Model Setup

A two-dimensional model (Flügge, 2009) was set up based on the schematic cross section and the geological and hydrogeological information given by numerous publications for the Gorleben site (Klinge et al, 2002; Klinge et al., 2007). The length of the area of interest is 16.4 km and it has a maximum depth of 400 m (Fig. 1). Three different units represent a lower and an upper aquifer (both 100 m thick) with an intercalated aquitard (50 m thick). The northwestern rim syncline is realized in the model with an additional thickness of the lower aquifer of 150 m. The aquitard is locally interrupted by two hydraulic windows; one close to the southern boundary of the model and a second one at the northern boundary of the northwestern rim syncline. The contact of the lower aquifer to the cap rock of the Gorleben salt dome is marked in red. The hydrogeological flow field is calculated with the code d3f. Boundary conditions are taken from different publications (Klinge et al., 2002, Klinge et al., 2007).

 

Fig. 1: Geometry of the groundwater flow and transport model (All figures of the model and simulation results are exaggerated by a factor of 10 here).

 

In a first step, the present flow field is modeled, which developed since the end of the Weichselian cold stage and the beginning of the Holocene 11,500 a ago. After 11,500 a model time, the results for the flow field and the salt distribution are compared to the present state. Initial conditions for the salt concentration are set according to the salt distribution at the beginning of the Holocene. The lower aquifer and the aquitard show saline conditions, while in the lower part of the upper aquifer, there is a transition zone of 30 m to fresh water conditions in the upper part of the aquifer. Present state boundary conditions are applied. In order to be able to compare the simulation results using the smart Kd‑concept to those employing conventional Kd-values, the flow field and salt concentration should not be too complex. Thus, constant climatic conditions with constant boundary conditions were assumed and results are evaluated at a model time of 160,000 a.

For transport simulations, the environmental parameters have to be quantified for the initial and boundary conditions. Based on the groundwater chemical data for the Gorleben area, the parameters pH, Ca, and DIC have been derived (Tab. 1). Based on the environmental parameters and the concentrations of the respective radionuclides, calculated for each point in time and space, the code r3t selects the appropriate smart Kd‑value from the multidimensional Kd‑matrix. At this stage the assignment of smart Kd‑values is only applied to the aquitard and the upper aquifer. For the lower aquifer – where a Pitzer formalism would be more appropriate to account for the ion-ion-interactions in highly saline groundwater – conventional Kd‑values are set.

 

Tab. 1: Precipitaion and initial groundwater compisiton according to [Klinge & Baharian-Shiraz, 2004].

Parameter

Unit

Upper Aquifer

Aquitard

Lower Aquifer

Precipitation water

pH

[-]

7.5

8.0

7.2

5.6

Ca

[mol m-3]

1.21

0.24

22.28

0.025

DIC

[mol m-3]

2.70

3.57

4.75

0.002

 

For the transport calculations, a Dirichlet condition is set for the inflow into the lower aquifer. The chemical composition of the inflowing groundwater is assumed to be identical to the formation water of the lower aquifer. An in- and outflow boundary condition is stated for the model surface, depending on the flow direction. The chemical composition of the outflowing water at the surface equals the calculated actual chemical composition of the groundwater. In case of an inflow, the chemical composition is defined equal to a typical composition of precipitating water. The mineral composition of the hydrogeological units is accounted for during the pre-calculation of the multidimensional smart Kd‑matrix and does not have to be specified in the r3t calculations (Noseck et al., 2012).

 

 

References:

Flügge, J., Radionuclide Transport in the Overburden of a Salt Dome – The Impact of Extreme Climate States, Verlag Dr.Hut, München 209 p (2009).
Klinge, H., Köthe, A., Ludwig, R.-R., Zwirner, R.: Geologie und Hydrogeologie des Deckgebirges über dem Salzstock Gorleben, Z. Angew. Geol., 2 7-15 (2002).
Klinge, H., Baharian-Shiraz, A.: Projekt Gorleben, Dokumentation hydrogeologischer Basisdaten, Hannover (Bundesanstalt für Geowissenschaften und Rohstoffe), 16 (2004).
Klinge, H., Boehme, J., Grissemann, C., Houben, G., Ludwig, R.-R., Rübel, A., Schelkes, K., Schildknecht, F., Suckow, A.: Standortbeschreibung Gorleben, Teil 1: Die Hydrogeologie des Deckgebirges des Salzstocks Gorleben, Geol. Jb., C 71, 199 p (2007).
Noseck, U., Brendler, V., Flügge, J., Stockmann, M., Britz, S., Lampe, M., Schikora, J., Schneider, A.: Realistic integration of sorption processes in transport codes for long-term safety assessments, Final Report GRS-297, (Gesellschaft für Anlagen- und Reaktorsicherheit (mbH)), Braunschweig, 293 p (2012).