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

• Published in: Ground water Vol. 58; no. 3; pp. 432 - 440
• Main Authors: Gonçalves, Roger D., Teramoto, Elias H., Engelbrecht, Bruno Z., Alfaro Soto, Miguel A., Chang, Hung K., Genuchten, Martinus Th
• Format: Journal Article
• Language: English
• Published: Malden, US Blackwell Publishing Ltd 01-05-2020; Ground Water Publishing Company
• Subjects: Air; Computational fluid dynamics; Computer simulation; Contaminants; Entrapment; Flow velocity; Fluid flow; Groundwater; Groundwater flow; Groundwater pollution; Groundwater recharge; Groundwater table; Mathematical models; Pore water; Porosity; Research Paper; Research Papers; Saturation; Specific yield; Water flow; Water table; Water table fluctuations
• ISSN: 0017-467X; 1745-6584
• DOI: 10.1111/gwat.12916

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 (Sy), 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 Sy. 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.