Quasi-Saturated Layer: Implications for Estimating Recharge and Groundwater Modeling.

Roger D Gonçalves,Martinus Th Van Genuchten,Elias H Teramoto,Hung K Chang,Bruno Z Engelbrecht,Miguel A Alfaro Soto

doi:10.1111/gwat.12916

Roger D Gonçalves, Martinus Th Van Genuchten + Show 4 more

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https://doi.org/10.1111/gwat.12916

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Abstract

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.

Highlights

IntroductionThe upward movement of a water table (WT) in response to groundwater recharge causes a displacement of air (the nonwetting phase) by water (the wetting-phase) in partially saturated vadose zone pores
The upward movement of a water table (WT) in response to groundwater recharge causes a displacement of air by water in partially saturated vadose zone pores
Based on the obtained entrapped air estimates, we strongly suggest a review and correction of recharge rates calculated using the water table fluctuation method (WTF) method

Summary

Introduction

The upward movement of a water table (WT) in response to groundwater recharge causes a displacement of air (the nonwetting phase) by water (the wetting-phase) in partially saturated vadose zone pores. Article impact statement: Quasi-saturation due to air entrapment near the water table has major effects on estimated recharge rates and modeling groundwater. Several studies have focused on reductions in the saturated hydraulic conductivity (K ) by air entrapment (Smith and Browning 1943; Christiansen 1944; Orlob and Radhakrishna 1958; Debacker 1967; Faybishenko 1995; Fry et al 1997; Sakaguchi et al 2005; Zlotnik et al 2007; Marinas et al 2013). Zlotnik et al (2007) verified a reduction of 50% due to entrapped air, whereas Faybishenko (1995) observed reductions of one to two orders of magnitude.

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