A Comprehensive Multi-Scale Study of the Impact of Capillary-Held Water Films on Fluid Motion, Transport and Mass Transfer in the Unsaturated Zone

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A Comprehensive Multi-Scale Study of the Impact of Capillary-Held Water Films on Fluid Motion, Transport and Mass Transfer in the Unsaturated ZoneThe knowledge resulting from the project could ultimately lead to improved models for a wide range of unsaturated phenomena important to practical hydrologic, agricultural, and environmental applications, including prediction of water infiltration into soils and flooding during heavy rainfalls, remediating polluted soils and groundwaters, and movement of fertilizers, pesticides, etc. When water drains out of a porous medium, films of water remain behind on solid grain surfaces. For this reason, water films are ubiquitous in the unsaturated zone under dry conditions, and they play a central role in a wide range of phenomena. Although the magnitude of water film area is substantial, sustained mass transfer at air-water interfaces depends on the extent to which films on solid surfaces are hydraulically connected and mobile enough to be replenished from bulk porewater. Recent information has shown that water films on solid grain surfaces are likely orders of magnitude thicker than previously thought, and that their configuration and thickness are dominated by the roughness of solid grain surfaces. The implication of this finding is that flow and transport in water films on solid grain surfaces are likely far greater than previously thought. However, very little quantitative information is available on how flow and transport occur in grain surface-associated water films. This project is aimed at addressing this critical knowledge gap. The proposed work will combine experimental work spanning a range of cutting-edge techniques with a modeling effort to arrive at a quantitative, predictive understanding of the behavior of capillary-held water films in the unsaturated zone. Experiments and simulations will be conducted at scales ranging from the nanoscale to the grain cluster scale. Stereoscopic SEM will be used to create highly detailed surface roughness maps of real grains and grain clusters, and confocal microscopy techniques will be used to study the impact of external inputs (capillary pressure, evaporation) on dynamic film configuration, flow, and advective transport. Work will be focused around four tightly-coupled tasks designed to test the hypotheses of the work, and then explore the implications of the observed sub-pore-scale film phenomena on REV and field scale behaviors.