Water flow and solute transport in unsaturated sand; a comprehensive experimental approach

Water flow and solute transport in unsaturated porous media are affected by the highly nonlinear material properties and nonequilibrium effects. This makes experimental procedures and modeling of water flow and solute transport challenging. In this study, we present an extension to the well-known multistep-outflow (MS-O) and the newly introduced multistep-flux (MS-F) approaches to measure solute dispersivity as a function of water content under well-defined conditions (i.e., constant pressure head and uniform water content). The new approach is termed multistep-transport (MS-T) and complements the MS-O and MS-F approaches. Our setup allows for applying all three approaches in a single experimental setting. Hence, it provides a comprehensive data set to parameterize unsaturated flow and transport processes in a consistent way. We demonstrate this combined approach (MS-OFT) for sand (grain diameter: 0.1-0.3 mm) and complemented the experimental results with an analysis of the underlying pore structure using X-ray computed tomography (CT). The results show that dispersivity is a nonlinear function of water content, and a critical water content (∼0.2) exists at which dispersivity increased significantly. The results could be explained by marked change in the geometry of the flow field as derived from X-ray CT measurements. It is characterized by a reduced connectivity of the water phase. The results demonstrate the potential of a combined approach linking pore structure, hydraulic functions, and transport characteristic.