New publication, published in the scientific journal "Water Resources Research". The work has been developed in the context of the SFB 1313 research project C05, associated research project C-X5, and A02.
"Non‐Invasive Imaging of Solute Redistribution Below Evaporating Surfaces Using Na‐MRI"
Authors
Abstract
Saline water evaporation from porous media is a key phenomenon in the terrestrial environment and is linked to problems such as soil salinization and weathering of building materials. Recent modeling studies suggest the development of local instabilities due to density differences during evaporation in case of saturated porous media with high permeability. To experimentally investigate this and improve our understanding of near surface solute accumulation, we performed evaporation experiments on two types of porous media (F36 and W3) with intrinsic permeabilities that differed by two orders of magnitude. Using magnetic resonance imaging (Na‐MRI), we monitored the development of solute accumulation and subsequent redistribution during evaporation under wicking conditions. The F36 sample showed an initial enrichment at the surface, but soon after a downwards moving plume developed that redistributed NaCl into the column. Average depth profiles of Na concentrations obtained from 3D imaging showed that the surface concentration reached only 2.5 mol L-1, well below the solubility limit. In contrast, the W3 sample with lower permeability showed enrichment in a shallow near‐surface zone reaching a concentration of over 6 mol L-1. No fingering occurred although the mean evaporation rate was similar to that of F36 sand. Comparison of experimental results with numerical simulations using DuMux for both samples showed qualitative agreement between measured and modeled solute concentrations. This study experimentally confirms the importance of density‐driven redistribution of solutes in case of evaporating saturated porous media, carrying implications for predicting evaporation rates and the time to start of salt crust formation.