An Assessment of LNAPL Mobility to Evaluate the ... - Info Ngwa

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An Assessment of LNAPL Mobility to Evaluate the ... - Info Ngwa

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)

Abstract

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.

Background

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.


Site LNAPL

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.

Figure 1

Site Map with LNAPL – May 2004 (off-site LNAPL is historical extent)

Off-Site LNAPL

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 Evaluation

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.

LNAPL Saturation

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)

1.50

1.25

1.00

0.75

0.50

0.25

0.00

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

LNAPL Saturation

Figure 2

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:

K

NAPL

k

rn

ki

o g


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:

PV

NAPL

KNAPL i

S


NAPL

where K NAPL is the LNAPL conductivity (cm/sec), i is the hydrocarbon gradient, is

the porosity, and S NAPL is the LNAPL saturation.

(1)

(2)


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)

1.50

1.25

1.00

0.75

0.50

0.25

0.00

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Estimated LNAPL Velocity (feet/year)

Figure 3

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

MW-A.

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)

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.0

Groundwater Flow Direction

MW-A MW-A + 1 year MW-B

(250 feet

downgradient)

Well ID

Figure 4

Comparison of Estimated Maximum LNAPL Velocities

MW-C

(550 feet

downgradient)


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

LNAPL.

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

Installations

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.

Current Conditions

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

LNAPL area.

Figure 5

Site Map with LNAPL – June 2007


Summary

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

off-site LNAPL.

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

wells.


References

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.

Table 1

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

BIOGRAPHICAL SKETCH

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.