1

EPA/540/S-92/004

April 1992

United States

Environmental Protection

Agency

Office of Solid Waste

and Emergency

Response

Office of

Research and

Development

Ground Water Issue

Robert S. Kerr Environmental

Research Laboratory

Ada, Oklahoma

Superfund Technology Support Center for

Ground Water

Evaluation of Soil Venting Application

Dominic C. DiGiulio*

Technology Innovation Office

Office of Solid Waste and Emergency

Reponse, U.S. EPA, Washington, DC

Walter W. Kovalick, Jr., Ph.D.,

Director

Introduction

The Regional Superfund Ground-Water Forum is a group of

scientists, representing EPA's Regional Superfund Offices,

organized to exchange up-to-date information related to

ground-water remediation at Superfund sites. One of the

major issues of concern to the Forum is the transport and fate

of contaminants is soil and ground water as related to

subsurface remediation.

The ability of soil venting to inexpensively remove large

amounts of volatile organic compounds (VOCs) from

contaminated soils is well established. However, the time

required using venting to remediate soils to low contaminant

levels often required by state and federal regulators has not

been adequately investigated. Most field studies verify the

ability of a venting system to circulate air in the subsurface

and remove, at least initially, a large mass of VOCs. They do

not generally provide insight into mass transport limitations

which eventually limit performance, nor do field studies

generally evaluate methods such as enhanced biodegradation

which may optimize overall contaminant removal. Discussion

is presented to aid in evaluating the feasibility of venting

application. Methods to optimize venting application are also

discussed.

For further information contact Dominic DiGiulio (405)332-

8800 or FTS 700-743-2271 at RSKERL-Ada.

Determining Contaminant Volatility

The first step in evaluating the feasibility of venting application

at a hazardous waste site is to assess contaminant volatility. If

concentrations of VOCs in soil are relatively low and the

magnitude of liquid hydrocarbons present in the soil is

negligible, VOCs can be assumed to exist in a three-phase

system (i.e., air, water, and soil), as illustrated in Figure 1. If

soils are sufficiently moist, relative volatility in a three-phase

system can be estimated using equation (1) which

incorporates the effects of air-water partitioning (Henry’s

constant) and sorption (soil-water partition coefficient).

 

C

C Kf h

g

t g oc oc

= +

1

( /) r f (1)

where:

Cg = Vapor concentration of VOCs in gas phase(mg/

cm3

 air)

Ct = Total volatile organic concentration (mg/cm3 soil)

ρg = Bulk density (g/cm3

)

Koc = Organic carbon-water partition coefficient

(cm3

/g)

f

oc = Fraction of organic carbon content (g/g)

Kh = Henry’s Constant (mg/cm3

air/mg/cm3water )

θ = Volumetric moisture content (cm3

/cm3)

φ = Volumetric air content (cm3

/cm3)

Caution must be exercised when using this approach since

this relationship is based on the assumption that solid phase

sorption is dominated by natural organic carbon content. This

assumption is frequently invalid in soils below the root zone

where soil organic carbon is less than 0.1%.

* Environmental Engineer, Robert S. Kerr Environmental

Research Laboratory
2

Air Water Soil

K = Soil-water partition coefficient 

K = Henry's Constant p

H

KH Kp

NAPL Water Soil

Air

Figure 1. Three phase system. Figure 2. Four phase system.

Equation (1) can be used to evaluate individual VOC

contaminant reduction trends and attainment of soil-based

remediation standards. Vapors should be collected from

dedicated vapor probes under static (venting system not

operating) conditions. This estimate is valid only for soils in the

immediate vicinity of the probe intake. This approach

minimizes sample dilution and collection of vapor samples

under nonequilibrium conditions. It, however, necessitates

periodic cessation of venting. When the vapor concentration

for a VOC approaches a corresponding total soil

concentration, actual soil samples can be collected to confirm

remediation. This approach has several benefits over

conventional soil samples collection and analysis. At lower

VOC concentration levels, collection of static vapor samples is

likely more sensitive than soil collection and analysis due to

VOC loss in the latter procedure. Siegrist and Jenssen (1990)

demonstrated substantial VOC loss during normal soil sample

collection, storage, and analysis. Also, comparing contaminant

reduction trends strictly with soil samples is difficult due to

spatial variability in soils. No two soil samples can be collected

at the exact same location. In addition, soil gas analyses can

be accomplished more quickly and inexpensively than soil

sample collection, thus enabling more frequent evaluation of

trends. A potential disadvantage of using this approach is

inability to distinguish VOC vapors emanating from soils as

opposed to ground water. Hypothetically, soils could be

remediated to desired levels with probes still indicating

contamination above remediation standards. This concern

could be alleviated to some degree by determining the

presence of a diffusion vapor gradient from the water table

using vertically placed vapor probes.

If soils are visibly contaminated or the presence of

nonaqueous phase liquids (NAPLs) is suspected in soils

based on high contaminant, total organic carbon, or total

petroleum hydrocarbon analysis, contaminants are likely

present in a four phase system as illustrated in Figure 2.

Under these circumstances, most of the VOC mass will be

associated with the immiscible fluid and assuming that the

fluid acts as an ideal solution, volatilization will be governed by

Raoult’s Law.

Pa

 = Xa Po

a (2)

where:

Pa = vapor pressure of component over

solution (mm Hg)

Xa = mole fraction of component in

solution

Po

a = saturated vapor pressure of pure

component (mm Hg)

In a four-phase system, contaminant volatility will be governed

by the VOC’s vapor pressure and mole fraction within the

immiscible fluid. The vapor pressure of all compounds

increases substantially with an increase in temperature while

solubility in a solvent phase is much less affected by

temperature. This suggests that soil temperature should be

taken into account when evaluating VOC recovery for

contaminants located near the soil surface (seasonal

variations in soil temperature quickly dampen with depth). For

instance, if conducting a field test to evaluate potential

remediation of shallow soil contamination in the winter, one

should realize that VOC recovery could be substantially

higher during summer months, and low recovery should not

necessarily be viewed as venting system failure.

As venting proceeds, lower molecular weight organic

compounds will preferentially volatilize and degrade. This

process is commonly described as weathering and has been

examined by Johnson (1989) in laboratory experiments.

Samples of gasoline were sparged with air and the

concentration and composition of vapors were monitored.

The efficiency of vapor extraction decreased to less than 1%

of its initial value even though approximately 40% of the

gasoline remained. Theoretical and experimental work on

product weathering indicate the need to monitor temporal

variation in specific VOCs of concern in extraction and

observation wells.

Evaluating Air Flow

Air permeability (ka) in soil is a function of a soil’s intrinsic

permeability (ki

) and liquid content. At hazardous waste sites,

liquid present in soil pores is often a combination of soil water

and immiscible fluids. Air permeability (ka

) can be estimated

by multiplying a soil's intrinsic permeability (ki) by the relative

permeability (kr

).

ka

 = ki kr (3)
3

The dimensionless ratio kr varies from one to zero and

describes the variation in air permeability as a function of air

saturation. Equations developed by Brooks and Corey (1964)

and Van Genuchten (1980) are useful in estimating air

permeability as a function of air saturation or liquid content.

The Brooks-Corey equation to estimate relative permeability of

a non-wetting fluid (i.e. air) is given by:

kr

 = (1 - Se)

2 (1 - Se(2+λ)/λ

) (4)

where:

Se = effective saturation

λ = a pore distribution parameter

The effective saturation is given by:

Se

θ 

ε

r θ 

ε

+

= r θ 

ε 1 - (5)

Where:

θ = volumetric moisture content

ε = total porosity

θr = residual saturation

The pore size distribution parameter and residual water content can be estimated using soil-water characteristic curves

which relate matric potential to volumetric water content.

When initially developing an estimate of relative permeability

for a given soil texture and liquid content, values for ε, θr

, Se,

and λ can be obtained from the literature. Rawls et al.

(1982) summarized geometric and arithmetic means for

Brook-Corey parameters for various USDA soil textural

classes. Figure 3 illustrates relative permeability as a function

of volumetric moisture content for clayey soils assuming ε =

0.475, θr

 = 0.090, and λ = 0.131.

The most effective method of measuring air permeability is by

conducting a field pneumatic pump test. Using permeameters

or other laboratory measurements provide information on a

0 0.1 0.2 0.3 0.4 0.5

0

0.2

0.4

0.6

0.8

Moisture ( θ)

Time

Re-Start Yield Spike

concentrations as illustrated in Figure 4 or by an asymptotic

increase in total mass removal with operation time. Typically,

when venting is terminated, an increase in soil gas

concentration is observed over time. Slow mass transfer with

respect to advective air flow is most likely caused by diffusive

release from porous aggregate structures or lenses of lesser

permeability as illustrated in Figure 5. The time required for

the remediation of heterogeneous and fractured soils depends

on the proportion of contaminated material exposed to direct

bulk airflow. It would be expected that long-term performance

of venting will be limited to a large degree by gaseous and

liquid diffusion from soil regions not exposed to direct airflow.

Regardless of possible causes, the significance of mass

transport limitations should be evaluated during venting field

tests. This can be achieved by pneumatically isolating a small

area of a site and aggressively applying vacuum extraction

until mass transport limitations are realized. Isolation can be

achieved by surrounding extraction wells with passive inlet or

air injection wells as shown in Figure 6. Quantifying the

effects of mass transport limitations on remediation time might

then be attempted by utilizing models incorporating mass

transfer rate coefficients.

The discrepancy frequently observed between mass removal

predicted from equilibrium conditions using Henry’s Law

constants and that observed from laboratory column and field

studies is sometimes reconciled by the use of “effective or

lumped” soil-air partition coefficients. These parameters are

Figure 4. Concentration vs. time.

Figure 3. Relative permeability vs moisture content of clay.

relatively small scale. Information gained from pneumatic

pump tests is vital in determining site-specific design

considerations (e.g., spacing of extraction wells). Selecting the

placement and screened intervals of extraction and

observation wells and applied vacuum rates during a pump

test is often based on preliminary mathematical modeling.

Evaluating Mass Transfer Limitations and

Remediation Time

The effects of mass transport limitations are usually

manifested by a substantial drop in soil vapor contaminant
4

Molecular Diffusion

Molecular Diffusion

Low Permeability Strata

Air Advection

Air Advection

Palmer et at., 1988

semivolatile organic compounds which cannot be removed by

physical stripping alone.

Venting circulates air in soils at depths much greater than are

possible by tilling, and oxygen transport via the gas phase is

much more effective than injecting or flooding soils with

oxygen saturated liquid solutions.

Hinchee (1989) described the use of soil vacuum extraction at

Hill AFB, Utah for oxygenation of the subsurface and the

enhancement of biodegradation of petroleum hydrocarbons in

soils contaminated with JP-4 jet fuel. Figures 7 and 8 illustrate

subsurface oxygen profiles at the Hill site prior to and during

venting. It is evident that soil oxygen levels dramatically

increased following one week of venting. Soil vapor samples

collected from observation wells during periodic vent system

shutdown revealed rapid decreases in oxygen concentration

and corresponding CO2 production suggesting that aerobic

biodegradation was occurring at the site. Laboratory

treatability studies using soils from the site demonstrated

increased carbon-dioxide evolution with increasing moisture

content when enriched with nutrients. It is worthwhile to note

that soils at Hill AFB were relatively dry at commencement of

field vacuum extraction indicating, that the addition of moisture

could perhaps stimulate aerobic biodegradation even further

under field operating conditions.

When conducting site characterization and field studies, it is

recommended that CO2

 and O2 levels be monitored in soil

vapor probes and extraction well offgas to allow the

assessment of basal soil respiration and the effects of site

management on subsurface biological activity. These

measurements are simple and inexpensive to conduct and

can yield a wealth of information regarding:

1. The mass of VOCs and semivolatiles which have

undergone biodegradation versus volatilization. This

information is crucial if subsurface conditions (e.g.,

moisture content) are to be manipulated to enhance

biodegradation to reduce VOC offgas treatment costs

and maximize semivolatile removal.

2. Factors limiting biodegradation. If O2

 and CO2 monitoring

Figure 6. Proposed Pilot Test Design. Figure 7. Oxygen concentration in vadose zone before venting.

M

Vent 

Well #7

X U S

Z W

0

10

20

30

40

50

60

70

0 10 20 30

Distance (feet)

40 50 60 70 80 90

K R Q

T

Y

5%

10%

1%

determined from laboratory column tests and are then used for

model input to determine required remediation times. While

this method does indirectly account for mass transport

limitations, problems may arise when one attempts to

quantitatively describe several processes with lumped

parameters. The primary concern is whether the lumped

parameter is suitable for use only under the laboratory

conditions from which it was determined, or whether it can be

transferred for modeling use in the field. Perhaps the most

direct method of accounting for mass transport limitations

would be to incorporate diffusive transfer directly into

convective-dispersive vapor transport models.

Enhanced Aerobic Biodegradation

With the exception of a few field research projects, soil

vacuum extraction has been applied primarily for removal of

volatile organic compounds from the vadose zone. However,

circulation of air in soils can be expected to enhance the

aerobic biodegradation of both volatile and semivolatile

organic compounds. One of the most promising uses of this

technology is in manipulating subsurface oxygen levels to

maximize in-situ biodegradation. Bioventing can reduce vapor

treatment costs and can result in the remediation of

Figure 5. Effect of Low Permeability Lenses.

Venting Probe Cluster

Passive Inlet Well

Borehole Sampling Locatio

Vent Well

0 24ft.

7.32 meters

AAAA

AAAA

AAAA

AAAA AAAA

AAAA

AAA AAA

AAA

AAA

DiGiulio, 198

n
5

15%

10%

5%

1%

M

Vent 

Well #7

X U S

Z W

0

10

20

30

40

50

60

70

0 10 20 30

Oxygen Concentration in Vadose Zone After Venting

Distance (feet)

40 50 60 70 80 90

K R Q

T

Y

15%

10%

5% 1% 1% 5%

10%

10%

Figure 8. Oxygen concentration in vadose zone after venting.

extraction well. Thus, only a limited volume of soil near an

extraction well will be effectively ventilated regardless of the

ROI. Johnson (J.J., 1988) describes how the addition of 13

extraction wells within the ROI of other extraction wells

increased blower VOC concentration by 4000 ppmv and mass

removal by 40 kg/day. They concluded that the radius of

influence was not an effective parameter for locating

extraction wells and that operation costs could be reduced by

increasing the number of extraction wells as opposed to

pumping at higher rates with fewer wells.

Determining the propagation of induced vacuum requires

conducting pneumatic pump tests in which variation in static

vacuum is measured in vapor observation wells at depth and

distance from extraction wells. Locating extraction and

observation wells along transects as illustrated in Figure 4

minimizes the number of observation wells necessary to

evaluate vacuum propagation at linear distances from

extraction wells. Pressure differential can be observed at

greater distances than would otherwise be possible in other

configurations.

Propagation of vacuum in soils as a function of applied

vacuum can be determined by conducting pneumatic pump

tests with incrementally increasing flow or applied vacuum.

Vacuum is increased after steady state conditions (relatively

constant static vacuum measurements in observation wells)

exist in soils from the previously applied vacuum. A step pump

test will indicate a significant increase in static vacuum or air

velocity with increasing applied vacuum near an extraction

well. However, at distance from an extraction well, a

significant increase in static vacuum will not be observed with

an increase in applied vacuum. Pneumatic pump tests allow

determination of radial distances from extraction wells in which

air velocity is sufficient to ensure remediation.

After the initial placement of extraction wells has been

established based on the physics of air flow, an initial applied

vacuum must be selected to ensure optimal VOC removal. In

regard to mass transfer considerations, the vent rate should

be increased if a significant corresponding mass flux is

observed. Even though an increased venting rate may not

substantially increase the propagation of vacuum with

distance, air velocity will increase near the extraction well. If

most contaminants are in more permeable deposits, an

increase in applied vacuum will increase mass removal

eventually to a point of diminishing returns or until the system

is limited by diffusion. Note that this strategy is for

optimization of volatilization not biodegradation. Optimizing insitu biodegradation often necessitates reducing air velocity in

soil. As a result, vapor treatment costs are minimized but

overall mass flux decreases. Thus, in-situ biodegradation of

VOCs minimizes overall costs but may extend venting

operation time.

During a field test, it is desirable to operate until mass

transport limitations are realized in order to evaluate the long

term performance of the technology. This can be achieved by

isolating small selected areas of a site by the use of passive

air inlet wells. When attempting to evaluate diffusion limited

mass removal in isolated areas, applied vacuum should

remain high and the distance between passive inlet and

extraction wells should be minimized. Too often, venting field

tests are conducted for relatively short periods of time (e.g.,

reveals low O2

 consumption and CO2 generation while

readily biodegradable compounds persist in soils, further

characterization studies could be conducted to determine

if biodegradation is being limited by insufficient moisture

content, toxicity (e.g. metals), or nutrients.

3. Subsurface air flow characteristics. Observation wells

which indicate persistent, low O2

 levels may indicate an

insufficient supply of oxygenated air at that location

suggesting the need for air injection, higher extraction

well vacuum, additional extraction wells, or additional

soils characterization which may indicate high moisture

content or the presence of immiscible fluids impeding the

flow of air.

Location and Number of Vapor Extraction Wells

One of the primary objectives in conducting a venting field

test is to evaluate the initial placement of extraction wells to

optimize VOC removal from soil. Placement of extraction

wells and selected applied vacuum is largely an iterative

process requiring continual re-evaluation as additional data

are collected during remediation. Vacuum extraction wells

produce complex three-dimensional reduced pressure zones

in affected soils. The size and configuration of this affected

volume depends on the applied vacuum, venting geometry

(e.g., depth to water table), soil heterogeneity, and intrinsic

(e.g., permeability) and dynamic (e.g., moisture content)

properties of the soil. The lateral extent of this reduced

pressure zone (beyond which static vacuum is no longer

detected) is often termed the radius or zone of influence

(ROI). Highly permeable sandy soils typically exhibit large

zones of influence and high air flow rates whereas less

permeable soils, such as silts and clays, exhibit smaller zones

of influence and low air flows.

Measured or anticipated radii of influence are often used to

space extraction wells. For instance, if a ROI is measured at

10 feet, extraction wells are placed 20 feet apart. However,

this strategy is questionable since vacuum propagation and

air velocity decrease substantially with distance from an
6

When constructing observation wells it is desirable to minimize

vapor storage volume in the screened interval and sample

transfer line. This will minimize purging volumes and ensure a

representative vapor sample in the vicinity of each observation

well. Analysis of soil gas in an on-site field laboratory is

preferred to provide real time data for implementation of

engineering controls and process modifications. It is

recommended that steel canisters, sorbent tubes, or direct GC

injection be used in lieu of Tedlar bags when possible

because of potential VOC loss through bag leakage or

diffusion within the teflon material itself. This problem may

lead to erroneous analytical results and the potential of a false

negative indication of soil remediation at low soil gas

concentrations.

Summary/Conclusions

While the application of soil vacuum extraction is conceptually

simple, its success depends on understanding complex

subsurface physical, chemical, and biological processes

which provide insight into factors limiting venting performance.

Optimizing venting performance is critical when attempting to

meet stipulated soil-based clean-up levels required by

regulators. The first step in evaluating a venting application is

to assess contaminant volatility. Volatility is a function of a

contaminant’s soil-water partition coefficient and Henry’s

constant if present in a three-phase system, and a

contaminant’s vapor pressure and mole fraction in an

immiscible fluid, if present in a four phase system. Volatility is

greatly decreased when soils are extremely dry. As vacuum

extraction proceeds, lower molecular weight organic

compounds preferentially volatilize and biodegrade.

Decreasing mole fractions of lighter compounds and

increasing mole fractions of heavier compounds affect

observed offgas concentrations. Understanding contaminant

volatility is necessary when attempting to utilize offgas vapor

concentrations as an indication of venting progress.

The significance of mass transport limitations should be

evaluated during venting field tests. Long term performance of

venting will most likely be limited by diffusion from soil regions

of lesser permeability which are not exposed to direct airflow.

Mass transport limitations can be assessed by isolating a

small area of a site and aggressively applying vacuum

extraction. Simplistic methods to evaluate remediation time

should be avoided. One of the most promising uses of vacuum

extraction is in manipulating subsurface oxygen levels to

enhance biodegradation. When conducting field studies, it is

recommended that CO2 and O2 levels be monitored in vapor

probes to evaluate the feasibility of VOC and semivolatile

contaminant biodegradation.

Air permeability in soil is a function of a soil’s intrinsic

permeability and liquid content. Relative permeability of air

can be estimated using relationships developed by Brooks

and Corey (1964) and Van Genuchten (1980). The most

effective method of measuring air permeability is by

conducting pneumatic pump tests. Information gained from

pneumatic pump tests can be used to determine site-specific

design considerations such as the spacing of extraction wells.

Measured or anticipated zones of influence are not particularly

useful in spacing extraction wells. Extraction wells should be

located to maximize air velocity in contaminated soils.

2 - 21 days) which only results in assessment of air

permeability and initial mass removal. Longer field studies

(e.g., 6 months - 12 months) enable better insight into mass

transfer limitations which eventually govern venting

effectiveness.

Screened Interval

The screened interval of extraction wells will play a significant

role in directing air flow through contaminated soils. Minimum

depths are recommended by some practitioners for venting

operation to avoid short-circuiting of air flow. However, the

application of venting need not be limited by depth to water

table since horizontal vents can be used in lieu of vertically

screened extraction wells to remediate soils with shallow

contamination. Often, it is desirable to dewater contaminated

shallow aquifer sediments for venting application. For

remediation of more permeable soils with deep contamination,

an extraction well should be screened at the maximum depth

of contamination or to the seasonal low water table,

whichever is shallowest, to direct air flow and reduce shortcircuiting. For less permeable soils, or for more continuous

vertical contamination, a higher and longer screened interval

may be useful. In stratified systems, such as in the presence

of clay layers between more permeable deposits, more than

one well will be required, each venting a distinct strata.

Screening an extraction well over two strata of significantly

different permeability will result in most air flow being directed

only in the strata of greater permeability. It is important to

screen extraction wells over the interval of highest soil

contamination to avoid extracting higher volumes of air at

lower vapor concentration.

During venting, the reduced pressure in the soil will cause an

upwelling of the water table. The change in water table

elevation can be determined from the predicted radial

pressure distribution. Johnson et al. (1988) indicated that

upwelling can be significant under typical venting conditions.

Water table rise will cause contaminated soil lying above the

water table to become saturated, resulting in decreased mass

removal rates. Ground water upwelling due to venting system

operation can be minimized with concurrent water table

dewatering.

Placement of Observation Wells

Observation wells are essential in determining whether

contaminated soils are being effectively ventilated and in the

evaluation of interactions among extraction wells. The more

homogeneous and isotropic the unsaturated medium, the

fewer the number of vapor monitoring probes required. To

adequately describe vacuum propagation during a field test,

usually at least three observation well clusters are needed

within the ROI of an extraction well. At least one of these

clusters should be placed near an extraction well because of

the logarithmic decrease in vacuum with distance. The depth

and number of vapor probes within a cluster depends on the

screened intervals of extraction wells and soil stratigraphy.

However, vertical placement of vapor probes might logically

be near the soil-water table interface, soil horizon interfaces,

and near the soil surface. As previous mentioned, the use of

air flow modeling can assist in optimizing the depth and

placement of vapor observation wells and in the interpretation

of data collected from these monitoring points.
7

Pneumatic pump tests with increasing applied vacuum may be

useful in determining radial distances from extraction wells in

which air velocity is sufficient to ensure remediation.

Screened intervals should be located at or below the depth of

contamination. In stratified soils, more than one well is

necessary to ventilate each strata. At least three observation

well clusters are usually necessary to observe vacuum

propagation within the radius of influence of an extraction well.

Logical vertical placement of vapor probes might be near the

soil-water table interface, soil horizon interfaces, and near the

soil surface.

References

(1) Brooks, R.H., and Corey, A.T., 1964. Hydraulic

Properties of Porous Media, Colorado State University,

Fort Collins, CO., Hydrol. Pap. No. 3, 27 pp.

(2) Hinchee, R.E., 1989. Enhanced Biodegradation through

Soil Venting, Proceedings of the Workshop on Soil

Vacuum Extraction, Robert S. Kerr Environmental

Research Laboratory, Ada, Oklahoma, April 27-28, 1989.

(3) Johnson, J.J., 1988. In Situ Air Stripping: Analysis of

Data from a Project Near Benson, Arizona, Master of

Science Thesis, Colorado School of Mines, Colorado.

(3) Johnson, P.C., Kemblowski, M.W., and Colthart, J.D.,

1988. Practical Screening Models for Soil Venting

Applications, NWWA/API Conference on Petroleum

Hydrocarbons and Organic Chemicals in Groundwater,

Houston, TX, 1988.

(4) Johnson, R.L., 1989. Soil Vacuum Extraction: Laboratory

and Physical Model Studies, Proceedings of the

Workshop on Soil Vacuum Extraction, Robert S. Kerr

Environmental Research Laboratory, Ada, Oklahoma,

April 27-28, 1989.

(5) Rawls, W.J., Brakensiek, D.L., and Saxton, K.E., 1982.

Estimation of Soil Water Properties, Transactions of the

ASAE, 1982, pp. 1316-1328.

(6) Siegrist, R. L., and Jenssen, P. C., 1990. Evaluation of

Sampling Method Effects on Volatile Organic Compound

Measurements in Contaminated Soils, Environ. Sci.

Technol., Vol. 24, No. 9, p. 1387-1392.

(7) Van Genuchten, M.T., 1980. A Closed-Form Equation for

Predicting the Hydraulic Conductivity of Unsaturated

Soils, Soil Sci. Soc. Am. J., 44:982-898.