Quantification of solute transport in a fracture-matrix system using geoelectrical monitoring

Solute transport in fracture is easy to diffuse into the matrix due to the permeability of the matrix. The quan-tification of solute concentration is particularly important during diffusion and is a challenge for the manage-ment of pollutants in a fracture-matrix system. In this paper, sodium chloride as solute, the geoelectrical technology was combined with high density electrical method and conductivity acquisition method to conduct a series of sandbox experiments. In the center of the sandbox is a fracture, and on both sides of the fracture is the matrix. The fracture and matrix in the sandbox form a fracture-matrix system. Loading-flushing process of so-dium chloride solution in fracture-matrix system was monitored in real-time and non-destructive. The effect of matrix porosity and flow velocity on solute transport in fracture-matrix system was investigated. The results showed that: 1) Mass transfer rate coefficient (alpha) and fracture/matrix porosity ratio (beta), and tailing exponent of residence time distribution (omega) were quantified by pairing bulk electrical conductivity (BEC) and fluid electrical conductivity (FEC), without the requirement of inverse model calibration. In contrast, the single electrical method still lacks resolution or is not representative in determining solute transport parameters in fracture -matrix system. 2) The matrix porosity had a significant effect on solute transport in fracture-matrix system. With the increase of porosity, the mass transfer rate coefficient (alpha) increased and the residence time in the matrix decreased. 3) An increase of flow velocity led to a decrease of residence time in the matrix and an increase in mass transfer rate coefficient (alpha). The solute transport in fracture-matrix system can be quantified by electrical conductivity hysteresis. The results can provide a basis for the management of solute transport in fracture-matrix system