Air and water flows induced by water level change in aquifers

Abstract

The study of air and water flows induced by water level changes is important for a better understanding of water release in response to pumping in unconfined aquifers and give new insights into pumping test data analysis. To improve the unsaturated flow and air-water two-phase flow numerical modeling of unconfined aquifer pumping tests, an improved unsaturated hydraulic conductivity model and a new relative air permeability model were proposed. A synthesis of the literature on subsurface airflow induced by natural forcings was presented to provide a better understanding of the generated airflow and its importance in environmental and engineering applications. A literature review was carried out to study the naturally occurring subsurface airflow. Analytical studies were carried out to derive the improved unsaturated hydraulic conductivity model and the new relative air permeability model. Experimental studies were conducted to investigate the air and water flows induced by drainage and pumping and numerical simulations were carried out using TOUGH2 and TOUGH2-MP to interpret the experimental data. Numerical models for unconfined aquifer pumping test analysis were built with TOUGH2-MP to investigate the effect of heterogeneity on pumping test results. The study of relative permeability models found that the relative permeabilities predicted by the proposed models are in better agreement with experimental data than existing models. Experimental and numerical investigations on air and water flows induced by drainage and pumping found that significant negative air pressure can be generated in a two-layered system with a low-permeability upper layer. The negative air pressure increases very fast after the start of pumping or drainage, reaches a maximum, and then gradually recovers to atmospheric pressure. The generated negative air pressure can significantly reduce the drainage of water from the column and increase the drawdown. Numerical studies of the effects of local heterogeneity on unconfined aquifer pumping test results found that the drawdown in an unconfined aquifer with a low-permeability zone is significantly greater than that of the homogeneous aquifers but the impact of the low-permeability zone diminishes in the long-term production period. This study presents state of knowledge on naturally occurring subsurface airflow and its implications, proposes improved and new relative permeability models for unsaturated flow and multiphase flow modeling, and offers new insights into the effect of air on water release and unconfined aquifer pumping test data analysis. The present study not only has theoretical significance but also has engineering implications.