Air and Water Flows Induced by Pumping Tests in Non-Uniform Unconfined Aquifer System

The impact of air flow on groundwater flow to wells has not been considered in most analytical and numerical models developed to analyze pumping test data. However, if there is a low permeability layer on the top of the unconfined aquifer or a low permeability lens in the aquifer, negative air pressure may be generated during pumping and may have impact on drawdown in the aquifer and subsequent aquifer parameter estimation. A transient, three-dimensional air-water two-phase flow model is employed to investigate the pumping-induced air and water flows in an unconfined aquifer with a low permeability layer on the top. The aquifer, initially in the condition of hydrostatic equilibrium, is pumped at constant rate. The results show that negative air pressure can be generated in the vadose zone of the aquifer. The negative air pressure starts to increase when the drawdown rate becomes significant, reaches a maximum, and gradually returns to zero when the drawdown rate approaches zero. Because of the negative air pressure in the vadose zone, water is sucked and gravity drainage is reduced. Consequently the drawdown is much larger than without the low-permeability layer. As a result, the drawdown versus time curve does not conform to the classical S-shaped curve for an unconfined aquifer. Various non-uniform scenarios such as local low-permeability zones above the pumping well and around the water level fluctuation zones near the observation well are simulated to explore the air pressure generated by low-permeability lens and its impact on drawdown.