Influence of Various Dispersion Coefficients on Contaminant Migration in a Fracture-matrix System with Skin Formation

Abstract

Subsurface contamination due to transient migration of chemicals through weathered fractures is a serious problem in hard rock terrains. In this paper, a numerical model has been used to analyse the effect of various dispersion coefficients on the movement of contaminants in a fracture-matrix syste m with skin formation. A constant continuous source of contaminants is considered at the inlet of a set of parallel fractures and an implicit finite difference technique has been used to solve the model. The transport of contaminants with constant dispersion coefficients were compared with those subjected to distance and time dependent dispersion. Sensitivity analysis has been conducted to study the effect of different fluid velocities, fracture-skin porosities, fracture-skin diffusion coefficients, fracture aperture, retardation factors, and dispersivity-distance ratio on solute transport mechanism within the system. Results suggest that the solute mass retained in the fracture is higher when time-dependent dispersion coefficients are used which may be due to seasonal conditioning of the rocks. When constant dispersion coefficient is considered, the reduction in the solute mass with increment in the fracture-skin porosity is gradual unlike other cases.The solute mass retained in the fracture increases with increase in dispersivity-distance ratio for time-dependent dispersion coefficients.