soluti on metal does not change). However, the actual rate of resupply is hig her in the treated soils due to the much higher concentration associat ed with the solid phase. For Ni and Cu there appears to be only one po ol of metal irrespective of the extent of sludge treatment.