Numerical modeling of contaminant transport in fractured crystalline rocks (FCRS)

Abstract

The prediction of contaminant transport in saturated fractured media is a difficult task, due to the complexity of fracture geometry, connectivity, heterogeneity and anisotropy of hydraulic conductivities and chemical processes. Generally, the simulation of contaminant transport in fractured media is by the discrete fracture network (DFN) approach. However, this approach is not appropriate for district to regional scale due to computational challenges and the extensive data required by discrete fracture models for fracture system characterisation. In this paper, a hybrid approach that combined the advective-dispersion equation (ADE) continuum model, and discrete fracture network (DFN) simulation is used for the prediction of the contaminant transport in the fractured crystalline rocks within southeast Ghana. The hydrogeology of the study area consists of crystalline rocks with negligible matrix porosity. The approximate geometry of the problem was represented as a horizontal aquifer without curvature, and no-flow boundaries existing at the top and bottom of the aquifer. A MATLAB code is developed for the finite difference method to obtain the numerical solution. The study demonstrates that, the major factors affecting contaminant migration in the study area include, hydraulic conductivity which depends on fracture connectivity, aperture and infill material, and the retardation factors of the various contaminants. Results of transport simulation indicate that, contaminants could travel between 1.0 to 1.7 m/day in the horizontal direction. This spreading effect of plume movement through the fractured rocks can pose critical environmental problems.