# How Insoluble Inclusions and Intersecting Layers Affect the Leaching Process within Potash Seams
## 2 Materials and Methods
### Scenario Analysis
Figure 6: Heterogeneous rock distributions examined: The homogeneous potash salt (red) shown in Figure 5 is veined by layers with (**a**) 0\({}^{\circ}\), (**b**) +10\({}^{\circ}\), (**c**) -10\({}^{\circ}\) inclination or inclusions of pure halite with correlation lengths of (**d**) 2:1, (**e**) 17:0.5 and (**f**) 34:1 that make up 13% to 15% of the potash seam. The 2D model height was 2 m, representing the typical thickness of potash seams in Germany, whereas the model width was 5 m with a discretization of 101 \(\times\) 41 cells (Figure 5). At the start of the simulation, the entire model consisted of dry potash salt with a porosity of 0.1%. Natural caverns and leaching zones are commonly formed in the vicinity of tectonic fault systems which enable fluid migration [28]. Due to the fact that the ascending solution has to cross several rock salt layers before it reaches the potash seam, it is usually NaCl-saturated. To represent such a fault zone, a Dirichlet boundary was used (Figure 5). It maintains a constant solution composition and porosity at the left model boundary, assuming a high fluid and mineral exchange rate within the fault zone. The other boundaries were considered as impermeable without any pre-defined pressure gradient applied. The 2D model made use of the horizontally symmetric expansion of leaching zones to be expected if a potash seam has the same composition in both horizontal directions. Seen from the fault zone, the starting point of the leaching process, only one direction was simulated by taking advantage of the symmetry to reduce the required computational time. If carmallite-bearing potash seams come into contact with NaCl-saturated brine, carmallite is always dissolved first, triggering the precipitation of sylvite and halite [29]. Equation (6) gives the overall reaction:  
\[K\!M\!g\!C\!I_{3}\times 6H_{2}O+\textit{brine (NaCl)}\to KCl+\textit{NaCl (brine)}+\textit{brine (MgCl_{2}, KCl, NaCl)} \tag{6}\]  
In contrast, halite is not dissolved from the potash seam [6; 29], resulting in areas of pure halite (further referred to as halitic areas), acting as barriers to fluid flow. To investigate their influence on leaching zone evolution and hazard potentials, six generic rock distributions were considered. As shown in Figure 6, the first three represented a potash seam (red) with intersecting layers of halite (grey). These layers were 30 cm thick and showed inclinations of 0\({}^{\circ}\) or \(\pm\)10\({}^{\circ}\), respectively (Figure 6a-c). To ensure that boundary conditions are comparable, the layers always started at the same height at the center of the fault zone. In the other three cases, the potash seam contained halite inclusions ranging from 10 cm \(\times\) 10 cm (Figure 6d) to nearly continuous, horizontal layers (Figure 6f). Their distribution was created with GSTools [30], using correlation lengths of 2:1, 17:0.5 and 34:1 in the horizontal direction. In all six cases, the halitic area made up between 13% and 15% of the potash seam and the surrounding potash salt was homogeneous with the composition shown in Figure 5. For comparison, an additional case without any halitic areas was considered. For flow and transport calculations, the porosity-permeability relationship presented by [PERSON] et al. [31] was applied. Carmallite is dissolved first and makes up \(>\)30% of the rock volume; therefore, a relatively large contact area between the solution and remaining minerals, sylvite and halite, can be assumed. Both minerals show the same maximum dissolution rate \(k_{max}\) as carmallite, facilitating a quick equilibration in the case of undersaturations. Therefore, non-equilibrated minerals were neglected in this study. Instead, sylvite and halite were immediately added to the equilibrated part \(Min_{eq}\) as soon as the carmallite in their surroundings was dissolved.