Method and apparatus for determining spatial distribution of fluids migrating through porous media under vacuum-induced pressure differential

A method and apparatus are provided which determine the spatial distribution of fluids in a porous media contained in a three-dimensional cell as such fluids migrate through porous media in response to a vacuum-induced pressure differential. Specifically, the method comprises: (a) providing a three-dimensional cell including a front side and a back side, each of which are at least semi-transparent; (b) installing at least two slotted casing wells at opposing ends of the cell, at least one first slotted casing well capable of being de-pressurized and at least one second slotted casing well capable of allowing air entry into the cell; (c) filling the cell with a porous media; (d) introducing at least one fluid into the filled cell and allowing the fluid to infiltrate the porous media; (e) sealing the infiltrated, filled cell; (f) de-pressurizing the first slotted casing, thereby creating a negative pressure gradient across the infiltrated, filled cell; and (g) measuring the saturation of the fluid in the porous media at each location of interest across the plane of the front side of the three-dimensional cell. A method of use is also provided for employing the present apparatus in a pilot-scale experiment for the feasibility of recovering non-aqueous phase liquids (NAPLs) by a vacuum-based technique, such as soil vapor extraction (SVE) and vacuum-enhanced recovery (VER), depending upon the volatility of NAPL of interest.

TECHNICAL FIELD 
The present invention relates generally to the measurement of fluid 
saturations in porous media, and more particularly, to the determination 
of fluid saturations of fluids migrating through porous media in a 
laboratory cell in response to a vacuum-induced pressure differential as 
well as the use of such determination to assess the feasibility of 
vacuum-induced recovery of undesirable fluids. 
BACKGROUND ART 
Non-aqueous phase liquids (NAPLs) such as hydrocarbon fuels, organic 
solvents, and other immiscible organic liquids are widely employed by 
industry. Through spills, leakage from underground storage tanks, and 
improper disposal practices, NAPLs may enter the vadose zone and possibly 
ultimately contaminate groundwater resources. In the event of a confirmed 
release of NAPLs, Underground Storage Tank (UST) regulations mandate 
investigation of the release, including an initial site characterization 
and development of a corrective action plan for free-product removal of 
NAPLs (e.g., diesel oil floating in wells or over the water table) before 
soil and groundwater remediation for the clean up of contaminants in the 
saturated or vadose one. Typically, free product is initially recovered by 
skimming or bailing of product or by pumping. Thereafter, residual NAPLs 
may be removed by technologies such as soil vapor extraction (SVE) and 
vacuum enhanced recovery (VER). The selection of the appropriate 
remediation technology for a given situation is governed by the type of 
NAPLs to be recovered and cost constraints, among other factors. 
SVE has generally been the remedial option used for the recovery of highly 
volatile hydrocarbon species from spills of non aqueous phase liquids, 
such as gasoline. SVE involves the recovery of volatile phase organic 
hydrocarbons vaporized by vacuum induced through slotted casing wells 
installed in the contaminated zone. Typically, a basic SVE system pairs 
vapor extraction recovery wells with vacuum pumps or blowers to remove 
vapors, while more complex recovery systems further include trenches, air 
injection wells, and passive wells. SVE is most effective for the 
remediation of hydrocarbons of high volatility, where air is induced at a 
low flow rate for the evaporation of volatile organic compounds (VOCs), 
and also to provide oxygen for in situ biodegradation. SVE technologies 
are not readily applicable remediation options for non-volatile 
hydrocarbons such as diesel oil, which has little if any volatile organic 
species, depending on the age of the spill. 
For the remediation of non-volatile NAPLs not amenable to removal by SVE, 
one may employ vacuum enhanced recovery (VER). VER is an in-situ 
remediation process that induces flow and recovery of NAPLs by vacuum 
extraction through slotted casing recovery wells. However, depending on 
the cost and feasibility of VER, remediation engineers and consultants may 
nonetheless be forced to opt for the excavation and offsite disposal of 
contaminated soils or leave the contaminants in place with no remedial 
action. 
Numerous numerical codes have been developed to evaluate and design various 
remediation strategies for the removal or extraction of NAPLs from 
contaminated sites with heterogeneous soil and complex hydrogeology. 
Three-phase models have been found to be very useful to estimate and 
predict NAPL flow and design remediation plans and assess the associated 
costs and risks involved in the process. A few of these three-phase models 
applicable for air-water-NAPL flow in three dimensions claim to be useful 
in the design of SVE and VER remediation systems. However, very few of 
these models are tested against experimental three-phase flow data and it 
is believed that none of these models are tested specifically against 
experimental three-phase vacuum-induced flow data. 
It is noted that although it would be preferred to test models against data 
from NAPL-contaminated field sites rather than against laboratory results, 
controlled field experiments for NAPL remediation would be very expensive 
and time-consuming. Using data from existing NAPL-contaminated sites would 
also be difficult due to the inability to accurately establish initial and 
boundary conditions such as duration, quantity, rate and time of NAPL leak 
or spill. 
There are known experimental techniques available for studying multiphase 
flow in 2-D cells used in qualitative visual or photo imaging techniques 
to track the movement of red-dyed NAPLs. For example, Ho et al conducted 
such an experiment for volatile liquid hydrocarbon mixtures. C. K. Ho et 
al, "An Experimental Investigation of Air Venting of Volatile Liquid 
Hydrocarbon Mixtures from Homogeneous and Heterogeneous Porous Media", 
Journal of Contaminant Hydrology, 11, 1992, pp. 291-316. The experiment 
was designed to investigate the vapor phase of volatile organics in soil 
using visual observations of the red dyed plume movement through 
heterogeneous media. Experimental results delineated the vaporization 
modes of single and binary hydrocarbon liquids held in homogeneous and 
heterogeneous porous media during air venting. A vacuum pump was used to 
induce airflow through the sand pack, and a flowmeter at the entrance was 
used to measure the flow rate. Gas samples were taken from a sampling bulb 
at the exit of the apparatus. 
However, none of the qualitative visual or photo imaging techniques 
directly measure the fluid saturations of NAPL and water in porous media. 
Therefore, these experimental techniques cannot be accurately used to 
predict the feasibility of VER or SVE in a particular application nor to 
validate mathematical models of VER or SVE. 
In contrast, there are experimental techniques that directly measure fluid 
saturations. An example of such experimental flow data used to validate a 
mathematical model is found in Parker et al, "Modeling Multiphase Organic 
Chemical Transport In Soils And Ground Water", U.S. EPA Report No. 
EPA/600/2-91/042, Ada, Okla., August 1991. Parker et al. conducted 2-D 
multiphase laboratory experiments using a dual-energy gamma attenuation 
system and hydrophobic and hydrophilic tensiometers for measuring 
two-phase pressures and saturations to develop multiphase flow and 
transport models. Specifically, Parker et al conducted laboratory studies 
in a 1.times.1.5.times.0.85 meter sand tank to validate the infiltration 
and redistribution of both light and dense NAPLs in a single coarse soil 
texture. Essentially, Parker et al performed the five stages of (1) water 
drainage; (2) NAPL infiltration; (3) NAPL redistribution; (4) water 
flushing; and (5) NAPL entrapment. 
The use of a dual-energy gamma attenuation system by Parker et al 
accurately assessed fluid saturations of NAPLs and water in porous 
media--this technology has been long established and is based upon the 
theory of Beer's law. Specifically, the attenuation of gamma radiation (I) 
through a soil with dry bulk density (.rho.) and volumetric water content 
(.theta..sub.w) can be written using the subscripts s, w and g to denote 
soil, water and gas phase, respectively: 
EQU I=I.sub.0 exp(-.mu..sub.s .rho..sub.s x-.mu..sub.w .rho..sub.w 
.theta..sub.w x-.mu..sub.g .rho..sub.g .theta..sub.g x) (1) 
where I.sub.0 is the empty cell count rate in counts per second (cps) 
I is the emergent count rate (cps) 
.mu..sub.i is the mass attenuation coefficient of phase i 
x is the path length of the gamma beam through i 
.rho..sub.i is the density of phase i 
.theta..sub.g is the volumetric gas phase content 
.theta..sub.w is the volumetric water content. 
When simulating a vacuum-enhanced recovery process, one substitutes U.sub.w 
for (.mu..sub.w .rho..sub.w) and ignores the contribution of the gas phase 
to the attenuation of the photons, such that equation (1) becomes 
EQU I=I.sub.0 exp(-.mu..sub.s .rho..sub.s x-U.sub.w .theta..sub.w x) (2) 
It is noted that the contribution of the gas phase would not be ignored in 
a simulation of SVE. 
Substituting subscripts (c) and (a) for the .sup.137 Cs and .sup.241 Am 
gamma radiation to form two equations and then solving these equations 
yields the bulk density (.rho..sub.s): 
##EQU1## 
While Parker et al employed .sup.137 Cs and .sup.241 Am in their 
dual-energy gamma attenuation system, any two gamma sources with gamma 
energies that are different such that their mass attenuation coefficients 
are sufficiently different may theoretically be employed. Specifically, 
.sup.137 Cs emits gamma radiation at about 662 KeV after .beta. decay, 
while .sup.241 Am gamma rays have an energy of about 60 KeV after .alpha. 
decay. Other applications have employed .sup.137 Cs and .sup.170 Tm for 
dual-energy gamma sources instead of .sup.241 Am and .sup.137 Cs for 
measuring fluid saturations. 
However, none of the experimental determinations of fluid saturations in 
porous media using dual-energy gamma attenuation systems, including Parker 
et al, conduct experimental flow data in which the infiltrated NAPLs and 
water are subject to a vacuum-induced pressure differential. It follows 
that there is presently no experimental method in use to accurately 
evaluate the effectiveness of such vacuum-based remediation techniques 
such as SVE and VER. 
A need remains for a method and apparatus to model the three-phase flow of 
NAPL, water, and air through porous media under the influence of a 
vacuum-induced pressure gradient, including the ability to directly 
measure the fluid saturations of the phases. Additionally, a need remains 
for a method for assessing the feasibility of VER and SVE remediation 
techniques as well as for calibrating mathematical models describing such 
techniques. 
DISCLOSURE OF THE INVENTION 
In accordance with the present invention, a method and apparatus are 
provided which determine the spatial distribution of fluids in a porous 
media contained in a three-dimensional cell as such fluids migrate through 
porous media in response to a vacuum-induced pressure differential. 
Specifically, the method comprises the steps of employing the 
below-described three-dimensional cell: 
(a) providing a three-dimensional cell including a front side and a back 
side, each of which are at least semi-transparent; 
(b) installing at least two slotted casing wells at opposing ends of the 
cell, at least one first slotted casing well capable of being 
de-pressurized and at least one second slotted casing well capable of 
allowing air entry into the cell; 
(c) filling the cell with a porous media; 
(d) introducing at least one fluid into the filled cell and allowing the 
fluid to infiltrate the porous media; 
(e) sealing the infiltrated, filled cell; 
(f) de-pressurizing the first slotted casing, thereby creating a negative 
pressure gradient across the infiltrated, filled cell; 
(g) collimating a beam of gamma or X-ray of radiation on the front side of 
the infiltrated, filled cell at each location of interest across the plane 
of the front side of said three-dimensional cell; and 
(h) measuring the amount of radiation exiting the back side of the cell at 
each location of interest and therefrom determining the saturation of the 
at least one fluid in the porous media at each location of interest across 
the plane of the front side of the three-dimensional cell. 
A method of use is also provided for employing the present apparatus in a 
pilot-scale experiment for the feasibility of recovering NAPL by a 
vacuum-based technique, namely SVE and VER, depending upon the volatility 
of the NAPL of interest. 
Therefore, the present method and apparatus provide an accurate means to 
determine in pilot-scale the three-phase saturations of an oil spill as it 
redistributes from the surface through the heterogeneous soils in the 
vadose zone to the water table, followed by vacuum enhanced recovery or 
soil vapor extraction of free product or volatile phase in a slotted 
casing extraction well and recovery system. Accordingly, remediation 
consultants and regulators are afforded the capability to assess and test 
vacuum enhanced recovery system designs and validate models under 
controlled laboratory simulations that represents site conditions before 
they are installed at soil and ground water remediation sites at extensive 
cost. The two step procedure also enables controlled laboratory scale 
verification and testing of three-phase models or codes by simulating 
sequential experimental stages for the vacuum-based recovery of NAPL and 
volatile hydrocarbons. 
Three-phase modeling codes that have been successfully tested with 
experimental data allow investigators to have confidence that the code 
will predict the field behavior reasonably well, provided that the site is 
accurately characterized and the model parameters are adequately 
calibrated to the site.

BEST MODES FOR CARRYING OUT THE INVENTION 
A method and apparatus are provided which determine the spatial 
distribution of fluids in a porous media contained in a three-dimensional 
cell as such fluids migrate through porous media in response to a 
vacuum-induced pressure differential. Specifically, the method comprises 
the following steps, each of which will be discussed in greater detail 
below: 
(a) providing a three-dimensional cell including a front side and a back 
side, each of which are at least semi-transparent; 
(b) installing at least two slotted casing wells at opposing ends of the 
cell, at least one first slotted casing well capable of being 
de-pressurized and at least one second slotted casing well capable of 
allowing air entry into the cell; 
(c) filling the cell with a porous media; 
(d) introducing at least one fluid into the filled cell and allowing the 
fluid to infiltrate the porous media; 
(e) sealing the infiltrated, filled cell; 
(f) de-pressurizing the first slotted casing, thereby creating a negative 
pressure gradient across the infiltrated, filled cell; and 
(g) measuring the saturation of the fluid in the porous media at each 
location of interest across the plane of the front side of the 
three-dimensional cell. 
FIG. 1 depicts a three-dimensional representation of the cell 10 employed 
in the method of the invention and as part of the present apparatus. The 
cell 10, although three-dimensional, approximates a two-dimensional cell, 
given its narrow width. In a preferred embodiment of the invention, the 
cell 10 is 1 meter high, 1.5 meters long, and 0.08 meters wide, such that 
its front side 12 and back side (identical to front side on opposite side 
of cell, not shown) have an areal extent of 1.5 m.sup.2. Although the 
front side 12 and back side of the cell 10 of the preferred embodiment are 
rectangular in shape, it is contemplated that the sides of the cell may 
assume any shape, so long as the resulting cell 10 is capable of 
containing the porous media; the cell thickness is such that fluid 
saturations within the porous media may be measured; and the cell 10 may 
be sealed to achieve a negative pressure gradient. For example, it is 
contemplated that the front and back sides might have curved or even 
circular outlines. 
It is contemplated that the front side 12 and back side of the cell 10 be 
substantially transparent such that the fluid saturations within the 
porous media may be visible in addition to being measurable, such as by a 
gamma radiation system. Specifically, the preferred method of measuring 
fluid saturations in the practice of the invention involves using a 
dual-energy gamma ray attenuation system, which may require that the cell 
10 be at least semi-transparent along the path of measurement. Thus, it is 
preferred that the front side 12 and back side of the three-dimensional 
cell 10 be made of a material that is at least semitransparent and strong 
enough to contain the porous media without buckling. Moreover, the front 
side 12 and back side should be resistant to attack by the fluids 
introduced into the porous media, such as certain organic chemicals. An 
example of a suitable material is plastic, such as an amber 
semitransparent plastic such as Ultem.RTM. Plexiglass.RTM., commercially 
available from Rohm & Haas. To further buttress the front and back sides 
of the cell 10, it is contemplated that a metal frame 16 and cross braces 
18 are installed about the cell 10. 
The cell 10 is equipped with at least two slotted casing wells at opposing 
ends. These wells are illustrated in the schematic of the laboratory 
apparatus presented as FIG. 2, which represents a plan view of the front 
side 12 of the cell 10. At least one well 20 is devoted to providing a 
negative pressure gradient to induce flow of the fluids contained within 
the porous media, thereby simulating fluid flow under the influence of a 
vacuum. Additionally, at least one well 22 is devoted to providing air 
entry. The slotted casings for the wells 20 and 22 preferably contain some 
means of preventing the well from plugging due to inflow of porous media. 
For example, the use of a gravel pack and wire mesh within the slotted 
casing is specifically contemplated in the practice of the invention to 
prevent such porous media as sand and clay from plugging the wells 20 and 
22. 
The vacuum induced in the slotted casing wells 20 may be accomplished by 
any available means. It is specifically contemplated that, to achieve a 
negative pressure gradient in the practice of the invention, a vacuum pump 
24 is connected to the slotted casing wells 20 and is operated after 
sealing the cell 10. Any degree of negative pressure gradient that may be 
safely withstood by the cell 10 may be employed in the practice of the 
invention. It is further contemplated that the negative pressure induced 
by the vacuum pump 24 will be controlled by a vacuum control 26 and that 
the pressure itself will be monitored, such as by a simple U-tube 
manometer 28. 
The porous media placed into the cell 10 may be any porous media through 
which the fluid of interest might flow in response to a negative pressure 
gradient. Specifically, it is contemplated that the porous media comprise 
a naturally-occurring earth material such that the porous media in the 
cell 10 simulates soil conditions of interest for fluid migration. 
Examples of naturally-occurring earth materials include clay, sand, and 
silt. It is also contemplated that the porous media will comprise a 
heterogeneous mixture of materials of known but different textures to 
establish boundary conditions within the cell 10. For example, layers of 
clay, sand, and silt might be employed to simulate soil conditions in a 
particular area of NAPL contamination. In the Example below, Areas 30 
represented Type 1 clay while Areas 32 and 34 represented Types 1 and 2 
sands, respectively, having characteristics as reported in Table 1 below. 
Thus, the schematic of FIG. 2 illustrates an essentially vertical layering 
of porous media within the cell 10. 
In addition to the air introduced into the cell 10 by means of the slotted 
well 22, it is contemplated that at least one additional fluid is 
introduced into the cell for infiltration into the porous media. While the 
fluid may be either gaseous or liquid in phase, it is preferred that at 
least one liquid be introduced into the cell 10. However, any fluid of 
interest may be introduced thereinto for response to the vacuum-induced 
negative pressure gradient. Examples of possible fluids of interest 
include NAPLs, although the invention is not so limited. It is 
specifically contemplated that the porous media mimic earth materials in 
situ, such that water is also introduced into the cell 10. Therefore, it 
is preferred that, in addition to the introduction of air, water and the 
NAPL of interest are introduced into the cell 10 to infiltrate the porous 
media. 
Typically, the introduction of a water and the NAPL of interest into the 
cell 10 involves four stages, each of which may be studied. First, the 
water is allowed to redistribute within the porous media, reflecting 
changes in water table and capillary fringe. Then, the NAPL is allowed to 
infiltrate and re-distribute within the porous media. Finally, the water 
and NAPL migrate through the porous media in response to a negative 
pressure gradient induced by vacuum at one of the slotted casing wells 20. 
In this manner, one essentially models the migration of a NAPL through a 
porous media that might be expected following a spill of the NAPL onto 
soil and the eventual remediation of the soil by vacuum, such as VER or 
SVE or combinations thereof. 
The manner of simultaneously and continuously measuring the fluid 
saturations of interest throughout the porous media is specifically 
contemplated to be a dual-energy gamma attenuation system, although the 
invention is not so limited. For example, an X-ray fluorescence system 
might also be employed, among other options. FIG. 3 represents a flow 
diagram of a typical dual-energy gamma radiation system 36. The mechanical 
part of the dual-energy gamma ray attenuation system 36 consists of the 
source 38/detector 40 assembly which is mounted on a movable platform 42 
on parallel steel rails about the cell 10, shown in a side view. A stable 
voltage source 41 provides necessary power to the detector 40. The 
interconnected sources 38 and detector 40 can be moved simultaneously, 
either horizontally or vertically, by horizontal 44 and vertical 46 
stepper motors connected to an arrangement of chain and gear drives (not 
shown). The platform 42 supports and maintains axial alignment of the 
radiation source 38 and detector 40. The horizontal 44 and vertical 46 
stepping motors are used to drive and position the system using a 
microprocessor-based controller 48. The stepper motor control units 50 can 
be activated manually or by computer control. The actuator 52 serves as an 
intermediate controlling device, interfacing between the stepper motor 
control units 50 and the data acquisition board 54 in the computer. In the 
preferred embodiment, the system 36 is capable of moving and taking repeat 
measurements at any position within its 2 meter horizontal or 1.1 meter 
vertical range to within 0.5 mm. A heavy steel table (not shown) is welded 
between the tracks to provide a stable support for clamping the soil cell 
10. 
In the preferred embodiment, the gamma radiation source 38 consists of two 
distinct energy ranges of 60 and 662 KeV mounted coaxially in a custom 
lead alloy shield (not shown). The sources 38 are standard sealed 
capsules; one preferably contains 200 mCi .sup.137 Cs and the other 100 
mCi .sup.241 Am. The Cs source is located in the center, while the Am 
source is placed at the front end of the shield. Typically a 
thallium-activated crystal of sodium iodide (not shown) is used to detect 
the transmitted radiation from the gamma detector 40. The scintillations 
produced from the incident gamma rays are contemplated to be converted 
into electrical pulses in a 10-stage photomultiplier tube (not shown) 
which is built into the detector crystal package 40. The detector package 
40 is mounted in a custom manufactured, lead alloy shield with collimation 
provided by cylindrical holes, preferably about 6.35 mm in diameter. 
Pulses leaving the photomultiplier tube are shaped and amplified in a 
preamplifier 56. These pulses are again amplified in a gain-stabilized 
amplifier 58. Once amplified, the electrical pulses are linearly 
transmitted through an automatic gain control unit 60, to a pair of single 
channel pulse height analyzers 62 (SCA). The SCAs 62 convert the shaped 
linear pulses into countable pulses by using its integral or differential 
discriminator (not shown). Radiation intensities are determined by first 
filtering out the pulses that represent energies outside of the window of 
interest. Counting and timing functions are performed in a 4-channel 
precision counter/timer module 64. Three of the channels are used for 
counting operations and the fourth is used in timing. This module 64 also 
contains the remote communications circuitry (not shown) to allow remote 
control of the counter/timer functions. The timer/multiscaler 64 is 
connected to the data acquisition board 54 via a buffered interface card 
66. The computer 48 is used to instruct the timer/multiscaler 64 to start 
and stop counting via the buffered interface 66 where data is stored 
temporarily in its buffer. 
It is noted that a plurality of ports 68 for tensiometers may be 
incorporated into the cell 10 in order to measure pressure at various 
locations of interest within the cell 10. A plurality of pressure 
measurement ports 68 is likewise depicted in FIG. 2 as being distributed 
across the plane of the cell 10 (see small circles). Tensiometers may be 
15 installed in the ports 68 as incorporated into the back and front 12 
sides of the cell 10, then being connected to transducers 70. The analog 
data from these transducers 70 is converted to digital data in an A to D 
converter 72 and is then sent to the microprocessor 48. These ports 68 for 
tensiometers may also be used to measure pressures in the cell locations 
by injecting an instrument called a Tensimeter.RTM. (available from Soil 
20 Measurement Systems of Tucson, Ariz.). Specifically, the needle of the 
Tensimeter.RTM. is inserted through a septum at the location of the 
tensiometer. 
The derivations of equations and procedures used for the determination of 
attenuation coefficients, path lengths, bulk densities and fluid 
saturations from the dual-energy gamma system data are detailed below, as 
demonstrated by Parker et al., in "Modeling Multiphase Organic Chemical 
Transport In Soils and Ground Water", U.S. EPA Report No. 
EPA/600/2-91/042, Ada, Okla., August 1991. 
First, the attenuation coefficients must be determined. Assuming .sup.241 
Am and .sup.137 CS sources, the attenuation coefficients for NAPL, soils 
and water, for example, may be determined by the following known procedure 
using a small Plexiglass cell with 4 compartments of known widths which 
represent the path lengths (x) (see, e.g. R. J. Lenhard et al., 
"Measurement and Simulation of One-Dimensional Transient Three-Phase Flow 
for Monotonic Liquid Drainage", Water Resources Research, Vol. 24, No. 6, 
pp. 853-63 (June 1988)). The method employs a compartmentalized 
calibration cell of known path lengths for taking exiting radiation 
measurements through the cell with varying number of chambers filled with 
the material of interest. Linear regression programs may be used to 
evaluate the product of the attenuation coefficient and density of the 
material (-.mu..sub.i .rho..sub.i) according to the linearized form of 
Beers Law for a radioactive beam (I) passing through a fluid I: 
EQU 1n(I)=1n(I.sub.0)-.mu..sub.i .rho.x (4) 
This method provides several points at different path lengths to determine 
-.mu..sub.i .rho..sub.i from the Cesium and Americium counts. 
Determinations of path lengths and bulk densities must be calculated for 
each location of interest across the front side 12 of the cell 10 at which 
fluid saturation measurements are desired. Due to variation of cell 
thickness after filling with soil and water, accurate path lengths (x) for 
each of the measurement locations are calculated from the .sup.241 Am and 
.sup.137 Cs counts for the empty cell and the water filled cell: 
EQU x=(1/.mu..sub.i .rho..sub.i).multidot.1n(I.sub.0 /I) (4a) 
where .mu..sub.i and .rho..sub.i are the known gamma radiation attenuation 
coefficient and mass density, respectively. 
Bulk densities and water contents at each measurement location in the cell 
can now be calculated from Equation 5 using the measured Americium and 
Cesium gamma radiation counts passing through a porous medium containing 
water and air as fluids, the corresponding path lengths (x), gamma 
radiation attenuation coefficients (.mu..sub.i) and empty experimental 
cell counts (I.sub.0). The attenuation of gamma radiation (I) through a 
soil with dry bulk density (.rho.) and volumetric water content 
(.theta..sub.w) can be written using the subscripts s, w and g to denote 
soil, water and gas phase, respectively: 
EQU I=I.sub.0 exp(-.mu..sub.s .rho..sub.s x-.mu..sub.w .rho..sub.w 
.theta..sub.w x-.mu..sub.g .rho..sub.g .theta..sub.g x) (5) 
where I.sub.0 is the empty cell count rate in counts per second (cps) 
I is the emergent count rate (cps) 
.mu..sub.i is the mass attenuation coefficient of phase i 
x is the path length of the gamma beam through i 
.rho..sub.i is the density of phase i 
.theta..sub.g is the volumetric gas phase content. 
Substituting U.sub.w for (.mu..sub.w .rho..sub.w) and ignoring the 
contribution of the gas phase to the attenuation of the photons, equation 
(5) becomes 
EQU I=I.sub.0 exp(-.mu..sub.s .rho..sub.s x-U.sub.w .theta..sub.w x) (6) 
Substituting subscripts (c) and (a) for the .sup.137 Cs and .sup.241 Am 
gamma radiation to form two equations and then solving these equations 
yields the bulk density (.mu..sub.s): 
##EQU2## 
The fluid saturations within the cell 10 may be determined from the 
.sup.137 Cs and .sup.241 Am counts using Beers Law relationship. The gamma 
ray attenuation (I) through a porous medium with water (w) , NAPL (n) and 
gas (g) can be written as: 
EQU I=I'.sub.0 exp(-.mu..sub.s .rho..sub.s x-.mu..sub.w .rho..sub.w 
.theta..sub.w x-.mu..sub.g .rho..sub.g .theta..sub.g x) (8) 
where I'.sub.0 is the count rate through the empty column in cps 
.mu..sub.n is the NAPL attenuation coefficient 
.rho..sub.n is the density of NAPL 
.theta..sub.n is the volumetric NAPL content. 
Assuming the contribution of the gas phase to be negligible (unless 
simulating an SVE process) and assuming the bulk density is constant with 
time, equation (8) can be rewritten as 
EQU I=I.sub.0 exp(-.mu..sub.w .rho..sub.w .theta..sub.w x-.mu..sub.n 
.rho..sub.n .theta..sub.n x) (9) 
where I.sub.0 =I'.sub.0 exp(-.mu..sub.s .rho..sub.s x). 
Substituting subscripts (c) and (a) for .sup.137 Cs and .sup.241 Am gamma 
radiation to form two equations and then solving these equations yields 
the water (.theta..sub.w) and NAPL (.theta..sub.n) contents: 
EQU .theta..sub.w =.mu..sub.na .rho..sub.n x1n(I.sub.oc /I.sub.c)-.mu..sub.nc 
.rho..sub.n x1n(I.sub.oa /I.sub.a)!/y (10) 
EQU .theta..sub.n =.mu..sub.wc .rho..sub.n x1n(I.sub.oc /I.sub.c)-.mu..sub.wa 
.rho..sub.n x1n(I.sub.oa /I.sub.a)!/y (11) 
where y=(.mu..sub.wc .rho..sub.w x)(.mu..sub.na .rho..sub.n x)-(.mu..sub.wa 
.rho..sub.w x)(.mu..sub.nc .rho..sub.n x). 
Thus, in a preferred embodiment, one may pack the cell 10 with porous 
media, introduce air, water, and the NAPL of interest into the cell 10, 
and measure the fluid saturations of the water and NAPL at desired 
locations across the back and front 12 sides of the cell 10 by 
substituting data measured by a dual-energy gamma radiation system 36 in 
equations (10) and (11) above. 
The fluid saturation data acquired in the practice of the invention is 
contemplated for use in several applications. For example, the porous 
media in the cell 10 may be selected and placed to simulate an actual 
contaminated zone and the fluids introduced into the cell 10 may be 
selected to match those actually spilled into such a zone, such that the 
feasibility of vacuum-based remediation techniques such as VER and SVE may 
be assessed on a pilot scale before committing to such clean-up operations 
on a large scale in an actual spill site. Specifically, one would be able 
to analyze the migration of fluids through the pilot-scale model as well 
as calculate an anticipated recovery of NAPL for a VER- or SVE-based 
clean-up operation. In a related application, one might study the fluid 
migration of oil and gas through porous media in response to 
vacuum-induced negative pressure gradients, which represents in 
pilot-scale the recovery of oil and gas via slotted casing wells. In 
addition to the simple primary recovery of oil and gas in porous media, 
one might also add various chemicals or fluids, such as carbon dioxide or 
steam, to study the secondary and tertiary recovery of oil and gas in 
porous media in a pilot-scale apparatus. 
Another contemplated application of the present pilot-scale apparatus is 
the assessment and optimization of computer models used to predict the 
behavior of fluids undergoing VER, SVE, or "bioslurping", which is a 
combination of SVE and VER. Essentially, the experimental results from the 
present pilot-scale apparatus, as operated under controlled boundary 
conditions, may be compared to the predictive results from a model, such 
that the model may be calibrated to match the experimental results. In 
this fashion, the accuracy of the model in regard to application to 
vacuum-based recovery of fluids in heterogeneous soils should be improved. 
Testing the effectiveness of three-phase models to simulate VER of 
non-volatile NAPL free-product such as diesel oil or SVE of volatile NAPLs 
in controlled 2-D laboratory scale investigations prior to installation at 
VER or SVE remediation sites could be used in lieu of expensive field 
testing to meet regulatory requirements or clean-up standards. 
The usefulness of the disclosed method and apparatus are contemplated to 
extend to many fields and applications, of which assessment of the 
feasibility of VER and SVE are two of many. Others include bioslurping, 
venting, bioventing, and air sparging. The method and apparatus of the 
invention are demonstrated below in the following example. 
EXAMPLE 
The method of the invention was used to study, in pilot-scale, the spatial 
distributions of three-phase fluid saturations after an oil spill 
simulation from the surface at the 100-cm elevation of the cell 10 as 
migration occurred through the vadose zone during the sequential stages of 
infiltration, redistribution and vacuum enhanced recovery (VER). 
Particularly, the laboratory experiments were performed in a 
three-dimensional cell 10 having the dimensions of 1 m high.times.1.5 m 
long.times.0.08 m wide, such as illustrated in FIG. 1. The front side 12 
and back side of the cell 10 were made of amber semitransparent plastic 
(Ultem) which is resistant to organic chemicals. The frame 16 was made of 
steel with cross braces 18 installed on both sides to prevent bulging of 
the cell 10. A strong frame 16 was necessary because the accuracy of 
measurements from the gamma system are affected by any variation to the 
path lengths (thickness of cell). Accurate path lengths were determined by 
using equations from the gamma system's Am and Cs data. An automated 
dual-energy gamma attenuation system 36 such as described above as the 
preferred embodiment was used for the simultaneous continuous measurement 
of NAPL and water saturations. 
In particular, the stepper motor control units 50 employed were Model SP 
155A from Superior Electric of Bristol, Conn.; the actuator 52 was Model 
Aston 800 from Aston Company of Georgia; the gamma detector 40 was from 
Harshaw Chemical of Solon, Ohio; the data acquisition board 54 was Model 
MBC IEEE 4888 from Keithley MetraByte Corporation of Taunton, Mass.; the 
buffered interface card 66 was Model TC 489 from Tennelec of Oak Ridge, 
Tenn.; and the Tensimeters.RTM. were manufactured by Soil Measurement 
Systems of Tucson, Ariz. Specifically, Tensimeters.RTM. are injection-type 
devices used to measure pressure. In contrast, tensiometers (which may be 
alternatively employed) are employed for hydrophilic and hydrophobic 
pressure measurement, having been coated and treated in a laboratory. 
FIG. 2 depicts the types of porous media employed in this example as well 
as the general layout of the heterogeneous soil packing placement in the 
cell 10. Horizontal clay layers were dispersed within layers of sand, with 
Areas 30 representing Type 1 Clay while Areas 32 and 34 represent Types 1 
and 2 sands, respectively. Specifically, Type 1 sand was packed at the 
bottom of the cell 10 to an elevation of about 13 cms to simulate the 
saturated zone and a 10-cm thick Type 1 clay layer was packed at an 
elevation of 30 cms across the entire length of the cell. Two smaller Type 
1 clay layers were placed on the left side of the cell 10 at an elevation 
of 55 cms and on the right side of the cell at an elevation of 67 cms. The 
remainder of the heterogeneous soil packed in the cell 10 constituted Type 
2 sand. The clay barriers were used to illustrate the experimental 
simulation capability and accuracy of NAPL and water saturation 
measurements in heterogeneous porous media. Table 1 below lists the 
initial moisture content, dry bulk density, porosity and particle density 
for the types of sands and clay used: 
TABLE 1 
______________________________________ 
Summary Of Moisture Content, Bulk Density, Porosity And Particle 
Density. 
Iden- 
Initial Moisture Content 
Dry Bulk Calculated 
tifica- 
Gravimetric 
Volumetric Density Porosity 
Particle 
tion (%, g/g) (%, cm.sup.3 /cm.sup.3) 
(g/cm.sup.3)(%) 
(g/cm.sup.3) 
Density 
______________________________________ 
Type 33.0 50.2 1.52 43.0 2.67 
Sand 
Type 23.7 38.7 1.63 40.2 2.73 
2 
Sand 
Type 47.4 55.6 1.17 57.0 2.73 
1 
Clay 
______________________________________ 
The gamma system was capable of moving and taking repeat measurements at 
any position within its 2 meter horizontal or 1.1 meter vertical range to 
within 0.5 mm. A heavy steel table (not shown) was welded between the 
tracks to provide a stable support for clamping the soil cell 10. A series 
of gamma radiation counts of 5 minutes in duration were taken at 96 
locations across the front side 12 of the cell 10 as programmed with a map 
file; each of these 96 programmed grid positions are shown in FIG. 2 as 
signified by squares 73. The gamma radiation counts were accomplished at 
of the 96 grid positions 73 by moving two interconnected parallel 
platforms 42, one supporting the radiation sources 38 and the other 
supporting the gamma detector 40, via stepper motors 44 and 46 under 
computer 48 control. In the automatic mode of operation, the computer 48 
read positions data from a pre-defined map file and moved the source 
38/detector 40 to the selected map location 73 on the cell 10, thereby 
making possible automatic, unattended system operation experiments for 
long durations and repetitious experiments at predetermined times. 
Two rectangular slotted casing wells 8 cms wide.times.5 cms were installed 
at each end of the cell. One well 20 was designed to enable vacuum 
enhanced recovery and the other well 22 was designed for air entry. The 
slotted casing was designed using a fine wire mesh and a gravel pack just 
outside the wells 20 and 22 to prevent the sand and clays from plugging 
the wells. The vacuum extraction system used a graduated flask 74 for NAPL 
and water retrieval hooked up to a variable vacuum pump 24. The top of the 
cell 10 was sealed during the VER stage of the experiment. 
The Americium and Cesium windows were first calibrated to correct for 
Compton scattering before a systematic calibration of the cell 10. The 
.sup.137 CS and .sup.241 Am spectra were calibrated by using the pulse 
height analyzer 62 to plot each spectrum and adjusting the high voltage 
setting and the coarse gain setting in such a way that the base line 
voltage was at the ideal energy ratio to the gamma radiation. Since the 
amplified output pulses had a broad energy spectrum it was necessary to 
discriminate these pulses for the identification of the energy spectra of 
both sources. The .sup.137 Cs spectrum was determined by using the pulse 
height analyzer 62 and plotting the data spectrum. 
Correction for Compton-scattered .sup.137 Cs photons detected in the 
.sup.241 Am window was necessary since a single detector 40 was used to 
simultaneously determine .sup.241 Am and .sup.137 Cs photons. 
Compton-scattered .sup.137 Cs photons cause interference in the .sup.241 
Am window. Compton scattering will cause photons to deflect from their 
original path while giving up some of their energy. The interference of Cs 
in the Am spectrum is caused by Compton scattering in the detector 40, and 
the amount of scattering at a given Cs intensity is independent of the 
absorbing material. To determine the amount of Compton scattering, which 
is the low energy Cs, the collimator of the source holder 38 was covered 
with thin brass plates and variable amounts of glass and plexiglass placed 
in the beam path and the count rates recorded in all 3 channels (Am, Cs 
and Integrated) for 300 seconds. The brass plates prevented Am radiation 
from reaching the detector 40. Count rates of the low energy window were 
plotted against those of the high energy. A regression program was used to 
calculate the correlation between the high energy Cs counts and the low 
energy Cs counts in the Americium window. The data points were fitted by 
using a third order polynomial, which was subsequently used to correct the 
.sup.241 Am count rates by subtracting the low energy Cs count from the 
total count observed in the low energy window. 
Following the completion of calibrating the gamma sources 38, a systematic 
calibration of the cell 10 was performed by the determination of Am and Cs 
counts for (1) the empty cell, (2) water-filed cell, (3) dry soil- and 
clay-packed cell, and (4) water saturated cell packed with soil and clay. 
The Am and Cs counts from this calibration sequence were used to determine 
the path length (thickness of cell), bulk densities, NAPL and water 
saturations at each of the 96 locations of interest 73 indicated in FIG. 2 
using Beer's Law relationship in the equations described above. 
The heterogeneous soils and clays were initially packed dry in the cell 10 
using the soil configuration shown in FIG. 2. Am and Cs counts were taken 
at the 96 grid locations 73 using the automated feature of the gamma 
system 36 to obtain the dry soil measurements required for calibration. 
The cell 10 was then water saturated completely by adding water through 
the bottom port in the cell until the water-table elevation coincided with 
the upper soil boundary. Am and Cs counts were again taken at the same 96 
grid locations 73 using the automated feature of the gamma system 36 to 
obtain the water-saturated soil measurements. During each of these 
calibration stages, gamma counts at the 96 grid locations 73 were taken 
for 300 seconds using two repetitions. The cell 10 was then considered 
ready for the experimental simulation of water drainage to establish 
initial conditions followed by the NAPL spill experiment. Table 2 below 
reports the attenuation coefficients determined for the apparatus of the 
example: 
TABLE 2 
______________________________________ 
Summary of Attenuation Coefficients (-.mu..sub.i .rho..sub.I). 
Identification .sup.241 Am 
.sup.137 Cs 
______________________________________ 
Water 0.197552 0.08458 
NAPL 0.760684 0.071456 
Soil 0.250384 0.078785 
______________________________________ 
Once the cell 10 was calibrated and initial conditions established with a 
water saturated soil cell, the NAPL spill evaluation was performed in four 
stages. Am and Cs counts were taken continuously for 90 seconds at each at 
the 96 programmed cell locations 73 to determine NAPL and water 
saturations during the experimental stages of (1) water redistribution, 
(2) NAPL infiltration, (3) NAPL redistribution, and (4) VER. The 
collection of data at the end of each stage was used as the representative 
boundary conditions for the start of the next stage during modeling 
exercises. This required documentation of the exact time when each of the 
sequential stages were completed and the next stage began in a continuous 
manner. 
Stage 1: Water Redistribution. The top of the cell 10 was covered to 
prevent water evaporation losses. The water in the cell 10 was then 
allowed to drain through the side ports 76 located 10 cms from the bottom 
until steady state conditions were obtained with the water table at 10 cms 
elevation. Am and Cs readings were taken at all 96 grid locations 73 
during the water redistribution to enable plotting the water saturation 
contours during the redistribution process through the heterogeneous 
soils. 
Stage 2: NAPL Infiltration. NAPL was infiltrated at the soil surface at the 
top center of the cell 10 over an area of 25 cm.sup.2 using gravity feed 
through a set of 6 distribution tubes from a calibrated bottle before the 
start of the experiment. Soltrol-170, a branched alkane solvent 
manufactured by Phillips Petroleum of Bartlesville, Okla. and having a 
density of about 0.83 g cm.sup.-3, was used to simulate light non-aqueous 
phase liquid (LNAPL) in the oil infiltration and VER phase of the 
experiment. To enable clear differentiation between the attenuation 
factors for LNAPL and water, the Soltrol-170 was spiked with the addition 
of 1-iodoheptane at a volumetric mixture at a ratio of 1:9. The addition 
of 1-iodoheptane to the Soltrol-170 increased the .sup.241 Am attenuation 
coefficient to 0.95 cm.sup.2 g.sup.31 1, a five fold increase compared to 
the original Soltrol-170 .sup.241 Am attenuation coefficient of 0.18 
cm.sup.2 g.sup.-1. 
More particularly, the LNAPL was introduced into the cell 10 at a steady 
flow rate of about 2 liters/hour, such that about 9 liters were introduced 
into the cell in 4.5 hours. The NAPL was fed using a graduated flask (not 
shown) fitted with an adjustable valve and 6 small flow tubes (not shown) 
to facilitate NAPL infiltration on the soil surface over an area of 
5cms.times.5 cms. Am and Cs counts were taken using a special map file 
that enabled the tracking of the plume close to the spill source during 
infiltration. 
Stage 3: NAPL Redistribution. The Am and Cs counts were continued following 
NAPL infiltration to track the redistribution of NAPL and water in the 
heterogeneous soils for 20 hours. 
Stage 4: Vacuum Enhanced Recovery. After NAPL redistribution reached steady 
state conditions, the top of the cell 10 was sealed air tight and the 
vacuum pump 24 hooked up to the VER well 20. Air entry was through the 
vent well 22 located on the opposite end of the cell 10. Vacuum extraction 
was started at 34 mb of vacuum. The vacuum was slowly increased to 57 mb 
which raised the water level in the extraction well 20. NAPL and water 
were recovered in a graduated flask 74 hooked up as shown in FIG. 2. After 
12 hours the extraction rate was increased to 67 mb. The final vacuum 
extraction rate was increased to 100 mb for a total of 28 hours. The rate 
of NAPL and water extracted in the graduated flask 74 was measured. The 
vacuum was measured using a mercury manometer 28. A Tensiometer.RTM. was 
used to measure the air vacuum in the soil by insertion of the measurement 
needle through the septum stopper located at a pressure measurement port 
68. 
The raw data consisting of Am and Cs counts at the 96 locations 73 at 
various time intervals were converted to NAPL and water saturations using 
the procedures described earlier. The average of two sets of data obtained 
for NAPL and water saturations, one each from the Am and Cs counts, was 
used. 
FIGS. 4A through 4D represent a series of contour plots showing NAPL and 
water saturations aerially across the back and front 12 sides of the cell 
10 as measured in centimeters. The location of the clay layers 30 were 
superimposed on the graphs as dotted lines. A smoothing algorithm was 
employed to smooth the plots. It is noted that one could also plot fluid 
saturations against time for each of the 96 grid locations 73. 
Stage 1: Water Redistribution. FIG. 4A represents a contour plot of 
measured water saturations for the experimental soil configuration after 
water redistribution. The water redistribution contours after 30 hours 
show the water table and capillary fringe at the 30-cm to 40-cm elevation 
with saturations from 1.0 to 0.4. The Type 1 clay layer 30 located on the 
left side of the soil cell at the 60-cm elevation retained moisture over 
0.80 even after 30 hours. The clay layer located on the right side of the 
cell at the 70-cm elevation retained moisture at about 0.40, as shown by 
the measured water redistribution contours. This contour plot of measured 
water saturations shows the significantly higher moisture retention 
characteristics of the Type 1 clays 30 compared to the sand layers 32 and 
34. These results agree with the high porosity clays at 60% compared to 
40% porosity for sands used in this experiment. 
Stage 2: NAPL Infiltration. FIG. 4B represents a contour plot of measured 
NAPL saturations and water redistribution for the experimental soil 
configuration after 3 hours of NAPL infiltration. The NAPL plume for 9 
liters of NAPL was found to spread above the clay layer 30 at the 40-cm 
elevation. The water retention in the Type 1 clay layer 30 at the 60-cm 
elevation on the left side of the cell seems to have prevented the NAPL 
from penetrating the clay to the left. NAPL saturations were found close 
to 0.80 in the center of the plume at the 45-cm elevation, where the clay 
barrier 30 prevented vertical downward migration. 
Stage 3: NAPL Redistribution. FIG. 4C represents a contour plot of measured 
NAPL saturations for the experimental soil configuration after NAPL 
redistribution after 9 hours. The clay layer 30 at the 30-cm elevation, 
which retained water at a saturation of 0.7 to 1.0, only allowed NAPL 
entry to saturations of 0.0 to 0.3. This illustrates the mass balance of 
volume occupied by the three phases NAPL-water-air in the soil pores. It 
appears that there was very little trapped air in these voids, based on 
the saturations of NAPL and water at these locations. No NAPL saturations 
were generally present below the clay layer 30 in the water table. 
Stage 4: Vacuum Enhanced Recovery. FIG. 4D represents a contour plot of 
measured NAPL saturations for the experimental soil configuration after 28 
hours of VER, which successfully extracted a relatively large percentage 
of the NAPL. The maximum retention of NAPL saturation is shown plotted at 
about 0.20 near the lower right side of the cell in FIG. 4D. The rest of 
the contour plot indicate a reduction in the NAPL saturation to below 
0.10. The vacuum extraction rate at 100 mb could attribute to the recovery 
of NAPLs. 
The accuracy of the gamma system 36 for the measurement of NAPL retention 
in the soil was evaluated by comparing the actual retention volume with 
that measured by the gamma system 36. This accuracy analysis was performed 
by comparing the measured NAPL retention volume calculated by integrating 
measured NAPL saturations at 96 locations 73 in the cell 10 with the 
actual NAPL retention derived from the volume recovered by the VER system 
in the flask 74. The actual retention volume was derived by subtracting 
the NAPL volume recovered in the VER extraction system from the original 
infiltration volume of 9,000 ml. The measured retention volume of NAPL was 
calculated by integrating the measured NAPL saturations at the 96 
locations 73, using the porosity of the soil and the volume represented 
for each location, as shown: 
##EQU3## 
where .PHI..sub.i, V.sub.i, and A.sub.i are the calculated soil porosity, 
incremental porous media volume and area, respectively, for each 
representative location i; .rho..sub.bi, x.sub.i, and q.sub.ni are the 
soil bulk density, path length and NAPL saturation, respectively, 
determined by the gamma system; .rho..sub.pi is the soil particle density 
measured in the laboratory; V.sub.n is the representative NAPL volume 
calculated for location i; and V.sub.n is the calculated total NAPL volume 
in the cell 10 shown as "measured retention volume" in Table 3 below. 
V.sub.t is the total volume of NAPL added (here, 9000 mls) as a spill on 
the surface of the soil filled cell 10 and V.sub.r is the volume of NAPL 
recovered by VER. V.sub.a is the NAPL retention volume determined by 
subtracting the NAPL recovery volume by VER from the total spill volume, 
i.e., (V.sub.a =V.sub.t -V.sub.r). 
The total volume is the summation of NAPL volume calculated for each of the 
96 positions 73. However, an area calibration factor for that represented 
by locations at the lower end of the cell 10 had to be made since not all 
96 locations 73 were equally spaced therein. The path lengths determined 
by the gamma system 36 provided an accurate measure of cells thickness at 
each of the 96 measurement locations 73. The NAPL saturations were 
measured at each of the 96 location 73 using a count time of 90 seconds. 
Allowing for the movement time for the gamma attenuation system 36 between 
each location 73, the total time lapse for a complete run to measure for 
all 96 locations was about 2.5 hours. Due to the continuous flow of NAPL 
during the infiltration and VER processes, and the measurements not being 
instantaneous, an error is introduced in the summation of the total NAPL 
from the measurement of NAPL during each run at all 96 locations 73. Table 
3 below shows the measured and actual (calculated by difference) NAPL 
volumes over the different stages of the experiment. The difference 
.DELTA. ranged between 0.04 to 5.08% with a mean of 0.97%. This comparison 
illustrates the accuracy of the methodology used for VER measurement and 
further verifies the gamma system approach for measuring fluid saturations 
in porous media. 
TABLE 3 
______________________________________ 
Summary Error Analysis of Measured NAPL Retention and VER. 
% 
mea- 
sured 
re- 
VER Gamma VER ten- 
Total measured measured 
measured 
tion 
NAPL recovery vol- 
retention 
retention 
vol- 
Time volume ume, V.sub.r 
volume volume ume 
Stage (hrs.) (V.sub.t, ml) 
(ml) (%) (V.sub.n, ml) 
(V.sub.a, ml) 
.DELTA. 
______________________________________ 
NAPL 9 9000 0 0 9364 9000 +4.04 
infil- 
tration 
NAPL 12 9000 0 0 8975 9000 -0.27 
redis- 
tribu- 
tion 
NAPL 15 9000 0 0 8600 9000 -4.44 
redis- 
tribu- 
tion 
NAPL 18 9000 0 0 8542 9000 -5.08 
redis- 
tribu- 
tion 
VER 28 9000 1700 18 7278 7300 -0.30 
at 
57 mb 
VER 38 9000 3480 42 5200 5220 -0.38 
at 
78 mb 
VER 58 9000 4780 53 4218 4220 -0.04 
at 
100 mb 
______________________________________ 
The rate of NAPL and water recovery at various vacuum rates versus time are 
depicted in the x-y plot of FIG. 5, wherein Curve 78 represents NAPL 
recovery in milliliters, Curve 80 represents water recovery in 
milliliters, and Curve 82 represents the vacuum rate in mb. A maximum 
free-product retrieval rate of 53% NAPL was achieved from a total spill 
volume of 9 liters, by progressively increasing vacuum rates in steps of 
0, 37 mb, 57 mb, 78 mb, and 100 mb over a total duration of 30 hours. 
Lower vacuum rates of 57 mb and 78 mb resulted in NAPL recovery rates of 
18% and 42% respectively. NAPL recovery in this pilot-scale experiment was 
performed by skimming free-product from the top of the simulated VER well 
20. FIG. 4D illustrates the effectiveness of NAPL removal by VER after 28 
hours of vacuum extraction. At 28 hours, the residual NAPL saturation over 
this area was generally reduced to 0.20. The rest of the cell 10 had NAPL 
saturations below the 0.10. About 0.10 in NAPL saturation was found in the 
water table. 
While the above data and figures are helpful in the assessment of the 
feasibility of a VER program for NAPL remediation, the present method and 
apparatus are also useful in the calibration of three-phase models for the 
optimization of vacuum enhanced recovery of NAPL from remediation sites 
with complex hydrogeology and heterogeneous soils. In general, a 
sequential approach would be used, in which data of increasing complexity 
was used to make predictions using the computer model, whereupon the 
predictions are compared with the measured results from the present method 
and apparatus. Using that comparison, refinements may be made to the 
computer model to improve model parameters, until satisfactory agreement 
with the experimental data is obtained. 
In sum, the above example illustrates the use of the pilot-scale laboratory 
soil cell 10 and an automated dual-energy gamma ray attenuation system 36 
for the feasibility assessment of VER and optimization of three-phase 
remediation models to remediate free-product NAPL spills. As illustrated, 
the dynamic laboratory method determines the spatial distributions of 
three-phase fluid saturations of an oil spill simulation from the surface 
as it migrates through the vadose zone during the sequential stages of 
infiltration, redistribution and VER. A mean difference of 0.97% was 
achieved by comparing gamma-system measured NAPL retention volumes in the 
soil with that derived by the VER system, thereby demonstrating the 
accuracy of the disclosed method. The above example therefore illustrates 
the usefulness of the disclosed method and apparatus in assessing the 
feasibility of vacuum-enhanced recovery of NAPL from a heterogeneous soil 
in the vadose zone and the optimization of three-phase remediation models 
for NAPL recovery and remediation. 
INDUSTRIAL APPLICABILITY 
The present method and apparatus are expected to find use in the 
remediation of subsurface contamination by NAPLs as predictors of the 
reliability and accuracy of vacuum-induced clean-up methods and models. 
Additionally, the present method and apparatus may also find use in the 
petroleum industry for the study of oil and gas recovery from hydrocarbon 
reservoirs. 
Thus, there has been disclosed a method and apparatus for determining the 
spatial distribution of fluids in a porous media contained in a 
three-dimensional cell as such fluids migrate through the porous media in 
response to a vacuum-induced pressure differential. Additionally, there 
has been disclosed a method of using the apparatus for the assessment of 
feasibility of vacuum-induced recovery of NAPLs from porous media. It will 
be readily apparent to those skilled in the art that various changes and 
modifications of an obvious nature may be made without departing from the 
spirit of the invention, and all such changes and modifications are 
considered to fall within the scope of the invention as defined by the 
appended claims.