In situ procedures for measuring chemical speciation and availability of trace metals in soils and sediments

The chemical forms of inorganic components in soils and sediments determines their mobility, their availability to plants and their geochemical interactions. When considering the availability of an element to plants, or to geochemical reactions such as precipitation leading to mineral formation, both the kinetic rate of supply and the distribution of species are important. To obtain a true representation of the distribution of chemical species in soil and sediment systems in situ chemical techniques are required. In situ procedures may be classified as: (a) continuous in situ measurement (e.g. ion selective electrodes), (b) in situ characterization and collection followed by in situ measurement (e. g. stripping voltammetry and flow injection analysis) and (c) in situ characterization and collection followed by laboratory measurement (dialysis, ion exchange). New procedures which fall into category (c) are DET and DGT. DET (diffusive equilibrium in thin films) uses a polyacrylamide gel rather than a dialysis cell to establish an equilibrium with the pore waters. Its advantages are chiefly associated with its thin-layer construction which allows the measurement of chemical gradients over sub-mm intervals. DGT (diffusive gradients in thin films) uses a polyacrylamide gel which is backed with a layer of binding agent, such as Chelex resin for trace metals. It differs from other procedures using resin in soils which only provide arbitrary comparative estimates of available metal. By precisely defining the geometry, mass transport to the resin is controlled, allowing a mean flux of metal in the soil or sediment to be calculated. Such an in situ flux is fully transferable between laboratories and should provide a sound quantitative chemical surrogate for bioavailability.