Effects of soil moisture and physical ch
Journal of Contaminant Hydrology, 11 (1992) 273-290
Elsevier Science Publishers B.V., Amsterdam
273
Effects of soil moisture and physical-chemical
properties of organic pollutants on vapor-phase
transport in the vadose zone
S.K. O n g a, T.B. Culver b, L.W. Lion b" and C.A. Shoemaker b
aBattelle Memorial Institute, Columbus, OH 43201, USA
bSchool of Civil and Environmental Engineering, Cornell University, Ithaca, N Y 14853, USA
(Received May 22, 1991; revised and accepted May 22, 1992)
ABSTRACT
Ong, S.K., Culver, T.B., Lion, L.W. and Shoemaker, C.A., 1992. Effects of soil moisture and
physical-chemical properties of organic pollutants on vapor-phase transport in the vadose
zone. J. Contam. Hydrol., 11: 273-290.
Vapor-phase transport of organic pollutants is one of the important pathways in the
distribution and attenuation of volatile organic compounds in the vadose zone. In this study,
the impact of vapor-phase partitioning and of the physical-chemical properties of organic
pollutants on vapor-phase transport was assessed. An experimentally derived relationship to
predict vapor sorption for a variety of soil types under varying soil moisture conditions was
incorporated into the two-dimensional finite-element model, VOCWASTE.The revised model was
then used to simulate the transport of volatile organics. Vapor-phase partitioning in the model
accounted for vapor uptake by sorption onto moist mineral surfaces as well as sorption at the
liquid-solid interface and dissolution into soil water. Under dry conditions, vapor-phase
sorption of volatile organic pollutants was shown to have a retarding effect on transport of
organic vapors. However, for shallow, contaminated soils, volatilization was controlled by
vapor diffusion, even under dry conditions where vapor-phase sorption was high. The influence
of Henry's law constant and of the aqueous-phase (solid-liquid) partition coefficient for volatile
organic pollutants was considered in the simulations. Volatilization of organic vapors was
shown to be favored for contaminants with high Henry's law constants and low aqueous-phase
partitioning coefficients. Because of the interdependence of these two physical-chemical
properties, individual properties of the contaminant should not be considered in isolation in the
evaluation of vapor transport.
Correspondence to: L.W. Lion, School of Civil and Environmental Engineering, Cornell
University, Ithaca, NY 14853, USA.
016%7722/92/$05.00
© 1992 Elsevier Science Publishers B.V. All rights reserved.
274
S.K. O N G ET AL.
INTRODUCTION
Volatilization from the soil surface has long been recognized as one of the
important pathways for the movement and attenuation of organic pesticides
in the subsurface (e.g., Spencer et al., 1973). Recently, with the discovery of
gross contamination of groundwater by volatile organic compounds (VOC's)
such as solvents and fuels, knowledge regarding the movement of organic
vapors in the subsurface has been extended to consideration of transport and
the interphase distribution of these compounds. The importance of organic
vapor movement in the subsurface is evidenced by the successful use of
shallow soil-gas sampling techniques to estimate the extent of groundwater
pollution (Marrin and Kerfoot, 1988) and the use of venting techniques to
remediate contaminated aquifers (Baehr et al., 1989).
Use of mathematical models to predict the movement of VOC's has become
a part of the planning effort for design of new waste disposal sites and can be
a component of remedial investigations at hazardous waste sites. A list of
some currently available vapor-phase transport models along with their physical-chemical and biological features is presented in Table 1. Although many
models on vapor transport are available, information are limited on physicalTABLE
1
Physical-chemical and biological features of various vapor transport models
References
Features*
1
Mohsen et a l . ( 1 9 8 0 )
Weeks et a l . ( 1 9 8 2 )
Jury et a l . ( 1 9 8 3 )
Stephanatos ( 1 9 8 5 )
Abriola and Pinder ( 1 9 8 5 )
Pinder and Abriola ( 1 9 8 6 )
Springer ( 1 9 8 6 )
Baehr and Coracioglu ( 1 9 8 7 )
Baehr ( 1 9 8 7 )
Hutzler et a l . ( 1 9 8 9 )
Metcalfe and Farquhar ( 1 9 8 7 )
Sleep and Skyes ( 1 9 8 9 )
Shoemaker et a l . ( 1 9 9 0 )
Culver et a l . ( 1 9 9 1 )
2
3
4
2
z
x
1
x
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1
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2
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x
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6
7
8
x
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9
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11
12
13
14
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15
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x
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x
x
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1
x
x
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x
x
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×
2
x
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x
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x
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×
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×
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3
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x
x
×
x
×
x
* 1 = number of dimensions; 2 = multicomponent (pollutant); 3 = diffusion, vapor phase; 4 = advection,
vapor phase; 5 = diffusion, aqueous phase; 6 = advection, aqueous phase; 7 = dissolution (Henry's l a w ) ;
8 = sorption at solid-liquid interface; 9 = vapor sorption other than 7 and 8 ; 10 = non-equilibrium
12 = temporal variability in moisture contents; 13 = spatial
mass-transfer effects; 11 = b i o d e g r a d a t i o n ;
heterogeneity; 14 = immiscible phase; 15 = density-dependent gas behavior
ORGANIC POLLUTANTS AND VAPOR-PHASE TRANSPORT IN THE VADOSE ZONE
275
chemical processes affecting the movement of organic vapors, such as the rate
of vapor-phase partitioning and/or mass transfer of vapors.
Movement of organic vapors in the subsurface is influenced by the
reactions between the vapor phase and the moist, porous media. Sorption in
a three-phase (gas-liquid-solid) system is complex and must consider the
interrelated distribution of the contaminant into pore water, soil matrix and
vapor phases, as well as the possibility of contaminant accumulation at the
water-vapor interface (Valsaraj, 1988). Several researchers have measured
elevated sorption of vapors onto unsaturated soils (Chiou and Shoup, 1985;
Chiou et al., 1985; Peterson et al., 1988; Ong and Lion, 1991a, b). The sorption
of trichloroethylene (TCE) vapor onto soils for a range of moisture contents
has recently been evaluated by Peterson et al. (1988) and Ong and Lion
(1991b). These authors showed that sorption reactions (in addition to those
occurring in the aqueous phase) such as vapor sorption at the water-air
interface and direct sorption of vapors to mineral surfaces may be important
under certain moisture conditions. For soils with moisture contents less than
those equivalent to surface coverage by eight layers of water (as in semi-arid
regions or surface soils), sorption of TCE vapor can exhibit partition coefficients two to four orders of magnitude larger than those at higher moisture
contents (Ong and Lion, 1991b). Similarly, equilibrium sorption results for
other organic vapors onto a variety of minerals and soils show that vaporphase partitioning is a function of moisture content and that, in the presence
of a mixture of organic vapors, interactions resulting in enhancement or
suppression of sorption of individual vapor may occur (Lion et al., 1990; Ong
et al., 1991).
The mechanisms controlling the sorption of vapors in the unsaturated zone
are not completely understood. It has been suggested that the mineral portion
of a dry soil dominates the sorption of organic vapors (Chiou and Shoup,
1985; Chiou et al., 1985). In fact, the variation in vapor sorption coefficients
for oven-dry soils is best explained by their surface areas rather than their
organic contents (Rhue et al., 1988; Ong and Lion, 1991a). Yet, for air-dried
soils, organic matter content is the best predictor of vapor sorption strength
(Ong and Lion, 1991a).
The impact of vapor-phase sorption on organic vapor transport in the
subsurface was investigated by Shoemaker et al. (1990) with a one- and
two-dimensional analytical model. Sensitivity analysis indicated that incorporation of vapor-phase sorption can significantly reduce model predictions
of organic vapor transport relative to those predicted when only the liquidphase sorption was considered. Culver et al. (1991) extended this study by
developing a two-dimensional numerical model, VOCWASTE,and applied it to
several different organic vapor transport scenarios. For their one-dimensional
sensitivity analysis, Culver et al. (1991) assumed vapor sorption to be inversely
276
s . K ONG ET AL.
and exponentially related to the percent soil saturation. Given this general
form of the vapor sorption profile, Culver et al. (1991) performed a systematic
evaluation of the sensitivity of vapor transport predictions to changes in the
shape of the vapor sorption profile given varying moisture profiles. Their
results demonstrated that accurate knowledge of the vapor sorption response
to soil moisture is essential for accurate prediction of volatile organic
transport if the soil profile includes regions of low soil moisture.
In this study, we incorporate the equilibrium vapor sorption results of Ong
and Lion (1991b) into VOCWASTEto investigate the impact of vapor sorption
on the transport of organic vapors under variable soil moisture contents.
Using the data base developed by Ong and Lion (1991b) this study is the first
to develop an empirical relationship to predict vapor partitioning over variety
of soil types given a range of soil moisture contents. The vapor sorption
relationship used in previous applications of VOCWASTE(Culver et al., 1991)
was based on a limited data base and was not generalizable across soil types
and moisture conditions. In this paper we also evaluate the influence on vapor
migration of two commonly determined chemical properties, the Henry's law
constant and the aqueous-phase partition coefficient.
DESCRIPTION OF TRANSPORT MODEL
Movement of organic vapors in the subsurface was calculated using the
two-dimensional transport model VOCWASTE(Culver et al., 1991). This model
is a modification of a finite-element model developed by Yeh (1981, 1987). The
original contaminant transport model consisted of two parts: FEMWATERand
FEMWASTE. The FE~WA'rER program (Yeh, 1987) predicts the changes in the
potential heads and in volumetric water contents due to water fluxes in
variably saturated systems. FEMWASa'E(Yeh, 1981) describes the movement of
the contaminant in the aqueous phase for both saturated and unsaturated
zones. Aqueous-phase adsorption and biodegradation of the contaminant are
also included in the FEMWASTEmodel. Volumetric water contents and Darcy's
velocities computed by FEMWATERare used as inputs to FEMWASTE.
FEMWASTEdoes not include vapor transport and the code was extensively
modified by Culver et al. (1991) to include vapor-phase diffusion and sorption.
The modified version of FEMWASTEis called VOCWASTE. VOCWASTEdoes not
include vapor advection. The common assumption that gas-phase advection
is negligible for modeling vapor-phase transport has been questioned by
Pinder and Abriola (1986). Kell (1987) observed that advective transport may
be significant in laboratory studies. However, work by Kreamer et al. (1988)
suggested that vapor-phase advection is small and can be neglected unless air
pumping is present or unless there is a rapidly fluctuating water table. Since
277
ORGANIC POLLUTANTS AND VAPOR-PHASE TRANSPORT IN THE VADOSE ZONE
VOCWASTEassumes advection occurs only in the aqueous phase, no fundamental changes were needed in the FEMWATERcode. The details of the modifications to FEMWASTEand example simulations which employ VOCWASTEhave
been described by Culver (1989) and Culver et al. (1991). The numerical
accuracy of VOCWASTE was found to be equivalent to that of the original
FEMWASTE (Culver, 1989). A brief overview of FEMWATER and VOCWASTEis
presented here.
Water flow in a variably saturated media is described in FEMWATER by
(symbols in the equations that follow are defined in the Notation):
dO 0h
- V'[K(O)'(VH)]+q
dh ~t
(1)
Assumptions made for this equation are that the soil matrix and the carrier
fluid are incompressible.
The modified equation for contaminant movement in VOCWASTE, which
includes diffusive vapor-phase movement, is given by:
~t
Oz
"++
ax
~---~(
~CT/RL+ ODHxz ~CT/RL
)
ODHxx Ox
~z
ODHzx
~x
+ ODm~
~z
+ M
(2)
where
CT = OCL +aCG + CLKdPb+ CGKsGPb
= (total mass of contaminant in a volume of soil)
CT = RsL CsL = RL CL = R c C c = RsGCsc
RL = PbKd +O+aKH +PbKHKsG
Rc, = pbKd/KH +O/K. +a+pbKsG
RSG = PbKd/K, Ksc + O/KHKsc + a/Ksc + Pb
and
RSL = Pb + O/Kd + aKH/Kd + PbKn Ksc/Kd
VOCWASTE assumes all phase partition processes obey local equilibrium.
Most subsurface transport models have assumed that local equilibrium is
278
S.K. ONG ET AL
NOTATION
Definition of terms
Symbol
Description
Dimension
h
H=h+z
K(O)
0
q
dO/dh
pressure head
total head
hydraulic conductivity tensor
volumetric water content
source/sink
specific moisture capacity
total concentration
aqueous concentration
gaseous concentration
mass sorbed per unit mass of sorbent due to
aqueous sorption
mass sorbed per unit mass of sorbent other
than aqueous sorption and dissolution into
soil water, e.g. vapor-liquid sorption or condensation
saturated soil-water partitioning coefficient
aqueous partitioning coefficient normalized
with respect to organic carbon
weight fraction of organic carbon in soil
vapor sorption coefficient (a variable function
of the water content)
Henry's law constant
volumetric air content
soil bulk density
Darcy's velocity in the ith direction
/jth component of the hydrodynamic dispersion
tensor including aqueous diffusion
~th component of the mechanical dispersion
tensor
aqueous-phase coefficient of molecular diffusion
vapor-phase coefficient of molecular diffusion
source/sink input term
overall vapor-phase partitioning coefficient
activity coefficient
density of water
moisture content in percent
specific surface area of the solid
[L]
[L]
[L T --j]
[L3 L 3]
[L3L 3 T 1]
[L-l]
[M L 3]
[M L 3]
[M L -3]
[M M - ~]
G
cL
c,:,
Csl.
C~,:,
Kd = KoJCo~ = CsL/CI~
Koc
foc
Ks 8
(4)
The first set of simulations illustrates the impact of vapor-phase sorption
under three different climatic conditions: (1) and (2) a wet and a humid
environment where the soil is moist at about field capacity and (3) a dry
environment, typical of a drought season, where the soil close to the ground
surface is fairly dry. A rainfall rate (above evaporation) of 0.4 cm day i (4.72
in m o n t h -l ) is used for the wet condition, 0.2cm day -~ (2.36 in month -~ ) for
the humid condition and 0.02 cm day-J (0.236 in m o n t h -1 ) is used for the dry
condition. The input parameters for the soil moisture characteristic curves
were adjusted appropriately for each region. Input data for the computer
simulations, including the soil moisture characteristic curves, are summarized
in Tables 2 and 3. Physical-chemical data for various VOC's were obtained
282
SK. ONG ET AL.
TABLE 2
Soil input parameters for computer simulation
Symbol
Definition
Units
Ph
Joc
Ksxx
soil bulk density
soil organic content
xx-component of saturated
conductivity
zz-component of saturated
conductivity
xz-component of saturated
conductivity
saturated volumetric water
content
residual water content
surface area of soil
pressure head scaling factor
wet region
humid region
dry region
characteristic curve exponent
wet region
humid region
dry region
g cm 3
g g- 1
cm day ~
1.35
0.0125
100.2
cm day 1
100.2
cm day 1
0.0
cm 3cm 3
0.45
cm 3 cm 3
m 2 g-~
cm
0.00
80.0
K,~,,z
Ks.~
0~a~
Or
S.A.
ha
/~
Value
3,500
3,200
2,100
1.2
1.0
0.5
Characteristic curves:
O(h)
K(O)
=
~'0s,, - (0~t - 0 r ) ( - h / h a ) s,
if
)~0sat
otherwise
Ks[(O--Or)/(Osa
t --
h < 0.0cm
0r)l 4
from Verschueren (1983) and O R N L (1989) while vapor and aqueous
diffusion coefficients were computed using the Hirschfelder-Bird-Spotz
equation and Wilke-Chang equation, respectively (Welty et al., 1984).
The second set of computer simulations illustrates the sensitivity of
transport of VOC's in the vadose zone to two commonly determined physicalchemical properties of contaminants. The two properties considered were the
Henry's law constant and the aqueous-phase partition coefficient. Four
c o m m o n components of gasoline were modeled, i.e. benzene, toluene, oxylene and n-octane (Table 3). A study domain similar to the first simulation
was used (Fig. 2) and it was assumed that the climatic condition was humid
with a rainfall of 0.2cm day -1 .
ORGANIC
POLLUTANTS
AND VAPOR-PHASE TRANSPORT
283
IN THE VADOSE ZONE
¢N
cq
o
N
o
N
-
oo
q
~
N
o°
cS
~d
oo-
d
,~
~.
0o
~D
Z
0
0o
•
I~
f'~
~
oo
o
,~.
I=
~I~
~1~
I~
c,,;
[.-
;>
I
0
o
>,
..0
.o
.
.
.
.
-~
~
~
= o~
e~
e~
<
=
o
o
~
~
o=
~'~
~
0
~
~
~
~
~
~
~
-~-"
284
S.K.
ONGAL.
ET
Ground Level
Rainfal~l -~~ 0R~infymal/Ida
~aY
-5'
-10
-15
0.0
0.1
0.2
0.3
0.4
0.5
Moisture Content (Vol./Vol.)
Fig. 3. Volumetric moisture content profile generated by the simulation model FEMWATER for three
rainfall patterns.
RESULTS AND DISCUSSION
Effects of vapor-phasepartitioning
In order to predict vapor-phase transport and sorption, the soil moisture
content profile associated with a given continuous precipitation rate must first
be computed. The moisture profiles generated by FEMWATEgfor three different
rainfall conditions are shown in Fig. 3. Under dry climatic conditions (0.02 cm
d a y - l ) , the volumetric moisture content ranges from 0.08cm 3 cm -3 (18%
saturation) at the ground level to ,-~0.36 cm 3 cm 3 at 15 m (80% saturation).
For a soil with a specific surface area of 80 m 2 g - l, the number of layers of
water at 0.08 cm 3 cm -3 volumetric moisture content is ~ 2.6. For a rainfall of
0.2 cm d a y - l , the moisture content at the ground level is ~ 22 cm 3 cm -3 (49%
saturation or ~ 7 layers of water) and increases to ~ 4 4 c m 3 cm -3 at 15m
(98% saturation). At this rainfall rate, vapor-phase sorption (KsG) is
negligible throughout the soil profile and does not influence vapor transport.
With TCE-contaminated soil assumed to be 10 m below the surface, volatilization of TCE over a period of 8 yr was computed for two different climatic
conditions. Fig. 4 shows that the a m o u n t of TCE that is volatilized after 8 yr
under dry conditions is less than that under wet conditions. The 10m of
relatively dry soil between the source and the volatilization flux at the surface
act as a "buffer" from the volatilization loss. This result indicates that
enhanced vapor-phase partitioning in a deep, dry, soil profile can play an
important role in the retardation of organic pollutants in the subsurface.
The volatilization simulations were repeated for a 4-yr period with the
source located at 4 m below the surface (Fig. 5). Three different climatic
ORGANICPOLLUTANTSAND VAPOR-PHASETRANSPORTIN THE VADOSEZONE
~
60-
285
Source at l0 m Below Ground Level
5o-
~
40"
~
:30
"~
2o
Rainfall
0.2 cm/day
o
2
/
"
4
6
T i m e (years)
Fig. 4. V O C W A S T E computer simulations of the effects of vapor-phase partitioning on the volatilization
of TCE for two rainfall patterns. The TCE source is at a depth of 10m.
conditions were evaluated. Contrary to the results when the source is deep
(Fig. 4), volatilization under dry conditions was greater than under wet
conditions for the 4-m-deep source. Even though vapor-phase partitioning
still occurs, the dominant physical parameter controlling volatilization in the
upper soil is vapor diffusion. Vapor diffusion is much greater in a dry soil than
a moist soil where vapor-phase diffusion is restricted by moisture (Fig. 5). The
increased partitioning from vapor sorption in the case of the 4-m-deep source
is not strong enough to counterbalance the increase in vapor diffusion and the
impact of volatilization near the surface. Vapor-phase sorption was not
40
4m
Source at
30
Below Ground Level
Rainfall
0.02 cm/dayj
~"
J
Rainfall
20
n~lHday
1
2
3
4
T i m e (years)
Fig. 5. V O C W A S T E computer simulations of the effects of vapor-phase partitioning on the volatilization
of TCE for three rainfall patterns. The TCE source is at a depth of 4 m,
286
SK. ONG ET AL.
important for the moisture profiles corresponding to rainfalls of 0.2 and
0.4 cm day- ~.
Cutver et al. (1991) performed simulations using VOCWASTE under a
different set of conditions, i.e. different moisture profiles, vapor-phase
sorption capacities and a contaminant source placed 21.5m below ground
level. These simulations also showed that vapor-phase sorption and soil
moisture contents may significantly retard the transport of VOC's.
Effects of physical-chemical properties of pollutants on volatilization
A m o n g the physical-chemical properties of pollutants that influence their
loss by volatilization are the vapor diffusion coefficient, Henry's constant (K~)
and aqueous-phase partition coefficient (Ka). Even though the diffusion of
VOC's in the vapor phase is about four orders of magnitude larger than that
in the aqueous phase, the diffusion coefficients for most volatile organic
compounds are of similar magnitude (Table 3). Therefore, differences in the
vapor transport between organic vapors based solely on differences in their
diffusion coefficients may be negligible.
To illustrate the effects of Henry's law constant, two organic pollutants
having similar Kocvalues but differing KH-values were selected: TCE (Koc =
61.1, KH = 0.397) and benzene (Koc = 65, KH = 0.222). For the following
simulations, the contaminated soil was placed at 4 m below ground level and
the rainfall was 0.2 cm day -~ . At these moisture levels, vapor-phase sorption
was negligible and was not a factor in the simulations. As expected, the
organic pollutant with the larger KH-value, i.e. favoring distribution of the
pollutant in the vapor phase, had greater loss by volatilization from the soil.
This result is shown in Fig. 6.
Three organic compounds with similar Henry's law constants but different
Koc-values were used to illustrate the effect'of aqueous-phase partitioning on
volatilization. Aqueous-phase sorption reduces the mass fraction of
pollutants in the aqueous phase and subsequently in the vapor phase, which
in turn affects volatilization. The three compounds used in this simulation
were: benzene (Koc = 65, KH = 0.222), toluene (Koc = 259, K H = 0.270) and
o-xylene (Koc -- 690, K H = 0.20). Results for volatilization of all three
compounds are shown in Fig. 7. As anticipated, o-xylene, which has the
largest Koc-value, was found to volatilize the least in comparison with the
other two compounds. This effect is similar to the chromatographic effect
commonly observed for the transport of organic pollutants under saturated
conditions.
In contrast, simulation runs with n-octane, which has large KH- and Kocvalues (121.1 and 73,000 respectively), predicted less volatilization than oxylene (Koc = 690, K H = 0.20). The reason for the low amount of vapor loss
ORGANIC POLLUTANTS AND VAPOR-PHASE TRANSPORT IN THE VADOSE ZONE
Rainfall
= 0.2 c m / d a y
TCE (K H = 0.397,
°.
287
~
J
/
Benzene (KIt = 0"222
Koc = 65.0)
2
3
Time (years)
Fig. 6. V O C W A S T E computer simulations of the effect of Henry's constant on volatilization of two VOC's.
The contaminant source is at a 4-m depth and the rainfall rate is 0.2cm day ~.
40
30 -
°.
Benzene (Koc=65,
KH = 0"~...,..,ak
~
20 •
J
Toluene (Koc = 259,
i
I
2
3
4
5
6
7
Time (years)
Fig. 7. V O C W A S T E computer simulations of the effects of Koc on the volatilization of three VOC's. The
contaminant source is at a 4-m depth and the rainfall rate is 0.2 cm day ~.
was that most of n-octane ( > 99.9%) was calculated to be sorbed from the
aqueous phase, in spite of the large Henry's constant which would favor loss
of this pollutant by volatilization. These simulations indicate the interdependent effects of the physical-chemical characteristics of pollutants on their
transport in the vadose zone.
CONCLUSIONS
Vapor-phase transport of organic pollutants is one of the important
288
sx. ONe ETAL.
pathways in the distribution and attenuation of volatile organic compounds
in the vadose zone. Vapor-phase sorption accounts for vapor uptake
processes other than sorption at the liquid-solid interface and dissolution into
soil water. Therefore, vapor-phase sorption would include direct sorption of
vapors to moist mineral surfaces and sorption at the water-air interface. In
this study, a VOC transport model was adapted to incorporate an empirical
relationship between the vapor sorption coefficient and the soil moisture
content based on the average surface coverage of the sorbent by water.
Computer simulations performed in this study show that, under dry
conditions, vapor-phase sorption of organic pollutants may significantly
retard the transport of volatile pollutants. As a result of sorptive vapor
uptake, dry soils may act as a reservoir of VOC's. However, for contaminated
soils that are close to ground level, the volatilization of organic vapors is
controlled by vapor diffusion, even in dry soils where vapor-phase sorption is
expected to be high.
The transport of vapors was found to be greatly affected by the aqueous
partition coefficient (Kd) and liquid-vapor partitioning (Henry's law
constant). Volatilization is favorable for contaminants with high Henry's law
constants and low aqueous partition coefficients. Because the physical-chemical properties of a contaminant act in concert to influence vapor behavior,
individual properties should not be considered in isolation in the evaluation
of vapor transport.
ACKNOWLEDGEMENTS
This research was supported, in part, by the U.S. Air Force Engineering
and Services Center under Contract No. F08635-85-C-0030.
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ORGANIC POLLUTANTS AND VAPOR-PHASE TRANSPORT IN THE VADOSE ZONE
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Culver, T.B., 1989. Modeling the transport of volatile organic chemicals in variably saturated
subsurface regions. M.S. Thesis, Cornell University, Ithaca, NY, 135 pp.
Culver, T.B., Shoemaker, C.A. and Lion, L.W., 1991. Impact of vapor sorption on the
subsurface transport of volatile organic compounds: A numerical model and analysis. Water
Resour. Res., 27(9): 2259-2270.
Hutzler, N.J., Gierke, J.S. and Krause, L.C., 1989. Movement of volatile organic chemicals in
soils. In: B.L. Sawhney and K. Brown (Editors), Reactions and Movement of Organic
Chemicals in Soils. Soil Sci. Soc. Am., Madison, WI, Spec. Publ. No. 22, pp. 373-403.
Jury, W.A., Spencer, W.F. and Farmer, W.J., 1983. Behavior assessment model for trace
organics in soil, 1. Model description. J. Environ. Qual., 12(4): 558-564.
Kell, R.F., 1987. Studies on hazardous vapor transport in soil. M.S. Thesis, Department of
Civil Engineering, University of Waterloo, Ont., 177 pp.
Kreamer, D.K., Weeks, E.P. and Thompson, G.M., 1988. A field technique to measure the
tortuosity and sorption-affected porosity for gaseous diffusion of materials in the unsaturated
zone with experimental results from Barnwell, South Carolina. Water Resour. Res., 24(3):
331-341.
Lion, L.W., Ong, S.K., Lindner, S.R., Swanger, J.L., Schwager, S.J. and Culver, T.B., 1990.
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Force Eng.-Serv. Cent., Tyndall Air Force Base, FL, Tech. Rep. No. ESL TR 90-05, Natl.
Tech. Info. Serv., Springfield, VA, 281 pp.
Marrin, D.L. and Kerfoot, H.B,, 1988. Soil-gas surveying techniques. Environ. Sci. Technol.,
22(7): 740-745.
Metcalfe, D.E. and Farquhar, G.J., 1987. Modeling gas migration through unsaturated soils
from waste disposal sites. Water, Air Soil Pollut., 32: 247-259.
Mohsen, M.F.N., Farquhar, G.J. and Kouwen, N., 1980. Gas migration and vent design at
landfill site. Water, Air Soil Pollut., 13: 79-97.
Ong, S.K. and Lion, L.W., 1991a. Effects of soil properties and moisture on the sorption of
trichloroethylene vapor. Water Res., 25: 29-36.
Ong, S.K. and Lion, L.W., 1991b. Mechanisms for trichloroethylene vapor sorption onto soil
minerals. J. Environ. Qual., 20(1): 180-188.
Ong, S.K., Lindner, S.R. and Lion, L.W., 1991. Applicability of linear partitioning relationships for organic vapors onto soil and soil minerals In: R.A. Baker (Editor), Organic
Substances and Sediments in Water, Vol. 1. Lewis, Chelsea, MI, pp. 275-289.
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diffusion on the fate of trichloroethylene in an unsaturated aquifer system. Environ. Sci.
Technol., 22(5): 571-578.
Pinder, G.F, and Abriola, L.M., 1986. On the simulation of non aqueous phase organic
compounds in the subsurface. Water Resour. Res., 22(9): 109S-119S.
Rhue, R.D., Rao, P.S.C. and Smith, R.E., 1988. Vapor-phase adsorption of alkylbenzenes and
water on soils and clays. Chemosphere, 17(4): 727-741.
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impact of two-phase sorption on subsurface transport of volatile chemicals. Water Resour.
Res., 26(4): 745-758.
Sleep, B.E. and Sykes, J.F., 1989. Modeling the transport of volatile organics in variably
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290
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Spencer, W.F., Farmer, W.J. and Cliath, M.M., 1973. Pesticide volatilization. Residue Rev.,
49: 1-47.
Springer, E.P., 1986. Modeling of organic vapor movement at Area L Disposal Site. Los
Alamos Natl. Lab., Los Alamos, NM, Tech. Rep. LA-UR-86-4359, 94 pp.
Stephanatos, B.N., 1985. Groundwater pollution by gas-phase transport of contaminants
through the vadose zone. M.S. Thesis, University of Illinois, Urbana, IL, 74 pp.
Valsaraj, K.T., 1988. On the physico-chemical aspects of partitioning of non-polar hydrophobic organics at the air-water interface. Chemosphere, 17(5): 875-887.
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Vilkner, V.L. and Parnas, R.S., 1986. Analysis of volatile hydrocarbons losses from quiescent
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Weeks, E.P., Earp, D.E. and Thompson, G.M., 1982. Use of atmospheric fluorocarbons F-I1
and F-12 to determine the diffusion parameters of the unsaturated zone in the Southern
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Transfer. Wiley, NY, 3rd ed., 803 pp.
Yeh, G.T., 1981. FEMWASTE: A Finite Element Model of WASTE transport through
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Elsevier Science Publishers B.V., Amsterdam
273
Effects of soil moisture and physical-chemical
properties of organic pollutants on vapor-phase
transport in the vadose zone
S.K. O n g a, T.B. Culver b, L.W. Lion b" and C.A. Shoemaker b
aBattelle Memorial Institute, Columbus, OH 43201, USA
bSchool of Civil and Environmental Engineering, Cornell University, Ithaca, N Y 14853, USA
(Received May 22, 1991; revised and accepted May 22, 1992)
ABSTRACT
Ong, S.K., Culver, T.B., Lion, L.W. and Shoemaker, C.A., 1992. Effects of soil moisture and
physical-chemical properties of organic pollutants on vapor-phase transport in the vadose
zone. J. Contam. Hydrol., 11: 273-290.
Vapor-phase transport of organic pollutants is one of the important pathways in the
distribution and attenuation of volatile organic compounds in the vadose zone. In this study,
the impact of vapor-phase partitioning and of the physical-chemical properties of organic
pollutants on vapor-phase transport was assessed. An experimentally derived relationship to
predict vapor sorption for a variety of soil types under varying soil moisture conditions was
incorporated into the two-dimensional finite-element model, VOCWASTE.The revised model was
then used to simulate the transport of volatile organics. Vapor-phase partitioning in the model
accounted for vapor uptake by sorption onto moist mineral surfaces as well as sorption at the
liquid-solid interface and dissolution into soil water. Under dry conditions, vapor-phase
sorption of volatile organic pollutants was shown to have a retarding effect on transport of
organic vapors. However, for shallow, contaminated soils, volatilization was controlled by
vapor diffusion, even under dry conditions where vapor-phase sorption was high. The influence
of Henry's law constant and of the aqueous-phase (solid-liquid) partition coefficient for volatile
organic pollutants was considered in the simulations. Volatilization of organic vapors was
shown to be favored for contaminants with high Henry's law constants and low aqueous-phase
partitioning coefficients. Because of the interdependence of these two physical-chemical
properties, individual properties of the contaminant should not be considered in isolation in the
evaluation of vapor transport.
Correspondence to: L.W. Lion, School of Civil and Environmental Engineering, Cornell
University, Ithaca, NY 14853, USA.
016%7722/92/$05.00
© 1992 Elsevier Science Publishers B.V. All rights reserved.
274
S.K. O N G ET AL.
INTRODUCTION
Volatilization from the soil surface has long been recognized as one of the
important pathways for the movement and attenuation of organic pesticides
in the subsurface (e.g., Spencer et al., 1973). Recently, with the discovery of
gross contamination of groundwater by volatile organic compounds (VOC's)
such as solvents and fuels, knowledge regarding the movement of organic
vapors in the subsurface has been extended to consideration of transport and
the interphase distribution of these compounds. The importance of organic
vapor movement in the subsurface is evidenced by the successful use of
shallow soil-gas sampling techniques to estimate the extent of groundwater
pollution (Marrin and Kerfoot, 1988) and the use of venting techniques to
remediate contaminated aquifers (Baehr et al., 1989).
Use of mathematical models to predict the movement of VOC's has become
a part of the planning effort for design of new waste disposal sites and can be
a component of remedial investigations at hazardous waste sites. A list of
some currently available vapor-phase transport models along with their physical-chemical and biological features is presented in Table 1. Although many
models on vapor transport are available, information are limited on physicalTABLE
1
Physical-chemical and biological features of various vapor transport models
References
Features*
1
Mohsen et a l . ( 1 9 8 0 )
Weeks et a l . ( 1 9 8 2 )
Jury et a l . ( 1 9 8 3 )
Stephanatos ( 1 9 8 5 )
Abriola and Pinder ( 1 9 8 5 )
Pinder and Abriola ( 1 9 8 6 )
Springer ( 1 9 8 6 )
Baehr and Coracioglu ( 1 9 8 7 )
Baehr ( 1 9 8 7 )
Hutzler et a l . ( 1 9 8 9 )
Metcalfe and Farquhar ( 1 9 8 7 )
Sleep and Skyes ( 1 9 8 9 )
Shoemaker et a l . ( 1 9 9 0 )
Culver et a l . ( 1 9 9 1 )
2
3
4
2
z
x
1
x
x
1
x
2
x
x
5
6
7
8
x
x
x
9
10
11
12
13
14
x
15
x
x
x
x
x
x
x
x
x
x
x
1
x
x
x
x
x
x
x
x
×
2
x
x
x
x
x
x
x
x
x
x
x
1
x
x
x
x
x
x
x
x
x
2
x
x
×
x
x
x
x
x
x
x
x
x
x
x
×
x
x
3
x
x
1
x
x
2
x
x
x
2
x
x
2
x
x
x
x
x
x
2
x
x
x
x
x
x
x
x
x
x
x
x
×
x
×
x
* 1 = number of dimensions; 2 = multicomponent (pollutant); 3 = diffusion, vapor phase; 4 = advection,
vapor phase; 5 = diffusion, aqueous phase; 6 = advection, aqueous phase; 7 = dissolution (Henry's l a w ) ;
8 = sorption at solid-liquid interface; 9 = vapor sorption other than 7 and 8 ; 10 = non-equilibrium
12 = temporal variability in moisture contents; 13 = spatial
mass-transfer effects; 11 = b i o d e g r a d a t i o n ;
heterogeneity; 14 = immiscible phase; 15 = density-dependent gas behavior
ORGANIC POLLUTANTS AND VAPOR-PHASE TRANSPORT IN THE VADOSE ZONE
275
chemical processes affecting the movement of organic vapors, such as the rate
of vapor-phase partitioning and/or mass transfer of vapors.
Movement of organic vapors in the subsurface is influenced by the
reactions between the vapor phase and the moist, porous media. Sorption in
a three-phase (gas-liquid-solid) system is complex and must consider the
interrelated distribution of the contaminant into pore water, soil matrix and
vapor phases, as well as the possibility of contaminant accumulation at the
water-vapor interface (Valsaraj, 1988). Several researchers have measured
elevated sorption of vapors onto unsaturated soils (Chiou and Shoup, 1985;
Chiou et al., 1985; Peterson et al., 1988; Ong and Lion, 1991a, b). The sorption
of trichloroethylene (TCE) vapor onto soils for a range of moisture contents
has recently been evaluated by Peterson et al. (1988) and Ong and Lion
(1991b). These authors showed that sorption reactions (in addition to those
occurring in the aqueous phase) such as vapor sorption at the water-air
interface and direct sorption of vapors to mineral surfaces may be important
under certain moisture conditions. For soils with moisture contents less than
those equivalent to surface coverage by eight layers of water (as in semi-arid
regions or surface soils), sorption of TCE vapor can exhibit partition coefficients two to four orders of magnitude larger than those at higher moisture
contents (Ong and Lion, 1991b). Similarly, equilibrium sorption results for
other organic vapors onto a variety of minerals and soils show that vaporphase partitioning is a function of moisture content and that, in the presence
of a mixture of organic vapors, interactions resulting in enhancement or
suppression of sorption of individual vapor may occur (Lion et al., 1990; Ong
et al., 1991).
The mechanisms controlling the sorption of vapors in the unsaturated zone
are not completely understood. It has been suggested that the mineral portion
of a dry soil dominates the sorption of organic vapors (Chiou and Shoup,
1985; Chiou et al., 1985). In fact, the variation in vapor sorption coefficients
for oven-dry soils is best explained by their surface areas rather than their
organic contents (Rhue et al., 1988; Ong and Lion, 1991a). Yet, for air-dried
soils, organic matter content is the best predictor of vapor sorption strength
(Ong and Lion, 1991a).
The impact of vapor-phase sorption on organic vapor transport in the
subsurface was investigated by Shoemaker et al. (1990) with a one- and
two-dimensional analytical model. Sensitivity analysis indicated that incorporation of vapor-phase sorption can significantly reduce model predictions
of organic vapor transport relative to those predicted when only the liquidphase sorption was considered. Culver et al. (1991) extended this study by
developing a two-dimensional numerical model, VOCWASTE,and applied it to
several different organic vapor transport scenarios. For their one-dimensional
sensitivity analysis, Culver et al. (1991) assumed vapor sorption to be inversely
276
s . K ONG ET AL.
and exponentially related to the percent soil saturation. Given this general
form of the vapor sorption profile, Culver et al. (1991) performed a systematic
evaluation of the sensitivity of vapor transport predictions to changes in the
shape of the vapor sorption profile given varying moisture profiles. Their
results demonstrated that accurate knowledge of the vapor sorption response
to soil moisture is essential for accurate prediction of volatile organic
transport if the soil profile includes regions of low soil moisture.
In this study, we incorporate the equilibrium vapor sorption results of Ong
and Lion (1991b) into VOCWASTEto investigate the impact of vapor sorption
on the transport of organic vapors under variable soil moisture contents.
Using the data base developed by Ong and Lion (1991b) this study is the first
to develop an empirical relationship to predict vapor partitioning over variety
of soil types given a range of soil moisture contents. The vapor sorption
relationship used in previous applications of VOCWASTE(Culver et al., 1991)
was based on a limited data base and was not generalizable across soil types
and moisture conditions. In this paper we also evaluate the influence on vapor
migration of two commonly determined chemical properties, the Henry's law
constant and the aqueous-phase partition coefficient.
DESCRIPTION OF TRANSPORT MODEL
Movement of organic vapors in the subsurface was calculated using the
two-dimensional transport model VOCWASTE(Culver et al., 1991). This model
is a modification of a finite-element model developed by Yeh (1981, 1987). The
original contaminant transport model consisted of two parts: FEMWATERand
FEMWASTE. The FE~WA'rER program (Yeh, 1987) predicts the changes in the
potential heads and in volumetric water contents due to water fluxes in
variably saturated systems. FEMWASa'E(Yeh, 1981) describes the movement of
the contaminant in the aqueous phase for both saturated and unsaturated
zones. Aqueous-phase adsorption and biodegradation of the contaminant are
also included in the FEMWASTEmodel. Volumetric water contents and Darcy's
velocities computed by FEMWATERare used as inputs to FEMWASTE.
FEMWASTEdoes not include vapor transport and the code was extensively
modified by Culver et al. (1991) to include vapor-phase diffusion and sorption.
The modified version of FEMWASTEis called VOCWASTE. VOCWASTEdoes not
include vapor advection. The common assumption that gas-phase advection
is negligible for modeling vapor-phase transport has been questioned by
Pinder and Abriola (1986). Kell (1987) observed that advective transport may
be significant in laboratory studies. However, work by Kreamer et al. (1988)
suggested that vapor-phase advection is small and can be neglected unless air
pumping is present or unless there is a rapidly fluctuating water table. Since
277
ORGANIC POLLUTANTS AND VAPOR-PHASE TRANSPORT IN THE VADOSE ZONE
VOCWASTEassumes advection occurs only in the aqueous phase, no fundamental changes were needed in the FEMWATERcode. The details of the modifications to FEMWASTEand example simulations which employ VOCWASTEhave
been described by Culver (1989) and Culver et al. (1991). The numerical
accuracy of VOCWASTE was found to be equivalent to that of the original
FEMWASTE (Culver, 1989). A brief overview of FEMWATER and VOCWASTEis
presented here.
Water flow in a variably saturated media is described in FEMWATER by
(symbols in the equations that follow are defined in the Notation):
dO 0h
- V'[K(O)'(VH)]+q
dh ~t
(1)
Assumptions made for this equation are that the soil matrix and the carrier
fluid are incompressible.
The modified equation for contaminant movement in VOCWASTE, which
includes diffusive vapor-phase movement, is given by:
~t
Oz
"++
ax
~---~(
~CT/RL+ ODHxz ~CT/RL
)
ODHxx Ox
~z
ODHzx
~x
+ ODm~
~z
+ M
(2)
where
CT = OCL +aCG + CLKdPb+ CGKsGPb
= (total mass of contaminant in a volume of soil)
CT = RsL CsL = RL CL = R c C c = RsGCsc
RL = PbKd +O+aKH +PbKHKsG
Rc, = pbKd/KH +O/K. +a+pbKsG
RSG = PbKd/K, Ksc + O/KHKsc + a/Ksc + Pb
and
RSL = Pb + O/Kd + aKH/Kd + PbKn Ksc/Kd
VOCWASTE assumes all phase partition processes obey local equilibrium.
Most subsurface transport models have assumed that local equilibrium is
278
S.K. ONG ET AL
NOTATION
Definition of terms
Symbol
Description
Dimension
h
H=h+z
K(O)
0
q
dO/dh
pressure head
total head
hydraulic conductivity tensor
volumetric water content
source/sink
specific moisture capacity
total concentration
aqueous concentration
gaseous concentration
mass sorbed per unit mass of sorbent due to
aqueous sorption
mass sorbed per unit mass of sorbent other
than aqueous sorption and dissolution into
soil water, e.g. vapor-liquid sorption or condensation
saturated soil-water partitioning coefficient
aqueous partitioning coefficient normalized
with respect to organic carbon
weight fraction of organic carbon in soil
vapor sorption coefficient (a variable function
of the water content)
Henry's law constant
volumetric air content
soil bulk density
Darcy's velocity in the ith direction
/jth component of the hydrodynamic dispersion
tensor including aqueous diffusion
~th component of the mechanical dispersion
tensor
aqueous-phase coefficient of molecular diffusion
vapor-phase coefficient of molecular diffusion
source/sink input term
overall vapor-phase partitioning coefficient
activity coefficient
density of water
moisture content in percent
specific surface area of the solid
[L]
[L]
[L T --j]
[L3 L 3]
[L3L 3 T 1]
[L-l]
[M L 3]
[M L 3]
[M L -3]
[M M - ~]
G
cL
c,:,
Csl.
C~,:,
Kd = KoJCo~ = CsL/CI~
Koc
foc
Ks 8
(4)
The first set of simulations illustrates the impact of vapor-phase sorption
under three different climatic conditions: (1) and (2) a wet and a humid
environment where the soil is moist at about field capacity and (3) a dry
environment, typical of a drought season, where the soil close to the ground
surface is fairly dry. A rainfall rate (above evaporation) of 0.4 cm day i (4.72
in m o n t h -l ) is used for the wet condition, 0.2cm day -~ (2.36 in month -~ ) for
the humid condition and 0.02 cm day-J (0.236 in m o n t h -1 ) is used for the dry
condition. The input parameters for the soil moisture characteristic curves
were adjusted appropriately for each region. Input data for the computer
simulations, including the soil moisture characteristic curves, are summarized
in Tables 2 and 3. Physical-chemical data for various VOC's were obtained
282
SK. ONG ET AL.
TABLE 2
Soil input parameters for computer simulation
Symbol
Definition
Units
Ph
Joc
Ksxx
soil bulk density
soil organic content
xx-component of saturated
conductivity
zz-component of saturated
conductivity
xz-component of saturated
conductivity
saturated volumetric water
content
residual water content
surface area of soil
pressure head scaling factor
wet region
humid region
dry region
characteristic curve exponent
wet region
humid region
dry region
g cm 3
g g- 1
cm day ~
1.35
0.0125
100.2
cm day 1
100.2
cm day 1
0.0
cm 3cm 3
0.45
cm 3 cm 3
m 2 g-~
cm
0.00
80.0
K,~,,z
Ks.~
0~a~
Or
S.A.
ha
/~
Value
3,500
3,200
2,100
1.2
1.0
0.5
Characteristic curves:
O(h)
K(O)
=
~'0s,, - (0~t - 0 r ) ( - h / h a ) s,
if
)~0sat
otherwise
Ks[(O--Or)/(Osa
t --
h < 0.0cm
0r)l 4
from Verschueren (1983) and O R N L (1989) while vapor and aqueous
diffusion coefficients were computed using the Hirschfelder-Bird-Spotz
equation and Wilke-Chang equation, respectively (Welty et al., 1984).
The second set of computer simulations illustrates the sensitivity of
transport of VOC's in the vadose zone to two commonly determined physicalchemical properties of contaminants. The two properties considered were the
Henry's law constant and the aqueous-phase partition coefficient. Four
c o m m o n components of gasoline were modeled, i.e. benzene, toluene, oxylene and n-octane (Table 3). A study domain similar to the first simulation
was used (Fig. 2) and it was assumed that the climatic condition was humid
with a rainfall of 0.2cm day -1 .
ORGANIC
POLLUTANTS
AND VAPOR-PHASE TRANSPORT
283
IN THE VADOSE ZONE
¢N
cq
o
N
o
N
-
oo
q
~
N
o°
cS
~d
oo-
d
,~
~.
0o
~D
Z
0
0o
•
I~
f'~
~
oo
o
,~.
I=
~I~
~1~
I~
c,,;
[.-
;>
I
0
o
>,
..0
.o
.
.
.
.
-~
~
~
= o~
e~
e~
<
=
o
o
~
~
o=
~'~
~
0
~
~
~
~
~
~
~
-~-"
284
S.K.
ONGAL.
ET
Ground Level
Rainfal~l -~~ 0R~infymal/Ida
~aY
-5'
-10
-15
0.0
0.1
0.2
0.3
0.4
0.5
Moisture Content (Vol./Vol.)
Fig. 3. Volumetric moisture content profile generated by the simulation model FEMWATER for three
rainfall patterns.
RESULTS AND DISCUSSION
Effects of vapor-phasepartitioning
In order to predict vapor-phase transport and sorption, the soil moisture
content profile associated with a given continuous precipitation rate must first
be computed. The moisture profiles generated by FEMWATEgfor three different
rainfall conditions are shown in Fig. 3. Under dry climatic conditions (0.02 cm
d a y - l ) , the volumetric moisture content ranges from 0.08cm 3 cm -3 (18%
saturation) at the ground level to ,-~0.36 cm 3 cm 3 at 15 m (80% saturation).
For a soil with a specific surface area of 80 m 2 g - l, the number of layers of
water at 0.08 cm 3 cm -3 volumetric moisture content is ~ 2.6. For a rainfall of
0.2 cm d a y - l , the moisture content at the ground level is ~ 22 cm 3 cm -3 (49%
saturation or ~ 7 layers of water) and increases to ~ 4 4 c m 3 cm -3 at 15m
(98% saturation). At this rainfall rate, vapor-phase sorption (KsG) is
negligible throughout the soil profile and does not influence vapor transport.
With TCE-contaminated soil assumed to be 10 m below the surface, volatilization of TCE over a period of 8 yr was computed for two different climatic
conditions. Fig. 4 shows that the a m o u n t of TCE that is volatilized after 8 yr
under dry conditions is less than that under wet conditions. The 10m of
relatively dry soil between the source and the volatilization flux at the surface
act as a "buffer" from the volatilization loss. This result indicates that
enhanced vapor-phase partitioning in a deep, dry, soil profile can play an
important role in the retardation of organic pollutants in the subsurface.
The volatilization simulations were repeated for a 4-yr period with the
source located at 4 m below the surface (Fig. 5). Three different climatic
ORGANICPOLLUTANTSAND VAPOR-PHASETRANSPORTIN THE VADOSEZONE
~
60-
285
Source at l0 m Below Ground Level
5o-
~
40"
~
:30
"~
2o
Rainfall
0.2 cm/day
o
2
/
"
4
6
T i m e (years)
Fig. 4. V O C W A S T E computer simulations of the effects of vapor-phase partitioning on the volatilization
of TCE for two rainfall patterns. The TCE source is at a depth of 10m.
conditions were evaluated. Contrary to the results when the source is deep
(Fig. 4), volatilization under dry conditions was greater than under wet
conditions for the 4-m-deep source. Even though vapor-phase partitioning
still occurs, the dominant physical parameter controlling volatilization in the
upper soil is vapor diffusion. Vapor diffusion is much greater in a dry soil than
a moist soil where vapor-phase diffusion is restricted by moisture (Fig. 5). The
increased partitioning from vapor sorption in the case of the 4-m-deep source
is not strong enough to counterbalance the increase in vapor diffusion and the
impact of volatilization near the surface. Vapor-phase sorption was not
40
4m
Source at
30
Below Ground Level
Rainfall
0.02 cm/dayj
~"
J
Rainfall
20
n~lHday
1
2
3
4
T i m e (years)
Fig. 5. V O C W A S T E computer simulations of the effects of vapor-phase partitioning on the volatilization
of TCE for three rainfall patterns. The TCE source is at a depth of 4 m,
286
SK. ONG ET AL.
important for the moisture profiles corresponding to rainfalls of 0.2 and
0.4 cm day- ~.
Cutver et al. (1991) performed simulations using VOCWASTE under a
different set of conditions, i.e. different moisture profiles, vapor-phase
sorption capacities and a contaminant source placed 21.5m below ground
level. These simulations also showed that vapor-phase sorption and soil
moisture contents may significantly retard the transport of VOC's.
Effects of physical-chemical properties of pollutants on volatilization
A m o n g the physical-chemical properties of pollutants that influence their
loss by volatilization are the vapor diffusion coefficient, Henry's constant (K~)
and aqueous-phase partition coefficient (Ka). Even though the diffusion of
VOC's in the vapor phase is about four orders of magnitude larger than that
in the aqueous phase, the diffusion coefficients for most volatile organic
compounds are of similar magnitude (Table 3). Therefore, differences in the
vapor transport between organic vapors based solely on differences in their
diffusion coefficients may be negligible.
To illustrate the effects of Henry's law constant, two organic pollutants
having similar Kocvalues but differing KH-values were selected: TCE (Koc =
61.1, KH = 0.397) and benzene (Koc = 65, KH = 0.222). For the following
simulations, the contaminated soil was placed at 4 m below ground level and
the rainfall was 0.2 cm day -~ . At these moisture levels, vapor-phase sorption
was negligible and was not a factor in the simulations. As expected, the
organic pollutant with the larger KH-value, i.e. favoring distribution of the
pollutant in the vapor phase, had greater loss by volatilization from the soil.
This result is shown in Fig. 6.
Three organic compounds with similar Henry's law constants but different
Koc-values were used to illustrate the effect'of aqueous-phase partitioning on
volatilization. Aqueous-phase sorption reduces the mass fraction of
pollutants in the aqueous phase and subsequently in the vapor phase, which
in turn affects volatilization. The three compounds used in this simulation
were: benzene (Koc = 65, KH = 0.222), toluene (Koc = 259, K H = 0.270) and
o-xylene (Koc -- 690, K H = 0.20). Results for volatilization of all three
compounds are shown in Fig. 7. As anticipated, o-xylene, which has the
largest Koc-value, was found to volatilize the least in comparison with the
other two compounds. This effect is similar to the chromatographic effect
commonly observed for the transport of organic pollutants under saturated
conditions.
In contrast, simulation runs with n-octane, which has large KH- and Kocvalues (121.1 and 73,000 respectively), predicted less volatilization than oxylene (Koc = 690, K H = 0.20). The reason for the low amount of vapor loss
ORGANIC POLLUTANTS AND VAPOR-PHASE TRANSPORT IN THE VADOSE ZONE
Rainfall
= 0.2 c m / d a y
TCE (K H = 0.397,
°.
287
~
J
/
Benzene (KIt = 0"222
Koc = 65.0)
2
3
Time (years)
Fig. 6. V O C W A S T E computer simulations of the effect of Henry's constant on volatilization of two VOC's.
The contaminant source is at a 4-m depth and the rainfall rate is 0.2cm day ~.
40
30 -
°.
Benzene (Koc=65,
KH = 0"~...,..,ak
~
20 •
J
Toluene (Koc = 259,
i
I
2
3
4
5
6
7
Time (years)
Fig. 7. V O C W A S T E computer simulations of the effects of Koc on the volatilization of three VOC's. The
contaminant source is at a 4-m depth and the rainfall rate is 0.2 cm day ~.
was that most of n-octane ( > 99.9%) was calculated to be sorbed from the
aqueous phase, in spite of the large Henry's constant which would favor loss
of this pollutant by volatilization. These simulations indicate the interdependent effects of the physical-chemical characteristics of pollutants on their
transport in the vadose zone.
CONCLUSIONS
Vapor-phase transport of organic pollutants is one of the important
288
sx. ONe ETAL.
pathways in the distribution and attenuation of volatile organic compounds
in the vadose zone. Vapor-phase sorption accounts for vapor uptake
processes other than sorption at the liquid-solid interface and dissolution into
soil water. Therefore, vapor-phase sorption would include direct sorption of
vapors to moist mineral surfaces and sorption at the water-air interface. In
this study, a VOC transport model was adapted to incorporate an empirical
relationship between the vapor sorption coefficient and the soil moisture
content based on the average surface coverage of the sorbent by water.
Computer simulations performed in this study show that, under dry
conditions, vapor-phase sorption of organic pollutants may significantly
retard the transport of volatile pollutants. As a result of sorptive vapor
uptake, dry soils may act as a reservoir of VOC's. However, for contaminated
soils that are close to ground level, the volatilization of organic vapors is
controlled by vapor diffusion, even in dry soils where vapor-phase sorption is
expected to be high.
The transport of vapors was found to be greatly affected by the aqueous
partition coefficient (Kd) and liquid-vapor partitioning (Henry's law
constant). Volatilization is favorable for contaminants with high Henry's law
constants and low aqueous partition coefficients. Because the physical-chemical properties of a contaminant act in concert to influence vapor behavior,
individual properties should not be considered in isolation in the evaluation
of vapor transport.
ACKNOWLEDGEMENTS
This research was supported, in part, by the U.S. Air Force Engineering
and Services Center under Contract No. F08635-85-C-0030.
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