Measuring bioavailable trace metals by diffusive gradients in thin films (DGT): soil moisture effects on its performance in soils.

Conventional methods of measuring labile chemical species of trace metals in soil solutions, such as chemical competition following centrifuging, are inadequate if the speciation changes during sampling and extraction. A new technique, diffusive gradients in thin films (DGT), measures labile species of trace metals in natural waters and sediments in situ. A well-defined diffusive gel layer distinguishes it from other resin-based techniques. It perturbs the soil in a controlled way by introducing an in situ local sink for metal ions. Resulting fluxes to the device are quantitatively measured, allowing assessment of re-supply kinetics and in some cases measurement of in situ soil solution concentrations. We used DGT to measure fluxes of Cd, Co, Cu, Ni, Pb and Zn in a sludge-treated soil at various moisture contents (27–106%). Replicate measurements showed that the precision of DGT-measured fluxes was within 10%. For moisture contents exceeding the field capacity (42%), the DGT response reflected soil water concentrations. At smaller moisture contents, changes related to tortuosity and dilution were reflected in the measurements. This technique has the potential for in situ measurements in the field where it should provide quantitative flux data on individual soils and provide a good surrogate for bioavailable metal