Chemical availability in soils and sediments interpreted by coupled transport–reaction models of DGT (Diffusive Gradients in Thin-films) fluxes: Roles of dissolved ligands and finite sorption capacity

• Improved model for rigorous simulation of DGT accumulation in soils and sediments. • The open-source code SSOCO computes accumulations and concentration profiles. • Closed-form solutions available for specific cases (e.g. labile complexes) • Time-independent available concentration in soils and sediments can be estimated. The passive sampler DGT (Diffusive Gradients in Thin-films) can assess the availability of various trace elements and pollutants—including transition metals (e.g. Hg, Ni, Cu, Cd, Zn or Pb), platinum group elements, organic contaminants (e.g. antibiotics, polycyclic aromatic hydrocarbons or pesticides), radionuclides (e.g. uranium or iodide), and metalloids (e.g. As or Sb), as well as nutrients (e.g. phosphate, nitrate or ammonium) —in a variety of media. This DGT assessment is particularly relevant for monitoring in sediment remediation, biogeochemical redox cycling or fertilization studies. In this work, we develop mathematical tools to interpret and predict the accumulation (of any of the previously mentioned analytes) in terms of characteristic parameters (e.g. the diffusion coefficients or the porosities) when DGT devices are deployed in soils or sediments. The open-source code SSOCO (“Sediments and SOils with COmplexation”) numerically computes accumulations, resupply parameters and concentration profiles. The impact of various characteristic parameters is studied. The derivation of an analytical solution (which can be computed with a provided spreadsheet) for the simple case of an analyte just diffusing in two media leads to: a) an extended solution for some cases where the analyte is adsorbed and/or interacts with other species in solution (e.g. when a metal forms a labile complex); b) approximate expressions for the peak position and value of the resupply parameter maximum; and c) very simple expressions for long term deployments and for the effective concentration in the porewater. Comparisons with the widely-used simulation program DIFS show its limited accuracy for short deployment times.