The Role of Diffusive Mass Transfer on Nonreactive Solute Transport in Fractured Volcanic Tuff
Timothy J. Callahan, Paul W. Reimus, and Robert S. Bowman
The objective of this study was to investigate the behavior of nonreactive solute diffusion in fractured, porous geologic media. We performed transport experiments in rock cores containing one mechanically-induced fracture at different flow velocities using two nonreactive anion tracers of different molecular diffusion coefficients in fractured cores of relatively porous volcanic tuff. A dual-porosity model was used to interpret the transport data that treated advection and dispersion processes as the dominant transport mechanisms in the fracture domain and molecular diffusion acting to distribute the tracer mass into the unfractured porous matrix. The purpose of conducting two experiments in each core at different flow velocities was to study the relative importance of matrix diffusion under different experimental flow conditions. Initial interpretations of the transport data show that the dual-porosity approach accurately represents tracer transport in the fractured rock cores; however diffusive mass transfer coefficients appeared to be much smaller at lower flow velocities. This was probably due to the finite pore volume in the unfractured matrix during the low velocity tests. Accounting for the finite pore volume in the matrix significantly reduced this apparent time scale effect. Conducting tests with two tracers of different diffusion coefficients effectively separated the relative importance of advection and diffusion in dual-porosity. Tracer tests run under different flow velocities provided further information about the role of diffusion in distributing solutes within heterogeneous systems.Data collected at the laboratory core scale show that diffusive mass transfer coefficients for solute tracers were up to ten times greater than those deduced from field tests of the same solutes in the same rocks. Therefore, laboratory experiments on rock cores overestimated the role of diffusion in these fractured porous rocks.