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)

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Hydrocarbon Fractions

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Petroleum Hydrocarbon Solubilities(Linear Scale)

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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)

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Hydrocarbon Fractions

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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

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Time (years)

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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)

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Leachate Concentrations

Groundwater Concentration

Target Concentration

Predicted Aromatic EC>7-8 ConcentrationsLeachate vs Groundwater

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Time (years)

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Leachate Concentrations

Groundwater Concentration

Target Concentration

Predicted Benzene Concentrations in the GroundwaterBefore and After Excavation to 12-foot Depth

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0.0007

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0.0014

0.0015

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Time (years)

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No Excavation

Excavation to 12 Feet

Thank You, for your attentionThis presentation can be found at:

www.trident-environmental.com