Transport characteristics of a 134 m thick Opalinus Clay formation in southern Germany obtained from its porewater noble gas profile

Clay-rich formations are internationally considered as ideal seals for underground disposal of radioactive waste [1, 2]. In the context of site exploration programs transport characteristics must - among other information – be locally quantified [3].

Here we present noble gas profiles from Hondingen, southern Germany [4], an area already disqualified as a siting region due to the shallow depth of the Opalinus Clay formation and an active fault zone. The sampling procedure generally followed guidelines for drill core sampling of the recent Nagra drilling campaign [5], refining procedures originally developed elsewhere [6 - 10].

We compare our data with analytical solutions [11] and numerical simulation results [12]. The ⁴He concentration profile has reached steady state. Within the clay barrier, the transport is diffusion dominated with the minimum porewater residence time calculating to some million years.

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