Experimental Study And Numerical Modeling On Coupling Between Water Table Change And Air Flow In Double-layer Media

The change of groundwater level would drive air flow in the vadose zone and theair flow will interact with groundwater flow. This kind of coupling betweengroundwater level change and air flow will become more apparent when theunconfined aquifer is covered by a low-permeability layer. Intake and drainageexperiments were carried out in a double-layer sand column with fine sand (thicknessis2-7.5cm) over coarse sand, which used the thin find sand layer as thelow-permeability confining layer.As the decline of the water level in the drainage experiment, significant vacuumcan be generated in the vadose zone of the column with a finer layer on the top and airflows from atmosphere into the column. Because of the effect of the vacuum in thevadose zone, water is sucked and the drainage speed is significantly smaller than thatwithout the fine sand layer, confirming the existence of vacuum decay effect. Incontrast to the drainage experiment, when the water level uplifts in the intakeexperiment, air pressure in the vadose zone increases and air flows outward. Thechange of vadose zone air pressure with time both shows a single peak in the drainageand intake experiments. The air pressure increases quickly in the earlier stage of theexperiments, reaches a maximum and gradually becomes zero. The maximum isaffected by the thickness of the fine sand layer, the more the thickness of the confininglayer, the value of the maximum is more essential and the time reach to the peak ismore longer.Based on the Darcy flow of groundwater in the saturated zone and the linearseepage of compressible air in the vadose zone, a simplified kinetic model is proposedto explain the air-water movement in the sand column and Runge-Kutta algorithm wasused to solve the model, the observed vadose zone air pressure was reproduced and themain kinetic characteristics of the sand column experiments is revealed. Simulationresults show that the maximum air pressure in the vadose zone increases nonlinearlywith the increasing of the thickness of the low-permeability layer. Based on thesimplified kinetic model, the approximate analytical solutions are obtained for the early and late periods of the drainage. In this paper, the kinetic model only need asmall amount of medium parameters to explain the changes of the pressure in thevadose zone and the location of the surface of saturation. The pressure in the vadosezone depends on the saturated permeability of the coarse, the air-permeability of thefine sand and its thickness. It is also found through parameter identifications that theair-permeability in the late period is significantly higher than that in the early period,which inflects the influence of water-content in the confining layer. This study showsthe significance of air flow in groundwater drainage from sands if a low permeabilitysoil exits on the top.Using the multi-physics simulation software COMSOL which based on the finiteelement method to simulate the drainage experiments of unconfined aquifer coveredby2cm or5cm fine sand layer on the top.Considering the compression of the air torewrite the air-water–two-phase flow control equation, the model suitable for theexperiment studied in this paper is established. The simulation results confirm thatvacuum can be generated in the vadose zone when the unconfined aquifer is coveredby a low-permeability layer. But the simulation results do not fit very well with themeasured data, the vacuum value is underestimated and the cumulative outflow isoverestimated so the specific reason need further studied.