The Impact of Capillary Trapping of Air on Satiated Hydraulic Conductivity of Sands Interpreted by X-ray Microtomography

Abstract

The relationship between entrapped air content and the corresponding hydraulic conductivity was investigated experimentally for two coarse sands. Two packed samples of 5 cm height were prepared for each sand. Air entrapment was created by repeated infiltration and drainage cycles. The value of K was determined using repetitive falling-head infiltration experiments, which were evaluated using Darcy’s law. The entrapped air content was determined gravimetrically after each infiltration run. The amount and distribution of air bubbles were quantified by micro-computed X-ray tomography (CT) for selected runs. The obtained relationship between entrapped air content and satiated hydraulic conductivity agreed well with Faybishenko’s (1995) formula. CT imaging revealed that entrapped air contents and bubbles sizes were increasing with the height of the sample. It was found that the size of the air bubbles and clusters increased with each experimental cycle. The relationship between initial and residual gas saturation was successfully fitted with a linear model. The combination of X-ray computed tomography and infiltration experiments has a large potential to explore the effects of entrapped air on water flow.

Surface Water and Groundwater Interactions in Salt Marshes and Their Impact on Plant Ecology and Coastal Biogeochemistry

“…For example, evapotranspiration and plant root rehydration may lower the marsh watertable continually between semidiurnal flooding tides, day and night (Dacey & Howes, 1984), but applying Darcy's Law to the head difference between the watertable and flood water would be mechanistically incorrect. Capillary rise may maintain the smaller pores of a fine‐textured marsh surface in a saturated condition (i.e., capillary fringe intersecting the marsh surface) even throughout low tides, whereas capillary suction may both enhance surface water infiltration in smaller pores and decrease overall hydraulic conductivity via air blocking by entrapped air accumulation in larger pores' preferential flow pathways (Princ et al., 2020; Snehota et al., 2015). This entrapment of an aerated layer, its pressures, gas composition, and compression dynamics during tidal flooding were documented decades ago by Chapman (1938) and Chapman (1940).…”

“…This entrapment of an aerated layer, its pressures, gas composition, and compression dynamics during tidal flooding were documented decades ago by Chapman (1938) and Chapman (1940). Although likely broadly understood in theory by coastal and wetland hydrogeologists, the specific application to the case of empirical and modeling study of salt marshes of distinctions between true saturated hydraulic conductivity, quasi-saturated hydraulic conductivity with entrapped air, and unsaturated hydraulic conductivity with atmospherically connected air fluid phase (Princ et al, 2020;Sakaguchi et al, 2005) has yet to be thoroughly developed. Notwithstanding reinvigoration of shallow marsh plant-groundwater studies by Ursino et al (2004), following earlier work (Dacey & Howes, 1984;Nuttle & Hemond, 1988), further investigation of relations among soil water and evapotranspiration in salt marshes is still warranted, as has been done in other wetlands (Schwärzel et al, 2006).…”

Performance of experimental bioretention cells during the first year of operation

“…The maximal volumetric water contents in the biofilter layers were nearly the same for both bioretention cells, slightly over 0, which is less than the θs content of the biofilter that is 0.46 cm 3 •cm -3 . The reason why θs was never reached can be explained by the air entrapment in the soil pores where air content can easily reach over 10%, even in homogeneous sand (Princ et al, 2020). The other reason for TDR not reaching the saturated water content can be the use of the general Topp equation (Topp et al, 1980) for TDR probes.…”

Water Flow, Solute and Heat Transfer in Groundwater

“…The results from single well and wellfield indicated that interferences between wells are important for WHPA delineation, and thus that only semi-analytical and numerical modelling are recommended for WHPA delineation at wellfields. Princ et al (2020) [7] investigated experimentally the relationship between the entrapped air content and the corresponding hydraulic conductivity for two coarse sands. The amount and distribution of air bubbles were quantified by micro-computed X-ray tomography (CT) for the selected runs.…”