In the first step the concept was developed for a typical sedimentary system in Northern Germany and described by a comprehensive data set for the Gorleben site. The model used was based on a schematic cross section through the Gorleben area (Fig. 1), which summarizes all main geological and hydrogeological characteristics of the site under present conditions (Klinge et al., 2007). It comprises three hydrogeological units, representing the upper aquifer, an underlying aquitard, and the lower aquifer. Based on lithological conditions two different hydrogeological units exist on site: The lower aquifer is characterized by high salt concentrations in the groundwater, resulting from the contact to the cap rock of the salt dome and leaching of salt. The upper aquifer is characterized by low salt concentrations (fresh water conditions).
Groundwater recharge takes place at the surface. Part of this water is drained to the lowlands of the river Elbe, while the other part may infiltrate through hydraulic windows of the aquitard down to the lower aquifer. At the area of contact between the salt dome and lower aquifer, salt can be dissolved increasing the salt concentration and the density of the groundwater. The water over the salt dome flows to the north and sinks down in the northwestern rim syncline. Apart from that, the water can also rise into the upper aquifer through the hydraulic windows or through the aquitard. The ascending salt water results in locally elevated layers of salt water underlain by less saline water in the lower aquifer and in salt water occurrences at the surface (upper aquifer).
Fig. 1: Schematic north-south striking cross section of the hydrogeological system at the Gorleben reference site (modified from Klinge et al., 2007; published with permission from BGR), lateral extension app. 16 km, horizontal extension up to 480 m.
Currently, a second representative sedimentary system is set-up, where the key selection criterion was the occurrence of a spatially and temporally changing redox conditions and the availability of reliable redox data from field explorations. The selected site denoted as Bourtanger Moor aquifer system is located in Northwestern Germany (Emsland) near the small town Haren (Houben et al., 2001).
Fig. 2: Simplified hydrogeological cross-section of the Bourtanger Moor aquifer system near Haren (Houben et al., 2001).
The hydrogeological system of the area comprises two aquifers composed of post-glacial outwash material (Houben et al., 2001, Fig. 1). Upper and lower aquifer are separated by a two to eight m thick argillaceous layer of low hydraulic conductivity sediment. The upper aquifer consists mainly of fine sands, whereas the lower aquifer is sandy to gravely. The latter one is recharged via leakage through the aquitard, where some hydraulic “windows” are also present. The system is characterized by sharp redox front in app. 11 m depth, which is mainly affected by a strong agricultural nitrate input oxidizing the authigenic pyrite minerals as the most relevant reactive and reducing constituent of the quaternary sediments (Houben et al., 2017). The system is well characterized with respect to mineralogy, groundwater chemistry (including redox pairs) and hydrogeological conditions. Using the information and data from this site it is intended to develop an approach for redox reactions to be implemented in the current concept (Concept).