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

Area of Science:

• Hydrogeology
• Environmental Science
• Geophysics

Background:

• The quasi-saturated layer, the dynamic upper zone of the saturated aquifer, is affected by water table fluctuations.
• Entrapped air within this layer can significantly decrease hydraulic conductivity (K) and fillable pore water.
• Air entrapment, often caused by rising water tables from groundwater recharge, impacts hydrological models.

Purpose of the Study:

• To extend the concept of quasi-saturation to a defined layer.
• To quantify the impact of entrapped air on groundwater recharge estimation and hydraulic conductivity.
• To assess groundwater flow dynamics and contaminant transport in the presence of entrapped air.

Main Methods:

• Numerical simulations were performed using the FEFLOW (Version 7.0) groundwater flow model.
• A quasi-saturated layer model was applied to a pilot area in Rio Claro, Brazil.
• Calculations focused on quantifying entrapped air volume and its effects on recharge and K values.

Main Results:

• The calculated groundwater recharge rate was 16% of average precipitation, about half of estimates using the water table fluctuation (WTF) method.
• Entrapped air represented a fillable porosity of 0.07, substantially lower than the experimental specific yield (Sy) of 0.17.
• The volume of entrapped air was significant (0.58 air fraction), impacting groundwater flow and recharge estimates.

Conclusions:

• Entrapped air in the quasi-saturated layer significantly affects groundwater recharge estimations and flow rates near the water table.
• Methods relying on specific yield (Sy), like the WTF method, may overestimate recharge due to unaddressed air entrapment.
• Accurate quantification of air entrapment is crucial for reliable groundwater resource management and contaminant transport modeling.