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@INPROCEEDINGS{Chaudhry:1037715,
      author       = {Chaudhry, Ali and Kiemle, Stefanie and Helmig, Rainer and
                      Pohlmeier, Andreas and Huisman, Johan Alexander},
      title        = {{N}on-invasive imaging of solute redistribution below
                      evaporating surfaces using 23{N}a-{MRI}},
      reportid     = {FZJ-2025-00875},
      year         = {2024},
      abstract     = {Evaporation from porous media is a key phenomenon in the
                      terrestrial environment and is linked to accumulation of
                      solutes at or near the evaporative surface. It eventually
                      leads to salinization, soil degradation and weathering of
                      building materials, topics with high economic impacts.
                      Although the detrimental effects manifest on different
                      scales from pores to the field, the key to understanding is
                      found on the pore scale since pore system connectivity and
                      structure control the solution behavior near the evaporation
                      surface. A thorough understanding requires the development
                      of physical models describing the most relevant processes
                      and their validation by experiments. Vice versa, new
                      experimental observations promote the further development of
                      the physical models.In this context, the current study aims
                      at the understanding of solute accumulation near evaporating
                      surfaces for model porous media at the cm-scale. Analytical
                      and numerical modelling predict the development of local
                      instabilities due to density differences during evaporation
                      in case of saturated porous media with high permeability,
                      which eventually causes density-driven backflow through
                      fingering [Bringedal et al. TPM 2022]. To experimentally
                      investigate this process, we performed experiments on sand
                      packings with a diameter of 3.1 cm and a height of 4 cm
                      prepared with two types of porous media: F36 (medium sand)
                      and W3 (fine sand/silt) with porosities of 0.37 and 0.39,
                      respectively. The intrinsic permeability of the two packings
                      differed by two orders of magnitude, i.e. 2.9×10-11 m2 for
                      F36 and 5.6×10-13 m2 for W3. Using magnetic resonance
                      imaging (23Na-MRI), we monitored the development of solute
                      accumulation and subsequent backflow with high spatial (1
                      mm) and temporal (1 hr) resolution during evaporation with a
                      continuous supply of water at the bottom of the samples
                      (wicking conditions).Significant differences between the
                      23Na enrichment patterns were observed for the two types of
                      sand. F36 sand produced an initial enrichment at the surface
                      within the first hour, but soon after a downwards moving
                      plume developed, hence redistributing NaCl back into the
                      column. This was attributed to density driven backflow made
                      possible by the high permeability. The backflow caused a
                      good mixing of the solute during the observation period of
                      120 h. 1D concentration profiles with depth obtained from
                      the 3D imaging showed that the average concentration reached
                      only 2.5 mol/L, well below the solubility limit of 6.13
                      mol/L. In contrast, for fine W3 sand with lower
                      permeability, enrichment only took place in a shallow
                      near-surface zone of a few mm with a maximum concentration
                      of 5.1 mol/L after 73 hours of evaporation. No fingering
                      occurred although the mean evaporation rate was similar to
                      that of the F36 sand. These results highlight the major role
                      that porous media properties play in solute redistribution
                      near evaporating surfaces, which was predicted by theory and
                      now confirmed experimentally. The findings encourage further
                      investigations involving different porous media with
                      systematic variation of hydrological properties and the
                      coupling of experimental results to numerical modelling.},
      month         = {May},
      date          = {2024-05-13},
      organization  = {Interpore2024, Qingdao (China), 13 May
                       2024 - 16 May 2024},
      subtyp        = {After Call},
      cin          = {IBG-3},
      cid          = {I:(DE-Juel1)IBG-3-20101118},
      pnm          = {2173 - Agro-biogeosystems: controls, feedbacks and impact
                      (POF4-217) / SFB 1313 C05 - Nicht-invasive Bildgebung von
                      Experimenten auf der REV-Skala zum Verständnis des
                      Einflusses von Fluid-Feststoff-Reaktionen auf Strömung und
                      Transport in porösen Medien (C05) (393212668)},
      pid          = {G:(DE-HGF)POF4-2173 / G:(GEPRIS)393212668},
      typ          = {PUB:(DE-HGF)6},
      doi          = {10.34734/FZJ-2025-00875},
      url          = {https://juser.fz-juelich.de/record/1037715},
}