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Transport of Petroleum Hydrocarbon Vapor Components in
the Subsurface; a Laboratory Soil Column Study
Elsy A. Escobar1, Paul Dahlen
1 and Paul C. Johnson
1
1
Ira A. Fulton School of Engineering, Arizona State University, P.O. Box 9309, Tempe, AZ
85287
ABSTRACT
The diffusive transport of volatile organic compounds (VOCs) in the subsurface at petroleum
spill sites can significantly affect vapor migration from sources to buildings and ground surface.
Thus, knowledge of compound-specific vapor transport and bio-attenuation is of great interests
to those who must identify risks and make corrective action decisions for petroleum spill sites. In
this work, the vapor transport of individual compounds in complex petroleum vapor mixtures is
being studied for idealized lithologies in 2-m (6-ft) tall laboratory soil columns. Six columns,
representing different geological settings were prepared using 40-60 mesh sand (medium
grained) and 16-minus mesh crushed granite (fine-grained). The contaminant vapor source is
composed by twelve petroleum hydrocarbons that typify weathered gasoline. The liquid
hydrocarbon mixture is placed in a chamber at the bottom of each column, and the vapors are
allowed to diffuse upward through the soil to a chamber at the top of the columns, which is
swept with humidified gas. The contaminant source vapor concentration is maintained constant
throughout the experimental period by periodic replacement of the liquid. The experiment is
conducted for two cases: i) anaerobic conditions, in which the sweep gas at the top of the column
is nitrogen and no biodegradation is expected to occur; and ii) aerobic conditions; in which, air is
the sweep gas; this phase will performed in the near future. Soil diffusion coefficients, and
oxygen, carbon dioxide and hydrocarbon vapor concentration of each chemical are monitored
over time in the column and the effluent sweep gas. The data allow determination of compound-
specific flux and times for steady profiles to be achieved. The anaerobic phase of the experiment
is ongoing. Results show that vapor transport is highly influenced by the chemical and physical
properties of the chemicals, soil moisture, soil effective diffusion coefficients and geological
settings.
INTRODUCTION
Any sites with groundwater or soil contamination pose a potential risk of vapor migration to the
surface (indoor or outdoor)1.At indoor locations, vapor intrusion can lead to risks such as
immediate flammability when concentration levels are high or health risks through long term
inhalation of low concentrations2. Understanding of the petroleum hydrocarbon vapor behavior
in the vadoze zone can help to identify and/or predict risks of vapor intrusion into buildings in
proximity to a spill site.
Hydrocarbon vapor transport to the surface depends on different factors including physical and
chemical properties of the contaminant; oxygen concentrations, soil physical properties such as
temperature, moisture content, porosity and gas permeability; and soil stratigraphy3. The latter is
one of the main reasons of spatial gas transport variability on a site and differences between one
site and another. As well known, the vadoze zone has regions with different soil characteristics
due to different type of soil or sections of soil with higher moisture content. As a consequence,
this affects the gas transport behavior for a number of reason including lower diffusion
coefficients 1,2,4
which decreases the mass flux of the contaminant to the surface and, during
transient conditions, increases the time to reach quasi-steady state.
There are few studies of hydrocarbon vapor transport for individual components of gasoline5,6,7,8
.
These studies focus on the effect of soil gas humidity in the vapor transport5 or the transport and
biodegradation extent of specific hydrocarbon vapor component in experimental soil columns
containing homogeneous media6,7,8
. Since soil conditions and lithology vary from site to site,
difficulties arise when trying to determine the state in a specific spill site. Thus, this study
includes the observation of gas transport behavior in different lithological settings and how the
lithology affects the gas flux and the time to reach quasi-steady state conditions, which can be of
use when assessing the extent of a recent contaminated site.
As stated by DeVaull et al.5, experiments are more accurate under actual field conditions;
however, their implementation is complex and expensive; therefore, it is proposed that the vapor
transient conditions and the time to reach near-steady state for a set of ideal lithological layouts
be studied in laboratory scale soil columns. This allows simultaneous experimental study of
idealized scenarios representing the range of conditions encountered at field sites. In this work,
there is particular interest in studying the diffusive vapor migration of a mix of twelve petroleum
hydrocarbon compounds that typify the vapor concentration of weathered gasoline; their vertical
vapor profiles and vapor fluxes, and how the later are affected by lithology, soil parameters and
source vapor concentration and composition under anaerobic conditions.
EXPERIMENTAL METHODS
The experiment consists in packing six soil columns each one containing different lithological
layouts in order to observe the gas transport behavior. Figure 1 shows the stratigraphic layout of
each column. Two different types of soils were utilized to create these settings: 20-40 mesh sand
(medium grained soil) and 16-minus mesh crushed granite (fine grained soil). Prior to packing,
the soils were moisturized until their water content was 2.5% v/v for the medium-grained soil
and 11% v/v for the fine-grained soil. The moisture contents were chosen based on previous tests
that demonstrated that water was not to redistribute in the soil once the columns were packed.
Once the soils had the desired moisture content, their diffusion coefficients were measured using
the Johnson et al. protocol9 using helium as tracer; results showed that the helium diffusion
coefficient of the sand was 1.2 cm2/s and the crushed granite 0.4 cm
2/s. The marked difference
between the diffusion coefficients of the sand and crushed granite helped to evaluate changes in
the flux and concentration profiles of gas transport in the geological layered setups.
COLUMN F:
Homogeneous
crushed granite
COLUMN B COLUMN CCOLUMN A:
Homogeneous
20-40 mesh Sand
Sa
nd
3 ft
Cru
sh
ed
Gra
nite
3 ft
Cru
sh
ed
Gra
nite
3 ft
Sa
nd
3 f
t
COLUMN D
Sa
nd
2 ft
Cru
sh
ed
Gra
nite
2 ft
Sa
nd
2 f
t
Cru
sh
ed
Gra
nite
2 ft
Sa
nd
2 ft
Cru
sh
ed
Gra
nite
2 ft
COLUMN E
Figure 1. Soil columns stratigraphic layout
The six soil columns are constructed of a stainless steel pipe of 6 feet length x 4 in. diameter.
They are sealed on both ends with aluminum covers having a square base that is 6 in. wide, and
that are sealed to the column using four 5-in compression bolts with a rubber seal in between the
cover and the ring to avoid vapor leaks. A coarse stainless steel support screen with a fine
stainless steel mesh screen sits within the base of the aluminum covers providing for a cavity
between the soil and the bottom or the top of the covers. Along the length of the column, there
are 17 stainless steel needle sampling ports (Pipetting needles blunt end standard hub 0.16”x4”,
Popper) coupled with three-way nylon Luer-type plastic valves (Kentos, Glass Company,
Vineland, New Jersey). Two sampling ports are also installed at the top and bottom caps to
monitor outlet and source concentrations. The needles are placed every 4 inches along the
column. Pressure and temperature are monitored through pressure transducers (Omega) placed
3.5 in. from the top and bottom and thermocouples installed at 3.5 in from the top, 3 ft and 3.5 in
form the bottom inside the soil respectively. A humidified sweep gas is passed through the top
cap of the column (Nitrogen) at a flow of 13 mls/min with the objective of maintaining the soil
moisture content constant throughout the experimental period. The gas flows from a gas cylinder
to a PVC column filled with water (bubbler) where it is humidified to a water content of 97 –
100%. Humidity sensors HM1500LF (Measurement Specialties Inc.) are placed in the gas outlet
pipe to monitor the humidity content of the sweep gas and ensure that is constant at all times.
Figure 2 shows a schematic of the basic apparatus.
Figure 2. Experimental apparatus
A liquid mix of twelve hydrocarbons in mineral oil was utilized as petroleum hydrocarbon vapor
source. The hydrocarbon chemicals and their concentration were chosen so that the vapor
concentration typifies weathered gasoline. Table 2 shows a comparison of the composition of
weathered gasoline and the vapor source solution. The solution is prepared in a 125 ml container
with a septum cap by adding pure liquid hydrocarbon chemicals into the mineral oil in which the
hydrocarbons do not dissolve or form liquid layers so they can volatilize easily. The chemicals
and the volumes utilized are shown in Table 1.Once the solution is made, the pressure built
inside the bottle is released so it equals atmospheric pressure and the solution is mixed with a stir
bar on a stir plate for 10 minutes before a quality check analysis is performed in a gas
chromatograph (SRI 8610C, SRI instruments) equipped with a flame ionization detector (GC-
FID) with a 60 m RTX-1 stainless steel column (Alltech Associates, Inc., Deerfield, IL, USA.
Concentrations of each of the chemicals are maintained constant during the experimental period.
The hydrocarbon vapor source mixture is replaced every 14 days so the vapor source is constant
during the entire experimental period. It was proven by previous experiments that the vapor
source is reduced by 13% for the most volatile compounds (i.e. n-pentane, 2-methyl-2-butene,
MTBE) and 6% for the heavy compounds (i.e. Toluene, n-octane, p-xylene) during this time
period. These percentages fall within the tolerance range for the experiment. Also, gas samples
from the vapor source (bottom cap of the columns) were taken daily to ensure that the
hydrocarbon concentrations were constant or within the tolerance range of the experiment (20%).
Table 1. Contaminant source mix composition
Chemical Formula Densityg/
ml
Molecular
Weight,
g/mol
Henry’s Law
Constant
L-H2O/
L-vapor
Experimental
Mass
Fraction
Mass in
vapor
source, g
Volume in
vapor source
mix, ml
n-Pentane C5H12 0.626 72.2 42.05 0.018 0.90 1.46
2-methyl-2-butene C5H10 0.662 70.1 9.13 0.011 0.55 0.86
MTBE C5H12O 0.742 88.2 0.02 0.002 0.10 0.11
n-Hexane C6H14 0.659 86.2 43.36 0.051 2.55 3.86
Benzene C6H6 0.879 78.1 0.18 0.005 0.25 0.29
Cyclohexane C6H12 0.779 84.2 7.82 0.059 2.95 3.79
n-Heptane C7H16 0.683 100.2 62.80 0.067 3.35 4.88
Toluene C7H8 0.865 92.1 0.21 0.038 1.90 2.17
p-xylene C8H10 0.87 106.2 0.19 0.043 2.15 2.49
Iso-Octane C8H18 0.688 114.2 0.100 5.00 7.27
n-Octane C8H18 0.703 114.2 93.35 0.056 2.80 3.97
1,3,5-
Trimethylbenzene C9H12 0.864 120.2 11.79 0.133 6.65 7.72
Mineral Oil - 0.84 - 0.500 25.00 24.81
TOTAL
100.0
(AVG)
1.000 50 g 63.69
Table 2. Comparison of experimental source to weathered gasoline vapor composition
Compound Alkanes Cycloalkanes Alkenes Aromatics Ether Total
Percentage in
weathered gasoline10 49.21 6.35 17.46 26.98 0.00 100.00
Percentage in
experimental source 41.67 8.33 8.33 33.33 8.33 100.00
Vapor Mass fraction in
weathered gasoline10 63.87 14.60 16.57 4.96 0.00 100.00
Vapor Mass fraction in
experiment 63.40 14.33 16.35 4.91 1.01 100.00
At the beginning of the experiment, the transient condition of each column was monitored by
taking samples from the top port of the column every two to four hours until the breakthrough
was observed and the near-steady state was achieved. Once the concentrations were stable, the
columns were sample once a day. Also, concentration profiles of each column are performed
approximately every two weeks. Hydrocarbon, methane (CH4), oxygen (O2) and carbon dioxide
(CO2) concentrations were determined by injecting the samples into a gas chromatograph (SRI
8610C, SRI instruments) equipped with a flame ionization detector (FID) with a 60 m RTX-1
stainless steel column (Alltech Associates, Inc., Deerfield, IL, USA); and, a thermal conductivity
detector (TCD) with a CTR I stainless steel column 6’x1/4”x120” (Alltech Associates, Inc.,
Deerfield, IL, USA) (GC-TCD-FID) to determine the oxygen, carbon dioxide and nitrogen
concentrations in each sample.
Table 3 shows the variables taken into account for this experiment.
Table 3. Experimental variables
Controlled Variables Measured Variables
- Vapor source concentration
(Typical of weathered gasoline)
- Pressure differential
- Soil temperature
- Soil stratigraphy - Sweep gas humidity
- Sweep gas oxygen concentration - Effective diffusion coefficient*
- Soil moisture content - Oxygen concentration profiles
- Components concentration profiles
- Component-specific flux profiles
- Component-specific bio-attenuation rates
*Performed by following the Johnson et al. (1998) protocol9 using helium as tracer gas.
RESULTS AND DISCUSSION
During the columns’ start-up, the transient state was monitored by taking gas samples from the
columns gas effluent (top cap) every two to four hours so the time at which the individual
hydrocarbon vapor components reach near-steady state could be determined. A hydrocarbon
vapor component was considered to be in near-steady state once its concentration at the top of
the column was not increasing over time. Results are presented in Table 4.
Table 4. Times at which each hydrocarbon component reached near-steady state
Column
Component
Time, days
A: Sand
B: Sand –
Crushed
Granite -
Sand
C: Sand -
Crushed
Granite
D: Crushed
Granite -
Sand
E: Crushed
Granite –Sand
-Crushed
Granite
F: Crushed
Granite
n-Pentane 7 7 17 11 10 27
2-Methyl-2-Butyl 7 7 17 13 11 26
MTBE 21
Transient
state > 83d
Transient
state > 83d
Transient
state > 78 d 39 46
n-Hexane 6 7 17 17 15 25
Benzene 7
Transient
state > 83d 24 30 22 56
Cyclohexane 6 7 13 10 15 22
Iso-Octane 6 12 19 13 20 21
n-Heptane 6 12 18 18 17 20
Toluene 7
Transient
state > 83 d 34 46 24
Transient
state > 70 d
n-Octane 7
Transient
state > 83 d 25 25 24 55
P-Xylene 18
Transient
state > 83 d 50 45 29
Transient
state > 70 d
1,3,5-
Trimethylbenzene 35
Transient
state > 83 d 60 62 40
Transient
state > 70 d
Note: The columns are currently being monitored. The compounds with higher Henry’s law
constant have not reached near-steady state conditions
The time at which the hydrocarbon vapors reached near-steady state depends in great extent on
the effective diffusion coefficient of the soils. Thus, since the crushed granite has the lowest
effective diffusion coefficient and accounting for the fact that the soil moisture content of each
type of soil is similar from column to column and no biodegradation is taking place, it was
expected that the time to near-steady state for a given hydrocarbon component increased with the
length of the crushed granite layer in the column. As can be seen in Table 4, most of the
columns, with the exception of B and E, behaved this way. Column B’s less volatile hydrocarbon
components are taking longer periods of time than expected since, after 83 days of running this
column, they are still in their transient state. Hydrocarbon components in Column E reached
near-steady state faster than expected (see Table 4). These inconsistencies might be due to
differences in the effective diffusion coefficients of the soil layers in these columns.
To ensure that no aerobic or anaerobic biodegradation is taking place during the experiment O2,
CO2, and CH4 were measured over time in the TCD-FID-GC. Results in all the columns showed
that the O2 concentrations are in the range of 0.2 to 0.8 % V/V; 0.1 to 0.5 % V/V for CO2 and
less than 5 ppm of CH4 in all the columns. These concentrations were constant during this
experimental period; therefore, no degradation (aerobic or anaerobic) is taking place in the
columns.
Figures 3a to 3f, show the mass flux behavior at the gas effluent of the columns (top cap) of n-
pentane, 2-methyl-2-butene, benzene and p-xylene over time by plotting normalized flux vs. time
graphs. The normalized flux was calculated as follows
(Eq. 1)
Where,
Qcolumn = Sweep gas flow at the top of the column [cm3/min]
Ci = Vapor concentration of the vapor hydrocarbon component i at the top cap of the column
[mg/cm3]
A = Transversal area [cm2]
Diair
= Diffusion coefficient of the hydrocarbon component i in air [cm2/s]
Ci,o = Vapor concentration of the vapor hydrocarbon component i in the vapor source [mg/cm3]
L = Column length [cm]
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0 20 40 60 80 100 120No
rmal
ize
d F
lux,
(Q
.Ci/
A)/
(Diai
r .C
i,o/L
)
Time, days
Figure 3a. Normalized Flux vs. TimeColumn A: Sand
Water accumulation at the bottom of the column
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0 10 20 30 40 50
No
rmal
ize
d F
lux,
(Q
.Ci/
A)/
(Diai
r .C
i,o/L
)
Time, days
Figure 3b.Normalized Flux vs. TimeColumn B: Sand-Crushed Granite-Sand
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0 20 40 60 80 100No
rmal
ize
d F
lux,
(Q
.Ci/
A)/
(Diai
r .C
i,o/L
)
Time, days
Figure 3c. Normalized Flux vs. TimeColumn C: Sand-Crushed Granite
Water accumulation at the bottom of the column
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0 20 40 60 80 100
No
rmal
ize
d F
lux,
(Q
.Ci/
A)/
(Diai
r .C
i,o/L
)
Time, days
Figure 3d. Normalized Flux vs. TimeColumn D: Crushed Granite-Sand
Water accumulation at the bottom of the column
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0 10 20 30 40 50 60 70 80No
rmal
ize
d F
lux,
(Q
.Ci/
A)/
(Diai
r .C
i,o/L
)
Time, days
Figure 3e. Normalized Flux vs. TimeColumn E: Crushed Granite-Sand-Crushed Granite
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0 20 40 60 80 100
No
rmal
ize
d F
lux,
(Q
.Ci/
A)/
(Diai
r .C
i,o/L
)
Time, days
Figure 3f.Normalized Flux vs. Time Column F: Crushed Granite
As can be observed in Figures 3a to 3f, after a period of time that near-steady state was achieved,
the more volatile compounds showed a decrease in their mass flux at the top of the columns,
especially in Figures 3a, 3c and 3d. Since no increase of CO2 or CH4 concentrations was
observed, the decrease cannot be attributed to biodegradation. However, water was collected at
the bottom cap sampling port of column A after 40 days of starting the column and water drops
were observed at inlet and outlet pipes of the humidified sweep gas. Hence, it was concluded that
the decrease in the flows is due to water accumulation at the soil base of the columns (between
the bottom cap port and first port in the soil). This is a consequence of water being condensed
from the sweep gas when it reaches the aluminum cap of the column due to changes in room
temperature. The condensed water dripped down into the soil causing a change in the soil
moisture content which at the same time produced a redistribution of the water in the column and
its accumulation at the soil base. A water mass balance in the sweep gas of column A determined
that approximately 2 mls of water per week were being condensed.
In order to study the effects of the soil lithology on the individual hydrocarbon vapor transport, a
comparison of current soil hydrocarbon vapor compounds mass emissions was performed and it
is presented in Table 5. The mass emissions were calculated as follows:
(Eq. 2)
Where,
qi = mass emission of hydrocarbon vapor component i [mg/s]
Table 5. Mass emission of each of the components in the column
Column
Components
Mass Emissions, mg/cm2-s
A: Sand B: Sand-Crushed
Granite-Sand
C: Sand-Crushed Granite
D: Crushed Granite-
Sand
E: Crushed Granite-Sand-
Crushed Granite
F: Crushed Granite
n-Pentane 3.78E-06 7.85E-07 1.93E-06 4.26E-06 1.23E-05 1.72E-06
2-Methyl-2-Butyl 7.16E-07 1.29E-07 4.74E-07 6.79E-07 2.68E-06 2.80E-07
MTBE 1.82E-07 3.24E-11 9.72E-09 8.27E-09 9.87E-08 2.73E-09
n-Hexane 8.52E-07 9.02E-08 3.27E-07 5.51E-07 2.21E-06 1.90E-07
Benzene 1.79E-07 4.18E-09 3.37E-08 5.19E-08 1.95E-07 1.46E-08
Cyclohexane 6.93E-07 7.03E-08 2.46E-07 4.05E-07 1.78E-06 1.52E-07
Iso-Octane 6.15E-07 5.50E-08 1.83E-07 3.77E-07 1.53E-06 1.11E-07
n-Heptane 4.06E-07 3.08E-08 1.25E-07 2.14E-07 1.03E-06 5.77E-08
Toluene 3.40E-07 6.42E-09 5.42E-08 9.50E-08 4.47E-07 1.77E-08
Octane 1.01E-07 4.05E-09 2.10E-08 3.28E-08 1.90E-07 5.23E-09
P-Xylene 9.68E-08 0.00E+00 1.17E-08 1.44E-08 1.20E-07 2.30E-09 1,3,5-Trimethylbenzene
8.27E-08 0.00E+00 5.98E-09 9.52E-09 8.63E-08 0.00E+00
As can be observed in Table 5, Column E is the column with the highest hydrocarbon mass
emissions followed in decreasing order by A, D, C, F and B. It was expected the emissions to
decrease from column A to F (the more layers of crushed granite in the soil the lower de
emission). Thus, taking into account that all the columns have the same vapor source
concentrations and it is kept constant over time, the inconsistency of the results can be attributed
to differences in the soil diffusion coefficients in the lithological layers from one column to the
other. To confirm this, the diffusion coefficients for the different soil layers in each column were
calculated with Eq. 3. Calculation results are presented in Table 6
(Eq. 3)
Where,
= Effective diffusion coefficient of vapor hydrocarbon component i in a type of soil (sand
or crushed granite [cm2/s]
= Length of soil layer [cm]
= Concentration gradient in the soil layer [mg/cm3]
Table 6. Diffusion coefficients in the columns layers
Column Effective Diffusion Coefficient, cm2/s
Sand Crushed Granite
A: Sand 0.0141 NA
B: Sand- Crushed Granite-
Sand
0.0155 (Bottom layer) 0.0002
(middle layer) 0.0120 (Top layer)
C: Sand – Crushed Granite
0.0307 0.0012
D: Crushed Granite-Sand
0.0225 0.0016
E: Crushed Granite -Sand-
Crushed Granite
0.0192 (middle layer)
0.0098 (Bottom layer)
0.0060 (Top layer)
F: Crushed Granite NA 0.0010
Table 6 confirms that the inconsistency in the mass emissions of columns B and E (see Table 5)
is due to differences in the effective diffusion coefficients in the soil layers from column to
column. As can be observed, the crush granite layers of column E have higher diffusion
coefficients than the rest of the columns which explain why it has the highest vapor hydrocarbon
mass emissions. Column B has a very low crushed granite effective diffusion coefficient; which
explain the low vapor mass emissions at the top of the column. The difference in the diffusion
coefficients are most likely due to differences in soil packing and/or differences in soil moisture
content between columns as a result of the condensation problem mentioned above.
Figures 4a to 4f show the concentration vertical profiles of n-pentane and benzene in each
column. This figure illustrates the influence of the lithology settings in the concentration profiles
of the hydrocarbon vapors along the length of the columns.
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
0.00 0.50 1.00 1.50
Vo
lum
n le
ngt
h, f
t
C/Co
Figure 4a. Concentration ProfileColumn A: Sand
n-Pentane
Benzene
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
0.00 0.20 0.40 0.60 0.80 1.00 1.20
Co
lum
n le
ngt
h, f
t
C/Co
Figure 4b. Concentration ProfileColumn B: Sand-Crushed Granite-Sand
n-Pentane
Benzene
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
0.00 0.20 0.40 0.60 0.80 1.00 1.20
Co
lum
n le
ngt
h, f
t
C/Co
Figure 4c. Concentration ProfileColumn C: Sand-Crushed Granite
n-pentane
Benzene
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
0.00 0.20 0.40 0.60 0.80 1.00 1.20
Co
lum
n le
ngt
h, f
t
C/Co
Figure 4d. Concentration ProfileColumn D: Crushed Granite-Sand
n-Pentane
Benzene
Columns A and F show the profiles for the homogeneous cases for sand and crushed granite
respectively. As expected, the profile of Column A is a straight line, trend broken by the point at
the bottom of the column (vapor source) due to water accumulation at the base of the soil caused
by the condensation at the top of the column; the water creates a diffusion resistance causing the
high gradient reflected in the plot. Column B plot is a line with a higher slope than the one for
column A. By the shape of the plot, it is evident that the hydrocarbon vapors are accumulating at
the bottom of the column which can be cause by regions with more compacted granite. Plots for
columns C and D are the two layer cases; consistently with the diffusion coefficients of the sand
and decomposed granite, the profiles show a small slope in the sand and a higher slope in the
crushed granite layers. Plots of columns B and E show the three layer cases and the results are
also consistent with the diffusion coefficient of the soil settings in the columns. As can be
observed in the plots the slope differences between the two types of soils in column E is less
marked than in the rest of the columns. This is due to lower differences in the soil diffusion
coefficients between sand and crushed granite than in the rest of the columns as observed in
Table 6.
SUMMARY
Soil column experiments were performed in order to study the petroleum hydrocarbon vapor
transport through soils and the effects different lithological layouts on vertical vapor profiles and
vapor diffusive flux. Six soil columns representing different stratigraphic settings were prepared
using 40-60 mesh sand (medium grained) and 16-minus mesh crushed granite (fine grained). The
vapor source is composed by twelve petroleum hydrocarbons in concentrations that typify
weathered gasoline and it is maintained constant throughout the experimental period. The
experiment is performed anaerobically. A humidified sweep gas is passed through the top cap of
the column to maintain the moisture content of soil constant. The transient stage of the columns
was closely monitored in order to determine the times the most volatile vapor hydrocarbon
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
0.00 0.20 0.40 0.60 0.80 1.00 1.20
Co
lum
n le
ngt
h, f
t
C/Co
Figure 4e. Concentration ProfileColumn E: Crushed Granite-Sand-Crushed Granite
n-Pentane
Benzene
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
0.00 0.20 0.40 0.60 0.80 1.00 1.20
Co
lum
n L
engt
h, f
t
C/Co
Figure 4f. Concentration ProfileColumn F: Crushed Granite
n-Pentane
Benzene
compounds reached near-steady state. Results showed that columns containing higher quantities
of crushed granite take longer to reach near-steady state. Time inconsistencies were found for the
case of column E, its hydrocarbon vapor compounds reached near-steady state faster than
expected due to differences in the soil effective diffusion coefficient from column to column as a
consequence of differences in soil packing and/or soil moisture content. Also, as expected, near-
steady state was reached faster by the more volatile hydrocarbon components such as n-pentane
and 2-methyl-2-butene than the heavier compounds; this demonstrates the effects of the physical-
chemical properties of the contaminants on gas transport through the unsaturated zone.
Carbon dioxide, oxygen and methane concentrations in the columns were measure to determine
if degradation inside of the columns was taking place. Results show that the concentrations of
these compounds do not change over time; therefore, it was concluded that the hydrocarbon were
not being degraded in the column and were not affecting the results.
Calculations of mass emissions at the top of the columns confirmed the effects of the physical-
chemical properties of the contaminants; the more volatile the compound the higher the
emission. Comparison of mass emissions showed that column E (composed of two layers of
crushed granite and a layer of sand in the middle) had the highest emissions and column B
(composed of two layers of sand and a layer of crushed granite) the lowest. This is inconsistent
with what it was expected; since the more crushed granite in the column, the lower the soil
diffusion coefficients and therefore, the lower the mass emissions. Since it was demonstrated that
not biodegradation was taking place, diffusion coefficients of each soil layer in the columns were
calculated in order to compare them; results of this calculation confirmed that column E had a
very low crushed granite diffusion coefficient which decreases the mass emissions and column B
has higher crushed granite diffusion than the rest of the columns, increasing the mass emission.
Differences in the effective diffusion coefficients of the soil layers from column to column can
be explained by differences in packing and/or changes in the moisture content in the columns as
a consequence of the condensation of the water contained in the sweep gas at the top cap of the
columns caused by room temperature changes. Changes in soil moisture content are more
evident in the plots of normalized flux vs. time (Columns A, C and D) where a decrease in the
vapor flux with time can be observed 5 to 30 days (depending on the columns) after near-steady
state had been reached.
Vertical snapshots of the concentration profiles were performed in each column in order to
illustrate the influence of the lithology settings in the concentration profiles of the hydrocarbon
vapors along the length of the columns. Plots of columns A, C and D (Figure 4) show a high
concentration gradient between the bottom concentration and the first sampling port in the soil of
the columns. This is caused by water accumulation at the base of the soil which is due to sweep
gas water condensing at the top cap of the soil column and dripping down into the soil, changing
in this way, the moisture content and water distribution along the column.
As mention above, this is the first phase of a project which objective is to gain knowledge of
compound-specific vapor transport and bio-attenuation in the vadoze zone. Future experiments
involve the study of the effects of natural attenuation on vapor migration; which is performed by
replacing the nitrogen sweep gas with breathing air. Experiments will focus on observing
changes in the vertical vapor concentration profiles and vapor fluxes in comparison to the
anaerobic phase (this experiment). Also, oxygen transport into the soil, preferential degradation
of vapor hydrocarbon compounds and biodegradation rates will be studied.
REFERENCES
1. Johnson, P. C.; Ettinger, R. A. Environ. Sci. Technol. 1991, 25(8), 1445-1452
2. American Petroleum Institute (API). Assessing the Significance of Subsurface Contaminant
Vapor Migration to Enclosed Spaces. Site-Specific Alternatives to Generic Estimates. Health
and Environmental Science Department. Publication No. 4674. 1998
3. Batterman, S.; Kulshrestha, A.; Cheng, H. Environ. Sci. Technol. 2005, 29, 171-18
4. Hong, L.; Johnson, P.C.; Peargin, T.; Creamer, T. Ground Water Monit Rem. 2009, 29(1),
81-91
5. Baehr, A.J.; Baker, R.J. Water Resour. Res.I1995, 31, 2877-2882
6. Davis, G. B., Rayner, J. L.; Trefry, M. G., Fisher, S. J.; Patterson, B. M. Vadose Zone J.
2005, 4, 225-239
7. Devaull, G.E.; Dortch, I.J.; Salanitro, J.P.; Ettinger, R. A.; Gustafson, J.B. Transport and
Aerobic Degradation of Gasoline Vapor Constituents in a Diffusive Soil Column – Theory
and Experiments. Technical Progress Report for the Petroleum Environmental Research
Forum (PERF) 1995-2004.
8. Jin, Y., Streck, T.; Jury, W. A. J. Contam. Hydrol.1994, 17, 111-127
9. Johnson, P. C.; Bruce, C., Johnson, R. L.; Kemblowski, M. W. Environ. Sci. Technol. 1998,
32(21), 3405-3409.
10. Johnson, P. C.; Kemblowski, M. W.; Colthart, J. D. Ground Water. 1990, 28(3), 413-429
11. Baehr, A.J.; Baker, R.J. Water Resour. Res.I1995, 31, 2877-2882.
12. McCarthy, K.A.; Johnson, R.L. Water Resour. Res. 1993, 29(6), 1675-1683