# How Insoluble Inclusions and Intersecting Layers Affect the Leaching Process within Potash Seams
## 5 Conclusions  
The scenario analysis for a generic potash seam has significantly improved the understanding of the influence of insoluble inclusions and intersecting layers on the evolution of leaching zones. It was shown that in the case of advection- and reaction-dominated systems (Pe \(>\) 2 and Da \(<\) 1), growth rate, shape and porosity of the leaching zone are basically identical in heterogeneous and homogeneous cases, meaning that heterogeneities can be neglected with regard to risk assessment. In advection- and transport-dominatedsystems (Pe > 2 and Da > 1), the amount of potash salt permeated over time is similar in both cases, but the shape of the leaching zones is different. As a result, the upper end of the dissolution front (s) moves forward 1.8 times slower in the heterogeneous case, regardless of the distribution of insoluble areas. Consequently, heterogeneities decrease the hazard potential. To account for this effect, only the volume ratio of insoluble minerals has to be known, representing a major advantage in practice because exact distributions are seldom known. Only in the case of intersecting layers with negative inclination, a smaller reduction in growth rate and an asymmetric expansion of the leaching zone need to be considered. Neglecting heterogeneities usually generates a safety margin in the overall assessment, unless the increase in permeation ratio endangers the mechanical stability of the leaching zone. If insoluble layers and/or large inclusions collapse, new fluid flow paths may be formed, significantly increasing leaching zone growth rates as well as hazard potentials. Literature data indicate that most natural systems are transport-dominated. However, the Damkohler number needs to be calculated individually for each mineral and, in the case of intersecting layers, also for each separate leaching zone to reliably assess the influence of heterogeneities and to predict further leaching zone evolution. Heterogeneities, especially inclined intersecting layers, influence local and average fluid flow velocities; therefore, the validity of a global Damkohler number is reduced. Instead, the occurrence of funnel shapes, barriers and sylvinitic zones (in the case of camallitic potash salt) should be used to determine if a system is dominated by reaction (Da < 1) or transport (Da > 1). The extension of the reactive transport model by variable saturation-dependent dissolution rates has improved its accuracy with regard to the leaching zone shape, growth rate and barrier formation. It is shown that constant dissolution rates overestimate barrier formation, and therefore underestimate hazard potentials in the long-term. In the next step, it is planned to not only distinguish between insoluble and highly soluble minerals, but to also include more slowly dissolving minerals such as anhydrite (CaSO4) or kieserite (MgSO4-H2O) into the assessment by using individual maximum dissolution rates. In doing so, the effects of mineral heterogeneity can be investigated in further detail. However, for many secondary minerals occurring in these complex quinary or hexary systems, \(k_{max}\) is unknown. Furthermore, a dependency on the local flow velocity is expected to intensify the coupling between chemical reactions and transport. Therefore, laboratory and field measurement data are required to calibrate \(k_{max}\). With these extensions, the reactive transport model can be principally applied to any study area for prediction of the preferential expansion of leaching zones along potash seams and assessment of their hazard potentials. Conceptualization, M.K. and S.S.; methodology, S.S. and T.K. ; software, S.S. and T.K. ; validation, S.S.; formal analysis, S.S.; investigation, S.S.; resources, M.K. ; writing--original draft preparation, S.S.; writing--review and editing, S.S., T.K. and M.K. ; visualization, S.S.; supervision, T.K. and M.K. All authors have read and agreed to the published version of the manuscript.