Understanding small-scale features in DGT measurements in sediments

Abstract

Environmental context. Observations, using the technique of diffusive gradients in thin-films (DGT), of pronounced, small-scale (millimetre) maxima in concentrations of sulfide and metals in the pore water of sediments, have emphasised the importance of processes occurring in microniches. Modelling of the interactions between microniche sources and DGT devices within a sediment environment demonstrates how these sharp features arise and provides a basis for their quantitative interpretation. Abstract. Measurements in sediments made using DGT (diffusive gradients in thin-films) have shown small-scale (millimetre and sub-millimetre) maxima in solute concentration (e.g trace metals and sulfide). The sediment–DGT system was simulated using a dynamic model, which incorporated a spherical microniche close to the DGT surface. DGT maxima could arise when the microniche was (1) a local source with associated elevated concentration in the pore water, and (2) when, within the microniche, the Kd for the relevant solute partitioning with exchangeable solute associated with the solid phase was much higher than for the rest of the sediment. Use of realistic values of Kd and comparison with existing data suggested that the latter mechanism was unlikely to be responsible for observed DGT maxima. Locally elevated concentrations will be reasonably accurately reproduced by DGT. Peak height measured by DGT will be between 62 and 87% of the true maxima in concentration within the sediment when DGT is not present, while peak widths will be similar. This work provides, for the first time, a means for confidently interpreting the two dimensional images of DGT-measured concentrations in sediments.