ANALYSIS OF A MODEL FOR CONTAMINANT TRANSPORT IN FRACTURED MEDIA IN THE PRESENCE OF COLLOIDS

A mathematical model has been developed to study the cotransport of co ntaminants with colloids in saturated rock fractures. The contaminant is assumed to decay, and sorb on to fracture surfaces and on to colloi dal particles, as well as to diffuse into the rock matrix; whereas, co lloids are envisioned to deposit irreversibly on to fracture surfaces without penetration into the rock matrix. The governing one-dimensiona l equations describing the contaminant and the colloid transport in th e fracture, colloid deposition on to fracture surfaces, and contaminan t diffusion into the rock matrix are coupled. This coupling is accompl ished by assuming that the amount of contaminant mass captured by coll oidal particles in solution and the amount captured by deposited collo ids on fracture surfaces are described by modified Freundlich reversib le equilibrium sorption relationships, and that mass transport by diff usion into the rock matrix is a first-order process. The contaminant s orption on to fracture surfaces is described by a linear equilibrium s orption isotherm, while the deposition of colloids is incorporated int o the model as a first-order process. The resulting coupled contaminan t transport non-linear equation is solved numerically with the fully i mplicit finite difference method. The constant concentration as well a s the constant flux boundary conditions have been considered. The impa ct of the presence of colloids on contaminant transport is examined. A ccording to model simulations the results show that, depending on the conditions of the physical system considered, colloids can increase or decrease the mobility of contaminants.