The fate of (ultra)trace elements along a steep redox gradient in mesoscale laboratory lysimeter experiments

Marcus Böhm1, Daniel Jara Heredia1, Arno Märten1, Francesca Quinto2, Thorsten Schäfer1
1 Lehrstuhl für Angewandte Geologie, Friedrich-Schiller-Universität Jena
2 Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology (KIT)

V 17.9 in Forum Junge Hydrogeologen

23.03.2022, 12:45-13:00, HS 3

In the context of the long-term safety assessment of a deep geological repository for high-level radioactive waste, the possibility of radionuclides transported into the geosphere and their accumulation in the far field up to the biosphere must be considered. Mesoscale laboratory lysimeter experiments were set up within the BMBF funded project TransLARA (www.trans-lara.de) with four different natural agricultural top soil substrates to investigate the upward transport of long-lived radionuclides and their homologues (e.g. rare earth elements) from groundwater through the unsaturated zone and ultimately into crop plants under different climate conditions.

The lysimeters were placed in a dark climate chamber at 10.5°C. Grimsel-Test-Site groundwater carrying actinide tracers 233U, 237Np, 242Pu and 243Am at ≤ ppq levels (Quinto et al., 2015, Quinto et al., 2017) was continuously fed to the packed lysimeters for over 750 days to their subsoil and maintained at a constant level in the evaporation driven system. A redox interface at the capillary fringe was observed after about 200 days by in-situ Eh probes and precisely located with a sub-mm resolution with CO2- and O2-chemical optical sensor foils (VisiSens TD). The redoximorphic features were also observable through the transparent lysimeter walls. An additional injection experiment with Br- and Cl- as conservative tracers and several target species such as SeO42-and I- was conducted. Pore water samples were periodically analyzed with ICP-MS/AES, ion chromatography, nanoparticle tracking analysis (NTA) and for DOC at three different depths. Along the physicochemical gradient with Eh differences of 500-700mV in a 7cm soil section, a change in porewater water hydrogeochemical composition was detectable. Conservative tracers showed a recovery of >80%, redox sensitive tracers showed significant lower recovery compared to injected concentrations. I- breakthrough was nearly identical to conservative tracers under reducing conditions, whereas after traversing the redox interface recoveries decreased to ≈24% under increased Eh values. Other species as well as AMS measurements of pore waters and solid phase analysis will be discussed in the presentation.



Quinto et al. (2015), Anal Chem, 87, 5766-73

Quinto et al. (2017), Anal Chem, 89, 7182-89



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