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pared to other sites with SOC-bearing sediments [11], a more detailed analysis of SOC from selected
samples of clay/lignite layers have been performed. This work represents a strong link between
RTDC5 and RTDC2. SOC was characterised in detail by
micropetrographical methods,
application of different extraction schemes,
degradability experiments, and
interaction experiments of humic acids with natural clay samples.
The micropetrographical study showed that the sedimentary organic matter at Ruprechtov site is
generally formed by slightly dispersed matter with low degree of coalification, which only reaches
brown coal or lignite degree. The main components of detritic and xylodetritic coal samples and
clay-lignite samples are mineral admixtures and huminite of the maceral group [5].
According to the results from SOC characterisation it seems that the low concentration of dissolved
organic matter in the Ruprechtov system is mainly caused by the low availability of organic matter
to the processes of degradation. Only a very small fraction of SOC is accessible to the groundwater.
An additional reason could be the strong sorption properties of the clay that fix humic acids on the
sediment matrix. This is indicated in first sorption experiments performed by NRI with standard HA
leonardite on the montmorillonite standard SWy-2 and on low TOC clay samples from borehole
Na11. The results are shown in Figure 6 and indicate significant sorption of HA on the clay samples
with higher sorption values on Ruprechtov samples compared to standard montmorillonite [5].
Adsorbed HA (mg per kg of clay)
8000
7000
6000
5000
4000
3000
2000
1000
0
-1000
-2000
0 50 100 150 200 250
Equilibrium concentration of HA (mg/L)
Figure 6: Sorption isotherm of HA on different clay samples
350
NA11 (465nm)
SWy-2 (465nm)
NA11 (280nm)
The integration of all results showed that organic matter did not play such an important role by direct
interaction with uranium, but SOC contributed and still contributes to maintain reducing conditions
in the clay/lignite layers. It can be concluded that SOC within the sedimentary layers was (and
to some extent still is) microbially degraded. By this process DOC is released, providing protons to
additionally dissolve SIC [2]. Moreover SO4 2- is reduced leading (and has lead in the geological
past) to the formation of iron sulphides, especially pyrite. Reducing conditions, being maintained
amongst others by sulphate reducing bacteria, caused the reduction of As, which sorbed onto pyrite
surfaces, forming thin layers of arsenopyrite. Uranium U(VI), originally being released from the
outcropping/underlying granite, was reduced to U(IV) on the arsenopyrite surfaces. UO2 and uranium
phosphates were formed by reaction of U(IV) with phosphates PO4 3- , released by microbial