2D DGT induced fluxes in sediments and soils (2D DIFS)

Diffusive gradients in thin films (DGT) is an emerging, dynamic, measuring technique that can provide diverse information on the concentrations and behaviour of solutes, including chemical speciation and partitioning between solid phase and solution in waters, sediments and soils. The DGT device, which accumulates solute in a binding layer after transport through a well-defined diffusion layer, is simple to construct and use. However, complete interpretation of the dependence of mass accumulation with time requires a numerical model of the transport and reactions occurring within the device and its deployment medium. We have developed a software tool that models the temporal dependence of DGT induced fluxes from soils or sediments by considering diffusion of solutes in two dimensions (2D) and incorporating first order exchange of solute between solid phase and solution. Microniches of elevated concentrations, that emulate zones of high microbial activity, can be created in the soil or sediment. The solver uses an advanced 2D FEM method and a flexible and user-friendly Graphical User Interface, which incorporates essential model calibration tools, is provided. This solution in 2D is shown to substantially increase the accuracy of the simulation, compared to that achieved with the established DIFS software that provides only a 1D solution, and yet it still has short calculation times on modern PCs. The 2D model is shown to provide a good approximation to the full 3D solution, obtained using FEMLAB, when the supply from the solid phase is larger than supply by diffusion. Example simulations are provided for the three major solute supply situations, namely diffusion only, very rapid and sustained supply from the solid phase and two intermediate cases of partial resupply.