This study shows an experimental study on solute removal through a large-scale rough fracture plane using a visualization technique. The removal process as well as the variations in solute ratio over time were captured using a high-resolution charged coupled device (CCD) camera, considering different hydraulic head differences. The results show that with increasing hydraulic head difference from 14.8 cm to 99.8 cm, the speed of solute removal increases. Since an inflow tank and an outflow tank are added on the two opposing ends of the model, the water can be theoretically injected into the model as a plane; however there are still some fluids that move faster than others, because of the roughness of fracture surface. The solute ratio over time decreases from approximately 1, which is actually less than 1 due to the existence of bubbles, to approximately 0, which is actually larger than 0 due to the folds that have been dealt as "solute". As the time increases, the variations in solute ratio among different hydraulic head differences increase and then decrease. For a fixed removal time, the larger flow rate of water, the smaller the solute ratio.
When the nuclear leaks occur, the nuclear materials will move into the ground and polute the underground water. Therefore, it is of special importance to study the removal process of solute/nuclear materials.
The previous studies commonly focused on the solute transport in the deep underground, by considering advection and hydrodynamic dispersion, sorption reactions, and matrix diffusion (Neretnieks, 1980; Tang et al., 1981; Bodin et al., 2003; Dou and Zhou, 2014; Zhao et al., 2014; Fiori and Becher, 2015; Ahmad et al., 2016; Guihéneuf et al., 2017). Grisak and Pickens (1980) described solute transport through a fractured media by combining fracture-dominated advective-dispersive transport and matrix-dominated diffusive transport. They found that the net effect of matrix that has a large diffusion coefficient can significantly reduce the effective solute velocity in the fracture. Kennedy et al. (1995) developed a control volume model for calculating solute transport in a single fracture, which can estimate matrix diffusion in the direction parallel to fracture axis and consider the influence of boundary condition on two-dimensional matrix diffusion. Chen et al. (2010) experimentally demonstrated contaminant removal from fractured rock through boiling. They reported that the chlorinated volatile compound is primarily removed by partitioning into vapor phase flow. Sund et al. (2015) extended the Spatial Markov model to upscale transport of a reacting solute across a diverse range of porous media flows, which can enforce the correlation between successive jumps and reproduce breakthrough curves that are measured from microscale simulations. Zhu et al. (2016) proposed an analytical solutions of solute transport in a fracture-matrix system that considers different reaction rates for fracture and matrix. They concluded that the first-order reaction and matrix diffusion in the fracture rocks decrease the solute peak concentration and shorten penetration distance into the fracture. Liu et al. (2018) derived a simple and robust solution for the problem of solute transport along a single fracture in a porous rock. This solution provided that the unchanged Peclet number with distance, which agrees with the observations in field experiments but cannot be predicted using the classical advection-dispersion equation.
