# How Insoluble Inclusions and Intersecting Layers Affect the Leaching Process within Potash Seams
## 4 Discussion
A similar phenomenon has been observed in solution mining: if several insoluble layers cross the salt body, the resulting cavern shape resembles an inverted Christmas tree [7]. The same shape is described by [PERSON] [10] for technical caverns crossing different (potash) salt layers. Furthermore, [PERSON] [10] implies that inclined layers cause an asymmetric shape, whereby the growth rate is faster in the direction of upward-directed inclination. These descriptions all correspond to the simulation results for a transport-dominated system with intersecting halite layers (Figure 12d-f). However, time frames for the evolution of natural leaching zonesare not available. Regarding insoluble inclusions, laboratory experiments conducted by [PERSON] et al. [36] and [PERSON] et al. [33] confirm that the cavern shape becomes increasingly distorted, and notches occur at the dissolution front if insoluble lenses impede regular fluid flow (Figure 12a-c). Additionally, findings that inclusions do not reduce the amount of dissolved/permeated rock per time, but only its distribution (Figure 7d), are consistent [36]. Reaction-dominated systems with (nearly) planar dissolution fronts only occur if the inflowing solution is already highly saturated with respect to the present salt minerals [33]. In this case, significantly slower growth rates as well as an increased influence of rock fabric were observed. The first finding agrees with our simulation results (Figure 7), whereas the second one could not be reproduced (Figure 10a-c). Overall, there is a good correlation between literature data and model results regarding the influence of heterogeneities. However, temporal scaling is still uncertain, and more data are required for comprehensive quantitative model validation. Finally, the results reveal how saturation-dependent dissolution rates affect barrier formation and leaching zone shape. In the case of fully reaction-dominated systems (Da \(<<1\)), there is no difference compared to constant dissolution rates because \(k_{max}\) is always reached. In contrast, transport-dominated systems show a more linear decrease in width from the top to the bottom, because the effective dissolution rate decreases along the dissolution front. Therefore, a longer distance is required until the solution is fully saturated and the transition from dissolved to undissolved sections of the potash seam is less sharp compared to constant dissolution rates [6]. Additionally, the barrier next to the inflow is smaller, resulting in a less decreasing flow velocity and growth rate over time. Thus, saturation-dependent dissolution rates lead to a wider extension, and therefore a higher hazard potential of the leaching zone in the long term. Furthermore, they intensify the coupling between chemical reactions and transport. Next to the dissolution front, where strong concentration gradients occur, changes in advection, diffusion or dispersion immediately affect the saturations, and therefore the effective dissolution rates. In return, the dissolved mineral amounts affect transport parameters such as brine density, viscosity and diffusion coefficients. An even stronger coupling would be achieved if \(k_{max}\) is treated as a function of flow velocities. Several studies indicate that for fast-dissolving minerals, such as most potash salts, the dissolution rate is controlled by the transport across a so-called diffusive boundary layer which is formed at the mineral-fluid interface [19; 23; 27]. Thereby, higher fluid flow velocities induce a decrease in thickness of the boundary layer, and therefore higher dissolution rates of the minerals. As a result, \(k_{max}\) varies locally and heterogeneities may have a stronger effect. However, the approach uses input parameters, such as the reaction rate constant, which are currently not known for many potash minerals. Furthermore, knowledge about the flow field at pore scale and the bulk concentrations outside the boundary layer are required [37].