Characterizing the contribution of diffusive mass transfer to solute transport in sedimentary aquifer systems at laboratory and field scales

The multi-tracer test method was used to investigate the impact of diffusion-mediated mass-transfer processes associated with physical heterogeneity on solute transport in sedimentary aquifer systems at different scales. Three systems were examined: (1) a column packed with homogenized aquifer material, (2) undisturbed aquifer cores, and (3) a section of a semi-confined alluvial aquifer encompassing approximately 2000 m². Tracer experiments were conducted using non-reactive tracers with different aqueous diffusion coefficients, based on the assumption that the impact of diffusion-mediated mass-transfer processes on solute transport will be dependent upon the magnitude of the diffusion coefficient. For the homogenized packed column, the breakthrough curves for the different tracers were very similar, and the attendant dispersivities were identical. These results indicate, as would be expected, that physical-heterogeneity related diffusive mass transfer did not measurably contribute to solute transport in the homogeneously packed column. Conversely, the results obtained for the undisturbed aquifer cores indicate that diffusive mass transfer, possibly associated with diffusion between zones of lower and higher advection, contributed to solute transport in this system. For the field test, the magnitudes of the dispersivity coefficients were different for the different tracers. Thus, solute transport at the field scale appeared to be influenced by diffusive mass transfer, most likely associated with mass transfer between regions of lower and higher permeability. These results illustrate that tracer tests using tracers with different diffusivities can be used to help elucidate the relative contribution of diffusion-mediated mass transfer to solute transport.