An Assessment of LNAPL Mobility to Evaluate the Potential for Commingling of Two
Large LNAPL Plumes
Robert J. Frank, R.G. (CH2M HILL, Tempe, Arizona, USA)
Historical and recent releases of fuel from underground storage tanks and pipelines at
a manufacturing and testing facility created a 45-acre zone of light non-aqueous phase liquid
(LNAPL) hydrocarbons beneath the Site. As part of the Site’s Corrective Action Plan (CAP)
in 2004, an LNAPL mobility assessment was performed to determine potential movement
and recoverability of the LNAPL to evaluate different remedial technologies. In addition, due
to the proximity of a large off-site LNAPL release located approximately 1,400 feet
downgradient of the Site, the mobility assessment was necessary to estimate the potential for
commingling of the two areas of LNAPL. Theoretical mobility values were compared to
measured hydrocarbon transmissivities and LNAPL recovery rates to check the validity of
the theoretical calculations.
Calculations using data from monitoring wells located near and at the downgradient
extent of the LNAPL at the time of the CAP resulted in LNAPL velocities ranging from zero
to approximately 6.6 feet/year, and zero to approximately 1.8 feet/year, respectively. By
comparison, the groundwater beneath the Site in the area of the LNAPL is estimated to flow
at approximately 800 feet/year to over 3,000 feet/year. The LNAPL mobility assessment was
used to show that the two LNAPL plumes would not commingle in any reasonable timeframe
based on conditions at the time of the CAP, distance between the plumes, and no future
releases of LNAPL.
Between 2004 and 2007, significant changes to the subsurface occurred that affected
the nature and extent of the LNAPL and the validity of the mobility assessment performed in
2004. This paper presents the methods used to generate the estimated LNAPL velocities for
the CAP in 2004, the changes that have occurred since the mobility assessment was
performed, and how those changes have resulted in the current potential for the two areas of
LNAPL to commingle.
The Site is a 118-acre, non-residential manufacturing and testing facility. Large
amounts of aviation fuel have been used at the Site for testing since operations began in the
1950s, and this fuel has been stored in a variety of aboveground and underground storage
facilities and moved about the Site via aboveground and underground pipelines. The Site is
located in an area zoned for industrial use, and the area surrounding the Site is a mix of
industrial, commercial, and residential property. To the south (hydraulically downgradient)
of the Site is a large, international airport.
LNAPL was first observed in monitoring wells at the Site in 1999 and was found to
consist primarily of a mixture of Jet-A and JP-4 aviation fuels. The nature and extent of the
LNAPL has been evaluated and delineated through the installation of over 50 groundwater
monitoring wells, with LNAPL encountered in 30 of the wells since 1999. The historical
extent of the Site LNAPL covers an area of approximately 45 acres and volume estimates
place the amount of mobile LNAPL in the range of 45,000 to 198,000 gallons.
Automated free-product skimming and manual recovery of the LNAPL has occurred
since 1999, with approximately 7,200 gallons of aviation fuel recovered to date. Figure 1
illustrates the extent of the Site LNAPL in 2004 when the CAP for the Site was developed.
Site Map with LNAPL – May 2004 (off-site LNAPL is historical extent)
In 1997, a leak was identified in the fuel hydrant system that supplies Jet-A aviation
fuel to the terminals of the airport. According to pressure tests, the leakage rate was
estimated at 110 gallons per hour. In 2000, consultants to the airport stated that the LNAPL
plume extended approximately 3,500 feet east to west and 1,500 feet north to south and
covered an area of approximately 80 acres. Another consultant to the airport estimated the
total product volume in the subsurface ranged from 1.2 to 6.6 million gallons.
The off-site LNAPL underwent extensive remediation efforts following its discovery
in 1997, including free-product skimming and multi-phase extraction. Consultants to the
airport estimated that approximately 49,400 gallons of Jet-A aviation fuel were recovered
between 1997 and 2003. The historical origin of the off-site LNAPL lies approximately
1,400 feet to the southwest of the Site LNAPL described above (Figure 1).
Rationale for Conducting the LNAPL Mobility Assessment
As part of the CAP to remediate the Site, it was necessary to determine the potential
movement and recoverability of the LNAPL to evaluate different remedial technologies. In
addition, due to the proximity of the off-site LNAPL to the Site, information regarding the
potential for the Site LNAPL to commingle with the off-site LNAPL was requested.
Therefore, a mobility assessment of the Site LNAPL was performed for the CAP, as well as
to evaluate the concerns over the potential commingling of the two LNAPLs.
Data were gathered from the laboratory analysis of soil and fluid samples collected
from the Site. All of the physical soil and fluid parameters necessary for the LNAPL mobility
assessment were generated from the results of these site-specific samples (shown in Table 1).
Soil samples were collected from borings at the Site and were analyzed for capillary
characteristics. The raw capillary pressure data from these samples were used to generate the
Van Genuchten parameters using the Van Genuchten equation (Van Genuchten, 1980). The
two-phase (air and water) Van Genuchten equation, as modified by the work of Farr et al.
(1990) and Lenhard and Parker (1990) to account for a third phase (LNAPL), was used to
correlate LNAPL saturation to elevation above the LNAPL/water interface. This correlation
was then used to determine the relationship between LNAPL saturation and mobility. The
Van Genuchten parameter values and LNAPL scaling factors used in this evaluation are
shown in Table 1.
The parameters shown in Table 1 were used to generate theoretical LNAPL saturation
and conductivity curves for the Site. A free-product thickness of 0.98 foot was used for the
analysis. This represented the thickness of product measured in monitoring well MW-A at
the time the initial analysis and LNAPL baildown tests were performed. Laboratorymeasured
values for LNAPL viscosity and the interfacial tension pairs (collected from nearby
monitoring well MW-B) were used to represent the LNAPL found at the Site.
The theoretical LNAPL saturation curve, based on an LNAPL thickness of 0.98 foot
is illustrated in Figure 2. The figure relates the saturation of LNAPL to the height above the
LNAPL/water interface as predicted by the modified Van Genuchten equation. As shown in
the figure, the soil and fluid data collected from the Site resulted in a theoretical maximum
LNAPL saturation (based on a 0.98-foot thickness) as predicted by the modified Van
Height Above LNAPL/Water Interface (feet)
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
Theoretical LNAPL Saturation Profile
Genuchten equation of 7.3 percent. This suggests that although there is about 1 foot of
product in a monitoring well for example, the formation (within that 1-foot zone)
theoretically consists of at most about 7 percent LNAPL and 93 percent water, meaning that
there is very little volume per unit area of LNAPL.
A limited set of LNAPL saturation data was collected from four soil borings located
in the downgradient portion of the LNAPL. The measured LNAPL saturations in these four
borings ranged from less than 0.1 percent to 10.7 percent. At the time of sampling, and in the
months following installation of monitoring wells in the borings, a good correlation between
measured LNAPL saturations and thickness (or presence) of LNAPL was not apparent. The
highest measured LNAPL saturations did not correlate to the thickest LNAPL observed in
the monitoring wells, and the detection of measurable LNAPL saturations in the soil borings
did not necessarily correlate to observed LNAPL in the well.
The theoretical saturation curve was compared to the laboratory-measured LNAPL
saturations to ensure that the saturations were within the same range prior to making
conclusions on potential mobility. Because measured saturations were not collected from
monitoring well MW-A, a direct correlation between the theoretical saturations (based on the
thickness of LNAPL in that well) and the measured saturations was not possible. As stated
above, the direct laboratory measurement of LNAPL saturations in the soil borings found a
ange from less than 0.1 percent to 10.7 percent LNAPL saturation, which was in the same
general range as the theoretical calculation (zero to 7.3 percent).
LNAPL Relative Permeability and Conductivity
The relative permeability of the LNAPL was calculated using the Mualem expression
(Parker, 1989). Due to the generally low LNAPL saturations predicted (and measured) at the
Site, it was expected that the LNAPL relative permeability would be similarly low. The
theoretical relative permeability of the LNAPL was calculated for a thickness of 0.98 foot,
similar to the hydrocarbon saturation profile. The theoretical maximum relative permeability
of the LNAPL at the Site was calculated to be 0.08.
The LNAPL conductivity (KNAPL; cm/sec), a measure of the soils’ ability to transmit
free-phase hydrocarbon, was calculated using:
where krn is the LNAPL relative permeability, ki is the intrinsic permeability (cm 2 ), o
is the LNAPL density (g/cm 3 ), g is the acceleration due to gravity (cm/sec 2 ), and is
the LNAPL viscosity (g/cm sec).
The resultant maximum theoretical LNAPL conductivity was calculated to be 7.2 x
10 -6 cm/sec. The theoretical LNAPL conductivity values over the entire range of elevations
above the LNAPL/water interface ranged from the maximum of 7.2 x 10 -6 cm/sec to 2 x 10 -12
cm/sec at a point approximately 1.3 feet above the LNAPL/water interface.
LNAPL Mobility Analysis
Analysis of LNAPL Mobility Using Theoretical Values
To estimate the potential mobility of the Site LNAPL, a range of theoretical LNAPL
velocities were calculated. The theoretical LNAPL velocity (PVNAPL) was calculated using:
where K NAPL is the LNAPL conductivity (cm/sec), i is the hydrocarbon gradient, is
the porosity, and S NAPL is the LNAPL saturation.
Because the LNAPL saturation varies vertically within the thickness of LNAPL
(Figure 2), the LNAPL velocity also varies from some minimum amount to a maximum
value near the LNAPL/air interface. In addition, LNAPL velocity varies laterally as well due
to changes in LNAPL saturation and soil characteristics throughout the subsurface.
The maximum LNAPL velocity based on the theoretical values presented above was
calculated using the maximum LNAPL conductivity of 7.2 x 10 -6 cm/sec (based on a freeproduct
thickness of 0.98 foot), a hydrocarbon gradient of 0.0160 (based on the LNAPL
surface elevations of three monitoring wells in the area), a porosity of 0.25, and a peak
LNAPL saturation of 7.3 percent. These values resulted in a maximum LNAPL velocity of
approximately 6.4 x 10 -6 cm/sec (6.6 feet/year). The relationship between LNAPL velocity
and elevation above the LNAPL/water interface is shown in Figure 3. The figure shows that
the LNAPL velocity ranges from zero at the LNAPL/water interface to 6.6 feet/year at 0.98
foot above the LNAPL/water interface, with a value of approximately 5.2 feet/year at 0.5
foot above the LNAPL/water interface.
Height Above LNAPL/Water Interface (feet)
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
Estimated LNAPL Velocity (feet/year)
Estimated LNAPL Velocity
Analysis of LNAPL Mobility Using Thicknesses Measured at the Time of the CAP From
Monitoring Wells Downgradient of MW-A
The theoretical LNAPL saturation and conductivity profile generated using the
thickness of LNAPL in monitoring well MW-A was used to extrapolate LNAPL velocities
for additional nearby monitoring wells using their measured LNAPL thicknesses.
Approximately 1 year after the initial evaluation was performed on monitoring well MW-A,
LNAPL was observed in monitoring well MW-A (0.53 foot), and two progressively
downgradient monitoring wells: MW-B (0.54 foot) located about 250 feet from MW-A, and
MW-C (0.04 foot) which was located on the leading edge of the LNAPL about 550 feet from
The maximum LNAPL velocity at monitoring well MW-A using the later thickness
data, an updated hydrocarbon gradient of 0.0156, and a peak LNAPL saturation of 3 percent
(based on a thickness of 0.53 foot) was calculated to be approximately 5.4 feet/year. By
comparison, monitoring well MW-C, located on the downgradient edge of the LNAPL at the
time of the CAP, contained a peak theoretical LNAPL saturation of 0.06 percent (based on a
thickness of 0.04 foot) and a corresponding maximum LNAPL velocity of approximately 1.8
feet/year. Because monitoring well MW-B, located midway between MW-A and MW-C,
contained a similar LNAPL thickness as MW-A, calculations showed that the maximum
velocity at monitoring well MW-B was similar in magnitude to that of monitoring well MW-
A. A comparison of the estimated LNAPL velocities along the direction of groundwater flow
is illustrated in Figure 4.
Maximum Estimated LNAPL Velocity (feet/year)
Groundwater Flow Direction
MW-A MW-A + 1 year MW-B
Comparison of Estimated Maximum LNAPL Velocities
Analysis of LNAPL Mobility Using Baildown Test-Generated Conductivities
For comparison to the theoretical values presented above, maximum LNAPL
velocities were also calculated using the results of baildown tests performed at eight different
monitoring wells at the Site. These monitoring wells all contained various product
thicknesses (ranging from 0.66 foot to 3.41 feet) and presumably different hydrocarbon
saturations at the time they were tested. The calculated LNAPL conductivities from the
baildown tests ranged from 1.7 x 10 -6 cm/sec to 3.0 x 10 -4 cm/sec. These values are greater
than the maximum theoretical LNAPL conductivity of 7.2 x 10 -6 cm/sec (which was based on
a free-product thickness of 0.98 foot).
For consistency, and due to a lack of multiple soil capillary characteristic samples, the
maximum LNAPL velocities were estimated using the same theoretical LNAPL saturation
profile described above. Based on historical LNAPL recovery rates at these wells, it was
clear that the saturation profiles differed greatly across the Site, so the velocity estimates
generated from the variety of baildown tests were difficult to evaluate and compare. Using
the theoretical LNAPL saturations associated with the thickness of LNAPL measured in each
of the baildown test wells, the calculated LNAPL conductivities from the baildown tests, and
a consistent hydrocarbon gradient (0.016) and porosity (0.25), the maximum LNAPL
velocities were calculated to range from 0.4 foot/year to 192 feet/year. The range of
estimated velocities had a median value of 21 feet/year and a standard deviation of +/- 73
feet/year, indicative of the tremendous variability across the Site.
Determination of the Commingling Potential of the Site LNAPL and Off-Site LNAPL
from the CAP Mobility Assessment
The above evaluation was used to estimate the relative velocities of the Site LNAPL
to assist in the evaluation of remedial alternatives as part of the Site’s CAP, as well as to
determine the potential for the Site LNAPL to commingle with the off-site LNAPL. As
described earlier, the off-site LNAPL was historically located approximately 1,400 feet
downgradient of the Site LNAPL (Figure 1).
Using site-specific soil and fluid data, the calculations described above resulted in
maximum estimated LNAPL velocities ranging from approximately 1.8 to 6.6 feet/year.
Given a distance of approximately 1,400 feet between the Site LNAPL and off-site LNAPL,
it would take more than 100 years at a consistent gradient, saturation, and conductivity for
the two LNAPL plumes to commingle. Therefore, it was estimated that the Site LNAPL
would not commingle with the off-site LNAPL in any reasonable timeframe based on
conditions at the time of the CAP, distance between the plumes, and no future releases of
The maximum estimated LNAPL velocity calculated from the baildown tests (192
feet/year) indicated that the two LNAPL plumes could commingle in a little more than 7
years, although the baildown tests were conducted on monitoring wells located within the
LNAPL rather than at its edges where the “migration” of the LNAPL is governed. Recovery
data from the Site indicate that there have been pockets within the extent of the LNAPL that
have appeared to be fairly mobile (LNAPL recovery at one monitoring well exceeded 3,500
gallons in approximately 2.5 months), meaning that these elevated velocities are not entirely
unexpected. However, these areas are not indicative of the movement of the LNAPL plume
as a whole.
Changes to the Subsurface Following the CAP Mobility Assessment
As explained above, the results of the LNAPL mobility assessment were applicable
based on conditions at the time of the CAP (2004), distance between the plumes, and no
future releases of LNAPL. However, between 2004 and 2007, significant changes to the
subsurface occurred that affected the nature and extent of the LNAPL and thus the validity of
the mobility assessment performed in 2004. The three changes that most significantly
affected the movement of the Site LNAPL and its potential to commingle with the off-site
LNAPL were, 1) substantial fluctuations of the water table, 2) an additional release of
LNAPL at the Site with subsequent additional monitoring well installations, and 3) on-going
remediation of the off-site LNAPL.
Substantial Fluctuations of the Water Table
Due to a wet winter in 2004/2005, there was an abnormal increase in surface water
flow near the Site during that timeframe. This increased surface water flow caused a rise of
the local water table by as much as 20 feet in the area of the LNAPLs, resulting in
measurable LNAPL disappearing from a majority of the monitoring wells for a period of
time. With a rising water table, the mobility of LNAPL can be limited by its inability to
displace water in the subsurface. Under certain conditions, the LNAPL can become “trapped”
and be virtually immobile. Due to additional surface water flow following the 2004/2005
winter, the local water table has remained elevated since that time, limiting the appearance of
the Site LNAPL in monitoring wells and its movement towards the off-site LNAPL.
Additional Release of LNAPL at the Site With Subsequent Additional Monitoring Well
An additional release of aviation fuel to the subsurface was identified in 2005 at the
Site. As a result, additional monitoring wells were installed in locations beyond the
downgradient extent of the LNAPL at the time of the CAP. Following their installation,
LNAPL was observed in some of these monitoring wells, indicating that the Site LNAPL
was further south than previously observed, but no closer to the historical origin of the offsite
LNAPL (the Site LNAPL was still approximately 1,400 feet from the historical origin of
the off-site LNAPL). Furthermore, the release of aviation fuel at the Site would have affected
local LNAPL saturations and velocities in and downgradient of the release location, thereby
resulting in potentially different values than those predicted in the 2004 mobility assessment.
On-Going Remediation of the Off-Site LNAPL
Continued remediation of the off-site LNAPL, in conjunction with the rise in the
water table as explained above, resulted in the disappearance of the off-site LNAPL from
monitoring wells as of June 2007. While this change alone does not affect the mobility
assessment of the Site LNAPL, remediation of the off-site LNAPL has completely removed
the potential for the Site LNAPL and off-site LNAPL to commingle.
The combined effect of the changes to the subsurface over the past 3 years is evident
in Figure 5 which illustrates the Site LNAPL based on June 2007 measurements. While the
location of measurable LNAPL in monitoring wells from the Site is observed further south
than it was in 2004 (Figure 1), the observance of LNAPL in monitoring wells has been
affected more by the dramatic fluctuations in the water table. These changes in the water
table have affected the appearance of the Site LNAPL to the point where there is no longer a
continuous presence in monitoring wells across the Site. In June 2007, as it has been for the
past 1 to 2 years, the Site LNAPL was limited to three, relatively small, disconnected areas.
In addition, there is no longer any measurable LNAPL in monitoring wells located in the offsite
Site Map with LNAPL – June 2007
The LNAPL mobility assessment prepared for the CAP showed that the estimated
velocity of the Site LNAPL near its downgradient extent at the time of the CAP ranged from
zero to approximately 6.6 feet/year, and zero to 5.4 feet/year using later thickness
measurements. By comparison, the groundwater beneath the Site in the area of the LNAPL is
estimated to flow at approximately 800 feet/year to over 3,000 feet/year.
The thickness of LNAPL measured at the downgradient extent at the time of the CAP
(MW-C) was used to give an indication of the potential movement of the leading edge of the
LNAPL. Assuming monitoring well MW-C had a similar LNAPL saturation and
conductivity profile as monitoring well MW-A, the measured LNAPL thickness of 0.04 foot
resulted in an LNAPL velocity range of zero to approximately 1.8 feet/year. Therefore, it was
estimated that the Site LNAPL would not commingle with the off-site LNAPL in any
reasonable timeframe based on conditions at the time of the CAP, distance between the
plumes, and no future releases of LNAPL. Based on the mobility assessment, it was
concluded that the Site LNAPL was generally not mobile at its downgradient extent.
Furthermore, none of the velocity estimates presented took into account mass removal via
skimming, volatilization, or biodegradation, which would prevent the LNAPL from moving
at the calculated maximum rates.
As alluded to above, the velocity and travel time estimates presented in the mobility
assessment assumed a consistent gradient, saturation, and conductivity for the migration rate
to be maintained. However, significant changes to the subsurface have occurred over the past
3 years that affected the nature and extent of the LNAPL and thus the validity of the mobility
assessment performed in 2004. The three changes that most significantly affected the
movement of the Site LNAPL and the potential to commingle with the off-site LNAPL were,
1) substantial fluctuations of the water table, 2) an additional release of LNAPL at the Site
with subsequent additional monitoring well installations, and 3) on-going remediation of the
Substantial fluctuations in the water table have affected the appearance of the Site
LNAPL to the point where there is no longer a continuous presence across the Site. In
addition, the water table fluctuations, along with continued remediation of the off-site
LNAPL, have resulted in the disappearance of the off-site LNAPL from monitoring wells (as
of June 2007). Therefore, while the estimated LNAPL velocities presented in the CAP are no
longer valid for the Site LNAPL, the dramatic changes in the subsurface have limited the
potential movement of the Site LNAPL (velocities calculated using the current subsurface
conditions would result in significantly lower mobility rates), and removed the potential for
the Site LNAPL and off-site LNAPL to commingle based on measurements in monitoring
Farr, A.M., Houghtalen, R.J., and McWhorter, D.B., 1990, “Volume Estimation of Light
Nonaqueous Phase Liquids in Porous Media,” Ground Water, 28, no. 1, 48-56.
Lenhard, R.J., and Parker, J.C., 1990, “Estimation of Free Hydrocarbon Volume From Fluid
Levels in Monitoring Wells,” Ground Water, 28, no. 1, 57-67.
Parker, J.C., 1989, “Multiphase Flow and Transport in Porous Media,” Review of
Geophysics, 27, no. 3, 311-328.
Van Genuchten, M. Th., 1980, “A Closed-Form Equation for Predicting the Hydraulic
Conductivity of Unsaturated Soils,” Soil Sci. Soc. Am. J., 44, 892-899.
Soil and Fluid Parameters Used for the Mobility Assessment
Soil Parameters Value Source
Porosity 0.25 Assumed uniform porosity
Van Genuchten "N" 1.625 Calculated from laboratory data
Van Genuchten "alpha" (1/cm) 0.03115 Calculated from laboratory data
Residual Water Content 0.125 Calculated from laboratory data
Bulk Density (kg/m 3 ) 1,820 Laboratory data
LNAPL Parameters Value Source
LNAPL Thickness (feet) 0.98 MW-A measurement prior to baildown test
LNAPL Density (g/cc) 0.81 Laboratory data
Air-Water Surface Tension (dyne/cm) 73.50 Laboratory data
Air-LNAPL Surface Tension (dyne/cm) 28.90 Laboratory data
LNAPL-Water Surface Tension (dyne/cm) 30.60 Laboratory data
Air-LNAPL Scaling Factor 2.543 Calculated from surface tensions
LNAPL-Water Scaling Factor 2.402 Calculated from surface tensions
Intrinsic Permeability (cm 2 ) 1.54E-9 Laboratory data
LNAPL Viscosity (g/cm sec)) 0.0142 Laboratory data
Robert J. Frank, R.G. is a senior hydrogeologist at CH2M HILL in Tempe, Arizona. He is a
registered geologist in the State of Arizona and holds an M.S. degree in hydrogeology from
San Diego State University where he received the American Petroleum Institute/National
Groundwater Association’s National Groundwater Scholarship. His research interests and
project experience focus on the distribution, mobility, and remediation of petroleum
hydrocarbons and chlorinated solvents in the subsurface. Robert can be reached at
CH2M HILL, 2625 South Plaza Drive, Suite 300, Tempe, Arizona 85282; telephone (480)
377-6263; email: Robert.Frank@ch2m.com.