An integrated modelling approach to reconstruct complex solute transport mechanisms – Cl and δ37Cl in pore water of sediments from a former brackish lagoon in The Netherlands

A one-dimensional transport model was developed to reconstruct historical conservative transport of chloride and δ37Cl in pore water of sediments from a former brackish lagoon in The Netherlands, an area now covered by the freshwater Lakes IJssel and Marken. Knowledge of the mechanism of historical solute transport in the sediments and environmental conditions during transport is critical in understanding observed pore water chemistry and will form a basis for simulating effects of changing environmental (climate change) conditions. The model synthesizes present knowledge of geology and historical information on storm surges in the area and takes into account processes such as erosion of sediments, mixing of pore water, sedimentation, and diffusion (EMSD). The chemistry of pore water from one particular boring in the area was found to be mainly controlled by alternating seawater and freshwater diffusion. Models with a constant (averaged) porosity (ϕ = 0.55) and tortuosity factor (τ = 0.3) showed similar results as models incorporating the measured bulk porosity variations (ϕ = 0.4–0.8) and variable tortuosity factors calculated with Archie’s law, τ = ϕ. The relatively small tortuosity factor either results from anion exclusion or from the heterogeneous build-up of the profile in which a peaty layer in the middle part may obstruct diffusion. Diffusion of 35Cl− was found to be 1.0017 times faster than of 37Cl−. Seawater diffusion into the sediments started at least 400 years ago and refreshening took place since the lagoon was isolated from the sea by a dam in 1932.