This document summarizes a study on the dynamics of fluids during transient air sparging of a heterogeneous unconfined aquifer contaminated by diesel fuel in Oman. The study monitored changes in the water table, dissolved oxygen, electrical resistivity, and air pressure during different air injection rates and operation scenarios of the air sparging and soil vapor extraction system, with and without an accompanying pump-and-treat system. Tracer tests using salt showed that groundwater velocity within the air sparging zone decreased with increasing air injection rate. Monitoring indicated the air sparging caused rapid rises and slower declines of the water table, and increased levels of dissolved oxygen and air pressure farther from the injection well over time.

1. ORIGINAL ARTICLE
Fluids’ dynamics in transient air sparging of a heterogeneous
unconfined aquifer
Rashid S. Al-Maamari • Akihiko Hirayama • Tsuyoshi Shiga • Mark N. Sueyoshi •
Mahfoodh Al-Shuely • Osman A. E. Abdalla • Anvar R. Kacimov
Received: 15 July 2009 / Accepted: 7 October 2010
Ó Springer-Verlag 2010
Abstract Water table dynamics, dissolved oxygen (DO)
content, electrical resistivity (ER) in monitoring wells and
air pressure in the vadose zone are monitored in air
sparging (AS) accompanied by soil vapor extraction (SVE)
at a hydrocarbon-contaminated groundwater site in Oman,
where a diesel spillover affected a heterogeneous uncon-
fined aquifer. The formation of a groundwater mound at the
early stage of air injection and potential lateral migration of
contaminants from the mound apex called for an additional
hydrodynamic barrier constructed as a pair of pump-and-
treat (P&T) wells whose recirculation zone encompassed
the AS and SVE wells. In all monitored piezometers the
phreatic surface showed a rapid and distinct peak, which is
attributed to the time of air breakthrough from the injection
point to the vadose zone and a relatively mild recession
limb interpreted as a decay of the mound. Tracer tests
showed a layer of a relatively low hydraulic conductivity at
an intermediate depth of the screened interval of the wells.
Increased levels of DO and borehole air pressure that have
been observed (as far as 50 m away) are likely mitigated by
SVE and P&T. Radius of influence can be indirectly
inferred from ER and DO changes in the AS operation
zone. Salt tracer tests have shown that groundwater
velocity within the AS zone decreases with the increase of
air injection rate.
Keywords Air sparging Á Heterogeneous formation Á
Groundwater Á Hydrocarbon contamination
Introduction
Air sparging (AS) is a groundwater remediation technique
widely used for treatment of VOCs (volatile organic
compounds) in contaminated aquifers (Johnson 1998;
Braida and Ong 2001). Air, injected under the water table,
ascends and breaks through the saturated zone to the
vadose zone, scavenging VOCs and releasing them to the
unsaturated soil, from which air ventilation (soil vapor
extraction, SVE) wells abstract the volatilized chemicals
(Fig. 1 depicts schematically the corresponding wells in a
vertical cross-section).
Laboratory, field, and modeling studies of AS have been
undertaken with a special focus on the effect of particle
size of the aquifer porous medium, its heterogeneity, geo-
logical and hydrogeological zonation, and engineering
parameters, including the depth of the injector suction
screens, distance between the wells, sizes of counter-
shortcut impermeable caps on the ground surface, the
injection-abstraction rates and regimes, maintained pres-
sures, etc. (e.g., Angell 1992; Brown and Jasiulewicz 1992;
Marley et al. 1992; Reddy et al. 1995, 2001; McCray and
Falta 1996; Ng and Mei 1996; Bass et al. 2000; Adams and
Reddy 2000; Berkey et al. 2003; Tomlinson et al. 2003;
Geistlinger et al. 2006; Kim et al. 2006; Selker et al. 2007).
Electronic supplementary material The online version of this
article (doi:10.1007/s12665-010-0793-y) contains supplementary
material, which is available to authorized users.
R. S. Al-Maamari (&) Á M. Al-Shuely Á
O. A. E. Abdalla Á A. R. Kacimov
Sultan Qaboos University, P.O. Box 33, Al-Khoudh,
PC 123, Sultanate of Oman
e-mail: rsh@squ.edu.om
A. Hirayama Á M. N. Sueyoshi
Shimizu Corporation, 1-2-3 Shibaura, Minato-ku,
Tokyo 105-8007, Japan
T. Shiga
Taisei Kiso Sekkei Co., Ltd., 3-43-3 Sendagi,
Bunkyo-ku, Tokyo 113-0022, Japan
123
Environ Earth Sci
DOI 10.1007/s12665-010-0793-y

2. Of crucial importance is the flow type in the air sparged
zone, which can be channel-type, pervasive (bubbly), or
chamber-type (with amoeba- and tabular-form air invaded
pockets) (Ji et al. 1993; Peterson 1999, 2001; Chao et al.
2008). The final output of AS–SVE is determined by the
drop in concentrations of chemicals to be removed from
groundwater–vadose zone. As regular sampling and
chemical analysis of soil and water is costly, indirect
indicators of remediation are used, in particular, the char-
acteristics of the oval-shaped sparging plume, which are
assessed by the air-pressure increment (air volumetric
content), dissolved oxygen (DO) content in groundwater,
VOC concentration in the vadose zone and groundwater
mounding. These characteristics are further used in models
for predictions of Darcian velocities of air, water, and free
products as well as of physical and chemical transforma-
tions (mostly volatilization) within the plume. One or
several characteristics are combined in the so-called radius
of influence (ROI) of the plume (McCray and Falta 1996;
Mei et al. 2002).
In modeling and conceptualization of AS–SVE two
major approaches are utilized. One-phase sharp-interface
model requires an intrinsic permeability and porosity as the
only parameters and assumes a distinct free boundary
between the air plume and groundwater (Nikolskii 1961).
Two–three phase flow models consider combined air and
groundwater (and sometimes NAPL) motion (McCray
and Falta 1996; Mei et al. 2002; Jang and Aral 2009)
that requires phase permeabilities and capillary pressure
functions, which are difficult to measure in the field. Any
intermittency of AS–SVE—as in our case—involves
imbibition-drainage cycles. The hysteresis of pressure–
saturation-permeability functions in these cycles exacer-
bates the complexity of multiphase modeling and under-
standing of the flow-transport dynamics. Consequently,
AS–SVE ‘‘multiphase’’ plumes are often assumed to be
steady-state and therefore only air motion is studied
(groundwater is hydrostatically stagnant) that reduces the
multiphase-flow models to apparently one-phase, albeit,
with air pressure, content and density varying from one
point to another (Philip 1998; Mei et al. 2002). Although
periodic agitation (cyclic flooding) of a multiphase sub-
surface system is widely practiced in secondary oil
recovery (Tsynkova et al. 1993) and has been recently
proved to be superior to steady air injection-suction (see for
e.g., Balcke et al. 2009), for unconfined aquifers AS–SVE
under these cyclostationary regimes has peculiarities,
which are not encountered by petroleum engineers.
Namely, groundwater is bounded from above by a fluctu-
ating phreatic surface, which periodically mounds and
slumps under the action of another free surface (the
injected air ‘‘macrobubble’’). Clearly, in groundwater
unconfined aquifers, the effect of the vadose zone where
gas abstraction wells are located makes another major
difference with deep and always confined porous pays. The
gas phase depressions caused by these wells have their own
effect on the phreatic surface similar to upcresting (upc-
oning) in reservoir engineering.
Saturated
zone
Monitoring
well 1
Monitoring
well 2
Air invaded
zone (plume)
Vadose zone
Transient
water table
Saturated
zone
Monitoring
well 1
Monitoring
well 2
Vadose
zone
Saturated
zone
Monitoring
well 1
Monitoring
well 3
Vadose
zone
(a)
(b)
(c)
A
B
C
SVE well (with vacuum
regulator/vacuum blower)
SVE-
induced
upwelling
AS-induced
upwelling
Air
compressor
Injection well
Original water table
Plume
Plume
A C
CA
Fig. 1 Dynamics of the sparged
zone and water table mound at
different stages of air injection:
a incipient stage with a slightly
bulging phreatic surface and
almost circular cross-section of
the bubble, b pre-breakthrough
stage with an oval-shaped
bubble almost reaching the
maximally-upwelled phreatic
surface, c post-breakthrough
stage with a slumping phreatic
surface and stabilized air-
flooded zone
Environ Earth Sci
123

3. Several AS–SVE systems in Oman have been operated
under essentially intermittent conditions when groundwater
and free/dissolved hydrocarbons are not only non-static but
undergo cyclostationary agitations interrupted by calm
periods. For example, the time regime of AS–SVE at
Al-Kamil site (fuel spillover from a filling station) is
practiced with a frequency of sparging varying from 1 to 2
days (with several hours of air injection per day).
Our experience with intermittent AS–SVE gave evi-
dence that free/dissolved hydrocarbons’ concentrations at
contaminated sites rebound during dormant stages. Corre-
spondingly, AS–SVE has been supplemented by a pump-
and-treat (P&T) system at a Rustaq site (Oman), which
served, during this study, as a benchmark for assessing the
effects of AS–SVE–P&T on the characteristics of the
plume. The remediation efforts at Rustaq site have shown
significant effect of AS–SVE–P&T in the removal of the
hydrocarbon contaminants from both groundwater and the
vadose zone as indicated by the analytical results of the
continuous monitoring (Al-Maamari et al. 2009). Apart
from their efficacy as remediation tools, AS–SVE–P&T
induces changes to the hydrodynamics of the remediated
aquifer. These changes vary with injection/abstraction rates
and may potentially cause an undesired migration of the
contaminant plume outside the radius of influence. During
the remediation process in Rustaq area variable air injec-
tion rates were used accompanied by multiple scenarios of
operation (e.g., AS alone, AS–SVE without P&T and AS–
SVE–P&T). The response of groundwater to these variable
injection rates will be of interest to remediation of other
sites adopting different combinations of AS, SVE, and
P&T. As indirect indicators of remediation, tracer tests
(measured by ER), DO, and air pressure were measured
during the tests which will also provide assessment on
groundwater velocity, degree of heterogeneity/anisotropy,
and radius of influence.
Therefore, the objectives of this study were to investi-
gate and analyze the effect of AS at different injection rates
on the fluids’ dynamics of a heterogeneous unconfined
aquifer and to implement tracer tests/ER, DO, and air
pressure as indirect indicators of remediation and tools for
assessment of aquifer’s heterogeneity, anisotropy, and
hydrodynamics. We present case-study data on imple-
mentation of AS–SVE–P&T at different air–water injec-
tion–abstraction rates. Indirect evidence of remediation by
DO and air pressure measurements and contained plume
(no lateral motion of groundwater from the agitated zone)
by ER measurements is collected. We also measure and
conceptualize pulse-type AS–SVE–P&T induced agitation
of the fluids. We start from almost stagnant conditions
(dormant stage) and monitor groundwater dynamics in
response to an induced high hydraulic gradient (though
short-duration) created by varying air injection rate.
Description of study site, equipment, and protocol
The study area is located in the Batinah Region of the
Sultanate of Oman, about 150 km west of the capital,
Muscat. It occupies approximately 7850 m2
of a low lying
elevated plateau surrounded by wadis, sloped gently from
south to north. Attributed to occasional rainfall, the wadis
flow to the north following the topographic pattern. There
is no irrigation or any other artificial input to the aquifer in
the study area. The average annual rainfall in the study area
is around 60 mm, direct recharge to the water table is
assessed to be 5–15 mm/year (Al-Mushikhi 1999) and the
evaporation rate for the existing groundwater depth
([10 m below the ground surface) is negligible. Therefore,
natural descending moisture fluxes from the vadose zone
can be neglected and the ascending air flow from AS wells
is a major factor determining the groundwater dynamics in
the remediation zone (Fig. 1).
Groundwater is hosted in an alluvium layer formed
during the Quaternary time and occupies the top of the
geologic succession. The thickness of the alluvium
increases toward the coast and is dominated by boulder/
gravels which are unconsolidated at the top and become
compacted downwards, and are unconformably underlain
by Tertiary limestone (Abdalla et al. 2010). The gravels are
driven from the weathering of the ophiolites and the Ter-
tiary limestones that are cropping out at the elevated pla-
teaus flanking the study area. The limestones deposited
under marine conditions whereas the ophiolites were
formed when the Tethyan oceanic crust abducted against
Arabian Plate in the Late Cretaceous between 90 and
105 Ma (Hanna 1995). The alluvium aquifer in the area lies
under unconfining conditions and the regional groundwater
flow is from the South to the North with varying hydraulic
gradient. The aquifer exhibits clear heterogeneity and
anisotropy owing to the spatial and vertical variations in
grain size and shape.
In November 1998, approximately 10,000 l of diesel
were accidentally released from an above-ground storage
tank, which was removed subsequent to the accident. As a
corrective measure, approximately 200 m3
of soil was
excavated to a depth of about 6 m below ground surface
and transported off-site. The site was backfilled with a
clean imported soil. In addition six polyvinyl chloride
(PVC) pipes (4 in. diameter, 6 m length) were installed for
venting the soil gas into the atmosphere. One AS well and
four surrounding SVE wells were eventually commis-
sioned. Additional ten boreholes (BHs) were drilled for
monitoring purposes in addition to the one existing BH.
Table 1 shows the distances of BHs with respect to the AS
well, the diameters of casing, BH depths, and well screen
parameters. Figure 2d indicates the positions of treatment
BHs at the site (plan view). P&T injection–abstraction
Environ Earth Sci
123

4. wells (BH-19 and -20) are 5.2 m apart symmetrically
located down gradient of AS BH-16 that comprises the
center of the contamination source. Pumped groundwater
was passed through activated carbon tanks and reinjected
into the aquifer up gradient at BH-41.
Drilling logs and visual inspection of collected core
samples showed the conglomerate–gravel–conglomerate–
gravel–conglomerate sequence from the depth of 11 to
20 m beneath the ground surface, i.e., heterogeneous rock
in the saturated zone. The hydraulic conductivity K deter-
mined from slug tests in piezometers was in the range of
0.01–1 m/day.
Water table measurements conducted before a sparging
event using different triads of piezometers have indicated
water depth of 13–15 m beneath ground surface. The
undisturbed natural groundwater heads (counted from
mean sea level) before AS operation are plotted in the map
(Fig. 3). Groundwater heads indicate a general flow
direction from the SSE to the NNW. Widely spaced head
contours in the north compared to relatively closely spaced
ones in the south indicate variation in hydraulic gradient
which was estimated to be 0.015 in the south and 0.003 in
the north. Variation in hydraulic heads results in variation
in the groundwater natural velocity which is assessed based
on an average effective porosity of 0.1 which was esti-
mated based on the lithological logs with reference to
Freeze and Cherry (1979) and Fetter (2000). Therefore, we
estimated the maximum natural groundwater velocity at the
macroscopic scale of the aquifer without consideration of
natural conduits to be 0.15 m/day in the south and 0.03 m/
day in the north based on the maximum K of 1 m/day.
Benzene of concentrations 5, 4, and 2.5 ppm was
observed at four out of the six pre-existing PVC venting
pipes located within the area between BHs 17–20. Soil
samples analyzed for total petroleum hydrocarbons (TPH)
gave 490 mg/kg (BH-09, at the depth of 310.5 m above
sea level (maSL)), 610 mg/kg (BH-09, 308.5 maSL),
809 mg/kg (BH-16, 308.6 maSL), 770 mg/kg (BH-20,
308.6 maSL), 1456-470-959 mg/kg (BH-41, 313.1-310.6-
308.2 maSL), and 1582 mg/kg (BH-42, 307.5 maSL).
Groundwater sampling and analysis started in February
2002, and indicated a contamination level of 1.3 mg/l at
BH-01. A thin free product was observed in this borehole.
AS–SVE plant was commissioned in November 2002, and
a full (continuous) AS operation was first conducted from
August 2004 to June 2005. Between November 2002 and
August 2004, several intermittent AS–SVE runs (active
periods lasting from several weeks to 2 months) were
implemented to optimize the recovery.
Figure 2a shows a contour map of groundwater con-
tamination (in mg/l) plotted in July 2004, i.e., before the
first continuous run (background contamination but—we
recall—after several trial cycles). An off-spillover point
shift of the contaminated zone in Fig. 2a is in agreement
with the natural groundwater gradient prevailing during the
dormant stages. Figure 2b illustrates the decontamination
pattern in June 2005, after continuous AS with the rate of
100 lpm combined with SVE and P&T. Figure 2c depicts
Table 1 Air sparging test borehole details
BH no. Function Distance
from AS (m)
Casing
diameter (mm)
BH GL
(maSL)
BH depth
(mbGL)
Top of screen
depth (mbGL)
Screen
length (m)
Screen open
area (%)
16 AS 0 20 319.57 20 18 2 4
41 MW only 1.1 100 319.61 20 8 12 20
09 TW RW 2.1 50 319.62 20 9 11 20
19 SVE/P&T 3.1 100 319.50 18 7 11 7
20 SVE/P&T 3.7 100 319.57 18 7 11 7
18 SVE 5.2 100 319.56 18 7 11 7
17 SVE 6.4 100 319.58 18 7 11 7
42 MW only 13.4 100 319.51 20 8 12 20
44 MW only 14.3 100 319.37 20 8 12 20
01 MW only 17.2 225 319.51 36 – – –
43 MW only 19.4 100 319.33 20 8 12 20
45 MW only 20.5 100 319.30 20 8 12 20
40 MW only 35.8 100 319.84 20 8 12 20
22 MW only 43.5 50 318.86 20 8 12 20
21 MW only 46.1 50 318.92 20 8 12 20
10 MW only 50.1 50 319.03 20 8 12 20
AS air sparging, MW monitoring well, SVE soil vapor extraction, TW RW treated water return well, P&T pump and treat, GL ground level, maSL
meters above sea level, mb meters below
Environ Earth Sci
123

5. contamination’s rebound in December 2005 (after
6 months of dormancy in AS remediation).
Changes in groundwater levels (GWLs), DO levels, and
borehole air pressures were all monitored at AS rates of 10,
25, 50, 100, and 200 l per minute (lpm). Groundwater
levels were measured using In Situ Minitroll and Solinst
Levelogger Model 3001 water level probes. For tracer tests
at AS rates 10, 25, and 50 lpm, table salt was introduced
into BH-09 and electrical resistivity (ER) was monitored in
the same borehole, up gradient of the AS BH-16. For tracer
tests at AS rate 100 lpm, salt was introduced into BH-19
and BH-43, and ER was monitored in the same boreholes,
down gradient of BH-16. (see Fig. 2 for BH locations.) Salt
was introduced by placing it in a small cloth sack attached
BH01
BH09
BH10
BH16
BH17
BH18
BH19
BH20
BH21
BH22
BH40
BH41BH42
BH43
BH44
BH45
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
(a) (b)
(c) (d)
0 10 20 30 40 m
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
13
BH01
BH09
BH10
BH16
BH17
BH18
BH19
BH20
BH21
BH22
BH40
BH41BH42
BH43
BH44
BH45
0 10 20 30 40 m
0
5
10
15
20
25
30
35
40
45
50
55
60
65
BH01
BH09
BH10
BH16
BH17
BH18
BH19
BH20
BH21
BH22
BH40
BH41BH42
BH43
BH44
BH45
0 10 20 30 40 m
BH09 Treated Water Return
BH16 AS
BH17 SVE
BH18 SVE
BH19 SVE/P&T
BH20 SVE/P&T
BH41
0 1 2 3 4 m
Fig. 2 Contour maps of
groundwater TPH
contamination (mg/l) at
different stages of treatment and
a plan view of the air sparging
area. a July 26, 2004-Before
AS100/SVE/P&T. b June 27,
2005-After AS100/SVE/P&T.
c Dec 24, 2005-Rebound after
P&T. d Details of AS area
Environ Earth Sci
123

6. to a weighted rope and moved up and down within the
saturated zone in the boreholes until all salt was dissolved.
Electrical resistance was measured by using digital
groundwater conductivity tester, Kyowa KCM-200C, at
50 cm depth intervals within the same boreholes where the
salt was introduced. Salt tracer was assumed to be flowing
out of the borehole as ER increased. Changes to ER under
natural conditions were compared with changes to ER
under different AS rates to indicate the effect of AS on
groundwater flow. DO measurements during two AS
experiments with rate 100 lpm, with and without SVE and
P&T were taken. DO was measured by in Situ MPTroll
9000 water quality probe. Borehole air pressure measured
directly by pressure gauge from borehole covers was
recorded during six tests at AS rates 50, 100, and 200 lpm
with and without SVE and P&T.
Results and interpretation
The GWLs were monitored in BHs 1 and 17–20, for AS
rate 200 lpm. GWLs rose to peak levels within the first
5 min of AS operation, after which they began to decline
after 15–30 min from the start, stabilizing after 1.5 h
(Fig. 4). The effect was found greatest in descending order,
for BH-18 (5 m from AS BH-16), for BHs 19–20, (3–3.5 m
from AS BH-16), for BH-17 (6.5 m from AS BH-16), and
for BH-01 (17 m from AS BH-16). Other than for BH-18,
for which there must be a special connectivity to AS BH-
16, the effect of AS was proportional to the distance from
BH-16. Stabilized GWLs after 1.5 h of operation ranged
from 0.2 to 0.6 m higher than the undisturbed GWL.
Similar rise and decay of the mound was monitored in BHs
1, 17, and 20, for AS rates 50 and 100 lpm (Fig. 5), and for
smaller injection rates (not shown).
The maximum rise of GWL during AS with a rate of
200 lpm was 1.1 m observed in BH-18 (Fig. 4) which is
located in the northern part that is characterized by a lower
hydraulic gradient (Fig. 3). Consequent to GWL rise, the
hydraulic gradient in the north changed to 0.013 (taking
BHs 18 and 10 as reference) resulting in a maximum
velocity of 0.13 m/day. Such velocity indicates a larger
travel time of groundwater significantly beyond the normal
duration of AS. Therefore, in case of a homogeneous iso-
tropic aquifer the uncontrolled migration of the plume due
to AS is unlikely. However, increased velocities through
conduits related to aquifer’s heterogeneity and anisotropy
shall be assessed (here we used tracer tests, ER, DO, and
air pressure as tools for such an assessment).
Realizing three possible types of motion of injected air
(indicated above) and multiphase dynamics of three
involved phases we upscale the monitored dynamics of the
water table in the manner related to observable (by avail-
able instruments) groundwater characteristics (Fig. 1). At
the very early stage (Fig. 1a), air plume ABC expands
around the screen of BH-16. The plume boundary can be
considered as a classical free boundary (Muscat 1949;
Nikolskii 1961; Polubarinova-Kochina 1977) similar to
oil–water and gas–water contacts in petroleum engineering
models of secondary recovery (Charny 1963). The free
boundary expands from a ‘‘point’’ source (the screen of
BH-16). Although water displacement in the plume is not
complete, the Muscat (1949) model postulates that flow
inside and outside the plume are one-phase with an abrupt
jump of viscosity and density of the moving phases (air and
groundwater) across the plume boundary. At this earliest
stage, outside ABC groundwater moves as a result of
almost radial displacement close to the expanding interface
between a high air-pressure zone and nearby groundwater
(at almost hydrostatic conditions). Far away from the AS
source, groundwater dynamics are affected by gravity
similarly to classical problems of sink-induced phreatic
surface flows (Polubarinova-Kochina 1977). What makes
our problem mathematically much more complicated than
the Muscat problem in reservoir engineering (Muscat 1949;
Polubarinova-Kochina 1977) is the existence of the second
free boundary, viz. the water table, which forms a transient
Fig. 3 Undisturbed natural groundwater heads in the study area
before AS operation (not to scale to show the individual boreholes in
the air sparging area)
Environ Earth Sci
123

7. (rising-decaying) mound. At a certain time (Fig. 1b) the
plume apex A and C reaches the water table and the
injected air breaks through into the vadose zone, i.e., points
A and C separate. We interpret this moment as the peak
time tp which is found to be 24.25, 11, 12.5, 24, and
11.9 min in BHs 01 and 17–20 (Fig. 4), respectively. After
tp, the BH hydrographs show a recession limb, which
corresponds to the slumping stage of the mound (Fig. 1c).
During this stage the air flow is almost steady-state from
the source to the vadose zone but groundwater is still
adjusting to the created excess air-pressure by transferring
the volume of water from the displaced plume (the interior
of ABC in Fig. 1c) i.e., groundwater spreads laterally from
the crater-shaped mound. The contaminants move radially
from BH-16 together with groundwater during the period
of effacing of the AS-up welled periphery of the ‘‘crater’’
(Fig. 1c). It explains the purpose of P&T as a hydrody-
namic barrier to this spread. As Fig. 4 peaks show the
created hydraulic gradients are very high (*0.1) but short-
lived due to the decay of the mound. The pulse-type
convective displacement of groundwater particles based on
the assessed hydraulic conductivity and corresponding
linear average velocity will not exceed several tens of
centimeters during one injection pulse. We note that the
most pronounced humping of the mound is detected in
different pairs of monitoring wells (1 and 2 in Fig. 1).
The maximum of GWL in BH-01 (Fig. 4) is delayed as
it should be in mound dynamics (Kacimov 1997) because
BH-01 is much farther from the source of disturbance than
other wells in Fig. 4. Clearly, the lower the injection rate
the smaller is the amplitude of the peak in all wells.
When AS is stopped the well hydrographs show a blip
with a minimum reached at tm (time minimum) = 7.3, 7,
9.5, 5, and 7.2 min in BHs 01 and 17–20, correspondingly
(Fig. 4). This is in qualitative congruity with the dynamics
of phreatic surface troughs, which is up well to the initial
flat position (Kacimov et al. 2009). The converging
hydraulic gradient—upon cessation of sparging—may be
responsible for the rebound of concentration observed in
the AS zone (Fig. 2c).
The GWLs from seven BHs 17–20, 41–42, and 44 were
observed when both AS and P&T were operated for AS
rates of 10, 25, 50, 100, and 200 and P&T pumping rate of
36 lpm. There was a high water level rise at the beginning
of AS/P&T operation similar in pattern to the water table
rise observed during AS operation but of lesser magnitude
attributed to the effect of P&T.
Salt (conservative tracer) was introduced and ER mea-
surements were taken without AS and at AS rates of 10, 25,
and 50 lpm, to investigate the effect of AS on groundwater
flow in BH-09 ER measurements started when GWL sta-
bilized, i.e., 60–90 min after commencing AS operation.
Figure 6 shows ER as a function of depth without AS
(solutes subject to the natural groundwater flow only). ER
Elapsed Time (minutes)
ChangeinGroundwaterLevel(m)
Fig. 4 Groundwater levels at
AS rate 200 lpm
Elapsed Time (minutes)
ChangeinGroundwaterLevel(m)
Fig. 5 Groundwater levels at AS rates 50 and 100 lpm
Environ Earth Sci
123

8. increases more rapidly at around 14–15 and 17 mbGL,
confirming the existence of a relatively rapid groundwater
flow at these depths and, consequently, layer-type hetero-
geneity found from core-samples and drilling logs in this
well.
The dynamics of ER in BH-09 at the depth of 15 mbGL
is shown in Fig. 7 for AS rates of 10, 25, and 50 lpm. For
no-AS operation, ER rises immediately and after some
70 min, the water in the BH is almost completely replaced
by natural groundwater. In contrast, this replacement does
not progress as rapidly during AS operation, indicating
reduced groundwater flow. The ER recovery becomes
slower as the AS rate increases. Similar ER recovery
curves were observed at every depth, including the layer of
17 mbGL that can be attributed to the effect of the air-flow
zone in Fig. 1c i.e., BH-09 depending on the AS rate can be
located either inside this zone (and then migration of salts
from the tracered well is basically diffusive through the
residual water) or close to the air–groundwater interface
ABC. The ovalic (in a vertical section of Fig. 1) sparging
zone acts as an apparent impermeable boundary for the
natural groundwater flow and thereby impedes the advec-
tive transport near the tracered well.
Salt as a tracer was introduced in BH-19 (3 m away
from AS BH-16) and BH-43 (19.5 m away from AS BH-
16), and ER measurements were taken at AS rate 100 lpm.
The objective of this experiment was to assess the potential
increase of groundwater velocity (dissolution of injected
salts) in the wells far from the AS point. The increase of
ER in BH-19 was decelerated by AS similarly to BH-09
(Fig. 7) while in BH-43 ER dynamics was unchanged
as compared with the natural conditions, indicating that
the ROI ends somewhere between 3 (BH-19) and 19.5 m
(BH-43).
The DO was measured at five different depths (0.05, 0.5,
1, 2, and 5 m below the water table (mbWT)) in fifteen
BHs 1, 9, 10, 17–22, and 40–45, without AS and at AS rate
100 lpm. DO increase with AS in all monitored BHs from
4–6 to 6–9 mg/l. The increase in DO levels was greater in
the BHs located closer to BH-16. ROI inferred on the basis
of DO extended as far as 50 m, as evident from the
increase of DO at BH-10.
For the most general case of AS–SVE–P&T
(100–150 lpm at four SVE BHs-26 lpm, correspondingly),
at BHs in proximity to the SVE and P&T, DO levels were
lower by 0.5–2.5 mg/l than those when AS was applied
without SVE and P&T. However, for BHs more distant
from the SVE and P&T BHs, SVE and P&T have little or
no reducing effect on DO levels compared with the AS-
only scenario.
Borehole air pressure was measured at fourteen BHs 9,
10, 17–22, 40–45, at AS rates of 50 and 200 lpm, both
without and with SVE–P&T (150 lpm at four SVE BHs-
26 lpm, correspondingly). For AS Rate 50 lpm without
SVE and P&T, pressure increase was observed at BHs 9
and 17 only, but not at any other BHs including BH-41, the
closest BH to AS BH-16 (1 m away). For AS rate of
200 lpm without SVE and P&T, pressure increase was
observed at BHs 9, 17–20, 21 (46 m away from AS BH-16)
and 45 (20.5 m away from AS BH-16). We surmise that in
a single-source AS operation of Fig. 1c (and without liner
covering the soil surface) air injected escapes more or less
uniformly through the vadose zone and such that only at
high AS rates the air pressure rises significantly in the
boreholes. With added SVE four-sink operation, pressure
in the contamination area became negative with an
exception of positive pressure at BHs 21 and 45, at AS rate
of 200 lpm.
Discussion and conclusion
In this article, experimentation at several AS rates in
combination with SVE and P&T at a diesel contaminated
site in Oman is presented. Conventionally, it is considered
that AS is applicable only in homogenous formations while
Depth(m)
Fig. 6 Tracer test: changes in ER in salt-infused borehole (BH-09)
with time (no AS)
Elapsed Time (Minutes)
Fig. 7 Tracer tests: ER in BH-09 (15 mbGL) for AS rates of 10, 25,
and 50 l/min
Environ Earth Sci
123

9. the tested site exhibited strong heterogeneity of the satu-
rated zone.
As debated by Mei et al. (2002) and McCray and Falta
(1996) even for steady-state AS or AS–SVE the definition
of ROI of the remediated zone is ambiguous. In transient or
intermittent AS–SVE regimes (switching on and off the
injection well and playing with the rates and combinations
of AS–SVE and P&T) this ambiguity calls for thorough
monitoring of direct indicators of remediation (TPH in
groundwater) at different stages of intermittency and in
different BHs and of indirect indicators of flow and
transport, viz. phreatic surface fluctuations, tracer move-
ment, and air/oxygen pressure/concentration.
Turning on an AS source under an almost flat phreatic
surface causes a strong wave-type GWL response. Crater-
shaped groundwater mound was detected with a rapid
growth of the water table and relatively slow decline after
an air plume had established from a continuously injected
point source of AS. The lateral hydraulic gradient caused
by the upwelling of groundwater on the periphery of the air
plume may generally induce contaminated water flow from
the original contamination zone faster than without AS but
in the tested regimes this was not the case because of a
short time of existence of a single-hump mound in Fig. 1a
and a relatively rapid (tens of minutes) recession in Fig. 1c.
Moreover, water table upwelling caused by SVE in Fig. 1c
may serve as another regional impediment to the natural
groundwater flow. Stauffer et al. (2009) upscaled injection
from individual paths of air, which can be monitored in
laboratory experiments only, to a macroscale (‘‘air
plume’’). Our upscaling is even cruder: it extends to
observation of free surface dynamics only. Tracer tests
(measured by ER) illustrated the replacement speed of
wellbore storage by groundwater in a BH-09 located 2 m
up gradient of the AS well. The dissolution rate of injected
salts decreases with an increase of the AS rate, i.e., AS
reduces the natural groundwater velocity in the vicinity of
the AS well (similar effect is reported by Wardwell
(1999)). DO levels increased with AS and DO-based ROIs
of at least 50 m were monitored. At AS rate of 50 lpm,
increases in vadose zone air pressure due to AS were
counteracted by SVE and P&T. However, at an AS rate of
200 lpm, even with SVE and P&T, increased pressure was
still evident in outlying BHs as far as 46 m from the
sparging source.
Remediation of strongly heterogeneous and anisotropic
aquifers can potentially cause uncontrolled release of
contaminants outside the ROI. Tracer tests, ER, DO, and
air pressure can be effective tools in defining the ROI
extension and at the same time to assess aquifer’s hetero-
geneity and anisotropy.
Overall, AS operations at a single air sparging well
drilled in a heterogeneous formation showed a significant
reduction of contamination in an unconfined aquifer.
Combined with SVE and P&T, any subcatchment-scale
spread in contamination due to AS is highly unlikely.
Acknowledgments The authors would like to thank Japan Coop-
eration Center, Petroleum (JCCP) for their generous sponsorship of
this project, and Ministry of Regional Municipalities and Water
Resources (MRMWR), Oman for their contribution to the project.
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