Quasi-Saturated Layer: Implications
for Estimating Rechargeand Groundwater Modelingby Roger D. Gonçalves1 , Elias H. Teramoto1 , Bruno Z. Engelbrecht1 , Miguel A. Alfaro Soto1 , Hung K. Chang2 , andMartinus Th. van Genuchten1,3,4Abstract
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 reductionsin the hydraulic conductivity (K ) and fillable pore water. Air entrapment is caused by a rising water table, usually as a result ofgroundwater recharge. The most significant effects of entrapped air are recharge overestimation based on methods that use specificyield (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 theeffects of entrapped air, numerical simulations with the FEFLOW (Version 7.0) groundwater flow model were carried out using aquasi-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 ofgroundwater recharge and groundwater flow rates near the water table.

Introduction
The upward movement of a water table (WT) in
response to groundwater recharge causes a displacementof air (the nonwetting phase) by water (the wetting-phase)in partially saturated vadose zone pores. Many studies
have shown that a significant amount of air can become
1 Centerfor Environmental Studies and Basin Studies
Laboratory, São Paulo State University, UNESP, Rio Claro, SP
13506-900, Brazil.
2 Department of Applied Geology, São Paulo State University,
UNESP, Rio Claro, SP 13506-900, Brazil.
3 Corresponding author: Center for Environmental Studies, SãoPaulo State University, UNESP, Rio Claro, SP 13506-900, Brazil;+55-21-887909055; rvangenuchten@hotmail.com
4 Department of Nuclear Engineering, Federal University of Riode Janeiro, COPPE, Rio de Janeiro, RJ 21941-972, Brazil.
Article impact statement: Quasi-saturation due to air
entrapment near the water table has major effects on estimatedrecharge rates and modeling groundwater.
Received August 2018, accepted May 2019.
© 2019 The Authors. Groundwater published by Wiley Periodicals, Inc. on behalf of National Ground Water Association.
This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution andreproduction in any medium, provided the original work is properly
cited.
doi: 10.1111/gwat.12916

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entrapped in the largest pores below the WT during thisascension (e.g., Smith and Browning 1943; Christiansen
1944; Debacker 1967; Tokunaga and Narasimhan 1987;
Faybishenko 1995; Fry et al. 1997; Williams and Oostrom
2000; Sakaguchi et al. 2005; Marinas et al. 2013). The
presence of entrapped air promotes considerable but
reversible changes in the aquifer properties. The term“quasi-saturated zone” was introduced by Faybishenko(1995) to describe the uppermost portion of the saturated zone partially filled with entrapped air. Althoughcontaining water and air in varying proportions, thequasi-saturated layer is distinct from the capillary fringein that the pressure head is positive, similar to a fully
saturated zone, and not negative like in the vadose zone.
Several studies have focused on reductions in thesaturated hydraulic conductivity (K ) by air entrapment(Smith and Browning 1943; Christiansen 1944; Orlob andRadhakrishna 1958; Debacker 1967; Faybishenko 1995;
Fry et al. 1997; Sakaguchi et al. 2005; Zlotnik et al.
2007; Marinas et al. 2013). The range of K reductions
due to pore clogging by entrapped air has been shown
to be very broad. For instance, Zlotnik et al. (2007)
verified a reduction of 50% due to entrapped air, whereas
Faybishenko (1995) observed reductions of one to twoorders of magnitude. They used the term quasi-saturated
Groundwater1

hydraulic conductivity (hereafter denoted by K quasi ) for
the resulting conductivity.
Several authors, including Peck (1969), Constantz
et al. (1988), and Fry et al. (1997), showed that theamount of entrapped air depends on soil type and theprevailing grain size distribution, as well as on the fluid
flow rate (Constantz et al. 1988; Fry et al. 1997), moisture
content (Fayer and Hillel 1986b), and pore geometry(Peck 1969; Fry et al. 1997). Furthermore, numerous
experiments have shown a broad range of measured
entrapped air saturation contents, between approximately
0.04 for clays and 0.55 for coarse sands (Christiansen
1944; Fayer and Hillel 1986a; Stonestrom and Rubin
1989; Faybishenko 1995; Wang et al. 1998; Sakaguchi
et al. 2005; Marinas et al. 2013).
Due to WT fluctuations (WTFs), entrapped air issubject to changes in hydraulic pressures, which resultsin the compression or expansion of entrapped air bubbles(Christiansen 1944; Gupta and Swartzendruber 1964;
Collis-George and Yates 1990; Bicalho et al. 2005;
Marinas et al. 2013). Marinas et al. (2013) observed
reductions between 18% and 26% in the amount ofentrapped air at a water pressure of 2.5 m, compared tozero pressure, with the amount of entrapped air decreasing
approximately linearly with the increase in water pressure.
Entrapped air will obstruct parts of otherwise saturated pores below the WT and as such can significantly
reduce local fluid flow velocities and estimated groundwater recharge rates (Christiansen 1944; Constantz et al.
1988; Faybishenko 1995; Heilweil et al. 2004). Regarding
shallow unconfined aquifers, the actual porosity filled bywater during a rising WT (the fillable porosity, θ f ) is often
smaller than the specific yield (S y ) due to entrapped air
below the WT (Kayane 1983; Maréchal et al. 2006). As
a consequence of this discrepancy, recharge rates calculated using the WTF method (Healy and Cook 2002) will
likely be overestimated since calculated values are very
sensitive to the assumed value of S y .
Considering the potential impact of entrapped air
on calculated WTFs, especially in shallow unconfined
aquifers, this work aims to introduce the concept ofa “quasi-saturated layer” as based on several earlier
studies, as well as on field evidence. The applicabilityof the quasi-saturated concept to practical problems
is illustrated by computing more realistic groundwater
recharge fluxes using the FEFLOW groundwater flow
simulator of Diersch (2014) as applied to an unconfined
aquifer in Rio Claro, Brazil.

Conceptual Model of the Quasi-Saturated
Layer
Figure 1 provides a schematic of the effects of air
entrapment during an imbibition event on the water
retention curve (WRC), when water invades the fillable
pore space and air bubbles become entrapped by snapoff and bypassing mechanisms. Faybishenko (1995)
suggested that entrapped air at and near the WT is
distributed into mobile and immobile parts. While the2

R.D. Gonçalves et al. Groundwater

Figure 1. Conceptual model illustrating the mechanism of
air entrapment due to hysteresis involving primary drainage
and imbibition WRCs (after Faybishenko 1995).

mobile part can be displaced, entrapped air remains as
an immobile and entrapped phase within the pores, and is
responsible for a hysteresis effect during alternating cyclesof fluid drainage and imbibition.
In our study we view the upper part of the aquifer
subject to air entrapment as a separate layer between
the vadose zone and the fully saturated zone, all in thevicinity of a fluctuating WT. Although the quasi-saturated
zone is ephemeral and dissipates when the WT moves
down, its recurrent formation when the WT moves up(Figure 2) allows one to consider this zone as a separate(seasonal) layer of the aquifer. Moreover, entrapped gases
within this zone may persist for a decade or longer,even without WTFs at this depth (Ryan et al. 2000;
McLeod et al. 2015). Although the pore system is filledwith different proportions of air and water, the mainfeature distinguishing the quasi-saturated zone from thecapillary fringe is that the quasi-saturated zone has apositive pressure head, similar to the fully saturated zone.
Moreover, the hydraulic properties (K quasi and θ f ) of thequasi-saturated zone are controlled by the volume of air
entrapped in the aquifer pores.
Table 1 presents results from laboratory tests related
to entrapped air saturations in various porous media. Air
saturation as used here is the volumetric entrapped aircontent divided by the porosity. The estimated values
exhibit a wide range that may reach relative air saturation
values above 0.50. Several studies (e.g., Constantz et al.
1988; Marinas et al. 2013) have noted that materials withlarge pores, such as sands and well-aggregated soils, tend
to have higher entrapped air proportions than media withfiner pore structures. Laboratory tests by Dzekunov et al.
(1987), Faybishenko (1995), and Fry et al. (1997), on
the other hand, showed that fine-textured media produced
more pronounced reductions in K than coarse-grainedsoils with increasing entrapped air saturation (Figure 3).
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Figure 2. Schematic showing how during recharge the water table rises from WT1 to WT2. The presence of significant
proportions of entrapped air reduces the hydraulic conductivity and fillable porosity.

Table 1
Entrapped Air Fractions Obtained Experimentally

References

MediumPoulovassilis
Glass beads(1970)
Stonestrom and Coarse sandRubin (1989)
Christiansen
Coarse sand(1944)
Wang et al.
Coarse sand(1998)
Marinas et al.
Fine to coarse(2013)
sands
Williams andFine sand
Oostrom(2000)
Sakaguchi et al. Sandy loam
(2005)
Sakaguchi et al. Aggregated clay
(2005)
Fayer and Hillel Loamy sand(1986a)Stonestrom and Loam
Rubin (1989)
Faybishenko
Loams(1995)

Proportion of
Pore Space Filledwith Entrapped Air
0.19
0.126
0.15 to 0.40
0.154 to 0.305
0.130 to 0.545
Figure 3. Relative hydraulic conductivity (K rel = K quasi /K s )of quasi-saturated soils as a function of entrapped air
saturation.

0.15

0.23

where K quasi is the quasi-saturated hydraulic conductivity;
K 0 is the minimum quasi-saturated hydraulic conductivity; K S is the saturated hydraulic conductivity without air
entrapment; ω is the volumetric fraction of entrapped air;ωmax is the maximum entrapped air content; and n an
exponent. The fillable porosity hence is related to specificyield (S y ) through

0.13
0.043 to 0.126
0.069
0.10 to 0.25

θf = Sy (1 − ω)Some of the experimental data further revealed
an approximately exponential decrease in the hydraulicconductivity with increasing entrapped air saturation(Figure 3). This relationship can be described also by apower law as proposed previously by Faybishenko (1995)
for loam soils:ω nKquasi = K0 + (Ks − K0 ) 1 −ωmax
NGWA.org(1)(2)

These findings are consistent with the general shapeof the unsaturated hydraulic conductivity as a function ofwater content, with or without the presence of entrappedair (Luckner et al. 1989), as shown by Carsel and Parrish(1988) among others for the van Genuchten (1980)
unsaturated soil hydraulic conductivity function. Although
Fry et al. (1997) reported the applicability of the van
Genuchten model for quasi-saturated systems, Marinas
et al. (2013) noted that the Faybishenko equation equally
R.D. Gonçalves et al. Groundwater3

Figure 4. Location of the study area and monitoring well (left) and a potentiometric map (Gonçalves 2016) showing also the
topography with elevation in meters above mean sea level.

or better captured the hydraulic conductivity changes fortheir data.
The vertical zone where the WT fluctuates periodically can be identified using monitoring wells or othermeans. That zone is considered here to be a distinct
and seasonal layer with reductions in the fillable porosity (θ f < S y ) and hydraulic conductivity (K quasi < K s ) due
to entrapped air. This separate layer is referred to here
as the quasi-saturated layer. Instead of assuming variably
saturated conditions typical for the vadose (unsaturated)zone with its negative pressure heads, this layer can be
simulated using aquifer conditions with positive hydraulicpressures.

Study Site
The study site is located at São Paulo State University
in the city of Rio Claro, Brazil (Figure 4). The site ispart of the Rio Claro Aquifer, which has been studied
and monitored extensively since 2000 (e.g., Ferreira
and Caetano-Chang 2008; Neto et al. 2016; Gonçalves
and Chang 2018). The Rio Claro Aquifer is a shallow
unconfined aquifer composed of Cenozoic sedimentaryrocks of the Rio Claro Formation overlaying the Permian
4

R.D. Gonçalves et al. Groundwateraquitard of the Corumbataí Formation. The aquifer covers
approximately 85 km2 on top of the Paraná Basin, with its
thickness varying from a few meters to up to 50 m (about
15 m at the study location). Sediments have fluvial origin
and are composed mostly of fine- to medium-grainedsands with a variable clay matrix (Gonçalves and Chang
2018).
The Rio Claro Aquifer presents a plateau morphology
with recharge occurring in most of the aquifer at relatively
high topographic locations, and discharge along streams
that cross the aquifer with their bottom near the lower
contact. Lateral hydraulic gradients at the site are very
low, less than 0.007, because of its location near agroundwater divide and functioning as an important
recharge zone. The WT generally varies between depths ofabout 6 to 10 m, but with a large seasonal variation due theoccurrence of alternating seasons of rainy summers anddry winters (Gonçalves 2016). Hydraulic conductivities
determined by slug tests ranged from 2.0 × 10−6 m/s to
2.0 × 10−4 m/s (Gonçalves and Chang 2018), while avalue of 0.17 for S y of the saturated soils was obtainedby analyzing the WRCs of undisturbed samples (Alfaro
Soto and Chang 2008). The precipitation data (Figure 5)
were obtained from daily total rainfall records provided by
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Figure 5. Monitored groundwater levels (black solid line)
showing the seasonal cycles of recharge and drawdown aswell as daily rainfall rates (gray bars).

the meteorological station of the Center for Environmental
Studies and Planning at UNESP, located in the study area.

Numerical Model
A three-dimensional saturated flow model was constructed to simulate transient WTFs in an unconfined
aquifer. We used for our simulations the FEFLOW(version 7.0) finite-element code of Diersch (2014), but
modified to include the effects of air entrapment during groundwater recharge periods. The model domain of
1.1 km2 was divided into 60,944 finite elements (30,807
nodes) with irregular spacing both in the horizontal andvertical directions. The domain was divided into twolayers with different hydraulic properties to reflect theexistence of entrapped air. The upper layer was a quasisaturated layer, defined by the vertical range where theWT was found to fluctuate periodically, while the lower
layer represented the fully saturated zone.
The adopted configuration can be seen in Figure 6.
The domain bottom is the contact with the aquitard, andhence was considered to be no-flow boundary, similarly
as the northern and eastern limits which represented
groundwater divides. In addition, seepage face boundary
conditions were introduced along the southeastern borderto reproduce discharge areas, thus allowing free outflow
from the model. Recharge rates were represented using
a transient fluid-flux boundary condition along the top
elements. Values of the saturated hydraulic conductivity(K s , as estimated from slug tests), and of S y (derived
from water retention data) were applied to the lower layer,
whereas K quasi and θ f values were calculated for the quasisaturated layer considering the entrapped air effects.
Monitored groundwater levels from 2002 to 2005
(1460 d) were used on a daily basis in order to calibrate
the transient model. The initial condition of the hydraulichead for the transient simulations was obtained using asingle calibrated steady-state run, while the bottom surfaceof the top layer was approximated by means of a steadystate run for the lowest WT period. Once the calculations
satisfactorily represented the transient hydraulic head data
for the first 4 years, an additional 4-year simulation was
carried out and compared with the monitored hydraulicheads from 2006 to 2009.

Numerical Validation of the Quasi-Saturated
Layer
In order to estimate the entrapped air content, a set oftransient-state simulations was performed representing thetwo main layers of the conceptual model: a quasi-saturated
layer on top of a fully saturated zone. For S y we used avalue of 0.17 as estimated from measured water retention
data, and for K s of the laterally homogeneous saturated
lower layer a value of 2 × 10−5 m/s, being the mean value
as derived from slug tests Due to air entrapment, thehydraulic parameters (K quasi and θ f ) of the quasi-saturated
layer were variable since they depend on air saturation.
The grain size distributions of the sands tested
by Marinas et al. (2013) were very similar to thoseof the Rio Claro Formation sediments (Gonçalves andChang 2018). Because of this similarity, we employed
an exponential function (rather than the van Genuchten
hydraulic function) fitted to the experimental data, with
K quasi being a function of the entrapped air content. The
following exponential function was fitted to the Marinas
et al. (2013) data shown in Figure 3:
Kquasi = 1.314 Ks e−3.969ω

(3)

We calibrated the numerical model using observed
transient WT data over a 4-year period as shown in

Figure 6. Three-dimensional (3D) view of the finite-element model setup.

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R.D. Gonçalves et al. Groundwater5

Figure 7. Comparison between observed (black solid line)
and simulated (red solid line) water levels for an 8-year-longsimulation using the quasi-saturated layer. Also represented
as gray bars are the estimated recharge rates during each
recharge period.

Table 2
Calculated Recharge Rates for the Time Period
2002 through 2009 Assuming a Quasi-Saturated
Layer, and Observed Annual Rainfall Rates (withTheir Ratio of Precipitation)Year

Accumulated
Precipitation(mm)

Calculated
Recharge (mm/year;% of Precipitation)

2002
2003
2004
2005
2006
2007
2008
2009

1026
1463
1820
1248
1514
1365
1318
1317

203 (19.8%)
265 (18.1%)
321 (17.6%)
161 (12.9%)
164 (10.9%)229 (16.8%)
174 (13.2%)
222 (16.9%)

Figure 7. The model could be run using a daily time
step. Adjustable parameters were the volumetric fractionof entrapped air, ω, in Equations 2 and 3, and the recharge
rate. The recharge rate was assumed to be uniform
laterally, and constant in time during each ascending anddescending WTF stage (Figure 7), without any constraints
on its values. A relatively high entrapped air fraction(0.58) was obtained, leading to values of 2.5 × 10−6 m/s
for K quasi (Equation 3) and 0.07 for θ f (Equation 2) of thequasi-saturated layer. Calculated hydraulic heads showedvery good correlation with the observed data as reflectedby a root mean square error (RMSE) of 0.0545 m and acoefficient of determination (R 2 ) of 0.96. Table 2 lists thecalculated recharge rates.
Figure 7 shows considerable variability in time ofthe observed and calculated heads, which we initiallythought would have affected the value of θ f during the
calculations. We found, however, that it was not necessary
to vary θ f in time to properly calibrate the heads.
Calibrated recharge rates were found to be significantly
lower than those previously estimated using the WTF
method (Neto and Chang 2008). The average calculated
recharge for the 2002 to 2005 period was 249 mm/year,
6

R.D. Gonçalves et al. Groundwaterin contrast to 491 mm/year as obtained using the WTF
method. The recharge rates were also estimated by
Neto et al. (2016) by simulating infiltration and variably
saturated flow in the soil profile, including root water
uptake, using the Richards equation. They found an
average value of 304 mm/year, which is much closer tothe value of 249 mm/year we obtained.
In order to confirm the reliability of the quasisaturated model, an additional 4 years were simulatedusing the calibrated model without further changes in thehydraulic parameters, including of θ f , except for setting
the recharge rates as listed in Table 2. The results in
Figure 7 show excellent agreement of the predicted values
with the observed WTFs, having a RMSE of 0.0497 m,
and a R 2 of 0.96.

Discussion
Effective porosities derived from laboratory experiments often lead to inaccurate predictions of the upwardand downward movement of a shallow WT in transientgroundwater models. Unrealistically large groundwater
recharge rates may then be needed to simulate observed
WT peaks correctly. Laboratory estimates of the porosity may similarly lead to poor predictions when the WTmoves downward. These difficulties may be resolved byusing lower hydraulic conductivities and effective porosities resulting from air entrapment at and below the WT.
Our study shows the importance of focusing on theprevailing mechanisms causing lower hydraulic conductivities and effective porosities. Several recent studies(e.g., Rama et al. 2018; Teramoto and Chang 2018) foundthat specific yield values calibrated using transient models
were significantly lower than expected for the lithologies
involved, or as estimated from the soil water retention
properties. Teramoto and Chang (2018) considered thisdeviation as a hysteresis effect, while Rama et al. (2018)
assumed the upper portion of the aquifer to remain unsaturated, thus using the Richards equation in conjunction withvan Genuchten (1980) type soil hydraulic functions to calculate the WTFs. Those two studies, as well as numericalsimulations carried out by Nogueira and Chang (2015) andthose presented in this paper, all required reductions in thefillable porosity to properly reproduce observed WTFs.
The decrease in the hydraulic conductivity and theeffective porosity of the quasi-saturated domain could
be described also using van Genuchten expressions forthe unsaturated hydraulic conductivity as a function of
air or fluid saturation (van Genuchten 1980; Luckner
et al. 1989; Fry et al. 1997). As demonstrated by Marinas
et al. (2013), and further used in our study, the original
equation by Faybishenko (1995) performed equally well.
We also note here a separate study by Neto et al. (2016)
who used the Richards equation for flow in variably
saturated vadose systems to simulate the WTFs and to
estimate recharge rates at the same site of the Rio Claro
Aquifer for the period from 2002 to 2005. Groundwater
recharge was well estimated on average (304 mm/year).
However, their model was not capable to properly predict
NGWA.orgthe transient WTFs. Moreover, their approach needed
far more data (including specific soil hydraulic and root
water uptake parameters), and is numerically far more
complex than a fully saturated model. For these reasons
we believe that the use of a quasi-saturated layer ingroundwater flow simulations as used in our study canbe extremely beneficial and opportune.
Several previous studies have shown that air entrapment can be significant and durable in the uppermost
portion of the saturated zone. Calculations using the WTF
method for this reason tend to overestimate recharge rates,
especially for shallow unconfined aquifers. This finding
has a major impact on water resources management since
water resource estimates are commonly sensitive to theassumed effective porosity, as well as for evaluating alternative groundwater exploitation scenarios, estimating fluid
flow velocities and/or contaminant migration rates, andpredicting the long-term effects of climate change.
In order to evaluate the effects of air entrapment
effects, and to moderate numerical simulation errors,
experimental tests should be conducted more systematically to estimate the air entrapment fraction during rising
WTs under different geological settings. Based on theobtained entrapped air estimates, we strongly suggest areview and correction of recharge rates calculated using
the WTF method. Hence, for correct recharge calculations that also consider entrapped air, the fillable porosity
term (θ f ) instead of S y may need to be used (i.e., R = θ fh/t, where R is the recharge rate, h is the WT height,
and t is time).
Despite the robustness of our numerical model calibration, certain aspects remain unanswered. Nonuniqueness continues to be a prominent nature of groundwatermodels, and a major source of error in recharge predictions by model calibration. Calibration may assure thepotential feasibility of a conceptual model, but does notnecessarily account for model singularity since calibration
may be obtained with different model parameterizations.
For example, Knowling and Werner (2017) demonstrated
that distinct combinations of S y , the recharge rate andK s can produce diverse goodness of fits of long-termtime series of water level data. This reflects the difficulty of quantifying time-varying groundwater recharge
rates through groundwater model calibration. Indeed, thefavorable calibration of our model using WT data is not
sufficient to prove the key role of entrapped air in estimating groundwater recharge. Still, our model suggests
the critical role of air entrapment controlling anomalous
WTFs in field-scale problems, and may explain the inability of calibrations employing laboratory derived S y values.
Since entrapped air and pore clogging leads to noticeable reductions in S y and K S , it represents another sourceof uncertainty to reliable quantification of groundwater
recharge. In summary, the quasi-saturated layer encompasses a phenomenological approach that can improve theperformance of groundwater flow models, particularly for
unconfined aquifers.
Finally, we note that our study focused on the physical effects of air entrapment in terms of groundwater
NGWA.orgmodeling, WT dynamics, and estimating recharge. The
effects of air entrapment, however, are not limited tothe physical effects of fluid flow at and below the WT.
Air entrapment in the uppermost part of the saturated
zone, as well as in the capillary fringe, is conducive toseveral physical, biological, and geochemical processes.
To exemplify, the concentration of noble gases below
the WT at levels above equilibrium with atmospheric air(also known as “excess air”) have been linked experimentally and numerically to entrapped air, in which thegases are transferred from entrapped air to groundwater by diffusional movement (Heaton and Vogel 1981;
Aeschbach-Hertig et al. 2000, 2008; Holocher et al. 2003;
Mächler et al. 2013). Likewise, Williams and Oostrom(2000), Mächler et al. (2013), Mcleod et al. (2015), andTeramoto and Chang (2019) demonstrated that the presence of entrapped air delivers oxygen to groundwater.
Aquifer oxygenation by entrapped air dissolution can haveimportant implications to biogeochemical process in thesaturated zone. For example, Teramoto and Chang (2019)
showed that entrapped air may be the most importantcause of aquifer oxygenation by driving the oxidation of
byproducts (such as CH4 and Fe2+ ) and affecting theredox state of hydrocarbon-contaminated aquifers. For
these reasons the possible physical, microbial, and geochemical effects of air entrapment should not be neglected
when studying the seasonal movement of groundwater in
the near surface.

Conclusions
The exact effects of air entrapment on recharge
calculations are uncertain and still being investigated since
it is very difficult, if not impossible, to assess theseeffects directly in field conditions, despite the numerous
laboratory and field experiments that have been carriedout over the years (Faybishenko 1997). The proposed
groundwater model using a quasi-saturated layer wascapable to satisfactorily predict WTFs observed underfield conditions considering air entrapment effects on
the hydraulic parameters in the upper portion of theaquifer. Recharge rates calculated using the WTF method
potentially may be overestimated significantly, which isespecially critical when the fillable porosity is much
lower than the specific yield due to high entrapped air
fractions. Our study shows that transient groundwaterflow modeling using a quasi-saturated layer is a very
effective approach for predicting WTFs and estimatingtransient recharge distributions in shallow unconfined
aquifers.

Acknowledgments
We acknowledge support for this study from theBasin Studies Laboratory (LEBAC) of the Department
of Applied Geology, associated with the Center for
Environmental Studies (CEA) of UNESP, the Foundation
for Development of UNESP (FUNDUNESP), the NationalCounsel for Technological and Scientific Development
R.D. Gonçalves et al. Groundwater

7

(CNPq), and CAPES-Brazil. We thank Boris Faybishenko
and an anonymous reviewer for their many helpfulcomments and suggestions.

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