E.F.A. I
Transport of Chlorinated Solvents Through Soil
Chlorinated solvents can enter the subsurface as a dissolved phase, a non-aqueous phase liquid (NAPL), or a
vapor. Non-aqueous phase liquids can exist as a separate phase within the subsurface and as a dissolved and/or
vapor phase plume originating from the NAPL. Care must be exercised in extrapolating pure phase properties,
such as fluid density, volatility, and viscosity, to a release that is a solvent mixture. As chlorinated solvents
travel through soil, they are distributed in the unsaturated zone and capillary fringe as residual saturation. Dense
non-aqueous phase liquid residual saturation values range from 2 to 40% of the soil pore space and are
generally higher in poorly sorted soils. The residual saturation of PCE in sand, for example, was reported to be
27%.
A vapor phase crown exists for some distance and concentration around this residual soil contamination. In the
case of a solvent dissolved in water that is released at the ground surface, the dissolved phase spreads vertically
and horizontally as a function of the soil suction gradient and gravity. This trend continues until the suction
gradient in the upper portion of the contaminant plume becomes negligible. Gravity eventually constitutes the
only remaining force to move the liquid vertically. If the liquid does not reach the capillary fringe, the fluid
become immobilized as residual saturation around the soil grains until it is remobilized by infiltrating water or a
fluctuating water table.
The movement of a free phase liquid through soil is controlled primarily by capillarity due to the interfacial
tensions present between different fluids (LNAPL/DNAPL, LNAPL/water, DNAPL/air, etc.) and the size of the
soil pore opening. The moisture content of the soil also influences the ability of the free phase liquid to move
into the open pore space or displace the water occupying this space. In most cases, water coats the soil grains,
thereby restricting the movement of the chlorinated solvent into the larger pore openings.
The ability of a chlorinated solvent to enter the smaller openings between the soil grains is a function of the
interfacial tensions between the chlorinated solvent and other liquids (air, water, another DNAPL/LNAPL). The
energy or pressure required for entry into these smaller openings is known as the entry pressure, which is
defined as the capillary pressure at which the free phase solvent becomes continuous at the macroscopic scale
and is capable of flowing through the material. For most soils, the entry pressure corresponds to water
saturation ranges from about 0.8 to 0.95.
The entry pressure is proportional to the interfacial tension between the free phase liquid and water and is
inversely proportional to the opening between the soil grains.
The entry of a free phase liquid into smaller openings between soil grains is dependent on the pore opening and
interfacial tension(s) between the liquids. Free phase liquids tend to become immobilized or trapped within finer
grained materials. Conversely, liquids introduced into the subsurface that reduce the interfacial tensions
between the liquids (e.g., surfactants) enhance the movement of the free phase liquid into these smaller
apertures. The primary forces driving free phase liquids through the vadose zone include;
Duration and volume of the release
Density and viscosity of the fluid
Pressures driving the liquid
Intrinsic permeability of the soil
Degree of free product saturation of the soil
Presence of preferential pathways such as fractured rock or slickensides found in silt or clayey soils
1 Transport of Chlorinated Solvents| Environmental Forensic Analysis I
Transport of Chlorinated Solvents Through Soil
Chlorinated solvents can enter the subsurface as a dissolved phase, a non-aqueous phase liquid (NAPL), or a
vapor. Non-aqueous phase liquids can exist as a separate phase within the subsurface and as a dissolved and/or
vapor phase plume originating from the NAPL. Care must be exercised in extrapolating pure phase properties,
such as fluid density, volatility, and viscosity, to a release that is a solvent mixture. As chlorinated solvents
travel through soil, they are distributed in the unsaturated zone and capillary fringe as residual saturation. Dense
non-aqueous phase liquid residual saturation values range from 2 to 40% of the soil pore space and are
generally higher in poorly sorted soils. The residual saturation of PCE in sand, for example, was reported to be
27%.
A vapor phase crown exists for some distance and concentration around this residual soil contamination. In the
case of a solvent dissolved in water that is released at the ground surface, the dissolved phase spreads vertically
and horizontally as a function of the soil suction gradient and gravity. This trend continues until the suction
gradient in the upper portion of the contaminant plume becomes negligible. Gravity eventually constitutes the
only remaining force to move the liquid vertically. If the liquid does not reach the capillary fringe, the fluid
become immobilized as residual saturation around the soil grains until it is remobilized by infiltrating water or a
fluctuating water table.
The movement of a free phase liquid through soil is controlled primarily by capillarity due to the interfacial
tensions present between different fluids (LNAPL/DNAPL, LNAPL/water, DNAPL/air, etc.) and the size of the
soil pore opening. The moisture content of the soil also influences the ability of the free phase liquid to move
into the open pore space or displace the water occupying this space. In most cases, water coats the soil grains,
thereby restricting the movement of the chlorinated solvent into the larger pore openings.
The ability of a chlorinated solvent to enter the smaller openings between the soil grains is a function of the
interfacial tensions between the chlorinated solvent and other liquids (air, water, another DNAPL/LNAPL). The
energy or pressure required for entry into these smaller openings is known as the entry pressure, which is
defined as the capillary pressure at which the free phase solvent becomes continuous at the macroscopic scale
and is capable of flowing through the material. For most soils, the entry pressure corresponds to water
saturation ranges from about 0.8 to 0.95.
The entry pressure is proportional to the interfacial tension between the free phase liquid and water and is
inversely proportional to the opening between the soil grains.
The entry of a free phase liquid into smaller openings between soil grains is dependent on the pore opening and
interfacial tension(s) between the liquids. Free phase liquids tend to become immobilized or trapped within finer
grained materials. Conversely, liquids introduced into the subsurface that reduce the interfacial tensions
between the liquids (e.g., surfactants) enhance the movement of the free phase liquid into these smaller
apertures. The primary forces driving free phase liquids through the vadose zone include;
Duration and volume of the release
Density and viscosity of the fluid
Pressures driving the liquid
Intrinsic permeability of the soil
Degree of free product saturation of the soil
Presence of preferential pathways such as fractured rock or slickensides found in silt or clayey soils
1 Transport of Chlorinated Solvents| Environmental Forensic Analysis I
