Boundary condition control on inter-aquifer flow in the subsurface of Berlin (Germany) – new insights from 3-D numerical modelling
We investigate the degree of hydraulic interconnection between the different (regional to local) groundwater compartments with respect to the choice of boundary conditions and their impact onto the groundwater safety beneath the urban centre of Berlin, capital city of Germany. To this end, we carry out a systematic study based on 3-D hydrothermal models differing in terms of imposed parametric set-ups of the hydrogeology, as well as different surface forcing with respect to their impact on fresh groundwater production. The study area is part of the Northeast German Basin and consists of a thick sequence (up to 5 km) of differently consolidated sedimentary deposits. This sedimentary succession features a sequence of alternating aquifers (reservoirs) and aquitards which are connected to different degrees, each one depicting a specific composition of its mineralised pore water. The uppermost aquifer system (made up mainly of poorly consolidated siliciclastic rocks) acts as the main freshwater reservoir used for groundwater production by the municipal water supplier. This compartment is incompletely sealed from the brackish to saline aquifers extending at greater depths by a regional clay-enriched aquitard, the Oligocene Rupelian Clay. The latter shows a heterogeneous thickness distribution due to erosion during the latest glacial periods resulting in local discontinuities. This aspect opens to the potential risk of contamination of the drinking water reservoir from mixing with the saline groundwater upconing, locally enhanced by shallow pumping activities. Based on our results and their correlation with available isotopic and chemical analysis of water samples, we demonstrate how hydraulic connection between the different compartments is indeed likely to occur thus supporting the possibility of a contaminant rise from the saline aquifers below through either natural or anthropogenic (pumping) forcing.