The host rock and confining units are sandwiched between aquifers that provide the hydraulic and hydrogeochemical boundary conditions to the low-permeability sequence (Section 4.5.3.1, Fig. 4‑84). Malm and Hauptrogenstein constitute the relevant aquifers above the host rock. Below the host rock, the local Keuper and the regional Muschelkalk aquifer are relevant. During the period under consideration, the flow systems in these deep aquifers can be affected by a variety of processes. In the following subsections, the impact of the most important drivers is briefly discussed.