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2 4 6 8 10 12 14 16 18 20 22 24 26 28
x (meters)
2
4
6
8 (
ete
s) Clay
Sand
Solvents and Fuel in Soil and Groundwater: When the Usual Hydrogeologist’s Toolbox is Insufficient
Walt W. McNab, Jr., Ph.D., P.G. Roux Associates, Inc., Oakland, CA
Why should you care about solvents and fuels?
2
Solvent and fuels = majority of groundwater contamination
Significant release volumes pose an enormous groundwater contamination potential
Neither mixes well with water
Sparingly soluble liquids represent long-term sources
Quantifying the movement of solvents and/or fuels coexisting with groundwater in an aquifer is problematic
One gallon of TCE can contaminate a 30-ft-thick aquifer across 119 acres (based on MCL of 5 parts per billion)
Solv
ent
What does “sparingly soluble” imply?
3
Both phases may require evaluation at some sites
Dissolved phase • Where “contamination” is defined
• Solubilities of common solvents and fuel constituents ~100s mg/L
• Dissolved contaminants move with groundwater
• Basic hydrogeology
Separate phase solvent of fuel • Ultimate source of contamination
• Where the large bulk of the mass resides
• Separate-phase liquids do not move with groundwater
• Beyond basic hydrogeology
Contamination = exceedance of drinking water standards by dissolved benzene, TCE, etc. at part-per-billion concentrations
Most of the chemical mass resides here, slowly dissolving into water; chronic source
Conventional Environmental Forensics Approaches Address the Dissolved Phase
When was it released?
Where was it released?
Where did it go?
Darcy’s law is often key to informing groundwater plume forensics
xhKv
∆∆
=φ
Pore velocity
Hydraulic conductivity
Hydraulic head gradient
Porosity
xh
∆∆ can be estimated from a
groundwater elevation map
K can be estimated from pumping tests (or aquifer texture)
v is used to trace path of dissolved contaminants back to source over time
C0
−−
+×
−−
+×
+−
×
+−=
xZz
xZz
xYy
xYy
Rvt
vR
vtx
vRxC
tzyxC
zzyy
x
x
x
x
αααα
α
αλαλ
α
2erf
2erf
22/erf
22/erf
2
41
erfc4
112
exp8
),,,( 0
Y
v
Solvent or fuel leak from a pipeline
Well
C(x, y, z, t)
For simple questions, textbook equations can sometimes suffice
e.g., what is the expected concentration far downgradient of source?
Easy to implement
Spreadsheet model
Typical hydrogeologist’s toolbox
Conventional Environmental Forensics Approaches Address the Dissolved Phase
Fuels, Solvents, and Environmental Forensics: the Conventional Approach Fails When the Fluids Don’t Mix
“How long did fuel spill take to reach groundwater?”
Definitions: • LNAPL = light non-
aqueous phase liquid (gasoline, bunker fuel)
• DNAPL = dense non-aqueous phase liquid (PCE, TCE, manufactured gas plant residual)
When forensic questions require an understanding of NAPL behavior, special approaches are required
“Did the spilled solvent move to this location from elsewhere?”
LNAPL
DNAPL
Unsaturated Zone
Saturated Zone
Groundwater flow
Clay
Plume
Plume
Multiple fluids share the pore spaces Water
NAPL: solvent or fuel
± Gas/air (unsaturated zone)
Fluid movement impacted by relative permeability Each fluid has a separate,
saturation-dependent effective permeability
Equations developed from oil industry experience
Mineral grain
Water NAPL
Fuels, Solvents, and Environmental Forensics: Why do NAPLs Behave Differently in an Aquifer?
NAPL and water blobs become discontinuous and cannot move easily when both are present
These are petroleum engineering concepts
0.00.10.20.30.40.50.60.70.80.91.0
0.0 0.2 0.4 0.6 0.8 1.0
Rela
tive
Perm
eabi
lity
NAPL Saturation
Water
NAPL
Low saturation = low mobility
Fuels, Solvents, and Environmental Forensics: Additional Reasons for NAPL Behavior
Trichloroethylene + water Oil + water
Density differences Lighter than water (LNAPL)
Tends to reside above water table
Denser than water (DNAPL) Tends to sink through aquifer to
confining layer
Viscosity differences “Runny” liquids
Water
Fresh, neat TCE or PCE
“Gooey” liquids
Heavy fuel oil
Manufactured gas plant residue
Two liquids with different viscosities
Fuels, Solvents, and Environmental Forensics: Re-visiting the Conventional Approach
xhKv
∆∆
=φPore velocity now separate
pore velocities, one for each fluid
Hydraulic conductivity now a complex function of density, viscosity, and relative permeability, for each fluid
Hydraulic gradient replaced by pressure gradients, one for each fluid, that depend on fluid saturations
X Simpler models no longer suffice
More sophisticated numerical models that solve the relevant equations are warranted
X
Tools to Quantitatively Address NAPL Behavior in Soils and Groundwater Added physics requires solving large
numbers of complex equations Numerical models = solvers for large sets of
complex equations
Simplified numerical models “Bare-bones” numerical models that consider only
a portion of the relevant physics
Simpler to work with and justified for limited data
Complex numerical models Sophisticated, multi-physics numerical simulators
(e.g., UTCHEM, TOUGH, oil reservoir simulators, etc.)
Multi-phase flow
Dependence of density and viscosity on conditions
Temperature effects
Geochemistry, biodegradation, surfactants
Oil industry pedigree
Vetted in the scientific literature
What types of data can inform numerical models for NAPLs?
Parameter values for models Soil properties
Intrinsic permeability
Relative permeability curves
Can be measured or assumed (based on texture)
NAPL characteristics Density
Viscosity
Boundary conditions, initial conditions, release history Postulate these to match the data
Data to validate model output Remedial investigation data
Soil, soil vapor, and aqueous concentrations
Physical descriptions M
odel
set
up
Mod
el te
stin
g
The Decision to Apply a Model
When is quantitative analysis of NAPL behavior warranted?
How quickly did NAPL travel from Point A to Point B?
Is NAPL expected to move under (fill-in-the-blank) conditions?
Three example applications:
1. DNAPL poured into a water-filled sandbox (demo)
2. NAPL migration to a nearby extraction well
3. A litigation problem involving vertical transport of LNAPL
Questions
Example #1 – DNAPL in a Sandbox Experiment
Demo simulations with UTCHEM simulator Modified lab-scale example
test problem supplied with code
Converted LNAPL example to DNAPL
Refined grid
Added heterogeneity
Tested effects of different DNAPL fluid properties
DNAPL source (short-duration)
Three scenarios: 1. Baseline 2. Reduced density contrast 3. Reduced density contrast + high DNAPL viscosity
3 ft. 1
ft.
Highly permeable sand
Low-permeability sand
Example #1 – DNAPL in a Sandbox Experiment
Example #1 – DNAPL in a Sandbox Experiment
Example #1 – Extension to a More Complex System
Highly heterogeneous geologic material
Impact of DNAPL physical characteristics on vertical movement NAPL #1 = manufactured
gas plant (MGP) residual
NAPL #2 = tetrachloroethylene (PCE)
Approach can be used to estimate vertical migration rate for variety of similar situations
100 m
Example #2 – NAPL Migration Toward an Extraction Well
NAPL source area (saturation = 0.5)
Sand
Clay
• What if the problem definition is more complex?
• A numerical model provides needed the flexibility
Question: will NAPL be encountered in the extraction well?
C0
Y
v
Solvent or fuel leak from a pipeline
Well
C(x, y, z, t)
Simple textbook model can’t handle migrating NAPL
Numerical multiphase flow model explicitly handles NAPL migration
Ambient groundwater flow paths (ignores effect of NAPL)
Example #2 – NAPL Migration Toward an Extraction Well
Numerical multi-phase flow model
Intrinsic permeability and relative permeability parameters vary in space
Relative permeability for both NAPL and water change dynamically
NAPL viscosity = 3 x water viscosity
Model run represents approximately 14 years
t = +50 days NAPL saturation = ~0.07 – ~0.13
Some spreading evident, stemming from initial condition
Example #2 – NAPL Migration Toward an Extraction Well
t = +100 days
More spreading, but rate has already slowed
Example #2 – NAPL Migration Toward an Extraction Well
t = +1,000 days
Spreading rate has slowed further
Example #2 – NAPL Migration Toward an Extraction Well
t = +5,000 days NAPL saturation = ~0.04 – ~0.05
Mineral grain
Water NAPL
Little additional spreading between 2.7 and 13.7 years
Very low NAPL saturation discontinuous blobs essentially no mobility
Answer: Little NAPL advancement toward extraction well can be expected.
Example #2 – NAPL Migration Toward an Extraction Well
Question: will NAPL be encountered in the extraction well?
Example #3 – LNAPL Infiltration from Sudden and Accidental Release Events
Setting: LNAPL releases at a large industrial facility
Litigation problem: associating groundwater impacts with specific release events
Can LNAPL from these events migrate through the thick unsaturated zone?
No way to quantify, except by modeling A complex numerical model for NAPL
percolation from two separate releases
?
1. Release size constrained (reports) 2. Subsequent migration depth not
constrained.
Separate phase
Dissolved phase
Water table
Example #3 – LNAPL Infiltration from Sudden and Accidental Release Events
Numerical modeling used to explore percolation scenarios, but…
…there is a paucity of data:
Hydraulic conductivity
Relative permeability parameters
Ambient precipitation recharge
Modeling with ranges of reasonable parameter values is a good approach when data are lacking
The coexistence of three phases (NAPL + water + air) complicates percolation rate assessment
Example #3 – LNAPL Infiltration from Sudden and Accidental Release Events
0
10
20
30
40
50
60
70
80
90
100
0.00 0.05 0.10 0.15 0.20
z
LNAPL Saturation
Scenario #1
Scenario #2
Scenario #3
Run large number of trials (e.g., n = 1,000) with simplified model
Each scenario based on random selections of parameter values
Intrinsic and relative permeability soil boring logs and textbook values
Precipitation recharge local rainfall, consideration of ground surface
Evaluate parameter sensitivity; forecast groundwater impact likelihood
• Each scenario represents a unique set of posited parameters
• Simplified physics (SciLab script) to expedite large number of simulations
Example #3 – When do unknown parameters (hydraulic conductivity, rainfall percolation) matter?
05
10152025303540
0.000 0.010 0.020 0.030 0.040Dep
th o
f NA
PL
Fron
t
NAPL Residual Saturation
05
10152025303540
0.0E+00 1.0E-09 2.0E-09 3.0E-09 4.0E-09 5.0E-09Dep
th o
f NA
PL
Fron
t
Water Recharge Rate (m/sec)
05
10152025303540
0.0E+00 2.0E-13 4.0E-13 6.0E-13 8.0E-13 1.0E-12 1.2E-12Dep
th o
f NA
PL
Fron
t
Absolute Permeability (m2)
05
10152025303540
1.50 1.70 1.90 2.10 2.30 2.50Dep
th o
f NA
PL
Fron
t
Van Genuchten "n" Parameter
Sensitive model parameter
Less-sensitive model parameter
Relative permeability curve shape factor
Example #3 – What is the likelihood of NAPL reaching groundwater?
0
20
40
60
80
100
120
140
160
180
200
5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0 32.5 35.0 37.5 40.0
Num
ber
(sce
nari
os)
Depth of NAPL Front at +50 Years
Approximate depth to groundwater
~4% of scenarios
Example #3 – Do field data confirm model predictions?
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
0% 5% 10% 15% 20%
z
LNAPL Saturation
0%
5%
10%
15%
20%
25%
30%
5.5 ft 10.5 ft
Abu
ndan
ce
Sample
TPH_C07
TPH_C08
TPH_C09-C10
TPH_C11-C12
TPH_C13-C14
TPH_C15-C16
TPH_C17-C18
TPH_C19-C20
TPH_C21-C22
TPH_C23-C24
0%5%
10%15%20%25%30%35%
60.5 ft 70.5 ft
Abu
ndan
ce
Sample
TPH_C07
TPH_C08
TPH_C09-C10
TPH_C11-C12
TPH_C13-C14
TPH_C15-C16
TPH_C17-C18
TPH_C19-C20
TPH_C21-C22
TPH_C23-C24
Fingerprinting analysis indicates pulses from separate releases have advanced and then stopped at multiple locations, consistent with model
Summary
Both LNAPLs and DNAPLs pose special challenges for environmental forensics
Large, residual source of contamination
Complex subsurface physics answers are not always obvious
Numerical models can provide useful insights into behavior of NAPLs that cannot be obtained otherwise
Pose the questions to be addressed by the model in a manner that matches the quantity/quality of available data