Residence time of chalk groundwaters in the Paris Basin and the North German Basin: a geochemical approach

The comparative geochemical and isotopic study of confined and unconfined Chalk groundwaters of the Paris Basin and the N German Basin proves a significant chemical evolution during groundwater how from the recharge zones to the deep confined aquifer. Different time dependent geochemical parameters have been tested as dating tools: Cation ratios(Sr2+/Ca2+, Mg2+/Ca2+), NNO3- noble gas contents as paleotemperature indicators (Ne, Ar, Kr, Xe), radiogenic He,C-13, C-14, O-18, H-2, H-3. Cation ratios and C-13 Show the importance of incongruent dissolution processes in the Chalk aquifer. Water-rock interactions were taken into account in a multi-step dissolution model to determine radiocarbon groundwater ages. The oldest waters in the confined part of the Paris basin Chalk with maximum C-14 ages of 14,000 a B.P. contain pleistocene recharge components as can be shown by a stable isotope depletion and noble gas temperatures significantly lower than in recent groundwaters. Chalk waters at the Lagerdorf site in Northern Germany show a distinct stratification with respect to residence times and hydrochemistry. (C) 1998 Elsevier Science Ltd. All rights reserved.