The Opalinus Clay constitutes a major aquitard of regional extent in Northern Switzerland that stands out from its confining units for the remarkable uniformity of its mineralogical, petrophysical and hydraulic properties (Section 4.2.6, Chapter 5). Along with adjacent units, the Opalinus Clay is an efficient barrier to flow between the regional aquifers, as supported by hydrogeochemical evidence (Sections 4.5.5, 4.6).
Hydraulic conductivities measured in situ (i.e. from hydraulic packer tests) and in laboratory tests are low (mainly < 10-12 m/s) and vary in a narrow range if the overburden thickness is larger than about 200 m (10-14 to 10-12 m/s for the hydraulic packer tests; Section 5.6). Observations from the boreholes in Northern Switzerland are well supported by hydrogeological investigations carried out in the Mont Terri rock laboratory (e.g. Marschall et al. 2004).
The Opalinus Clay is characterised by its high and uniform clay-mineral content, which is typically well above 40 wt.-% (Fig. 4‑28, Section 5.2). The clay-mineral-rich matrix is fundamental for many properties, notably self-sealing. The in-depth characterisation of the Opalinus Clay is presented in Chapter 5.