# How Insoluble Inclusions and Intersecting Layers Affect the Leaching Process within Potash Seams
## 4 Discussion
Additionally, it has to be noted that complex systems containing several minerals can be reaction- and transport-dominated at the same time: if the maximum dissolution rate \(k_{max}\) or the saturations of the inflowing solution vary a lot for different minerals, some of them will show Da \(>\) 1, whereas others show Da \(<\) 1. Accordingly, the Damkohler number has to be calculated individually for each mineral and an internal contact area between equilibrated and non-equilibrated minerals has to be defined (Figure 4) to reproduce the formation of several dissolution fronts, as observed by [PERSON] et al. [26]. In this case, heterogeneities are expected to mainly influence the dissolution pattern of minerals with Da \(>\) 1. Heterogeneities also affect the hazard potential of leaching zones by influencing the growth rate, shape and permeation ratio. In this context, higher growth rates are associated with faster expansion of the leaching zone and an increasing risk of mine flooding or integrity loss of a cavern. Higher permeation ratios or porosities also increase the hazard potential because they result in a lower mechanical stability as well as larger solution amounts stored per meter leaching zone. The results show that for Da \(>>\) 1, insoluble inclusions mainly affect the shape: at the top, the dissolution front is progressing relatively slowly, whereas at the bottom, dissolution rates are higher compared to the homogeneous case (Figures 11 and 12a-c). As a result, the permeation speed is basically the same (Figure 7d), but the dissolution front is steeper, and it takes 1.65-1.8 times longer until the right model boundary is reached. It is important to note that the distribution of insoluble inclusions does not have a significant influence if their volume ratio is identical. Although broader inclusions increasingly split up the convection cell into smaller ones and lead to a more irregular average flow velocity, the overall evolution is always quite similar (Figures 7d, 8d and 12a-c). Thereby, the leaching zone growth in the horizontal direction is slower compared to the homogeneous case, but the permeation ratio is higher at the end of the simulation, which can become more critical with regard to mechanical stability. In the case of one continuous insoluble layer, the leaching zone consists of two independent convection cells, showing the same regular dissolution front, sylvinitic zone and barrier as in the homogeneous case. However, flow velocities and permeation speed are more similar to those of insoluble inclusions (Figures 7c, 8c and 12d). Inclined, intersecting layers particularly influence the flow velocity within the lower half (Figure 8c). Thereby, negative inclinations increase the flow velocity, resulting in faster leaching zone growth compared to the upper half (Figure 12f), and vice versa. The evolution of the upper half is basically not influenced (Figure 12e,f). However, the amount of undissolved potash rock is larger with increasing height, resulting in relatively small permeation ratios (Figure 7c). In summary, an intersecting layer with negative inclination is the most critical distribution of insoluble inclusions, leading to higher permeation ratios and only slightly smaller growth rates in the horizontal direction compared to the homogeneous case. Field observations indicate that the formation of caverns and leaching zones in salt rock is usually dominated by transport. According to [PERSON] and [PERSON] [29], the upper half of a potash seam is often preferentially dissolved, and natural leaching zones within carnallitic rock are mostly divided into a halite zone near the inflow region and a sylvinitic zone close to the dissolution front. These observations agree with the results for a transport-dominated system (\(k_{max}\) = 5 \(\times\) 10\({}^{-4}\) cm/s) of homogeneous rock composition (Figure 11). In the case of intersecting layers from pure halite, [PERSON] and [PERSON] [29] assumed a split into several dissolution fronts with different penetration depths.