Non-reactive solute diffusion in unconfined and confined specimens of a compacted soil

The effect of specimen confinement on the determination of the effective diffusion coefficients, D*, for chloride, a non-reactive (non-adsorbing) solute, diffusing in a compacted soil was evaluated. The diffusion tests were performed by placing an acetic acid/sodium acetate buffer solution containing ZnCl2 (pH approximately 4.8) in a reservoir in contact with unconfined and confined specimens of a compacted sand-clay mixture for test durations of 7 or 14 d. The concentrations of chloride in the reservoir were measured as a function of time during the test, as well as a function of depth within the specimen at the end of the test. The resulting concentration distributions were analyzed using two models to Fick's second law for non-reactive solute diffusion in porous media, viz., (1) an analytical model assuming the porosity distribution could be represented by a single, weighted mean porosity and (2) a commercially available model, POLLUTE, that directly accounted for the measured porosity distribution. The D* for unconfined specimens based on the analytical model tended to be overestimated by a factor ranging from 1.13 to 1.59 relative to the D* using POLLUTE, whereas the D* values based on both methods for confined specimens typically were more consistent. In addition, the D* for unconfined specimens was greater than the D* for confined specimens when soil concentrations were used for the analysis, presumably due to the higher porosity for the unconfined specimens relative to the confined specimens. Analyses based on reservoir concentrations were inconsistent and contradictory in some cases, suggesting that the D* values based on soil concentrations were more reliable.