Karst aquifer evolution in fractured rocks

Abstract

Abstract. We study the large-scale evolution and flow in a fractured karst aquifer by means of a newly developed numerical method. A karst aquifer is discretized into a set of irregularly spaced nodal points, which are connected to their set of natural neighbors to simulate a network of interconnected conduits in two dimensions. The conduits are allowed to enlarge by solutional widening. The geometric flexibility of this method, along with a simplified model for the dissolution kinetics within the system water-carbon dioxide-calcite, enables us to study both laminar and turbulent flow in a karst aquifer during its early phase of evolution. A sensitivity analysis is conducted for parameters such as conduit diameter, hydraulic pressure differences, and recharge conditions along the surface of the aquifer and shows that passage evolution depends strongly on the recharge condition and the amount of water available. Under fixed hydraulic head boundary conditions an early single-passage system develops under laminar conditions and is transformed into a maze-like passage system after the onset of turbulence. Fixed recharge boundary conditions are more likely to result in a branchwork-like passage system, although the addition of distributed recharge may lead to a maze-like system of secondary passages. IntroductionA mature karst landscape is characterized by the virtual absence of surface flow and by subsurface drainage through cave systems. Subsurface flow is often established between sinking streams along the boundaries of insoluble and soluble rocks and major entrenched valleys serving as base level for resurgences. The dissolution of limestone within the system water-carbon dioxide-calcite is a prominent geomorphic process for the evolution of both surface and subsurface morphological features in a karst landscape, and our understanding of the dissolution process and its implications on flow and passage patterns in the karst aquifer is important for the interpretation of large-scale karst landscape evolution.The The widely different passage patterns characterizing the different types of caves lead to several questions, for example, what processes are responsible for cave development, and under which conditions do each of the specific cave passage patterns result. We focus our attention on the two most important cave types, branchwork and network caves, and follow Palmer [1991] in arguing that groundwater recharge is the most important factor controlling cave passage enlargement in these cases. Palmer showed that 67% of the total cave passage length originates from recharge through valleys and sinking streams, and 10% evolved around recharge points located along boundaries between soluble and insoluble rock. He concluded that recharge through discrete, small-catchment sinkholes is likely to result in branchwork caves. On the other hand, network (or 3223

Modelling karst geomorphology on different time scales

“…Several numerical models on single fracture growth have been published (Dreybrodt, 1990(Dreybrodt, , 1996Palmer, 1991Palmer, , 2000Groves and Howard, 1994a;Kaufmann and Braun, 1999;Dreybrodt and Gabrovšek, 2000). These models simulate the evolution of a single fracture as a simple analog of flow along a fracture or a bedding plane, and the evolution of the fracture width is controlled by the dissolution rate.…”

Phreatic drainage conduits within quartz sandstone: Evidence from the Jurassic Precipice Sandstone, Carnarvon Range, Queensland, Australia

“…Groundwater flow in carbonate and fractured rocks is mainly via discrete pathways; through either cracks or solutionally enlarged joints or conduits (Jennings, 1985;Domenico and Schwartz, 1997;Kaufmann and Braun, 1999;Ford et al, 2000;Ford and Williams, 2007). On the other hand, groundwater movement through relatively homogeneous porous sediments such as near-horizontally bedded quartz sandstones is generally accepted to either be concentrated into fractures where non-Darcian flows may prevail, or diffuse and governed by the rock's permeability and by hydraulic principles that are thought to be well understood (Domenico and Schwartz, 1997).…”

Influence of aquifer heterogeneity on karst hydraulics and catchment delineation employing distributive modeling approaches

“…Karst genesis simulation would provide process-based information about conduit widening towards a karst spring. Such simulations were employed for instance by Kaufmann and Braun (1999), Liedl et al (2003), Bauer et al (2003), and Hubinger and Birk (2011). They simulate the temporal evolution of a small fracture or fracture network due to solution with coupled transport and hydraulic models.…”