A DFN based 3D numerical approach for modeling coupled groundwater flow and solute transport in fractured rock mass

Abstract

Understanding of water and mass transfer in fractured rock mass is essential to the safety and environmental problems associated with rock engineering. It is still challenging to precisely predict the transport behaviors of water and solute in rock mass due to the large computational requirements when considering the presence of massive fractures and the mass exchange between rock matrix and fractures. Based on the discrete fracture network (DFN) model, a three-dimensional (3D) numerical approach is proposed to investigate the coupling behavior of groundwater flow and solute transport in fractured rock mass. Utilizing a stochastically generated fracture-matrix system, numerical simulation for groundwater flow and solute transport is implemented for both rock matrix and fractures with finite element method (FEM). With rock matrix discretized as tetrahedral elements and discrete fractures represented by zero-thickness elements, the modeling complexity is significantly reduced. The proposed approach is validated with various means including analytical, experimental and numerical methods. It is further employed to simulate the mass transport process in rock mass containing large-scale fracture network, predict the solute concentration distribution and estimate the dominant factors influencing the solute field. From the results, the main features of mass transfer in fractured rock mass are captured. Water convection in fractures could greatly affect the distribution of the solute concentration, indicating that the discrete fractures play an important role in accelerating the mass transport. The diffusion coefficient of the rock matrix also alters the overall transport characteristics of solute species, which should not be ignored for a precise prediction. The findings of this study could provide a direct reference for optimizing the pollutant treatment plan and the construction design of the radioactive waste repository.