Hydrodynamic modelling, environmental tracers and hydrochemistry of a confined sandy aquifer (Kędzierzyn-Głubczyce Subtrough, SW Poland)

Sarmatian sands and bur ied Pleis to cene val leys of the Kedzierzyn-Głubczyce Subtrough represent one of the main aquifers in southern Poland ( MGWB-332 - Major Ground Water Basin). This aquifer is intensively exploited, supplying tap water for the human population and for industry in the whole area; but, being confined, it has no influence on the ground water ecosystems. Two Ground Water Bodies (GWB-128 and GWB-129), introduced by administrative decisions according to EUDirectives, approximately cover the area of MGWB-332. The present study is related to the eastern part, the Sarmatian and buriedval eys sands of MGWB-332, i.e. to the most im - portant part of the multiaquifer GWB-129 which in profile consists of Holocene and Pleistocene sands, confined Sarmatian and Pleistocene buried valley sands, and Badenian sands. The presence and influence of deeper permeable formations is not addressed. The Sarmatian and Badenian aquifers are recharged and drained mainly by vertical seepages. Hydrodynamic modelling of the whole Kędzierzyn-Głubczyce Subthrough and tracer data indicate modern ages at the out crops of the Sarmatian under the Pleistocene deposits and mid to early Holocene ages close to the Odra River valley. Waters are of the HCO3-Ca type, changing at the centre of the Sarmatian to the SO4-Ca type due to the contribution of ascending sulphate water from the Badenian strata, whereas water in the buried Quaternary valley is of the HCO3-Na type, which means no significant contribution of ascending waters. Polluted modern waters occur only at the north west ern boundary in the area of the hydrogeological window. The quality of waters and their hydrochemistry result from water-rock in teractions and seepage exchanges with overlying and underlying aquifers belonging to the same GWB. Natural distributions of most major, minor and trace constituents are very wide, exemplifying difficulties in defining the quality of water in aunique way for the whole aquifer and particularly for the investigated multi-aquifer GWB.