Longitudinal dispersivity and saturation of sand-clay mixtures: Impact of clay content, initial degree of saturation, and swelling potential

Dispersion plays a critical role in predicting solute transport through unsaturated soils. Therefore, this study comprehensively investigated the unsaturated longitudinal dispersivity of sand-clay mixtures using laboratory soil-column experiments. The effects of clay content, average degree of saturation, flow path length, and swelling potential on the unsaturated longitudinal dispersivity were examined. The longitudinal dispersivity was evaluated based on the observed breakthrough curves using the advection-dispersion equation. It was found that an increase in illite content, initial degree of saturation, flow path length, and swelling potential led to an increase in longitudinal dispersivity. In addition, the longitudinal dispersivity under saturated conditions was lower than when the initial average degree of saturation was 80 % whereas an increase in unsaturated longitudinal dispersivity as the initial degree of saturation increased from 20 to 80 % was observed. This observation led to a bilinear trend of longitudinal dispersivity as a function of the initial degree of saturation in sand-clay mixtures. The trend in the longitudinal dispersivity of sand-clay mixtures observed in this study differs from that reported in the literature for sand, suggesting the need for incorporating clay content, swelling potential, and initial degree of saturation when predicting the unsaturated longitudinal dispersivity of clay-containing soils.