Solute transport through fractured media: 2. Column study of fractured till

Abstract

A tracer solution containing Cl and Ca was passed through a large intact cylindrical sample of fractured clayey glacial till obtained from a depth of 5 m in a fresh excavation. The sample was intersected by two orthogonal sets of fractures oriented parallel to the long axis of the column. Breakthrough curves for Cl and Ca show that these species were significantly retarded in relation to the average fracture flow water velocity as they passed through the column. The retardation is attributed to the effect of diffusion of the solutes from the fractures into the porous matrix of the till. The relative positions of the Ca and Cl breakthrough curves are attributed to the combined influence of a smaller effective diffusion coefficient for Ca than for Cl and adsorption of Ca in the matrix during the diffusion process. Relatively good agreement with the experimental data was achieved in mathematical simulation using the finite element solute transport model described by Grisak and Pickens (1980). Effective diffusion coefficients for Cl and Ca in the till matrix were found to be 5.0 × 10 −7 and 1.9 × 10 −7 cm 2 /s, respectively. The lack of total continuity between fracture sets was accounted for in the model by using an effective fracture spacing and effective aperture width based on the concepts of flow in planar fractures in sets of simple geometry.