The Wildegg Formation is subdivided into the Birmenstorf Member and the much thicker Effingen Member (Section 4.2.8). The latter is considered an aquitard of regional extent with strongly decreasing thickness from the west (more than 200 m in JO) to the east (below 10 m in ZNO) (Fig. 4‑85). It was studied in detail during SGT Stage 2 as a potential host rock for low- and intermediate-level waste disposal in the former Jura-Südfuss siting region (Nagra 2014f, Dossier VI). To the east of the occurrence of the Hauptrogenstein aquifer, in the NL and ZNO siting regions, it is part of the upper confining units of the Opalinus Clay.

The Effingen Member consists of sandy calcareous marls and argillaceous limestone interbeds. The regional packer test dataset of SGT Stage 2 shows hydraulic conductivities < 10^-11 m/s at an overburden thickness larger than ca. 300 m (Nagra 2014f, Dossier VI). At shallower depths, higher hydraulic conductivities are measured and indicated by water inflows into tunnels or by ground­water samples. In SGT Stage 2, notably the thicker limestone beds were assessed critically regarding the potentially increased permeability related to tectonic overprint (lack of self-sealing properties).

In SGT Stage 3, the hydrogeological database for the Effingen Member was complemented in view of its contribution as a confining unit by packer tests (Fig. 4‑85), fluid logging partly covering this unit and by the profiles of natural tracers (Section 4.6). The investigations confirmed the generally low hydraulic conductivities, e.g. expressed by the absence of relevant excursions in the profiles of the natural tracers. A transmissive zone was detected related to the tectonic overprint of the most prominent limestone bed (Gerstenhübel Bed; Section 4.2.8) at around 310 m depth in BOZ1; the transmissivity of 3 × 10^-8 m²/s is well within the values expected in SGT Stage 2. Note that the thickness of this limestone bed strongly decreases towards the east (BOZ1 ~ 37 m, BAC1 ~ 6 m).

The Birmenstorf Member is a thin carbonate-rich horizon located at the base of the Wildegg Formation. In Northern Switzerland, its thickness decreases from west to the east (e.g. OFT ca. 15 m, BOZ2 ca. 4 m, BAC1 0.7 m, TRU1 ca. 2.6 m) and the lithofacies changes. In the NL and ZNO siting regions it occurs at depths greater than 500 m and is dominated by marly lithologies. In this area, there is no indication of water flow in this unit, and it is part of the natural barrier. In contrast, it occurs in the Aarau – Solothurn area as a much thicker limestone (with sponges or sponge bioherms). In the Internal Jura, the Birmenstorf Member is an aquifer at shallow depth (e.g. Herold 1997) and in boreholes in the Aarau and Gösgen area groundwater could be sampled up to ca. 340 m depth.