Kinetics Of Dissolution Of Trapped Air Bubble In Quasi-saturated Porous Media And The Effect On The Hydraulic Conductivity

The presence of trapped air has been shown to affect groundwater processes extensively,especially the dissolution of trapped air can significantly reduce the hydraulic conductivity of aquifers,as a result of groundwater level fluctuations leading to the trapping of air bubbles in the upper layer of the phreatic water.In order to investigate the dissolution process of trapped gas in porous media and its effect on soil permeability in detail,this paper,the indoor onedimensional vertical trapped gas dissolution experiments were carried out,mainly achieved the following results:(1)Obtained the basic law of dissolution of trapped air.The dissolution rate of trapped air was first fast,then slow and finally tended to a stable value,because the oxygen concentration gradient at the air-liquid interface,which was the driving force of mass transfer,was larger in the early stage of the experiment.With the diffusion and consumption of oxygen molecules,the concentration gradient gradually decreased,and the mass transfer rate slowed down until it reached equilibrium with the liquid concentration.(2)A method for estimating the number and radius of trapped bubbles was proposed.The number of trapped bubbles was the basis for calculating the mass transfer coefficient and specific surface area.This paper assumed that the solid particles were arranged in a relatively stable cube with equal particle size in space,and the number and radius of the trapped bubbles were defined by the measured particle number,porosity and trap gas saturation.The mass transfer parameters between gas-water such as mass transfer coefficient and specific surface area are further estimated using empirical equations.(3)A kinetic model of oxygen steady-state dissolution was established by using the mass transfer rate coefficient inversed from the solute concentration at the dissolution equilibrium.(4)The law of hydraulic conductivity change in the process of trap gas dissolution was clarified,and a quantitative demonstration was carried out.The pore space previously occupied was released during the dissolution of trapped air,which made the hydraulic conductivity of the porous media increase.The increasing speed of hydraulic conductivity increased from fast to slow,and finally tended to the value of full saturated hydraulic conductivity which was consistent with the bubble dissolution rate.The change of hydraulic conductivity values of sand samples at different heights showed that the dissolution of trapped air bubbles started from the position near the water inlet and then gradually dissolved upward.The volume of trapped air decreased continuously in the process of dissolution.When the gas saturation is small,the position occupied by bubbles is the position that affects the water flow.At this time,the dissolution of bubbles has a great influence on hydraulic conductivity.