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A synchrotron X-ray Computed Tomography investigation of solute transport under unsaturated conditions

A synchrotron X-ray Computed Tomography investigation of solute transport under unsaturated conditions

Session

Stream 4: Diamond Light Source Session 2

Authors

Dr. Sharul Hasan (5), Dr. Vahid Joekar-Niasar (1), Dr. Nikolaos Karadimitriou (4), Dr. Jose R. A. Godinho (3), Dr. Nghia T. Vo (2), Senyou An (1), Arash Rabbani (1), Prof. Holger Steeb (4)

Affiliations

1. Department of Chemical Engineering and Analytical Science, University of Manchester
2. Diamond House, Harwell Science and Innovation Campus
3. Helmholtz Institute Freiberg for Resource Technology
4. Institute of Applied Mechanics (CE), University of Stuttgart
5. School of Chemical and Energy Engineering, Universiti Technologi Malaysia

Keywords

synchrotron, X-ray Computed Tomography,  solute transport,  unsaturated conditions, non-Fickian transport, advection, diffusion, dispersivity

Abstract text

Solute and contaminant transport is a process occurring in the subsurface with a vast socioeconomic impact. Despite the significant literature existing for single-phase solute transport, there is a significant scarcity of experimental data regarding multi-phase, commonly referred to as “unsaturated”, transport. This scarcity of experimental data has led to the adaptation of formulations from the single-phase transport case, leading to erroneous results. One of the characteristics of transport under unsaturated conditions is this of the early arrivals and elongated tails for the solute concentration, which is characteristic for non-Fickian transport. This is mostly attributed to the competition of two transport mechanisms, advection and diffusion, as the result of the interplay between stagnant and flowing zones.

This study focuses on the experimental investigation in three dimensions of solute transport under unsaturated conditions in glass-beads columns. Potassium iodide (KI) water solutions were used to displace the non-wetting phase, namely Fluorinert FC-43. The process was monitored using time-lapse computed tomography at a spatial resolution of 3.25 microns every  6 seconds, with imaging taking place at the tomography beamline I12 (27) at the Diamond Light Source, UK (Hassan et al., 2020). Various KI concentrations and flow rates were realized, and detailed information was acquired on solute transport at different carrier-phase saturation.

Our results show that the spatial distribution of phases, and the formation of the stagnant and flowing zones within water (the carrier phase), are critical in the development of non-Fickian transport, which should be included in the Darcy-scale transport models. We also experimentally verify the numerical speculation that a full-mixing assumption in a single pore is not consistent with our findings, and that even within a single pore, transport can exhibit two distinct time scales even after the introduction of multiple pore volumes. Finally, we show that dispersivity is also related to saturation. Dispersivity nearly doubled in the case of a half-saturated sample compared against this of a fully saturated one.

This experimental work is the first one to provide the scientific community with concrete evidence three-dimensional evidence, that the formulations describing solute transport under unsaturated conditions must be accordingly updated. Additionally, it provides indisputable proof that the use of the formulations developed for saturated (single-phase) conditions are not suitable to characterize unsaturated transport.

References

S. Hasan, V. Niasar, N. Karadimitriou, J. Godinho, N. Vo, S. An, A. Rabbania, and H. Steeb. Direct characterization of solute transport in unsaturated porous media using fast X-ray synchrotron microto- mography. Proceedings of the National Academy of Sciences, 117(38):23443–23449, 2020.