The mass of pore water present in an argillaceous rock formation is significant and varies between a few % to several tens of % depending on the geologic history of the formation. Due to the low permeability of argillaceous formations, pore water cannot be sampled by conventional groundwater sampling techniques. Thus, information about tracer concentrations in pore water can only be obtained by applying indirect extraction techniques based on fresh, saturated rock material, or directly by long-term and cost-intensive in-situ experiments in underground facilities. Indirect techniques are described in Sections A2.4 to A2.9, and direct techniques follow in Section A2.10.