MASS-TRANSFER EFFECTS ON TRACER TESTS CONDUCTED IN A HETEROGENEOUS ROCK FRACTURE

A methodology based on a combination of numerical simulation of conser vative transport and an analytical solution for combined matrix diffus ion and surface sorption is presented for the study of tracer tests co nducted under non-uniform flow conditions in heterogeneous fractures. The methodology is based on a Monte Carlo technique with multiple real izations; hence uncertainty in transport estimates can be assessed. In an illustration example, the methodology is implemented for dipole pu mping conditions (i.e. one injection borehole and one retrieval boreho le) in a single fracture. The effect of experimental conditions such a s pumping rate and tracer properties on matrix diffusion are investiga ted. Due to the kinetic character of the mass transfer process, it is shown that for high flow rates with consequent high advection velociti es and short residence times relative to the mass transfer rate, matri x diffusion only develops to a minor extent. The effect of fracture he terogeneity is also exemplified. The results indicate that both hetero geneity in transmissivity and matrix diffusion may be manifested as ta iling in solute arrival. Hence it is generally difficult to distinguis h these processes under experimental conditions.