Page 180 - Materials Chemistry, Second Edition

Vadose Zone Soil Remediation 163

5.2.3 Radius of Influence and Pressure Profile
Selecting the number and locations of vapor extraction wells is one of the
major tasks in design of in situ soil vapor extraction systems. The decisions
are typically based on the radius of influence (R ), which can be defined as
I
the distance from the extraction well where the pressure drawdown is very
small (P @ R ≈ 1 atm). The most accurate and site-specific R values should
I
I
be determined from pilot testing. The pressure drawdown data at the extrac-
tion well and the observation wells can be plotted as a function of the radial
distance from the extraction well on a semilog plot to determine the R of the
I
extraction well. The approach is similar to the distance–drawdown method
for aquifer tests, as described in Section 2.4.3. The R is commonly chosen to
I
be the distance where the pressure drawdown is less than 1% of the vacuum
in the extraction well.
The field test data can also be analyzed by using the flow equation, which
describes the subsurface air flow. The subsurface is usually heterogeneous,
and the air flow through it can be very complex. As a simplified approxima-
tion, a flow equation was derived for a fully confined radial gas flow sys-
tem in a permeable formation having uniform and constant properties [1–4].
These references are the basis for much of the discussion on soil venting in
this chapter.
For the steady-state radial flow subject to the boundary conditions (P = P
w
@ r = R and P = P atm @ r = R ), the pressure distribution in the subsurface can
I
w
be derived as:
rR )
2
2
P r − P w ( P RI − P w) ln(/ w (5.3)
=
2
2
RR )
I
ln(/ w
where
P = pressure at a radial distance r from the vapor extraction well
r
P = pressure at the vapor extraction well
w
P = pressure at the radius of influence (= atmospheric pressure or a pre-
RI
set value)
r = radial distance from the vapor extraction well
R = radius of influence where pressure is equal to the atmospheric pres-
I
sure or a preset value
R = well radius of the vapor extraction well
w
Equation (5.3) can be used to determine the R of a vapor extraction well, if the
I
pressure drawdown data of the extraction well and that of a monitoring well
(or drawdown data of two monitoring wells) are known. As shown in Equation
(5.3), the flow rate and the permeability of the formation are not included in this
equation. The R can also be estimated from the vapor extraction rate and the
I
pressure drawdown data in the extraction well (see Section 5.2.4).