14C dating of Gorleben groundwater

Previous attempts to apply C-14 for dating of groundwater in the Gorleben aquifer system has given results with conflicting H-3 and stable isotope data and hydrological estimates. C-14 model ages of 1-10 ka have been found for 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 this paper it is shown, that the reasons are assumption of to high C-14 concentration in recharge groundwater and not correcting for the influence of C-14 dilution by dissolved inorganic C (DIC) from microbiologically mediated mineralization of organic components in deep sediments. To overcome these difficulties a new approach is applied evaluating the site-specific C-14 source term (including the influence of nuclear atmospheric testing), and the already previously used overall dilution of DIC. Closed system conditions are assumed and no isotopic fractionation is considered. For most of the groundwaters, the C-14 ages achieved by the present method are in agreement with 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 also discussed, either where the 14C method is not applicable (shallow peat-bog like groundwater) or does not yield reliable groundwater ages (brines at < 200 m depth). (C) 2000 Elsevier Science Ltd. All rights reserved.