GROUNDWATER FLOW RE-ORGANIZATION UNDER THE LAST SCANDINAVIAN ICE SHEET IN NORTHERN POLAND

Isotope Stage 2 (Weichselian, Wisconsinan) ice sheet advanced into the northern Polish lowland ca. 25 ka ago and reached a maximum ca. 20 ka ago. This significant environmental change followed a long ice-free period and resulted in profound re-organization of the hydrogeological system, especially groundwater flow directions, velocity, and fluxes.

We have numerically simulated the subglacial groundwater flow dynamics in two and three spatial dimensions using finite differences (FD) code MODFLOW for steady-state and transient flow conditions under various combinations of boundary conditions and hydrogeological parameters to test the response of aquifer systems to ice sheet overriding.

The ice sheet loading caused a pronounced rise of the hydraulic head both underneath the ice and some distance in front of it. All models show enhanced groundwater discharge at the ice forefield driven by the down-ice decreasing pressure head. A transient 3D model shows significant changes in groundwater flow directions in the regional aquifer ca. 90 m below the ice sole and up to 40 km in front of the glacier, and up to 5 times flow velocity increase as compared to non-glacial times. Discharge from this aquifer likely fed the Notec–Warta ice marginal spillway, a major conduit evacuating meltwater along the Scandinavian Ice Sheet margin to the North Sea. Simulation shows that on average only about 15% of the meltwater formed at the ice sole could have drained through the bed as groundwater flow. The rest drained in short, repeated and violent episodes through subglacial channels (tunnel valleys). Low drainage capacity of the substratum suggests occasional decoupling of the glacier from its bed and phases of instability possibly recorded in highly lobate outlines of terminal moraines in the area.