Air and water flows in a large sand box with a two-layer aquifer system

The pumping of water from a sand box with a finer layer on the top was studied theoretically and experimentally. The sand box, saturated at its lower portion and initially in hydrostatic equilibrium, was pumped at constant rate. The results show that significant negative air pressure can be generated in the vadose zone during pumping. The negative air pressure increases quickly in the earlier stage of pumping, reaches a maximum, and then gradually becomes zero. The initial water-table depth has a significant effect on the generated negative air pressure. The shallower the initial water table, the larger the vacuum, and the longer the time to reach the maximum vacuum. A transient, three-dimensional model was constructed using TOUGH2-MP to simulate the air-water two-phase flow processes in the sand box. The reasonable match between the numerical solutions and the experimental data indicates that the numerical model can reproduce the dynamic process of air and water flows. The study has implications in pumping test analyses. If the air pressure in the two-layer system is ignored, the drawdown in the system will be underestimated, especially when the upper layer has low permeability and the initial water table is close to the interface of the two layers.