Application of Multiphase Concepts to LNAPL Site Investigations

Transcription

Application of Multiphase Concepts to LNAPL Site Investigations
Application of Multiphase
Concepts to LNAPL Site
Investigations
Don A. Lundy, PG
ES&T, a Division of Groundwater &
Environmental Services, Inc.,
Atlanta, GA, USA
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Conference on Design of Free Phase Removal –
Theoretical and Practical Aspects, 16 May 2011,
Israel Water Authority, Tel Aviv, Israel
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Topics to Cover
• What Controls LNAPL Recovery
• Basic Multiphase Concepts
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• Characterizing LNAPL Sites
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Controls on LNAPL Recovery
• What controls our ability to recover LNAPL?
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– Selection and design of engineered
structures, pumps, electrical controls, etc.
– Characterization of the spatial distribution,
mobility, and recoverability of the LNAPL
– Understanding of site hydrogeology
– Environmental regulations and policies
– Financial resources to carry out recovery
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Site Objectives
• Comply with environmental regulations
• Design or evaluate a recovery system
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• Negotiate an endpoint to remediation
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Overview of Basic Concepts
• Terminology
• The Saturation Profile conceptual model
• Parameters that control LNAPL behavior
• Effects of water-table changes
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• LNAPL plume migration and stability
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Terminology
• LNAPL = Light non-aqueous phase liquid
• LNAPL is sparingly soluble in water
• LNAPL is synonymous with “oil, free
product, and free phase product.”
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• LNAPL conductivity = Oil conductivity
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Saturation Profile Conceptual Model
• Basic Assumptions
–
–
–
–
Unconfined aquifer
Geologic materials are homogeneous
Fluids are homogeneous
LNAPL has an intermediate wettability between
air and water
– The fluids are in static equilibrium
– Hysteresis (imbibition) is ignored
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• Fluid and capillary properties, and fluid pressure
heads are the primary controlling variables for oil
distribution.
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Saturation Profile Conceptual Model
Oil Saturation
Profile in Soil
Pc
Vertical Elevation Soil Column
Air-Oil Interface
Theoretical Air-Water
Interface
Capillary
Pressure
Prediction
Observed
Monitoring
Well
Thickness
10
20
30
40
50
60
< 1 atm
1 atm
Pressure
> 1 atm
Oil-Water Interface
Hydrocarbon Saturation (%)
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Basic Conceptual Model
• The “oil saturation profile model” defines the
basic architecture for LNAPL distribution near
the water table.
• The basic architecture can be modified as model
assumptions are changed to fit site conditions.
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• Departures from the idealized assumptions are
identified during the site investigation phases.
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Controlling Parameters & Variables
• Fluid Properties
• Aquifer/matrix Properties
• Multiphase Interactions in
Subsurface
• Relative Permeability
• Specific Oil Volume
• Inherent Oil Mobility
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• Oil Conductivity and Transmissivity
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LNAPL Physical Properties
• Density and Specific Gravity
– Density is the mass per unit volume (g/cm3)
– Specific Gravity is oil density/water density, [d]
– Density is inversely related to temperature
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– Density increases with aging and weathering
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LNAPL Specific Gravity Ranges
• Approximate Ranges for Common LNAPLs:
–
–
–
–
–
–
–
Gasolines
Jet Fuels
Diesel Fuels
Fuel Oils
Lube Oils
Crude Oils
Waste Oils
0.74 to 0.77
0.77 to 0.80
0.82 to 0.85
0.82 to 0.92
0.86 to 0.95
0.83 to 0.95
0.85 to 0.97
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• Remember: Water is ~ 1.0 at 25 deg C
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Product Physical Properties
• Viscosity
– Viscosity is a measure of a fluid’s resistance to flow
– Two general forms
• Dynamic Viscosity = shear stress/shear rate [centipoises (cP)]
• Kinematic Viscosity = dynamic viscosity/density [centistokes (cSt)]
– Viscosity is inversely related to temperature
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– Viscosity increases with aging and weathering
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LNAPL Viscosity Ranges
• Approximate Ranges of Common LNAPLs:
–
–
–
–
–
–
–
Gasolines
Jet Fuels
Diesel Fuels
Fuel Oils
Lube Oils
Crude Oils
Waste Oils
0.5 to 0.8 cp
0.7 to 1.2 cp
2.0 to 4.5 cp
2.5 to 6.0 cp
50 to 200 cp
10 to 200 cp
50 to 500 cp
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• Remember: Water is ~ 1 cp at 25 deg C
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LNAPL Interfacial Tensions
• Surface Tensions
– Surface tension is the interfacial tension between a liquid (water
or LNAPL) and its own vapor
• Air-Water surface tension at 25°C = 72 dynes/cm
• Air-Impacted Groundwater surface tension is lower and ranges from
~57 to 70 dynes/cm
• Air-Oil surface tension ranges from ~ 20 to 30 dynes/cm
• Interfacial Tensions
– Interfacial tension between two liquids are lower than surface
tensions between air and liquids
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• Oil-Water interfacial tension ranges from ~ 8 to 25 dynes/cm
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Soil-Water Characteristic (SWC) Curves
Residual
Water
Contents
Note:
Capillary
Pressure =
“Suction”
Water Table
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SWC Curves – Air/Water
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Three Empirical vG Parameters
• van Genuchten α (vG-α)
– Accounts for the largest connected pores
– Larger values mean a smaller capillary fringe height
• van Genuchten n (vG-n)
– Accounts for pore size distribution
– Small values mean broader pore-size distributions
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• Residual (“irreducible”) Water Saturation
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SWC Curves for USDA Soils
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Note:
Calculated
using USDA
soil properties
taken from the
Carsel &
Parish (1988)
Database.
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Saturation Profile Model
Oil Saturation Profiles for Diesel within Different Soil Types
1.2
1
Silt
0.8
Sand
0.6
Silty Sand
0.4
0.2
0
0%
10%
20%
30%
40%
50%
Oil Saturation
60%
70%
80%
90%
100%
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Elevation above the Oil-Water Interface (m)
Air/Oil Table
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Specific Oil Volume
Specific Oil Volume = Vertical integration of oil
saturation profile x porosity over a unit area.
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Equivalent to the oil volume per unit area, in
units of length (e.g., m3/m2 = m)
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Volume Comparison
Calculated LNAPL volume for a constant in-well product
thickness of 1 meter in a 100 square meter area
10,000
Pancake
Conceptualization
8,000
7,000
6,000
Saturation Profile Conceptualization:
Mobile Oil Volume Function of Soil Type
5,000
4,000
3,000
2,000
1,000
0
Coarse Sand
Fine Sand
Silt
Sand
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Mobile Oil Volume (gallons)
9,000
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Volume Comparison
Oil Saturation Profiles for Same Volume of Gasoline
in Different Soil Types
6
Clayey Silt
4
Silt
3
2
Fine Sand
Coarse Sand
1
0
0%
10%
20%
30%
40%
50%
60%
Oil Saturation
70%
80%
90%
100%
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Elevation above the Oil-Water Interface (ft)
5
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Relative Permeability
• LNAPL flows in the larger pores and water flows
in the smaller pores
• Fluid flow is an average through the pore sizes
and volume
• When more than one fluid occupies a pore
space, each fluid has a relative permeability
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• Relative permeability (kr) is the ability of the
porous media to allow flow of a fluid when other
fluid phases are present
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Relative Permeability
Residual Oil
Saturation
Relative Permeability
1
100%
A function of saturation
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Defined as the ratio of the fluid
permeability at a given
saturation to the fluid
permeability at full saturation
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A dimensionless fraction
between 0 and 1
•
Calculated with the effective
total fluid and water saturations
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Relative permeability of LNAPL
(kro) and relative permeability
of water inversely related
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Below residual saturation, flow
decreases exponentially
NAPL
Water
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Water Saturation
NAPL Saturation
100%
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Irreducible
Water
Saturation
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Relative Permeability
• Center of LNAPL Plume
Irreducible
Water
Saturation
Residual Oil
Saturation
NAPL
• Edge of LNAPL Plume
– Low LNAPL saturations
– Minimal Kro
– Low flow rate
Water
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0
100%
Water Saturation
NAPL Saturation
100%
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• As LNAPL saturation
approaches residual
saturation, relative
permeability for LNAPL
approaches zero
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Relative Permeability
1
– Highest LNAPL saturations
– Highest Kro
– Highest flow rate
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Oil Conductivity
• Oil Conductivity (Ko) is the amount of LNAPL that
will flow across a unit area per unit time under a
unit LNAPL potentiometric gradient (dimen. L/T)
• Not just a function of the aquifer, but varies with
saturation and conditions of hydrostatic state
• Equation:
Ko = ko(ρο /μο) Kw ,
ko = relative permeability of the oil
ρο = specific gravity of the oil
μο = oil viscosity relative to water
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Kw = hydraulic conductivity of soil/rock
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Oil Transmissivity
• Oil Transmissivity (To) is the average oil conductivity
multiplied by thickness of free oil zone (dimen. L2/T)
T o = K ob ,
Ko = hydraulic conductivity of oil
b = thickness of free oil zone
• Describes the bulk movement and recovery, but not
discrete velocity at the pore scale at plume fronts
• Dependent on the hydrostatic conditions
(not a constant)
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– In general, the more dispersed an LNAPL plume becomes
vertically and laterally, the smaller the transmissivity
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Oil Transmissivity
a) OIL TRANSMISSIVITY
6
5
4
3
2
1
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2
4
6
8
10
Oil Thickness, ft
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2
Oil Transmissivity, ft /day
Transmissivity
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Inherent Oil Mobility
Defined as the ratio of oil transmissivity to specific
volume, but equivalent to…
Ratio of oil conductivity to the product of average oil
saturation and porosity:
Mo = To / Vo = bKo / bSoφ = Ko / Soφ,
Tο = transmissivity of the oil
Vο = specific oil volume
Ko = hydraulic conductivity of the oil
So = average oil saturation
b = LNAPL impacted aquifer thickness
Φ = porosity
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Mo = mobility of the oil
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Inherent Mobility
• Inherent oil mobility
increases with
LNAPL thickness
Inherent Mobility of Gasoline in Different Soils
80
Sand
70
– Hydraulic
conductivity
– LNAPL viscosity
– Relative oil
permeability
50
40
30
20
Sandy Loam
10
Loam
0
0
1
2
3
4
5
Apparent Well Product Thickness (feet)
Clay
Loam
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• Most sensitive to:
60
Mobility (ft/day)
• Reaches a
maximum value for
given soil and fluid
properties
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Mobility and Recoverability
Remedial Effort
Observed Well Product Thickness
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Mobility, Cost, or Time
Inherent Mobility
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Summary of Attributes of the
Saturation Profile Conceptual Model
• Key parameters:
– 5 soil properties (Kw,Srw,vG-α, vG-n, porosity)
– 4 fluid properties (ρο, μο, air/oil surface tension,
and oil/water interfacial tension)
– Observed in-well LNAPL thickness
• Oil saturation profile calculated with these
parameters
• Oil saturation controls oil volume and relative
permeabiltity
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• Ko, To, and Mo can then be estimated
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LNAPL Plume Velocity
80
70
70
60
60
LNAPL Velocity (ft/day)
50
40
Times Gradient = 0.01
30
Oil Velocity
50
40
30
20
20
10
10
0
0
0
1
2
3
Apparent Well Product Thickness (feet)
4
5
0
1
2
3
4
5
Apparent Well Product Thickness (feet)
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Inherent Mobility (ft/day)
Inherent Oil Mobility
80
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Effects of Water Table Fluctuations
• May “smear” the mobile LNAPL
• Transfer of LNAPL mass to a residual
saturation
• Reduces the oil mobility and transmissivity
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• May control LNAPL plume spreading
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Effect of Water Table Changes
• Increased understanding of trapping of oil in
pore network that creates residual oil as water
table fluctuates
Fall in Water Table
Rise in Water Table
Depth
Depth
Oil Trapped by
Capillary Forces
Oil Trapped by
Water Displacement
% Oil in Pore Space
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% Oil in Pore Space
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Plots of Water Table Changes
Sand
400,000
41.5
41
40
200,000
39.5
100,000
Water Table Changes
39
0
100
200
300
400
500
600
700
800
900
38.5
1100
1000
Silty Clay
Days
1,400
41.5
1,200
1,000
40.5
800
40
600
39.5
400
Water Table Elevation
LNAPL that could be
observed in a well
41
39
200
0
0
100
200
300
400
500
600
700
800
900
1000
38.5
1100
Days
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Oil Volume (gal)
Oil Volume (gals)
40.5
Water Table Elevation
300,000
Unsaturated Residual
Saturated Residual
Mobile Phase
Water Table
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LNAPL Thickness in Well versus
Water Table Elevation
Confined Aquifer
Conditions
Matched potentiometric
surface and LNAPL thickness
response (must factor in
density ratio of the two fluids)
•
i.e., LNAPL thickness
increases as water table
rises
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Adapted from ITRC 2009
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Confined LNAPL Conditions
•
LNAPL thickness in well increases with increase in water level
•
Bottom filling of well
•
Monitoring well acts like giant pore
Clay
Clay
Gravel
LNAPL
LNAPL
Water
Water
LNAPL
Gravel
Water
Water
Water
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Clay
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LNAPL Plume Stability
• Free LNAPL migration effects migration of
dissolved and vapor plumes
• Processes that control LNAPL spreading
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• How to recognize a stable LNAPL plume
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Section View of Hypothetical LNAPL and Anoxic GW Plume
Volatilization and
Biodegradation in
Vadose Zone
Greater Smearing
within distal flanks
Oil Table
Water
Table
Dissolution and
Biodegradation in
Anoxic/Methanogenic
Groundwater Zone
LNAPL flow
Groundwater flow
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Mass depletion/flux
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Plan View of Hypothetical LNAPL and Anoxic GW Plume
Three zones of Weathered LNAPL:
1 - Least weathered near release point
2 – Intermediate zone in central area
Anoxic/Methanogenic
Groundwater Plume
1 - Least
2 - Intermediate
3 - Most
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3 – Most weathered in distal flanks
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LNAPL Plume Migration
Large LNAPL head is sufficient
to overcome LNAPL entry
pressure and LNAPL plume
moves
Once the LNAPL head
dissipates, no longer sufficient
to overcome LNAPL entry
pressure and LNAPL plume
migration ceases
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Internal plume movement and
LNAPL can be observed in
wells until all LNAPL is residual
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History of Leading Edge Movement
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LNAPL Site Investigations
• Delineate the lateral and vertical extent
• Measure the fluid and matrix properties
that control oil saturations
• Perform field tests for oil transmissivity
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• Develop an LNAPL Conceptual Site Model
(LCSM) per ASTM guidance (2007)
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Delineation of the LNAPL Body
• Occurrence of oil in borings, excavations, and
monitoring or recovery wells
• Direct sensing of oil with LIF and MIP tools
• Soil/rock core collection with field observations
• High dissolved concentrations in groundwater
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• Elevated groundwater temperatures (2 to 3 deg C)
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Historical Fluid Levels
• Water table fluctuations and gradients
• LNAPL thickness variations in space and
time
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• Thickness and lateral extent of the LNAPL
“smear zone”
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Map of Water Table
FMW-20
FPZ-5
FPZ-3
FPZ-1
FPZ-2
DEM-MW7
FMW-19
FMW-21
FMW-23
FMW-24
DEM-MW6
PMW-3
DEM-MW5
SBMW-2
FMW-22
FMW-18
SBMW-1
SMW-5
DEM-MW4
PMW-7
AP-9
AP-3
DEM-MW2
AP-10
PMW-6
AP-5MW-3
PL
FMW-17
AP-2
AP-4
MW-4
AP-11
FMW-16
PL
PL
AP-12
TW-3
FMW-13
AP-1
PL
FPZ-4
MW-20
FMW-14
PMW-4
FMW-15
PMW-8
50
100
200
300
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Scale in feet
Source: Lundy (2002)
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Map of Free Oil Thickness
LNAPL
Thickness
(feet)
7.5
7
6.5
6
5.5
5
PL
4.5
4
3.5
PL
PL
3
2.5
PL
2
1.5
1
0.5
0.01
50
100
200
Scale in feet
300
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Groundwater BTEX Concentrations
Elevated groundwater
concentrations indicate
residual LNAPL source
Total BTEX
(μg/L)
50,000
10,000
1,000
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100
10
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Example Cross-Section
52,310
0
15,788
596
402
31,739
9,183
6,098
0
NS
1.3
0
69
9,264
10
840
453
0
94
69 Dissolved BTEX
Concentration (μg/L)
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NS Not sampled
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MIPs Equipment
• Geoprobe rig to push MIPs
• Hydrocarbons are vaporized inside probe
• Vapors are analyzed by an on-site GC
• Various detectors
• Data can be uploaded to a
website for retrieval and mapping
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– PID
– FID
– ECD
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Operating Principle of the MIP
Gas Return Tube
Permeable Membrane
VOCs in Soil
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Carrier Gas Supply
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MIPs Example
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CPT-ROST Sonde
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Combined CPT-ROST Log
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Benefits/Limitations of Geophysics
• Fastest way to map the extent of LNAPL
• But geophysical data are qualitative –
for screening purposes only
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• Data collection is limited to poorly
consolidated earth materials
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Soil/Rock Coring
• Equipment options
– Geoprobe or CPT – Acetate sleeve
– Auger - Shelby tube
– Mud or Air Rotary rig – Core barrel
– Sonic (rotary-vibratory) – NOT recommended
• Freeze cores in field to lock up the fluids
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• Ship to petroleum-type lab for analysis
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Quality of Cores
“Disturbed”
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“Undisturbed”
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Source: PTS Laboratories
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Core Photography
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• Sawed vertically in lab
• High resolution photo
• White light photo shows
details of texture
• UV light shows presence of
LNAPL
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Core Sampling
WALL DAMAGE and/or FLUID
INVASION: May occur during
coring.
VERTICAL SAMPLE: Sample
diameter is limited by core
diameter.
Source: PTS Laboratories
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HORIZONTAL SAMPLE:
Sample must be long enough
to meet Darcy flow
requirements.
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Lab Analyses of Core Samples
• Fluid saturations – air, oil, water
• Capillary properties – air-water drainage test
• Hydraulic conductivity
• Effective and total porosity
• Residual LNAPL saturations for vadose or
saturated zones
• Grain-size analyses, moisture content, bulk
density
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• Dual porosity assessment of fractured porous
media
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Pilot Testing Objectives
• Evaluate one or more remedial
technologies for screening alternative
• Generate design criteria for full-scale
implementation of the best alternative
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• Evaluate the performance of an existing
recovery system to improve it or to
negotiate an endpoint
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Hydraulic Properties
• Slug and pump test results
• Baildown test records – may need to be
re-analyzed with up-to-date tools
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• Ranges of hydraulic conductivity for the
strata impacted by LNAPL
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LNAPL Field Recovery Tests
• LNAPL Slug Withdrawal Tests
– Single or multiple slug (“baildown”) tests
– Oil is primary fluid removed
• LNAPL Pilot Recovery Tests
– Skimming Tests -- only oil removed
– Vacuum-Enhanced Skimming Tests – air and oil removed
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– Multiphase Extraction Tests – air, oil, water removal
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Compiling Baildown Test Results
Oil Transmissivity in Soil Zones
2
Oil Transmissivity, ft /day
12
Calculated
with Average
Parameters
9
Baildown
Results
6
3
0
0
2
4
6
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Free Oil Thickness, feet
Source: Lundy (2002)
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Cumulative LNAPL Volume,
gallons
Comparison of Oil Volumes Skimmed
to Model Prediction
35
MW-6
MW-15
Model Calculation
30
25
20
15
10
5
0
0
10
20
30
40
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Elapsed Time, days
Source: Lundy, Potter, White, and Ferrell (1998)
&
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Environmental Systems & Technologies
A Division of Groundwater & Environmental Services, Inc.
Summary Points
Identify the site objectives – recovery system design vs. closure
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Use historical hydrogeological data to characterize LNAPL smear zone
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Delineate the full extent of the LNAPL zone
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Collect and analyze fluids and cores from LNAPL zone
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Perform and analyze baildown/pilot tests in LNAPL zone
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Characterize the range of oil mobility and transmissivity
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Prepare a site-wide LCSM for meeting objectives
R
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&
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Environmental Systems & Technologies
A Division of Groundwater & Environmental Services, Inc.

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