Changes in Entrapped Gas Content and Hydraulic Conductivity with Pressure

Abstract

Water table fluctuations continuously introduce entrapped air bubbles into the otherwise saturated capillary fringe and groundwater zone, which reduces the effective (quasi-saturated) hydraulic conductivity, K(quasi), thus impacting groundwater flow, aquifer recharge and solute and contaminant transport. These entrapped gases will be susceptible to compression or expansion with changes in water pressure, as would be expected with water table (and barometric pressure) fluctuations. Here we undertake laboratory experiments using sand-packed columns to quantify the effect of water table changes of up to 250 cm on the entrapped gas content and the quasi-saturated hydraulic conductivity, and discuss our ability to account for these mechanisms in ground water models. Initial entrapped air contents ranged between 0.080 and 0.158, with a corresponding K(quasi) ranging between 2 and 6 times lower compared to the K(s) value. The application of 250 cm of water pressure caused an 18% to 26% reduction in the entrapped air content, resulting in an increase in K(quasi) by 1.16 to 1.57 times compared to its initial (0 cm water pressure) value. The change in entrapped air content measured at pressure step intervals of 50 cm, was essentially linear, and could be modeled according to the ideal gas law. Meanwhile, the changes in K(quasi) with compression-expansion of the bubbles because of pressure changes could be adequately captured with several current hydraulic conductivity models.

Numerical study on the influence of entrapped air bubbles on the time-dependent pore pressure distribution in soils due to external changes in water level

“…Laboratory tests on different soil types [1,4,6,7] show a decrease in the hydraulic conductivity with increasing entrapped air content. Usually this decrease in hydraulic conductivity is described by a power law.…”

“… max is the volumetric content of entrapped air, which corresponds to the minimum quasi-saturated hydraulic conductivity k 0 . n is a fitting parameter [4]. The relative hydraulic conductivity k rel in Equation (1) is the ratio between the quasi-saturated hydraulic conductivity and the saturated hydraulic conductivity.…”

“…A comparison of different laboratory tests on the hydraulic conductivity of quasi-saturated soils [4] showed a significant difference in the relative hydraulic conductivity between coarse soils, like sand, and finegrained soils -see Figure 2. The entrapped air saturation is the volumetric air content divided by the porosity.…”

“…Observations in the field [3,1] and laboratory tests [1,4] show that soils below the ground water table are often in a quasi-saturated state. This has an influence on the behaviour of the pore fluid.…”

“…This has an influence on the behaviour of the pore fluid. The air bubbles in the soil can occur due to different reasons, for example fluctuating water tables [4]. Such fluctuating water tables are possible, amongst others, in storage basins of pumped storage power plants and in ship canals.…”