Dale T Littlejohn Senior 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

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 = ft/ft Precipitation = 10.5 in/yr Source Area = 2,000 ft 2 3-Foot Sample: TPH = 23,510 mg/kg 11-Foot Sample: TPH = 20,060 mg/kg Benzene = mg/kg (also TPH Fractions) 15-Foot Sample: TPH = 6,270 mg/kg Benzene = mg/kg Distance = 50 feet Soil parameters: Lithology = Sand Bulk Density = 1.7 g/cm 3 Eff. Porosity = 0.38 Water Content = 0.12 Fraction Organic = 0.01

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

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 Leachate Volume Groundwater Volume Leachate Concentration x Groundwater Concentration =

Comparing “leachate” concentration to “groundwater” concentration A - is the source area (ft 2 ) 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) T aq – is the mixing zone thickness (ft) W – is the width of the source area parallel to the gradient (ft) Dilution Equation

Target Concentration

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