In situ air sparging is developing into a widely used
technology in the field of water contaminate removal. It involves
the injection of air into the saturated sub-surface coupled with
soil vapor extraction (SVE). SVE has been used for many years,
but air sparging seems to improve results. The process has proven
most effective for the removal of hydrocarbons such as gasoline
and chlorinated solvents.
The air sparging system involves many components. There is an air
injection well, an air compressor, air extraction wells, a vacuum
pump, piping and valving, and a gas treatment system. Injection
wells are placed two or three meters below the water table
forcing contaminants into the vapor phase and out of the
groundwater zone. It is believed that the injected oxygen aids in
the aerobic microbial degradation of the contaminants.
Drawing illustrating the components of an in situ air
sparging/SVE system.
Many aspects of air sparging technology are discussed below,
but it is important to note that this system affects and is
affected by numerous complex physical, chemical, and biological
systems. Sub-surface interactions are not completely understood,
so air sparging has varied results. Any application of this
technology should be paired with a very good understanding of the
specific conditions that exist at the remediation site.
There are two main processes that control the effectiveness of
In Situ Air Sparging (IAS). They are volatilization and aerobic
biodegradation. These processes are interrelated and are
controlled by similar factors. In volatilization, the pollutants
are evaporated in direct contact with air, while in aerobic
biodegradation, the pollutants are broken down into harmless
products due to microbial activity.
Treatment by volatilization is similar to SVE and is most
effective for those contaminants that are located within the air
channels. Where a NAPL is located directly in an air channel, the
contaminant will volatilize upon direct contact with air.
However, this process has its drawbacks. In most cases, the
majority of the contaminant will not be located in the air
channels, but in the water saturated zones. For volatilization to
be effective in this type of scenario, the injected air must
diffuse across the air-water interface and this is an extremely
slow process. There is the possibility that the injection of air
will enhance the mixing at this interface, thereby increasing the
effectiveness of IAS. However, this theory can to be confirmed
with available data.
Illustration showing air channels formed during in situ air
sparging.
IAS can also serve to enhance the effectiveness of
biodegradation. Biodegradation is an effective way of treating
hydrocarbon fuel contaminants. However, at most sites containing
hydrocarbon fuel spills, oxygen is the main factor limiting
biodegradation rates. There are several methods to enhance the
oxygen concentration below ground, including bioventing and the
use of oxygen releasing compounds (ORCs). In situ air sparging is
also an effective method to increase oxygen levels in the
saturated zone. By pumping air underground, microbes will receive
increased oxygen levels, thereby stimulating their productivity.
But as in the case of volatilization, the effectiveness of this
method is limited by the rate of diffusion.
All in situ air sparging systems should be designed and
operated to optimize volatilization and biodegradation processes
and to minimize the probability of adverse consequences, such as
off-site migration of vapor or contaminated ground water. Since
air sparging systems are extremely site-specific, there is a high
variability of design and operation from site to site. Currently,
both flow rate and well placement design parameters are based
highly on operating experience. Case studies have also lead to
large variations in parameters such as the screen depth below the
water table, number of injection and extraction wells, air
injection flow rate, vapor extraction flow rate, and air
injection pressures.
However, certain criteria must be met. The injection well screen
(area of air injection) must be located entirely within the water
table. The flow rate for the injected air must be significantly
less than that of the extracted air. Also, the placement of the
air injection wells should maximize contact with the contaminated
zone and therefore, the well screen should be placed below the
lowest observed contaminant depth. Due to the limited knowledge
of injected air paths, the "radius of influence" is
unknown and can only be guessed.
Illustration showing a close-up view of the air flow in a
well screen.
Air injection and vapor extraction wells can be installed in
situ either horizontally or vertically. After the Savannah River
Integrated Demonstration was completed, the United States
Department of Energy decided that the use of horizontal wells is
preferred over vertical wells. Horizontal well installation has a
higher initial cost compared to vertical installation, but the
operation costs of horizontal wells is much lower. Savannah River
test results indicated a five-fold increase in contaminant
removal for horizontal wells in comparison to vertical wells.
This higher efficiency of reaching the contaminated zone is most
likely due to the thin but extensive qualities of the contaminant
layer.
One of the concerns regarding in situ air sparging is the
potential for the volatile organic contaminants to disperse and
accumulate in nearby basements of buildings. Vaporous
contaminants will likely accumulate in these areas since they
present a low-pressure zone to the travelling vapors. This hazard
can be prevented by proper placement of extraction wells.
Similar concerns exist due to the possibility of the formation of
an "air bubble", which is air trapped when a low
permeable soil exists below the water table but above the zone of
air injection. The presence of an "air bubble" will
cause the vaporous contaminants to distribute and spread,
decreasing the efficiency of the removal system (Brown).
Illustration showing an "air bubble" below the
area of contamination.
System efficiency would also decrease with the presence of an
overly high pressure zone. In an effective system the introduced
air pressure would be high enough to spread the air horizontally,
removing the maximum amount of volatile contaminants. Increases
in injection pressure promote greater horizontal flow. However,
inducing air pressures that are too high may cause the airflow
into water to change from smooth to turbulent. At these high
pressures, a plume of dissolved contaminants will form which will
travel down-gradient in the aquifer.
The potential danger of water table mounding, caused by the
displacement of water due to injected air, is of lesser concern.
Normally, mounding at the water table would cause an increase in
the spread of contaminants. However, the mixing of injected air
and water combine to form a lower density fluid. This lower
density decreases the spread of contaminants, effectively
offsetting any mounding effects.
In most applications of in situ air sparging, monitoring of the
soil and groundwater systems has been achieved by means of
"air detectors" which can be used to detect levels of
various components of air. These detectors are placed in situ in
various locations and changes in levels between initial and
specific times during the process of air injection are measured.
This method allows the tester to be able to "see" where
the injected air has flowed. This type of monitoring has been
fairly successful in terms of allowing the location and flow
rates of injection and extraction wells to be based on site
specific information.
When air is injected into the saturated zone, groundwater is
displaced. Where the displacement of the water table has a
vertical component, there will be a rise in the water table; this
is termed mounding. This mounding can be used to determine a
radius of influence of a sparging well because the rise in the
water table is localized. Mounding can be a concern due to the
fact that it is the main force behind lateral movement of
groundwater and can lead to spreading of a contaminant plume.
There are two main stages that lead to a steady-state flow
pattern. During the first stage, air injected into the saturated
zone exceeds the airflow out of the saturated zone. This can be
compared to the inflation of a balloon. Once air breaks through
the vadose zone, the balloon begins to shrink. During the second
stage, pathways of higher air permeability to the vadose zone can
be compared to a leak in the balloon of air. The air pocket
shrinks until the amount of air injected equals the amount of air
that leaks out, and mounding decreases. The maximum mounding will
occur at the sparging well and it dissipates radially, spreading
wavelike. Lateral diversion of the airflow due to subsurface
heterogeneity is likely if the maximum extent of mounding does
not decrease regularly with increased distance from the sparging
well, especially if there is only a small amount of mounding in
the immediate vicinity of the well.
For sites that illustrate air sparging, see:
For case studies using air sparging, see:
For information on companies dealing with air sparging equipment, see:
Brown, R., Jasiulweicz, F. "Air Sparging: A New Model for
Remediation," Pollution Engineering, July 1, 1992,
pp. 52-55.
Delhaye, J. Measuring Techniques in Gas-Liquid Two-Phase
Flows. Springer-Verlag, Berlin. 1988.
Gvirtzman, H., Gorelick, S. "The Concept of In-Situ Vapor
Stripping for Removing VOCs from Groundwater," Transport
in Porous Media 8: 71-92. Kluwer Academic Publishers,
Netherlands. 1992.
Johnson, R., Johnson, P., McWhorter, D., Hinchee, R., Goodman, I.
"An Overview of In Situ Air Sparging," GWMR.
Fall, 1993, pp. 127-135.
Lundegard, P. "Air Sparging: Much Ado About Mounding." In
Situ Aeration: Air Sparging, Bioventing, and Related Remediation
Processes. Ed. Robert Henchee et al. Columbus: Battelle
Press, 1995. 21-30.
U.S. Department of Energy, Environmental Restoration and Waste
Management Office of Technology Development. "Cleanup of
VOCs in Non-Arid Soils: The Savannah River Integrated
Demonstration." WSRC-MS-91-290, Rev. 1.
Send comments or suggestions to:
Faculty Advisor: Naraine Persaud, npers@vt.edu
Copyright © 1998 Daniel Gallagher, dang@vt.edu
Last Modified: June 7, 1998
