Age and recharge pattern of water in the Oligocene of the Mazovian basin (Poland) as indicated by environmental tracers

Abstract

Controversies exist about the recharge pattern, age and renewal rate of water in the Oligocene sands of the Mazovian basin, Poland. Hydrodynamic modelling yields ages of about 1.5ka whereas earlier estimates based on environmental tracer methods suggested from 30ka to more than 200ka. This age discrepancy mainly resulted from apparent inconsistency of environmental isotope data and partly from unidentified flow paths, sparse data, and poor knowledge about the hydrodynamic parameters of the overlying Pliocene sediments. However, in spite of δ18O and δD values only slightly lighter than those typical for local Holocene waters, low inferred noble gas temperatures obtained within this study (0.3–4.9°C in comparison with the present air temperature of 7.5°C) and low 14C contents (0–7pmc), indicate a dominant role of glacial recharge. The isotope shifts to heavier values and to the right of W.M.W.L., which decrease the difference between the Holocene and glacial waters, are caused by evaporation prior to recharge via numerous paleolakes, and by admixture of ascending saline water. Though several earlier 36Cl analyses suggest ages of the order of 200ka, such great ages are excluded by 40Ar/36Ar ratios close to the atmospheric equilibrium and relatively low 4He excess values obtained within the present study. A good correlation of 4He excess with Cl− content indicates its origin to be related mainly to local ascension of older waters, and to be little depended on the radiogenic production within the aquifer, or to the accumulation of crustal flux along the flow paths. Therefore, the noble gas data have appeared to be indispensable for the identification of the glacial age of water far from erosion windows, in spite of weak δ18O and δD climatic signal, and for confirmation of a conceptual model invoking significant vertical recharge by leakage through Pliocene sediments.