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

Controversies exist about the recharge pattern, age and renewal rate of wat er in the Oligocene sands of the Mazovian basin, Poland. Hydrodynamic model ling yields ages of about 1.5 ka whereas earlier estimates based on environ mental tracer methods suggested from 30 ka to more than 200 ka. This age di screpancy mainly resulted from apparent inconsistency of environmental isot ope data and partly from unidentified flow paths, sparse data, and poor kno wledge about the hydrodynamic parameters of the overlying Pliocene sediment s. However, in spite of delta(18)O and delta D values only slightly lighter than those typical for local Holocene waters, low inferred noble gas tempe ratures obtained within this study (0.3-4.9 degrees C in comparison with th e present air temperature of 7.5 degrees C) and low C-14 contents (0-7 pmc) , indicate a dominant role of glacial recharge. The isotope shifts to heavi er values and to the right of W.M.W.L.. which decrease the difference betwe en the Holocene and glacial waters, are caused by evaporation prior to rech arge via numerous paleolakes, and by admixture of ascending saline water. T hough several earlier Cl-36 analyses suggest ages of the order of 200 ka, s uch great ages are excluded by Ar-40/Ar-36 ratios close to the atmospheric equilibrium and relatively low He-4 excess values obtained within the prese nt study. A good correlation of He-4 excess with Cl- content indicates its origin to be related mainly to local ascension of older waters, and to be l ittle depended on the radiogenic production within the aquifer, or to the a ccumulation of crustal flux along the flow paths. Therefore, the noble gas data have appeared to be indispensable for the identification of the glacia l age of water far from erosion windows, in spite of weak delta(18)O and de lta D climatic signal, and for confirmation of a conceptual model invoking significant vertical recharge by leakage through Pliocene sediments. (C) 20 00 Elsevier Science B.V. All rights reserved.