The Effects of Disconnected Entrapped Air on Hydraulic Conductivity in the Presence of Water Table Fluctuations

Abstract

<p>The hydraulic conductivity of a groundwater system can possess high spatial and temporal variability in the presence of an entrapped air phase (quasi -saturated soils) which is a key factor in controlling hydraulic behaviour. Previous studies have provided evidence of reduced hydraulic conductivity caused by entrapped air; however, these did not address the dynamic behaviour of the entrapped gaseous phase. In this study, the hypothesis that the decreases in hydraulic conductivity caused by entrapped air are sensitive to fluctuations in the water table was tested using laboratory experiments. The effects of applying increasing confining pressures (water table elevation) on the nature of entrapped air and its effects on quasi-saturated hydraulic conductivity was investigated for a range of sands and for air entrapment by both upward water flow and ponded infiltration (downward flow). Laboratory experiments were conducted using the constant flux method on saturated/ quasi-saturated horizontally positioned sand columns using 1% bleach as the solution. Induced pressures ranged from 0 to 250 cm with changes in hydraulic conductivity calculated using collected timed-interval outflow discharge and the pressure gradient measured by a differential pressure transducer. The sand core was also instrumented to measure volumetric moisture (or air) content with time domain reflectometry (TDR) probes. Results show that a 250-cm increase in water pressure above atmospheric pressure induced changes in the volume of entrapped gas according to the ideal gas law indicating the primary and tertiary roles played by air phase compression and capillary pressure, respectively. The reduction in air content at the 250-cm pressure increased the quasi-saturated hydraulic conductivity by a factor of 1.20 to 1.64. The results were fitted to the van Genuchten (1980) and Faybishenko (1995) functions for unsaturated and quasi-saturated hydraulic conductivity, respectively. The changes in quasi-saturated hydraulic conductivity due to changes in air content are expected to ubiquitously occur in the presence of a fluctuating water table. Thus the understanding of this fundamental mechanism and its incorporation into cunent general models of flow and transport will aid in better understanding the unique role of entrapped air in groundwater systems.</p>