Key points:

• The low-permeability sequence around the Opalinus Clay is delimited at the top by the regional Malm (NL, ZNO) and Hauptrogenstein (JO) aquifers. The lower limit is formed by the complex Keuper aquifer or, where it is absent, by the regional Muschelkalk aquifer.
• The Na-Cl type Malm groundwaters in NL are remnants of seawater from the Upper Marine Molasse Group (OMM) and are characterised by particularly high residence times
  > 500 kyr. The quasi-stagnant conditions can be explained by slow groundwater recharge through seepage from the Molasse. In ZNO, the seawater component of the Malm groundwater is increasingly displaced by freshwater recharging further to the north.
• The limestones of the Hauptrogenstein constitute the aquifer above the host rock in JO. Because of a lithological facies transition and a thickness change the hydrogeological importance decreases from west to east.
• The local Keuper aquifer is characterised by a large variation in hydraulic conductivity that results from the lithological complexity of these sediments deposited in a mainly continental setting. While it probably represents a continuous aquifer in JO and ZNO, water flow is constricted to locally occurring fluviatile sandstone channels in NL.
• In the ZNO and NL siting regions, the Muschelkalk aquifer hosts a continuous dynamic flow system with ⁸¹Kr model ages of the individual groundwater samples ranging from 23 – 115 kyr. In JO, there is a more local flow system related to the local topographic and structural situation.