Experimental and Numerical Investigation of Evaluation of Grain Size–Based Porosity Models for Solute Transport through Porous Medium

Abstract

Uncertainties associated in tracer transport arising through various geological formations are often sorted by laboratory experiments on representative elementary volume (REV). To understand the effect of grain size and packing on plume behavior, tracer transport experiments were performed in fully saturated condition for four different geological materials (glass beads, sand, and coarse and fine natural soil). A mini-aquifer setup having inner dimensions of 150 × 10 × 50 cm was used with sodium chloride (NaCl) as a tracer. The materials were selected to gradually increase the heterogeneity from relatively homogeneous (glass beads) to heterogeneous (natural Himalayan foothill field soil). The aim of the experimental investigation was to study contaminant transport and to examine applicability of various porosity models for particles for a range of geological materials. Simulation of observed breakthrough curves (BTCs) were carried out using a mobile–immobile model (MIM) and continuous time random walk (CTRW) model. Observed experimental breakthrough curves demonstrated relatively symmetrical spreading in glass beads, while natural soil exhibited the presence of preferential flow (physical nonequilibrium). The estimation of porosity from empirical relations deviated significantly from in situ measured porosity. Comparing CTRW and MIM with various porosity model, it is concluded that MIM provided a better representation of observed BTCs.