C-14 dating of Gorleben groundwater

Previous attempts to apply C-14 for dating of groundwater in the Gorleben a quifer system has given results with conflicting H-3 and stable isotope dat a and hydrological estimates. C-14 model ages of 1-10 ka have been found fo r H-3 containing recharge water, up to 31 ka for groundwater with Holocene stable isotope signatures and 6-10 ka for groundwater at 35 m depth. In thi s paper it is shown, that the reasons are assumption of to high C-14 concen tration in recharge groundwater and not correcting for the influence of C-1 4 dilution by dissolved inorganic C (DIC) from microbiologically mediated m ineralization of organic components in deep sediments. To overcome these di fficulties a new approach is applied evaluating the site-specific C-14 sour ce term (including the influence of nuclear atmospheric testing), and the a lready previously used overall dilution of DIC. Closed system conditions ar e assumed and no isotopic fractionation is considered. For most of the grou ndwaters, the C-14 ages achieved by the present method are in agreement wit h H-3, stable isotopes and hydrological estimates. It is shown that down to approximately 140 m depth no C-14 decay can be detected. Situations are al so discussed, either where the 14C method is not applicable (shallow peat-b og like groundwater) or does not yield reliable groundwater ages (brines at < 200 m depth). (C) 2000 Elsevier Science Ltd. All rights reserved.