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How insoluble inclusions and intersecting layers affect the leaching process within potash seams
(2021)
Potash seams are a valuable resource containing several economically interesting, but also highly soluble minerals. In the presence of water, uncontrolled leaching can occur, endangering subsurface mining operations. In the present study, the influence of insoluble inclusions and intersecting layers on leaching zone evolution was examined by means of a reactive transport model. For that purpose, a scenario analysis was carried out, considering different rock distributions within a carnallite-bearing potash seam. The results show that reaction-dominated systems are not affected by heterogeneities at all, whereas transport-dominated systems exhibit a faster advance in homogeneous rock compositions. In return, the ratio of permeated rock in vertical direction is higher in heterogeneous systems. Literature data indicate that most natural potash systems are transport-dominated. Accordingly, insoluble inclusions and intersecting layers can usually be seen as beneficial with regard to reducing hazard potential as long as the mechanical stability of leaching zones is maintained. Thereby, the distribution of insoluble areas is of minor impact unless an inclined, intersecting layer occurs that accelerates leaching zone growth in one direction. Moreover, it is found that the saturation dependency of dissolution rates increases the growth rate in the long term, and therefore must be considered in risk assessments.
Mehrphasige DNAPL-Pools zählen zu den häufigsten Ursachen für Grundwasserkontaminationen und sind bekannt für ihre Langlebigkeit. Obwohl Untersuchungen bereits gezeigt haben, dass die Phasen sich in ihrer Wasserlöslichkeit gegenseitig beeinflussen, werden diese Interaktionen von bisherigen Modellen vernachlässigt. Aus diesem Grund wurde ein semi-analytisches Berechnungsmodell entwickelt, welches die Poolzusammensetzung als zeitlich variabel behandelt. Basierend auf dem Raoult’schen Gesetz werden für jede Komponente Molanteil, effektive Wasserlöslichkeit und schließlich der Schadstoffaustrag infolge Advektion, Dispersion und Diffusion bestimmt. Die Ergebnisse unterscheiden sich deutlich von Studien an einphasigen Pools. So wird gezeigt, dass Schadstofffrachten über die Zeit sowohl zu- als auch abnehmen können und dass ohne Berücksichtigung des Raoult’schen Gesetzes sowohl die Langlebigkeit von DNAPL-Pools als auch die Dauer bis zur Unterschreitung von Grenzwerten teils deutlich unterschätzt wird. Eine Sensitivitätsanalyse zeigt zudem, dass schwer lösliche Nebenbestandteile nicht vernachlässigt werden dürfen, leicht lösliche hingegen schon.
Leaching zones within potash seams generally represent a significant risk to subsurface mining operations and the construction of technical caverns in salt rocks, but their temporal and spatial formation has been investigated only rudimentarily to date. To the knowledge of the authors, current reactive transport simulation implementations are not capable to address hydraulic-chemical interactions within potash salt. For this reason, a reactive transport model has been developed and complemented by an innovative approach to calculate the interchange of minerals and solution at the water-rock interface. Using this model, a scenario analysis was carried out based on a carnallite-bearing potash seam. The results show that the evolution of leaching zones depends on the mineral composition and dissolution rate of the original salt rock, and that the formation can be classified by the dimensionless parameters of Peclet (Pe) and Damkohler (Da). For Pe > 2 and Da > 1, a funnel-shaped leaching zone is formed, otherwise the dissolution front is planar. Additionally, Da > 1 results in the formation of a sylvinitic zone and a flow barrier. Most scenarios represent hybrid forms of these cases. The simulated shapes and mineralogies are confirmed by literature data and can be used to assess the hazard potential.