Deep saltwater in Chalk of North-West Europe: origin, interface characteristics and development over geological time

High-concentration saltwaters occur in many places in the regional Chalk aquifers of North-West Europe; to investigate deep occurrences, profiles of interstitial porewater chemistry have been studied from three 250-450m deep cores drilled in the eastern parts of Zealand, Denmark. At the studied location, saline water in the Chalk resides at depths from 40 to 80m and salinity increases with depth. Concentrations of chloride up to ca. 30,000ppm have been observed at depths of 400m. Measured vertical hydraulic heads in open boreholes suggest that advective groundwater flow is now restricted in deeper parts of the Chalk formation and diffusive transport is thus the predominant transport mechanism. Laboratory-measured porosity and effective diffusion coefficients were used as input to a numerical 1D diffusion model of the interface between freshwater in an upper, fractured aquifer and modified connate formation water below. The model satisfactorily simulated the observed chloride and delta O-18 profiles. The diffusive refreshening of the Chalk formation has been going on for about 0.9 million years. The connate water in the Chalk of parts of the sedimentary basin seems to have been modified by transport of saltwater from underlying Mesozoic and Paleozoic sediments during compaction, which presumably ceased around 4 million years ago.