Dale T Littlejohn Senior Geologist

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Dale T LittlejohnSenior Geologist
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What is fate and transport in the vadose zone?
Surface
Hydrocarbon release from buried pipeline
Vadose Zone
Initial plume size and shape is determined by the
release volume, gravity, and soil permeability
Aquifer
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What is fate and transport in the vadose zone?
Precipitation
Surface
Fresh water from precipitation comes into contact
with the hydrocarbons in the soil and forms
leachate, defined as meteoric water containing
soluble substances
Vadose Zone
Aquifer
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What is fate and transport in the vadose zone?
Precipitation
Surface
As meteoric water moves through the
hydrocarbon-impacted soil, the most soluble
components are transported downward while
diluting the overall plume concentration
Vadose Zone
Aquifer
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What is fate and transport in the vadose zone?
Precipitation
Surface
Eventually the leachate may come into contact
with the underlying groundwater.
Vadose Zone
Groundwater impact is determined by the volume
(rate) of leachate relative to the volume (rate)
of available groundwater at the source area
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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

• Impacted soil too deep to excavate
• Surface structures prevent excavation access
• In Situ remediation not cost effective
• Hydrocarbon composition has low transport
  potential

Groundwater is not impacted or impact is unknown
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Solubility lt RBSL
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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
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What information is required to model the
transport of hydrocarbons in the vadose zone?
HARD DATA
Length and width of impact area
Maximum hydrocarbon concentration of plume (TPH
Fractions) and knowledge of the contaminant
chemical parameters relative to transport
including
Hydrocarbon concentration of soil below the plume

• Water Solubility
• Organic Carbon Partition Coefficient
• Henrys Law Constant
• Free Air Diffusion Coefficient

Approximate groundwater depth
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What information is required to model the
transport of hydrocarbons in the vadose zone?
SOFT DATA
Recharge rate(conservative estimate)
General understanding of average vadose zone soil
type (below the plume) and a conservative
estimate of the fate and transport parameters,
including

• Bulk Density
• Effective Porosity
• Water Content
• Fraction of Organic Content

Width of source area parallel to the gradient
direction
Estimated (conservative) groundwater flow
parameters, including

• Hydraulic Conductivity
• Groundwater Gradient

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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

• 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

The VLEACH model does not predict current or
future groundwater concentrations
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Example Site
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
Soil parameters Lithology Sand
Bulk Density 1.7 g/cm3 Eff.
Porosity 0.38
Water Content 0.12 Fraction
Organic 0.01
Distance 50 feet
Groundwater parameters Depth 45 ft
k 33 ft/day
i 0.005 ft/ft
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Solubility lt RBSL
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Case Settings
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Soil Concentrations
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Chemical Parameters
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Soil Parameters
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Benzene in leachate at groundwater depth
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Aromatic ECgt7-8 in leachate at groundwater depth
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Benzene in soil (no excavation)
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Aromatic ECgt7-8 in soil (no excavation)
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What would the soil concentration profile look
like if the top 12 feet were removed?
Limited source area removal
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Benzene in soil (excavated)
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Comparing leachate concentration to
groundwater concentration
Leachate concentration(not measurable and not
regulated) must be predicted by modeling
GroundwaterConcentration

Soil and groundwater concentrations(measurable
and regulated)
LeachateVolumeGroundwaterVolume
LeachateConcentration
x
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Comparing leachate concentration to
groundwater concentration
Dilution Equation
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)T 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)
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Target Concentration
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Target Concentration
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Thank You, for your attention
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