As shown in Table 3.1-1, maximum Cl- contents vary strongly among the sites considered between values close to that of sea water (Couche Silteuse at Marcoule – MAR203 and MAR402, Opalinus Clay at Mont Russelin) to contents of less than 1 % of that of sea water (Boom Clay at Mol). Many of the formations lost most of their original salinity. Maximum Cl- contents are often found in the central parts of the low-permeability sequences, with negative concentration gradients towards both the underlying and the overlying aquifers (Table 3.1-1). Particularly high gradients for Cl- are found in the Couche Silteuse at Marcoule, in the Opalinus Clay at Mont Terri and Mont Russelin and in London Clay at Bradwell and most probably indicate geologically young interactions with the aquifers and/or high transport resistance (in particular, low diffusion coefficients) in the lowpermeability sequence. At some sites (e.g. Callovo-Oxfordian at Bure – EST311/312, Boom Clay at Essen), the Cl- concentrations in the low-permeability sequence show linear trends connecting the values in the aquifers (i.e. the concentration gradient is negative towards one aquifer and positive towards the other). As a first hypothesis, such Cl- distributions can be interpreted as steady-state diffusion profiles.