Mass Transfer Effects on Tracer Tests Conducted in a Heterogeneous Rock Fracture

A methodology based on a combination of numerical simulation of conservative transport and an analytical solution for combined matrix diffusion and surface sorption is presented for the study of tracer tests conducted under non-uniform flow conditions in heterogeneous fractures. The methodology is based on a Monte Carlo technique with multiple realizations; hence uncertainty in transport estimates can be assessed. In an illustration example, the methodology is implemented for dipole pumping conditions ( i.e. one injection borehole and one retrieval borehole) in a single fracture. The effect of experimental conditions such as pumping rate and tracer properties on matrix diffusion are investigated. Due to the kinetic character of the mass transfer process, it is shown that for high flow rates with consequent high advection velocities and short residence times relative to the mass transfer rate, matrix diffusion only develops to a minor extent. The effect of fracture heterogeneity is also exemplified. The results indicate that both heterogeneity in transmissivity and matrix diffusion may be manifested as tailing in solute arrival. Hence it is generally difficult to distinguish these processes under experimental conditions.