Entrapped air effects on dipole flow test in sand tank experiments: Hydraulic conductivity and head distribution

Zlotnik, Vitaly A.; Eisenhauer, Dean E.; Schlautman, D.J.; Zurbuchen, Brian R.; Peursem, Dan Van

doi:10.1016/j.jhydrol.2007.03.013

Cited by 12 publications

(14 citation statements)

References 34 publications

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“…The reduction in hydraulic conductivity following initial air entrapment determined in these experiments (Table 2) fits well with many other studies involving sandy soils (Christiansen 1944;Orlob and Radhakrishna 1958;Constantz et al 1988;and Zlotnik et al 2007), which reported K quasi values of 2 to 5 times less than K s . In contrast, Faybishenko (1995), Bicalho et al (2005), Sakaguchi et al (2005), and Fry et al (1997) reported reductions in hydraulic conductivity following air entrapment of ≥10 times.…”

Section: Hydraulic Conductivity Behaviorsupporting

confidence: 89%

“…This pretreatment is a widelyused effective and accepted practice to attain a state of complete water saturation in soil columns and tanks (Faybishenko 1995;Zlotnik et al 2007). This pretreatment is a widelyused effective and accepted practice to attain a state of complete water saturation in soil columns and tanks (Faybishenko 1995;Zlotnik et al 2007).…”

Section: Experimental Method: Saturated Hydraulic Conductivitymentioning

confidence: 99%

“…Each sand column was flushed with 50 pore volumes (each equal to 1 L) of carbon dioxide (CO 2 ) and then a minimum of 25 pore volumes of de-aired water (1% bleach solution). This pretreatment is a widelyused effective and accepted practice to attain a state of complete water saturation in soil columns and tanks (Faybishenko 1995;Zlotnik et al 2007). The saturated hydraulic conductivity was determined using the same constant flux method as described above, except that measurements were only taken for water pressure steps 0, 150, 200, and 250 cm.…”

Section: Experimental Method: Saturated Hydraulic Conductivitymentioning

confidence: 99%

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Changes in Entrapped Gas Content and Hydraulic Conductivity with Pressure

2012

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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.

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Section: Hydraulic Conductivity Behaviorsupporting

confidence: 89%

Section: Experimental Method: Saturated Hydraulic Conductivitymentioning

confidence: 99%

Section: Experimental Method: Saturated Hydraulic Conductivitymentioning

confidence: 99%

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Changes in Entrapped Gas Content and Hydraulic Conductivity with Pressure

2012

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“…There is a wealth of experimental evidence showing the extent of the impact of entrapped air on the flow. Zlotnik et al [2007] reviewed early experiments and found that K QS of sand samples can be reduced by 5-50% due to the presence of entrapped air when compared to the fully saturated state. Even more pronounced K QS changes due to the presence of entrapped air were observed on intact samples of sandy loam soils by Faybishenko [1995].…”

Section: Introductionmentioning

confidence: 99%

Water and entrapped air redistribution in heterogeneous sand sample: Quantitative neutron imaging of the process

Sněhota

Jelínková

Sobotková

et al. 2015

Water Resources Research

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Saturated flow in soil with the occurrence of preferential flow often exhibits temporal changes of saturated hydraulic conductivity even during the time scale of a single infiltration event. These effects, observed in a number of experiments done mainly on heterogeneous soils, are often attributed to the changing distribution of water and air in the sample. We have measured the variation of the flow rates during the steady state stage of the constant head ponded infiltration experiment conducted on a packed sample composed of three different grades of sand. The experiment was monitored by quantitative neutron imaging, which provided information about the spatial distribution of water in the sample. Measurements were taken during (i) the initial stages of infiltration by neutron radiography and (ii) during the steady state flow by neutron tomography. A gradual decrease of the hydraulic conductivity has been observed during the first 4 h of the infiltration event. A series of neutron tomography images taken during the quasi-steady state stage showed the trapping of air bubbles in coarser sand. Furthermore, the water content in the coarse sand decreased even more while the water content in the embedded fine sand blocks gradually increased. The experimental results support the hypothesis that the effect of the gradual hydraulic conductivity decrease is caused by entrapped air redistribution and the build up of bubbles in preferential pathways. The trapped air thus restricts the preferential flow pathways and causes lower hydraulic conductivity.

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“…Several studies have focused on reductions in the saturated hydraulic conductivity ( K ) by air entrapment (Smith and Browning ; Christiansen ; Orlob and Radhakrishna ; Debacker ; Faybishenko ; Fry et al ; Sakaguchi et al ; Zlotnik et al ; Marinas et al ). The range of K reductions due to pore clogging by entrapped air has been shown to be very broad.…”

Section: Introductionmentioning

confidence: 99%

Quasi‐Saturated Layer: Implications for Estimating Recharge and Groundwater Modeling

et al. 2019

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This study presents an extension of the concept of ''quasi-saturation'' to a quasi-saturated layer, defined as the uppermost dynamic portion of the saturated zone subject to water table fluctuations. Entrapped air here may cause substantial reductions in the hydraulic conductivity (K ) and fillable pore water. Air entrapment is caused by a rising water table, usually as a result of groundwater recharge. The most significant effects of entrapped air are recharge overestimation based on methods that use specific yield (S y ), such as the water table fluctuation method (WTF), and reductions in K values. These effects impact estimation of fluid flow velocities and contaminant migration rates in groundwater. In order to quantify actual groundwater recharge rates and the effects of entrapped air, numerical simulations with the FEFLOW (Version 7.0) groundwater flow model were carried out using a quasi-saturated layer for a pilot area in Rio Claro, Brazil. The calculated recharge rate represented 16% of the average precipitation over an 8-year period, approximately half of estimates using the WTF method. Air entrapment amounted to a fillable porosity of 0.07, significant lower that the value of 0.17 obtained experimentally for S y . Numerical results showed that the entrapped air volume in the quasi-saturated layer can be very significant (0.58 of the air fraction) and hence can significantly affect estimates of groundwater recharge and groundwater flow rates near the water table. Article impact statement: Quasi-saturation due to air entrapment near the water table has major effects on estimated recharge rates and modeling groundwater.

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Entrapped air effects on dipole flow test in sand tank experiments: Hydraulic conductivity and head distribution

Cited by 12 publications

References 34 publications

Changes in Entrapped Gas Content and Hydraulic Conductivity with Pressure

Changes in Entrapped Gas Content and Hydraulic Conductivity with Pressure

Water and entrapped air redistribution in heterogeneous sand sample: Quantitative neutron imaging of the process

Quasi‐Saturated Layer: Implications for Estimating Recharge and Groundwater Modeling

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