Environmental Engineering Reference
In-Depth Information
with cement grout to prevent the direct inflow of air
from the surface into the screened well intake. The
typical radius of a SVE well is in the range of 1.3-5 cm
(0.5-2 in). If the vadose zone is relatively thin, less than
3 m (10 ft) in depth, or if the contaminated soil is near
the ground surface, extraction wells can be placed hori-
zontally in vapor extraction trenches which are exca-
vated through the vadose zone to just above the seasonal
high water table. Airflow in the soil can be enhanced at
strategic locations by the installation of air inlet or injec-
tion wells. Inlet wells allow air to be drawn into the
ground at specific locations, whereas injection wells
force air into the ground and can be used in closed-loop
systems. Both inlet and injection wells are constructed
similar to extraction wells. Vapor treatment of the
extracted air may not be required if the emission rates
of chemicals are low or if they are easily degraded in
the atmosphere. SVE is the leading method for cleaning
up fuel spills and industrial solvents from unsaturated
settings, and it provides one of the few alternatives to
soil removal in the vadose zone where pure organic
liquids often occur at residual saturation. Compounds
that have been removed successfully by vapor extrac-
tion include trichloroethene, trichloroethane, tetrachlo-
roethene, and most gasoline constituents; compounds
that are less amenable to removal include trichloroben-
zene, acetone, and heavier petroleum fluids. Typical
treatment systems for extracted soil vapor include liquid
vapor condensation, granular activated carbon adsorp-
tion, catalytic or thermal oxidation, and biofilters.
Darcy's law can be assumed valid for the flow of air
in coarse-grained soils composed of sands and gravels
(massmann, 1989). However, in applying Darcy's law to
the flow of air, the density of air is not constant, and the
same definition of the hydraulic head as for incompress-
ible fluids cannot be used. The more general fluid poten-
tial , Φ *, must be used instead, where Φ * is defined as
(Hubbert, 1940)*
where q is the bulk air flow velocity, k a is the intrinsic
permeability of the porous matrix for airflow, and µ is
the dynamic viscosity of air. Intrinsic permeabilities of
porous media are typically given for cases in which the
entire pore space is available for fluid flow, and in the
case of airflow must generally be adjusted to account
for soil moisture and nAPL residual saturation. The
intrinsic permeability, k a , for airflow can be derived from
the intrinsic permeability, k , for groundwater flow in
saturated porous media using the relation (Stylianou
and DeVantier, 1995)
k
=
k
(
1
S
S
)
3
(5.90)
a
NAPL
water
where S nAPL is the residual saturation of the nAPL and
S water is soil moisture saturation. For most airflows, the
density, ρ , is small, and the force associated with the
pressure gradient is much larger than the gravity force,
hence Equation (5.89) can be closely approximated by
k
a
µ
q = −
p
(5.91)
and Equation (5.91) is exact for horizontal airflows. The
continuity equation for air in porous media can be
written as
p
t
0
(5.92)
θ
+ ∇⋅
(
ρ
q
)
=
a
where θ a is the volumetric air content, and sources and
sinks of air have been neglected. Assuming that the
transport process is isothermal, the ideal gas law holds
and
p
0
ρ ρ
(5.93)
p
=
p
o ρ
dp
+
where p 0 and ρ 0 are the pressure and density at some
reference state. Combining Equations (5.91) to (5.93)
gives
Φ * =
gz
(5.88)
p
where the density, ρ , is considered to be a function only
of the fluid pressure, p , and p 0 is a reference pressure.
Using Φ */ g instead of the piezometric head ϕ (= p / γ + z )
in the Darcy equation gives
p
t
p k
=
a
0
(5.94)
θ
+ ∇⋅
p
a
µ
Equation (5.94) is nonlinear in p and difficult to
solve. However, in applications associated with SVE
systems, Equation (5.94) can be linearized by assuming
that the pressure, p , is equal to atmospheric pressure,
p atm , plus a small perturbation, p *, where
k
a
µ
q
= −
(
∇ +
p
ρ
g
k
)
(5.89)
* Hubbert (1940) included an additional kinetic energy term, v 2 /2,
where v is the seepage velocity. The kinetic energy term is relatively
small and is usually neglected, leading to Equation 5.88.
p
=
p
+
p
*
(5.95)
atm
 
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