Fracture evolution in soluble rocks: From single-material fractures towards multi-material fractures

Abstract

IzvlečekUDK 551.435.8 Georg Kaufmann, Franci Gabrovšek & Douchko Romanov: Odvisnost kraškega razvoja razpoke od vrste in zaporedja vodotopnih kamnin, skozi katere poteka Z numeričnim modelom smo raziskovali razvoj sekundarne poroznosti v enodimenzionalni razpoki v apnencu, sadri in anhidridu. Cilji raziskav so bili: 1) ugotoviti razlike med razvojem plitvih in globokih razpok v različnih kamninah; 2) oceniti učinek morebitnega izločanja enega od mineralov v razpoki in 3) dinamika rasti razpoke, ki gre skozi plasti različnih kamnin. Rezultati kažejo na primerljiv razvoj razpok v apnencu in sadri, pri čemer je čas razvoja v obeh kamninah precej različen. V anhidridu je zaradi drugačne kinetike raztapljanja razvoj še hitrejši. Izločanje ob spreminjajočih se geokemičnih pogojih lahko hitro zamaši razpoke in posebej v večjih globinah spremeni poti razvoja mreže kraških prevodnikov. V primeru, ko so razpoke v apnencu prekrite s plastjo sadre, kar v naravi pogosto opažamo, je hitrost širjenja najhitrejša, kar daje vodnim potem vzdolž takih razpok izrazito primerjalno prednost pri zajemanju razpoložljivega toka. Ključne besede: vodotopne kamnine, enostavna razpoka, raztapljanje in izločanje, incepcijski horizont. We employ a numerical model describing the evolution of secondary porosity in a single fracture embedded in soluble rock (limestone, gypsum, and anhydrite) to study the evolution of isolated fractures in different rock types. Our main focus is three-fold: The identification of shallow versus deep flow paths and their evolution for different rock types; the effect of precipitation of the dissolved material in the fracture; and finally the complication of fracture enlargement in fractures composed of several different soluble materials. Our results show that the evolution of fractures composed of limestone and gypsum is comparable, but the evolution time scale is drastically different. For anhydrite, owing to its difference from calcite in the kinetical rate law describing the removal of soluble rock, the evolution is even faster. Precipitation of the dissolved rock due to changes in the hydrochemical conditions can clog fractures fairly fast, thus changing the pattern of preferential pathways in the soluble aquifer, especially with depth. Finally, limestone fractures coated with gypsum, as occasionally observed in caves, will result in a substantial increase in fracture enlargement with time, thus giving these fractures a hydraulic advantage over pure limestone fractures in their competition for capturing flow.

Three‐Dimensional Hydrochemical Model for Dissolutional Growth of Fractures in Karst Aquifers

“…The chemical kinetics for gypsum has similar formulations to limestone, expressed as equations ((27)) and ((29)), in which the relevant parameters are listed in Table 2. The dissolution kinetics of natural gypsum changes from a linear to a nonlinear rate law at a very close chemical (Birk et al, 2003;Kaufmann et al, 2017 (Birk et al, 2003). In addition, the equilibrium concentration is approximately 15.0 mol/m 3 (Kaufmann et al, 2017).…”

“…We simply prescribe a constant head difference of 37 m imposed on the circulation system for both scenarios. Table 3 Parameters Used for Case Studies (Birk et al, 2003;Hanna & Rajaram, 1998;Kaufmann, 2009;Kaufmann et al, 2017;Kaufmann & Dreybrodt, 2007…”

“…However, gypsum has a much larger Ca 2+ equilibrium concentration (~15.0 mol/m 3 ), but a very limited expansion of Ca 2+ concentration with aggressive power for nonlinear kinetics (i.e., 0.94-1.0C eq ; cf. Figure 14; Birk et al, 2003;Kaufmann et al, 2017). In this comparative study, we focus on the role that the kinetic trigger mechanism and reactive-infiltration instabilities play in dissolution propagation in rough fractures.…”

Modelling speleogenesis in soluble rocks: A case study from the Permian Zechstein sequences exposed along the southern Harz Mountains and the Kyffhäuser Hills, German

“…Here, experimental data are rare, and only precipitation with calcite and limestone is based on experimental data (Buhmann & Dreybrodt 1985b;Dreybrodt & Buhmann 1991). For precipitation rates of anhydrite and gypsum, we simply assume a linear relation (see Kaufmann et al 2017 for more details). This assumption rests on the fact, that during dissolution impurities accumulate on the interface solution-rock, and are responsible for the non-linear rates close to equilibrium, while during precipitation the pure mineral species will deposit, which we assume can be described by a linear rate law.…”

Effects of the vadose zone on groundwater flow and solute transport characteristics in mountainous karst aquifers – the case of the Javorniki–Snežnik massif (SW Slovenia)

“…In recent years, infiltration, flow and transport through karst aquifers has been thoroughly studied using various methods and techniques including natural and artificial tracers observed at cave drips or at aquifer outlets, and modelling (e.g., Emblanch et al 2003;Perrin et al 2003;Birk et al 2004;Pronk et al 2009;Doummar et al 2012;Kogovšek & Petrič 2014;Kübeck et al 2013;Mudarra et al 2014;Biondić et al 2016;Fehér et al 2016;Kaufmann et al 2017). These studies are very important, since they all contribute to better understanding of the aquifer's intrinsic properties, which is a prerequisite for proper interpretation and prediction of their behaviour, and for test hypotheses or assumptions.…”