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.

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