Sorption process plays a significant role for solute retardation in rock fractures. In this paper, for the aim to investigate the effect of sorption on solute transport in a single rough fracture, a 2D model of representative single rock fracture was built and its roughness was statistically characterized based on the measured data of rock surface topography by laser scanning. A Finite Volume Method (FVM) code was developed to solve the Navier-Stokes (NS) equations and transport equation for numerical modelling the process of fluid flow and solute transport in the rock fracture model. Two groups of simulations were conducted: with and without the consideration of the sorption process with different average flow velocities. The results show that the surface roughness increased the complexities of flow fields, and the non-linear sorption process plays a significant role in the retardation of solute transport through rock fractures. The sorption process caused an obvious lagging time in both the solute concentration fields (plumes) and corresponding breakthrough curves. This lagging time increases with the distance from the inlet boundary, and relatively decreases with the increase of mean velocities.
Fluid flow and solute transport in fractured rocks is an important research topic for performance and risk assessments for rock engineering projects concerning geo-environment safety, such as radioactive waste disposal, mining, geothermal extraction, reservoir engineering and other contaminant transport processes in geosphere (Berkowitz 2002).
The solute transport in the rock fractures mainly involve the advection, surface sorption, matrix diffusion, dispersion, radioactive decay and chemical reactions, governed by general transport equations (e.g. Bodin et al., 2003). Among these transport phenomena, the sorption on the fracture walls can alter solute migration between the fractures, and play an important role in natural retention of solute transport (Weber et al., 1991; Wels et al., 1998; Kumar 2007; Dai et al., 2012). The aim of this study is to investigate the effect of sorption on the solute transport in a single rock fracture with rough surfaces. More complex transport processes, such as matrix diffusion, dispersions, radioactive decay and chemical reactions, are not included in this study.
For modeling fluid flow in a single fracture, the fluid flow was traditionally assumed to be laminar between a pair of smooth parallel plates, in order to get an analytical solution of the fluid flow velocity field, named Cubic Law (e.g. Witherspoon et al., 1980). In this way, transport equations can be solved analytically in one dimension (e.g. Tang et al., 1981). However, in reality, the rock fractures usually consist of walls with different orders or scales of roughness (ISRM, 1978; Zou et al., 2014), which have critical effects on the fluid flow and solute transport behaviors inside the fractures, such as on the sorption process on the fracture walls (e.g. Thompson 1991; Yeo 2001; Cardenas et al., 2007). In addition, the flow through rock fractures is not generally laminar, for example, in mountain areas with high water head, field pump tests and some laboratory tests, fluid flow can have large values of Reynolds numbers. Hence, this assumption may over-simplify the transport process both in geometry and physics in realistic situations. Therefore, for enhancing our understanding of the process of fluid flow and the mechanisms of solute transport through natural rock fractures, it is necessary to solve the complete Navier-Stokes (NS) equations of fluid flow and solute transport equation directly in natural rock fractures with the complex geometry of rough surfaces.
