Origin and mobility of fulvic acids in the Gorleben aquifer system: implications from isotopic data and carbon/sulfur XANES

Abstract

The Gorleben aquifer system, overlaying a Permian salt dome, has been under investigation for more than two decades for the potential to host a nuclear waste repository. Groundwater in the system shows a range of compositions, especially with respect to salt content and dissolved organic carbon (DOC) concentration. An uncertainty for safety analysis is the mobility of metal-complexing dissolved organic acids. Hence, isotopic data and carbon/sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy have been used in order to determine the mobility of fulvic acids (FAs). Isotopic data ( 13C, 14C, 3H) show that FAs from the recharge zone are mobile over the entire depth, including into the salt dome influenced brines. 14C and δ 34S (up to 34‰) analysis shows furthermore that enhanced DOC (mainly humic and fulvic acids) concentrations originate from microbiologically mediated turnover of lignite intercalations in sandy Miocene sediments (“in situ generation”). XANES revealed that these in situ generated FAs have a high C arom/C aliph ratio (∼2.8), a decreased carboxyl/carbonyl content (less hydrophilic), a red shift in the C arom = C arom peak at 285.2eV, indicating heteroatom substitution and aromatic ring distortion, and feature a high reduced S content (∼69%). Shallow recharge groundwater and deep brine derived FAs exhibit a similar C arom/C aliph ratio (1.1–1.4), indicating invariance in the backbone structure against higher residence times and variation in geochemical conditions. XANES data also suggest that only heteroatom-substituted, destabilized aromatic ring structures of the FAs are stable in the brines and revealed 43% reduced S in recharge FAs and high (61%) reduced S with higher sulfate content in the channel brine FAs. Sulfur redox speciation therefore reflects geochemical conditions/reactions and shows a high stability of reduced sulfur species in more aerobic channel brine environments. The application of carbon and sulfur XANES shows that key structural information can be obtained from small sample amounts. The high mobility of FAs over a range of groundwater conditions and residence times verifies the potential for dissolved humic substances to enhance radionuclide transport.