Isotopic characterization of humic colloids and other organic and inorganic dissolved species in selected groundwaters from sand aquifers at Gorleben, Germany

The aim of this work was to determine the significance of groundwater colloids in far-field radionuclide migration. To this purpose, isotopic data presented in this paper were obtained from several selected Gorleben groundwaters as part of the colloid characterisation programme. The contents of major and minor ions, light isotopes (2H, 3H, 13C, 14C, 18O and 34S), and the U/Th isotopes were measured. Radiocarbon and 13C were measured in dissolved inorganic carbon (DIC), ion the humic acid (HA-colloids) and fulvic acid (FA-solution) fractions of dissolved organic carbon (DOC). The 18O and 34S were also determined in dissolved sulphate phase. The U/Th isotope measurements were carried out on total and surface solid phases, colloid fraction (1-1000 nm particle size, HA) and solution (<1.5 nm, FA). The 3H data using a piston-flow model have yielded groundwater ages of less than 40a before present (BP) for a near surface groundwaters and much older ages for all other deeper organically-rich Gorleben groundwaters. The 14C content of DIC is low and probably influenced by strong dilution with mineralised fossil carbon. The resulting high 14C-model ages (up to 27 ka BP) can only be interpreted as upper limits of the true groundwater ages. The 14C content of the HA fractions is partly very low or partly below detection limit yielding very high 14C-model groundwater ages (up to >30 h BP) whereas, some of the FA fractions contain higher 14C yielding model groundwater ages more consistent with post-glacial recharge (<l2 h BP). These groundwater ages are consistent with Holocene recharge inferred from the 2H and 18O isotope data. These differences between the HA and FA fractions are also confirmed by the U/Th isotope ratios suggesting different source/histories of the two organic carbon fractions. Thus, it is possible that the FA fraction has originated mainly from the near-surface (soil) source, while the HA fraction derives mainly from organic matter from older sediments such as lignite lenses or from a mixture of an enhanced input of organically-rich water younger than some 12 ka BP and much older lignite source.