GEOCHEMISTRY AND C-14 DATING OF GROUNDWATERS FROM JURASSIC AQUIFERS OF NORTH AQUITAINE BASIN (FRANCE)

Twenty-seven samples from a confined Lower-Middle Jurassic aquifer and an unconfined Oxfordian aquifer of the North Aquitaine Basin (France) have been analysed for their major elements, Br-, O-18, H-2, C-13 and C-14 contents. Hydrochemistry indicates (1) a dissolution of carbonat e and anhydrite near the recharge zone and (2) a dilution of a saline water derived from a seawater/halite mixing in the deeper part of the aquifer. The mixing is also visible in a delta(18)O vs Cl- diagram in which two different groups appear: recent waters and old waters indica ting a mixing process between fresh and saline groundwaters. The compo sition of the saline water is likely to be 34,100 +/- 11,200 ppm in CI , 70 +/- 20 ppm in Br and more than -3.5 +/- 07 parts per thousand vs SMOW in O-18. C-13 contents indicate(1) a C exchange with CaCO3 matrix for groundwaters near the recharge zone and (2) a participation of or ganic matter in the deep part of the aquifer. Residence times for wate rs near the area of the aquifer outcrop correspond to Holocene and Lat e Pleistocene periods. The depletion in stable isotopes of 10 to 15,00 0 y B.P. waters show a late glacial period infiltration to the aquifer . After a distance of about 10 km in the aquifer, the C-14 activities are 0 pmc showing the presence of 'old' groundwaters. Copyright (C) 19 96 Elsevier Science Ltd.