A sequence of sedimentary rocks from the Lower Chalk to the Oxford Clay has been studied as part of a reasearch programme into the feasibility of the disposal of low- and intermediate-level radioactive wastes in argillaceous strata. The preservation of aragonite in such rocks suggests that little groundwater movement has occurred through the bulk of these clays and this would imply that most groundwater movement is likely to occur along any fractures that may be present. Such fractures may interconnect with adjacent aquifer units along which most transport of radionuclides would occur. In these sediments the presence of smectitic clay minerals would strongly influence their retention of radionuclides by sorption and ion-exchange reactions. Compaction, calcite cementation, calcite dissolution and the diagenesis of biogenic silica have influenced the porosity and permeability of the rocks. These processes have also controlled the chemistry of many pore surfaces in contact with groundwater and these in turn will affect groundwater chemistry, and the nature of interactions with transported radionuclides.