Impact of high‐permeability preferential flow zones on pumping‐induced airflow in unconfined aquifer

Abstract

When the unsaturated zone of the unconfined aquifer is covered by a lowpermeability upper layer, significant airflow will be generated in the unsaturated zone during water pumping. However, high permeability preferential flow zones (PFZs) such as fractures and macropores are frequently present in the unsaturated zone, forming the preferential fluid flow paths, which may change the original airflow pattern in the unsaturated zone during the pumping test and consequently affect the precision of obtained aquifer hydraulic parameters. The main objective of this paper is to investigate the effect of PFZs in low-permeability upper layer on pumpinginduced airflow in the unsaturated zone by numerical simulations of transient threedimensional air-water two-phase flow and to quantify errors in the aquifer hydraulic parameters obtained during pumping test. The results demonstrate that a large amount of air flows quickly from the atmosphere into the unsaturated zone through the PFZs, and that the PFZs can draw some air from the nearby low-permeability soils as well. The significant influx of air through PFZs also reduces the negative air pressure in the unsaturated zone and decreases the drawdown in the saturated zone at intermediate times, which are nevertheless still greater than the results obtained in the homogeneous aquifer. Estimations of the aquifer hydraulic parameters reveal that errors of these parameters obtained are smaller when the PFZs with favourable combinations of permeability, width and quantities facilitate more air to flow into the unsaturated zone. K E Y W O R D S air-water two-phase flow, preferential flow zone, pumping test, two-phase numerical simulation, unconfined aquifer 1 | INTRODUCTION Hydraulic parameters of aquifers play an important role in the studies of hydrological modelling, climate change, and civil engineering, which include, but are not limited to, rainfall infiltration, groundwater resources utilization, groundwater impacts on the climate system, and