Sand column experiments and modeling study on coupling between groundwater level change and air flow

Abstract

The change of groundwater level drives air flow in the vadose zone, and the air flow in turn interacts with groundwater flow. This kind of coupling between groundwater level change and air flow becomes more apparent when the unconfined aquifer is covered by a low-permeability layer. Intake and drainage experiments were carried out in a double-layer sand column with fine sand over coarse sand, using the thin fine sand layer as the low-permeability confining layer in this study. As the water level declines in the drainage experiment, significant vacuum can be generated in the vadose and air flows from atmosphere into the column. In contrast to the drainage experiment, when the water level uplifts in the intake experiment, air pressure in the vadose zone increases and air flows outward. The change of vadose zone air pressure with time shows a single peak and is affected by the thickness of the fine sand layer. Based on the Darcy flow of groundwater in the saturated zone and the linear seepage of compressible air in the vadose zone, a simplified kinetic model is proposed to explain the air-water movement in the sand column and Runge-Kutta algorithm was used to solve the model, the observed vadose zone air pressure was reproduced. Simulation results show that the maximum air pressure in the vadose zone increases nonlinearly with the increasing of the thickness of the low-permeability layer.