Influence of sedimentary organic matter on dissolved fulvic acids in groundwater. Significance for groundwater dating with 14C in dissolved organic matter

Radiocarbon dating of 28 groundwaters with dissolved fulvic acids (FA) from three different aquifer systems in Germany was carried out. In the case of the Munich limegravel aquifer and the Keuper sandstone aquifer at the Franconian Albvorland (both Bavaria), which are practically free of sedimentary organic carbon (SOC), the groundwater dating by FA resulted, to a large extent, in plausible groundwater ages. As initial 14C contents 75-85 per cent modern carbon (pmC), depending on the study area, were assumed. In the Gorleben aquifer system (Lower Saxony), which is rich in organic deposits, the groundwater ages obtained are partly unrealistically high and contradictory to other isotope data. This is attributed to the influence of 14C free SOC, which reduces the 14C content of the dissolved FA. In order to identify a possible SOC contribution, the dissolved organic carbon (DOC) and corresponding FA were characterized. The concentration and composition of DOC revealed that the FA and humic acids (HA) concentration in the aquifer as well as their concentration ratio are indications for a SOC contribution. The FA characterization by spectroscopic methods (UV/V is absorption and fluorescence spectroscopy) and gel permeation chromatography (GPC) resulted in significant differences of FA influenced by SOC from those not so influenced. In accordance with these results, FA originated from SOC had larger hydrodynamic molecular sizes and showed higher absorber and fluorescence intensities. By means of this DOC and FA characterization, the SOC contribution to the dissolved Gorleben FA fractions can be identified. It is possible to correlated the amount of admixed SOC with the change in spectroscopic characteristics. Thus in groundwater dating with FA, criteria for a better interpretation and a starting point for the correction of the initial FA 14C content were ascertained. (author). 28 refs, 7 figs