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Dale T LittlejohnSenior Geologist
What is fate and transport in the vadose zone?
Vadose Zone
Hydrocarbon release from
buried pipeline
Aquifer
Surface
Initial plume size and shape is determined by
the release volume, gravity, and soil
permeability
What is fate and transport in the vadose zone?
Vadose Zone
Fresh water from precipitation comes into
contact with the hydrocarbons in the soil and
forms “leachate”, defined as meteoric water containing soluble
substances
Aquifer
Surface
Precipitation
What is fate and transport in the vadose zone?
Vadose Zone
As meteoric water moves through the hydrocarbon-
impacted soil, the most soluble components are transported downward
while diluting the overall plume concentration
Aquifer
Surface
Precipitation
What is fate and transport in the vadose zone?
Vadose Zone
Groundwater impact is determined by the volume (rate) of leachate relative to the volume (rate) of available groundwater at the source area
Surface
Precipitation
Eventually the leachate may come into contact
with the underlying groundwater.
When is it desirable to model the transport of hydrocarbons in the
vadose zone?
Soil impact exceeds regulatory levels and is too
costly to remediate
Groundwater is not impacted or impact is unknown
• Impacted soil too deep to excavate
• Surface structures prevent excavation access
• In Situ remediation not cost effective
• Hydrocarbon composition has low transport potential
Petroleum Hydrocarbon Solubilities(Log Scale)
0.000001
0.00001
0.0001
0.001
0.01
0.1
1
10
100
1000
10000B
en
zen
e
Aro
ma
tic 5
-7
Aro
ma
tic 7
-8
Aro
ma
tic 8
-10
Aro
ma
tic 1
0-1
2
Aro
ma
tic 1
2-1
6
Aro
ma
tic 1
6-2
1
Aro
ma
tic 2
1-3
5
Alip
ha
tics
5-6
Alip
ha
tics
6-8
Alip
ha
tics
8-1
0
Alip
ha
tics
10
-12
Alip
ha
tics
12
-16
Alip
ha
tics
16
-21
Hydrocarbon Fractions
Aq
ue
ou
s S
olu
bili
ty (
mg
/L)
Petroleum Hydrocarbon Solubilities(Linear Scale)
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Be
nze
ne
Aro
ma
tic 5
-7
Aro
ma
tic 7
-8
Aro
ma
tic 8
-10
Aro
ma
tic 1
0-1
2
Aro
ma
tic 1
2-1
6
Aro
ma
tic 1
6-2
1
Aro
ma
tic 2
1-3
5
Alip
ha
tics
5-6
Alip
ha
tics
6-8
Alip
ha
tics
8-1
0
Alip
ha
tics
10
-12
Alip
ha
tics
12
-16
Alip
ha
tics
16
-21
Hydrocarbon Fractions
Aq
ue
ou
s S
olu
bili
ty (
mg
/L)
Solubility < RBSL
When is it desirable to model the transport of hydrocarbons in the
vadose zone?
To determine the benefit (to the groundwater) from near surface source removal or the placement
of a leach barrier
What information is required to model the transport of
hydrocarbons in the vadose zone?
Maximum hydrocarbon concentration of plume (TPH
Fractions) and knowledge of the contaminant chemical parameters
relative to transport including:
HARD DATA
Hydrocarbon concentration of soil
below the plume
Approximate groundwater depth
Length and width of impact area
• Water Solubility
• Organic Carbon Partition Coefficient
• Henrys Law Constant
• Free Air Diffusion Coefficient
What information is required to model the transport of
hydrocarbons in the vadose zone?
General understanding of average vadose zone soil type (below the plume) and a conservative estimate of the fate and transport parameters, including:
SOFT DATA
Estimated (conservative) groundwater flow parameters, including:
Recharge rate(conservative estimate)
• Bulk Density
• Effective Porosity
• Water Content
• Fraction of Organic Content
• Hydraulic Conductivity
• Groundwater Gradient
Width of source area parallel to the gradient direction
What are the most important results available from the vadose zone fate
and transport modeling?
The VLEACH model, used in this example, provides the following output information:
The VLEACH model does not predict current or future groundwater concentrations
• Concentration (or mass) of hydrocarbons in the GAS phase for each selected depth and time unit
• Concentration (or mass) of hydrocarbons in the SOIL (sorbed) for each selected depth and time unit
• Concentration (or mass) of hydrocarbons in the LEACHATE for each selected depth and time unit
• Total MASS in the vadose zone for each selected depth and time unit
Example Site
Groundwater parameters: Depth = 45 ft k = 33 ft/day i = 0.005 ft/ft
Precipitation = 10.5 in/yr
Source Area = 2,000 ft2
3-Foot Sample: TPH = 23,510 mg/kg
11-Foot Sample: TPH = 20,060 mg/kg Benzene = 0.212 mg/kg (also TPH Fractions)
15-Foot Sample: TPH = 6,270 mg/kg Benzene = 0.044 mg/kg
Distance = 50 feet
Soil parameters: Lithology = Sand Bulk Density = 1.7 g/cm3
Eff. Porosity = 0.38 Water Content = 0.12
Fraction Organic = 0.01
TPH Fraction of Test Case Soil Sample(11 feet below the surface)
0
100
200
300
400
500
600
700
800
900B
en
zen
e
Aro
ma
tic 7
-8
Aro
ma
tic 8
-10
Aro
ma
tic 1
0-1
2
Aro
ma
tic 1
2-1
6
Aro
ma
tic 1
6-2
1
Aro
ma
tic 2
1-3
5
Alip
ha
tics
6-8
Alip
ha
tics
8-1
0
Alip
ha
tics
10
-12
Alip
ha
tics
12
-16
Alip
ha
tics
16
-21
Alip
ha
tics
21
-35
Hydrocarbon Fractions
Co
nc
en
tra
tio
n (
mg
/kg
)
Solubility < RBSL
Case Settings
Soil Concentrations
Chemical Parameters
Soil Parameters
Benzene in leachate at groundwater depth
Aromatic EC>7-8 in leachate at groundwater depth
Leachate Concentration at Groundwater Depth
0.0
1.0
2.0
3.0
4.0
5.0
0 50 100 150 200
Time (years)
Co
nc
en
tra
tio
n (
mg
/L)
Benzene
Aromatic EC>7-8
Aromatic EC>8-10
Benzene in soil (no excavation)
Aromatic EC>7-8 in soil (no excavation)
What would the soil concentration profile look like if the top 12 feet were removed?
Limited source area removal
Benzene in soil (excavated)
Comparing “leachate” concentration to
“groundwater” concentration
Soil and groundwater concentrations
(measurable and regulated)
Leachate concentration(not measurable and
not regulated) must be predicted by modeling
LeachateVolume
GroundwaterVolume
LeachateConcentration
x
GroundwaterConcentration =
Comparing “leachate” concentration to
“groundwater” concentration
A - is the source area (ft2)R - is the source recharge (ft/yr)k – is the hydraulic conductivity (ft/day)i – is the groundwater gradient (ft/ft)Θ – is the effective porosity (unitless)Taq – is the mixing zone thickness (ft)W – is the width of the source area parallel to the gradient (ft)
LmgLEACHATE
yeardayWTik
RALmgGW Conc
aqT
Conc ..
365
)/(
Dilution Equation
Predicted Benzene ConcentrationsLeachate vs Groundwater
0.000
0.010
0.020
0.030
0.040
0 50 100 150 200
Time (years)
Co
nc
en
tra
tio
n (
mg
/L)
Leachate Concentrations
Groundwater Concentration
Target Concentration
Predicted Aromatic EC>7-8 ConcentrationsLeachate vs Groundwater
0
5
10
15
20
25
0 50 100 150 200
Time (years)
Co
nc
en
tra
tio
n (
mg
/L)
Leachate Concentrations
Groundwater Concentration
Target Concentration
Predicted Benzene Concentrations in the GroundwaterBefore and After Excavation to 12-foot Depth
0.0000
0.0001
0.0002
0.0003
0.0004
0.0005
0.0006
0.0007
0.0008
0.0009
0.0010
0.0011
0.0012
0.0013
0.0014
0.0015
0 50 100 150 200
Time (years)
Co
nc
en
tra
tio
n (
mg
/L)
No Excavation
Excavation to 12 Feet
Thank You, for your attentionThis presentation can be found at:
www.trident-environmental.com
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