Numerical analysis of solute transport in variably saturated bimodal heterogeneous formations with mobile–immobile-porosity

Considering flow and transport in three-dimensional, variably saturated, composite bimodal heterogeneous formations, the main purpose of this study was to extend the previous analyses [37], restricted to the one-region case in which the entire water-filled pore space is mobile, to the two-region case in which part of the water-filled pore space of each of the sub-soils of the composite formation is stagnant, and to investigate the effect of the interaction between the mobile and the immobile regions on solute transport in these formations. Following Russo [37], formations with fine- and coarse-textured embedded soils (FTES- and CTES-formations, respectively), were considered in the analyses. Main results of the present study suggest that mass exchange between the two regions masks features of the transport that exist in bimodal, one-region flow domains, related to characteristics of the unsaturated hydraulic conductivity in variably saturated bimodal, heterogeneous formations. In particular, the crossover behavior (i.e., that under relatively wet conditions, solute spread is larger in the FTES-formations than in the CTES-formations, while the opposite occurs under relatively dry conditions) characterizing one-region, bimodal flow domains disappears in two-region, bimodal flow domains. The latter attributes to the transfer of mass from the mobile region to the immobile region and the extension of the capture zone for the solute particles associated with the fine-textured embedded soil to lower water saturations. Consequently, for both steady state- and transient-flows, as water saturation decreases, the response of the composite formations is essentially independent of the texture of the embedded soil.

► Mass exchange between the two regions (ME) increases the solute spread.
► ME masks features of the transport which exit in bimodal, one-region flow domains.
► Response of the bimodal flow domain becomes independent of the embedded soils’ texture as water saturation decreases.