Direct Characterisation of Solute Transport inUnsaturated Porous Media using 4D X-raySynchrotron Microtomography


Direct Characterisation of Solute Transport inUnsaturated Porous Media using 4D X-raySynchrotron Microtomography

Sharul, H.; Vahid, J.-N.; Nikolaos, K. K.; Da Assuncao Godinho, J. R.; Nghia, T. V.; Senyou, A.; Arash, A.; Holger, S.

Solute transport in unsaturated porous materials is a complex pro-cess, which exhibits some distinct features differentiating it fromtransport under saturated conditions. These features emerge mostlydue to the different transport time scales at different regions of theflow network, which can be classified into flowing and stagnantregions, predominantly controlled by advection and diffusion, re-spectively. Under unsaturated conditions, the solute breakthroughcurves show early arrivals and very long tails, and this type of trans-port is usually referred to as non-Fickian. This is the first studywhich directly characterise transport through an unsaturated porousmedium in three spatial dimensions at the resolution of 3.25μm andthe time resolution of 6s. Using advanced high-speed, high spa-tial resolution, synchrotron-based X-Ray Computed Microtomogra-phy (sCT) we obtained the first detailed information on solute trans-port through a glass-bead packing at different saturations. A largeexperimental dataset (>50TB) was produced, while imaging the evo-lution of the solute concentration with time at any given point withinthe field of view. We show that the fluids’ topology has a critical sig-nature on the non-Fickian transport, which has not been addressedin the theories of transport through unsaturated porous media. Wedemonstrate that the "fully-mixing" assumption at pore scale is notvalid, due to the significant impact of the no-slip boundary of thesolid walls. Results demonstrate that dispersivity, as a major trans-port parameter, is changing with saturation, being two-fold larger atsmaller saturations compared to that at high saturations.

Keywords: contaminant transport; X-ray microtomography; stagnant saturation; non-Fickian transport; unsaturated flow

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Publ.-Id: 31297