Pore waters in aquitards reflect the past geochemical evolution of the adjacent aquifers. Solute transport in aquitards occurs predominantly via diffusion and is therefore slow. Depending on the aquitard thickness and the diffusion coefficient, the memory of the geochemical archive may reach back tens of thousands to millions of years. While conservative solutes such as Cl-, Br-, d18O, d2H or He directly reflect the diffusive response to changing boundary conditions, SO42- and all cations are also affected by rock-water interactions, such as solubility control by carbonate or sulphate minerals or cation exchange on clay-mineral surfaces.
We use a number of methods to extract pore water from low-permeability rocks, such as claystones or crystalline rocks. The spatial distribution of solute concentrations, in particular the trends towards water-conducting zones (e.g. sandstone beds in sedimentary sequences, faults in crystalline rocks), yields information on the rates of diffusive exchange and on past conditions in the water-conducting zone. We quantify the exchange process by numerical modelling, mostly targeted at conservative constituents but using the whole major-ion chemistry in more recent times.
