RESIDENCE TIME OF CHALK GROUNDWATERS IN THE PARIS BASIN AND THE NORTHGERMAN BASIN - A GEOCHEMICAL APPROACH

The comparative geochemical and isotopic study of confined and unconfi ned Chalk groundwaters of the Paris Basin and the N German Basin prove s a significant chemical evolution during groundwater how from the rec harge zones to the deep confined aquifer. Different time dependent geo chemical parameters have been tested as dating tools: Cation ratios(Sr 2+/Ca2+, Mg2+/Ca2+), NNO3- noble gas contents as paleotemperature indi cators (Ne, Ar, Kr, Xe), radiogenic He,C-13, C-14, O-18, H-2, H-3. Cat ion ratios and C-13 Show the importance of incongruent dissolution pro cesses in the Chalk aquifer. Water-rock interactions were taken into a ccount in a multi-step dissolution model to determine radiocarbon grou ndwater ages. The oldest waters in the confined part of the Paris basi n Chalk with maximum C-14 ages of 14,000 a B.P. contain pleistocene re charge components as can be shown by a stable isotope depletion and no ble 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.