Geochemical profiles in the hydrogeological system of the Opalinus Clay

Opalinus Clay is the chosen host rock for the deep geological disposal of nuclear waste in Switzerland and is also being considered for this purpose in Germany. For the long-term integrity of the disposal site, temporally and spatially stable geochemical conditions are essential. Adjacent aquifers can induce changes into the system. It is therefore essential to investigate if and how the geochemistry and mineralogy of the sediments is influenced by the hydrogeology. Changes in the Opalinus Clay and the surrounding formations provide information about geochemical processes in the past and thus enable an assessment for the future.

In this context, a 58 m long borehole was drilled at the Swiss Rock Laboratory in Mont Terri. Drilling was conducted from the Opalinus Clay (Toarcian) through the entire Staffelegg Formation (Toarcian-Sinemurian), which contains two water-bearing sections. The groundwater, the members and their transitions were characterised with a variety of analytical methods.

Groundwater was found locally in the Beggingen (Gryphaea Limestone) and Rietheim (Posidonia Shale) members, depending on the presence of pathways in open fractures and with differences in their chemical composition. The groundwater in the Rietheim member is not directly connected to the surface, but seems to be continuously recharged by an adjacent aquifer. At the transition from the Gross Wolf to the Rietheim Member, pyrite increases and many trace elements are enriched. The Fe/Al paleoredox proxy and the enrichment of trace-metals like uranium reveal prevailing anoxic conditions in the sediment and groundwater. This is more indicative for a depositional than a mobilization feature. The influence of the groundwater can therefore be classified as limited due to its reducing conditions at this transition.

The aim is to establish a method for analytically distinguishing between features of deposition, diagenesis, alteration and mobilization. This will allow assessment of the long-term integrity of the Opalinus Clay as a host rock and the surrounding formations. The gained understanding of the hydrogeological influence on the geochemical conditions within a system is to be transferred to other potential repository sites.