Groundwater flow modelling of multi-aquifer systems for regional resources evaluation: the Gdansk hydrogeological system, Poland

In order to evaluate groundwater regional resources, a mathematical model of the Gdansk hydrogeological system (Poland) was developed. The research area covers about 2,800 km². Groundwater occurs in Cretaceous, Paleogene and Neogene formations. The recharge zone is situated in the glacial upland of the Kashubian Lake District. The discharge zone spreads over the lowlands of Gdansk and beneath the Bay of Gdansk (Baltic Sea). The MODFLOW program was used to develop a three-dimensional steady-state model on the basis of data from over 1,700 boreholes. The research area was digitalized as a square network of sides 200 m in length. Simplification of groundwater occurrence allowed four aquifers to be distinguished: upper Pleistocene, Pleistocene-Miocene, Oligocene-Eocene, and Cretaceous and also four intervening aquitards. The mathematical model calculations showed that the system is recharged mainly by precipitation (infiltration recharge is 136 mm/year). The precipitation recharge amounts to 1,045,440 m³/day. The other part of the inflow constitutes lateral flows from beyond the area of research and also to some extent from losing rivers and lakes. The runoff from the system takes place mainly through gaining streams and through lateral outflow including under-sea and lake discharge.