Info

Subatmospheric \ pressure ^

1 Super atmospheric ^ pressure

Negative pressure potential

Positive

Positive pressure potential

FIGURE 18.2 Superatmospheric and subatmospheric pressures below and above a free-water surface.

cryo-scanning electron microscopy, and nuclear magnetic resonance. For more information, see references 26 to 28.

Wettability measurements show that most soil constituents are water wettable or hydrophilic,28 although calcium carbonates [calcite, CaCO3, and dolomite, CaMg(CO3)2] are slightly hydrophobic; for example, the contact angle of water and heptane is 100 to 105°. Therefore, carbonaceous reservoirs are usually oil-wet.

In practice, evaluating the wettability of a soil is far more uncertain, because soil is a mixture of gravel, sand, silt, and clay particles, as well as other chemical precipitates. The mineral components of soil particles include quartz, feldspar, carbonates, and clay. These components have different wettability by water and oil. Therefore, the retention of oil or water in a soil matrix is heterogeneous and variable. The general wettability of soil or liquid retention in a soil is reported on a statistical basis.

The relative wettability of soil by oil and water determines the relative affinity of soil to oil and water, which in turn determines the level of retention of oil or water in the soil. A soil is hydrophilic (i.e., it has water affinity) if water has more affinity to the soil than the oil, although soil may also to a certain degree be somewhat wetted by oil. A soil is called hydrophobic if oil has more affinity to the soil than water.

When water-affinity soil that is originally saturated with oil is rinsed with water, most of the oil will be displaced by water; however, some oil will remain. This remaining amount of oil that can no longer be displaced by the flowing water is defined by the petroleum industry as the "residual oil saturation."28 This term is used to measure the upper limit of the microscopic efficiency of the displacement of oil by water. There is no appreciable lower limit, as at a given pore level the wetting fluid should be spread over the mineral surface as a continuous wetting film, which might thin itself until the externally applied capillary pressure (or pressure difference across the wetting fluid/ nonwetting fluid interface) is balanced by the thin film disjoining pressure.

If an open space of a water-affinity porous medium filled with oil is brought into contact with a reservoir of water, the oil will be spontaneously displaced by the water. Conceivably, a symmetric behavior can be observed if an oil-affinity medium at residual oil saturation is brought into contact with oil. The driving force for this spontaneous flow is the high initial capillary pressure (which equals the pressure in the nonwetting fluid minus the pressure in the wetting fluid) inside the medium compared to its value of zero outside, where the oil-water interface has no curvature at equilibrium. Capillary pressures, like relative permeabilities, are a function of saturations, the geometric properties of the porous medium, the fluid-fluid interfacial tension, and the wettability.28 It is generally observed that if all other parameters of a system are maintained unchanged, but the wetting properties of the solid are changed, the nonwetting fluid will be displaced more easily by the wetting fluid than vice versa.

Surfactants are used to rinse oil from soil more effectively. Surfactants have higher soil affinity, and so reduce the interfacial tension between the oil and the soil. The replacement of the oil film by a surfactant solution is dependent on the contact angle between the oil-solution interface and the soil. As long as the contact angle (in the solution) is acute, the solution will tend to advance, displacing oil from the soil. Mechanical agitation would assist the spreading by compensating to some extent for the immobility of the molecules on the surface of the soil. Processes for the removal of surfactants in a contaminated liquid are described in works by Wang and colleagues29 and Hrycyk and colleagues.30

18.4.5 Adsorptive Force Affecting Underground Liquid Movement

Adsorption results from bonding forces between the solute and soil particles. These forces are generally electrostatic, although entropy generation and magnetic forces can be involved. Bonding forces range from relatively weak to strong with respect to bond formation.31

Adsorption can be attributed to the following interactions: van der Waals-London interactions, charge transfer/hydrogen bonding, ligand exchanges, ion exchange, direct and induced ion-dipole and dipole-dipole forces, chemisorption, hydrophobic bonding, and magnetic bonding. These interactions are the result of electrostatic magnetic or entropy-generating forces. Most often, physical adsorption is due to the electrostatic interactions between atoms, ions, and molecules resulting from instantaneous dipoles. Van der Waals force, charge transfer/hydrogen bonding, ligand exchange, ion exchange, direct and induced ion-dipole and dipole-dipole force, and chemisorption interactions are all the result of electrostatic forces, variation in energy level, and method of interaction. Hydrophobic bonding is due to entropy-generating forces, and magnetic bonding is due to magnetic forces.31

Van der Waals-London interactions are due to fluctuations in electron distribution as the electrons circulate within their orbits. These instantaneous dipoles are usually weak, but are, regardless, the most common interaction resulting in adsorption.31 Stronger interactions result from charge transfer.

18.4.6 Combination of Capillary and Adsorptive Forces Affecting Underground Liquid Movement

Both adsorptive and capillary forces play an important part in soil-liquid interaction (see Figure 18.3). This is very important for unsaturated soil. The total force (i.e., the sum of capillary force and adsorptive force) is termed the matrix potential, which has a negative gage pressure relative to the external gas pressure on the soil water (more often the gage pressure is referred to as the atmospheric pressure).

In fact, an unsaturated soil has no pressure potential, only a matrix potential (expressible as a negative pressure). The negative pressure causes water to move toward the soil with a higher suction potential, in contrast to the saturated flow where water moves from a high pressure potential to a low pressure potential. For soils with the same properties but with different saturation, the less saturated soil has more excessive suction force, causing water to move towards it.

The presence of water in films as well as under concave menisci is most important in clayey soil and at high suctions, because clay minerals have high specific surface area and often have a

Adsorbed water

Adsorbed water

Particles

FIGURE 18.3 Water in an unsaturated soil is subject to capillarity and adsorption, which combine to produce a matric suction.

Particles

FIGURE 18.3 Water in an unsaturated soil is subject to capillarity and adsorption, which combine to produce a matric suction.

high cation exchange capacity. In sandy soil, adsorption is relatively unimportant and the capillary effect predominates. A combination of capillary effects and adsorption results in negative pressure potential.

18.4.6.1 Viscosity and Shearing Stress

Viscosity is the property of a fluid that offers resistance to the relative motion of fluid molecules. The energy loss due to friction in a flowing fluid is due to its viscosity.

As a fluid moves, shearing stress develops in it. The magnitude of the shearing stress depends on the viscosity of the fluid. Shearing stress can be defined as the force required to slide one unit area layer of a substance over another. Considering a fluid moving along a fixed surface, the velocity is highest along the moving surface, and zero at the fixed surface. The shearing stress in a fluid is directly proportional to the velocity gradient:

where Ss = shearing stress (M/LT2), y = distance (L) between the moving and fixed surfaces, Vs = the velocity along the surfaces (L/T), dVs/dy = velocity gradient across the surfaces, and | = proportionality constant = dynamic viscosity (or absolute viscosity) of the fluid (M/LT).

Equation 18.1 is known as Newton's Law of Friction. In the SI system, the dynamic viscosity units are N-s/m2, Pa-s, or kg/m-s. Here 1 Pa-s = 1 N-s/m2 = 1 kg/m-s. The dynamic viscosity (or absolute viscosity) is also often expressed in the metric CGS (centimeter-gram-second) system as g/cm-s, dyne-s/cm2, or poise (P) where 1 poise = 1 P = 1 dyne-s/cm2 = 1 g/cm-s = 0.1 Pa-s = 100 centipoises = 100 cP. Water at 20.2°C (68.4°F) has a dynamic viscosity of 1 cP.

Kinematic viscosity is the ratio of dynamic viscosity and density, and can be obtained by dividing the dynamic viscosity of a fluid with its mass density, as shown by Equation 18.2:

where v = kinematic viscosity (L2/T), | = dynamic viscosity (M/LT), and p = density (M/L3 ).

In the SI system, the theoretical unit of v is m2/s or the commonly used Stoke (St) where 1 St = 0.0001 m2/s = 100 cSt = 100 centiStoke. Similarly, 1 centiStoke = 1 cSt = 0.000001 m2/s = 0.01 Stoke = 0.01 st. The specific gravity of water at 20.2°C (68.4°F) is almost 1. The kinematic viscosity of water at 20.2°C (68.4°F) is for all practical purposes equal to 1 cSt. For a liquid, the kinematic viscosity will decrease with higher temperature. For a gas, the kinematic viscosity will increase with higher temperature. Another commonly used kinematic viscosity unit is Saybolt universal seconds (SUS), which is the efflux time required for 60 mL of petroleum product to flow through the calibrated orifice of a Saybolt universal viscometer, as described by ASTM-D88. Therefore, the relationship between dynamic viscosity and kinematic viscosity can be expressed as v = 4.63 |/Sg (18.3)

where v = kinematic viscosity (SUS), | = dynamic or absolute viscosity (cP), and Sg = specific gravity (dimensionless).

The viscosities of water and gasoline increase with decreasing temperature. Gasoline has lower viscosity than water, and fuel and crude oil have a much higher viscosity that increases dramatically when temperature decreases.32 The ease with which a fluid pours is an indication of its viscosity. It is observed that cold oil has a high viscosity and pours very slowly. The viscosity properties of various potential pollutants are discussed in Section 18.9.

18.4.6.2 Electrokinetic Effects

Flow movement also has a relationship with the electrokinetic phenomenon, which can promote or retard the motion of the fluid constituents. Electrokinetic effects can be described as when an electrical double layer exists at an interface between a mobile phase and a stationary phase. A relative movement of the two phases can be induced by applying an electric field and, conversely, an induced relative movement of the two will give rise to a measurable potential difference.33

Four phenomena are classified as electrokinetic effects:

1. Streaming potential

2. Electroosmosis

3. Sedimentation potential

4. Electrophoresis

The streaming potential (Dorn effect) relates to a movement of liquid that generates electric potential, and electroosmosis occurs when a direct electric potential causes movement of the liquid. The sedimentation potential relates to sedimentation (directed movement) of charged particles that generates electric potential, and electrophoresis occurs when a direct electric potential causes a movement of charged particles.

With regard to the movement of liquid versus particles under direct current, electrophoresis is the reverse of the effect of electroosmosis.33 If particles move through a liquid that is stationary, this is called electrophoresis; conversely, if the liquid moves through particles that are stationary, that is called electroosmosis.

The potential governing these electrokinetic effects is clearly at the boundary (the face of shear) between the stationary phase (the fixed double layer) and the moving phase (the solution). This potential is called the electrokinetic potential or the zeta potential. An electrokinetic phenomenon in soil involves coupling between electrical, chemical, and hydraulic gradients.

Initially the electrical potential difference is distributed linearly across the specimen. The changing chemistry across the cell may result in variations in electrical potential difference in time and space.

It is suggested that the movement of the front by migration (electrical potential), diffusion (chemical potentials), and advection (hydraulic potentials) will cause desorption of cations and other species from clay surfaces and facilitate their release into the fluid.34

The relationship of electrokinetic phenomena and the movement of petroleum constituents is not of high importance; however, it can be important for the transport of some solutes related to a remedial technology such as electroosmosis remediation.

18.4.7 Energy Conservation Affecting Underground Liquid Movement

Equation 18.4 describes the energy conservation when water moves between two points (1 and 2)

where P1 = pressure at point 1 (M/L2), P2 = pressure at point 2 (M/L2), Pj/y = pressure head at point 1 (L), P2/y = pressure head at point 2 (L), y = density of liquid (M/L3), Z1 = elevation of point 1 (L), Z2 = elevation of point 2 (L), v2/2g = velocity head at point 1 (L), v2/2g = velocity head at point 2 (L), g = gravitational acceleration (L/T2), hA = the head added to the fluid with a mechanical device such as a pump (L), hR = the head removed from the fluid with a mechanical device such as fluid motor (L), and hL = the head losses from the system due to friction (L). Equation 18.4 reduces to the familiar Bernoulli's equation when there is no pump (hA = 0), no motor (hR = 0), and where the head loss (hL) between the two points is negligible.

18.4.8 Water Movement in Saturated Zone of Soil Formation

Hydraulic conductivity is one of the characteristic properties of a soil relating to water flow. The movement of water in soil depends on the soil structure, in particular its porosity and pore size distribution. A soil containing more void space usually has a higher permeability. Most consolidated bedrocks are low in permeability. However, rock fractures could create a path for water movement.

Groundwater flowing through an aquifer is influenced by gravitational force, but the rate at which the groundwater moves can vary significantly. Depending on the permeability of an aquifer and the flow gradient, groundwater can move at a velocity varying from only a few meters per year to several meters per day.

The most important factor for movement in the saturated zone is the hydraulic gradient. The velocity head, which is generally more than ten orders of magnitude smaller than the pressure and gravitational head, may be neglected because of the slow water movement. Equation 18.4 can therefore be simplified to

The relative importance of pressure and gravitational heads depends on whether the water formation is in a free water table condition or in a confined aquifer condition.

18.4.8.1 Water Table Condition

When considering two points on a water table 1 and 2, p can be regarded as equal to P2, because the external pressure is the same as the atmospheric pressure. If there is neither addition nor loss of head by mechanical devices (i.e., hA = 0 and hR = 0), then Equation 18.5 reduces to

The cause of flow between these points is the difference in elevation head between them, that is, (Zj - Z2), denoted as dh, which is contributed by the gravitational potential. If dl is the distance between the two points on the water table, then the dh/dl ratio is known as the hydraulic gradient.

Comparing the results from the above paragraphs, it is apparent that the elevation difference dh causes the flow between the two points and the energy is lost by friction (i.e., hL ) during the movement.

18.4.8.2 Confined Aquifer

For the confined aquifer, the pressure head becomes more important than the elevation head. As can usually be seen in an artesian aquifer condition, the groundwater may flow from a lower elevation to a higher elevation if the water pressure at the lower elevation is higher.

18.4.9 Water Movement in Unsaturated Zone of Soil Formation

In an unsaturated zone, the capillary force becomes predominant, and the pressure gradient becomes a suction gradient. Hydraulic conductivity is no longer constant, but is a function of the water content or suction, which is greatest in value when the soil is saturated and decreases in value steeply when the soil water suction increases and the soil loses moisture.

If we consider the water transport between two points, water movement would increase when suction increases and moisture decreases. That is, water tends to move from higher moisture to lower moisture points, because the point with lower moisture has a higher suction force.

Both vapor and liquid movement can be important where appreciable temperature gradients occur.

18.5 PROPERTIES OF GASOLINE AND ITS MOVEMENT IN SOIL 18.5.1 Properties of Gasoline and the Forms of Release Underground

Gasoline is a mixture of different compounds. A typical blend contains nearly 200 different hydrocarbons and additives such as antioxidants and antiknock agents. Thirteen of the chemicals commonly found in gasoline (nine hydrocarbons and four additives) are regulated as hazardous substances under CERCLA. Table 18.1 lists the chemicals along with the values of toxicity, water solubility, vapor pressure, and biodegradability.19

In general, there are four major forms of released gasoline underground:

1. Free product

2. Solutes dissolved in groundwater

3. Gases in the vapor phase in the soil void

4. Adsorbates adsorbed by the soil matrix table 18.1

Physical and Chemical Properties of Toxic Gasoline Components

Mass Prevalence Fate and Transport Toxicity

%

Water

Gasolines

Solubility

Vapor

% Volume

% Weight

Containing

at 20°C

Pressure

Degree of

Final RQ

Compound

in Gasoline

in Gasoline

Chemical

(mg/L)

(torr)a

Biogradability

(kg)b

Benzene

1-2

0.81

>99

1,780

75.0

Some

4.54

Toluene

4.0

12.02

>99

515

22.0

Some

454.00

Xylene-M

5-8

3.83

>99

175

5.0

Some

454.00

Xylene-O

5-8

1.93

>99

162

6.0

Some

454.00

Xylene-P

5-8

1.58

>99

198

6.5

Some

454.00

Ethylbenzene

2-5

1.70

>99

152

7.0

Some

454.00

Naphthalene

0.7

0.10

>90

31.1

1.0

Readily

45.40

Phenol

>90

66,667

0.5

Readily

454.00

EDB

0.01

0.024

<40

4,310

11.0

Some

4.54

EDC

0.01

<0.024

<40

8,690

61.0

Some

45.40

Tetraethyl lead

<40

0.08

0.2

Some

4.54

Dimethylamine

1,345.0

Readily

454.00

Cyclohexane

<0.7

0.17

66.5

77.0

Some

454.00

Source: U.S. EPA, Cleanups of Releases from Petroleum USTs: Selected Technologies, EPA/530/UST-88/00l, U.S. EPA, Washington, 1988.

EBD, ethylene dibromide; EDC, ethylene dichloride; RQ, reportable quantity. a At 20°C.

b The lower the RQ value, the more toxic the chemical is in pure product form.

Source: U.S. EPA, Cleanups of Releases from Petroleum USTs: Selected Technologies, EPA/530/UST-88/00l, U.S. EPA, Washington, 1988.

EBD, ethylene dibromide; EDC, ethylene dichloride; RQ, reportable quantity. a At 20°C.

b The lower the RQ value, the more toxic the chemical is in pure product form.

Water table

. Rainfall

Ground surface

Free product

Unsaturated zone

. Rainfall

Ground surface

Free product

Unsaturated zone

Water table

Dissolved product

Rainfall

Ground surface

^ Free product Uns

Unsaturated zone

Dissolved product

Water table

^ Free product Uns

Unsaturated zone

Dissolved product

Water table

Saturated zone

FIGURE 18.4 Schematic of contaminant plumes showing methods by which groundwater can be contaminated.

Figure 18.4 shows schematically the methods by which groundwater can be contaminated. Most of the gasoline components are immiscible with water—these are called the free product. The density of gasoline free product ranges from 0.72 to 0.78 g/mL with a viscosity less than that of water. Gasoline free product floats on and moves faster than groundwater. The density of crude oil and fuel oil ranges from 0.86 to 0.97 g/mL, with a viscosity greater than that of water.

There are many components of gasoline that readily dissolve in water and are transported as solutes in the groundwater. Most gasoline products are volatile and can release gas into the soil void in gaseous form, particularly in the unsaturated zone. Besides these three forms, gasoline components can be adsorbed by the soil matrix and exist in the soil as adsorbates.

Some gasoline constituents, particularly those that are highly volatile or soluble, are readily biodegraded in the presence of soil bacteria and oxygen. Gasoline constituents underground, specifically in the unsaturated zone, belong to the four forms or phases mentioned above. The released gasoline can be transported in the soil matrix in three forms: gas, liquid (free product), or solute. The distribution among these forms may change due to adsorption by soil, desorption from soil, and the extent of degradation.

18.5.2 Fates of Gasoline Underground: Adsorption and Degradation of Gasoline and the Effect on Gasoline Movement

18.5.2.1 Adsorption of Gasoline by Soil

The forces associated with adsorption of gasoline by the soil are the same as those for adsorption of water by soil. The difference is in the adsorptive strengths of gasoline and water, because gasoline and water have different affinity to soil. Moreover, different gasoline constituents may also have different extents of adsorption by soil. For example, tetraethyl lead and naphthalene have relatively low mobility values and are likely to be adsorbed to the soil. Toluene, xylenes, benzene, and phenol have high mobility values and are therefore more likely to appear in either the dissolved or gaseous phases than being adsorbed. Table 18.2 lists the adsorption coefficients for common gasoline compounds.

The soil above the water table in a gasoline release site is most likely to have the highest concentration of adsorbed gasoline. The soil may be flushed by groundwater when the level of the water table fluctuates, or by infiltrating water, thus changing the adsorbed concentration. In gasoline movement, the gasoline constituent will transfer between the moving phases and the soil adsorptive sites. The extent of transfer depends on the concentration of gasoline in these phases and the distribution coefficient among these phases. Generally, in the release case, gasoline will be adsorbed more by the soil matrix when passing through a pristine soil. During remediation, the gasoline

TABLE 18.2

Adsorption Coefficients for Gasoline Compounds

Chemical Koc Value (mL/g)

Tetraethyl leada

(n) Heptane

(n) Hexane

Naphthalene15

(n) Pentane

Ethylbenzene1

Toluene1

1-Pentane

(o) Xylene1

Benzene1

Phenolb

Ethylene dibromide

4900 2361 1097 976 568 565 339 280 255 50 50 44

Source: U.S. EPA, Cleanups of Releases from Petroleum USTs: Selected Technologies, EPA/530/

UST-88/00l, U.S. EPA, Washington, 1988. a Koc is a measure of the tendency for organic compounds to be adsorbed by soil. The higher the Koc value for each compound, the lower the mobility and the higher the adsorption potential. b Toxic compound.

constituents will be released from the soil, because the condition is manipulated to have a lower concentration than the previously partitioned concentration.

18.5.2.2 Degradation of Gasoline

Gasoline compounds are also subjected to chemical and biological processes.19 Biodegradation and biotransformation are two basic biotic processes. Biodegradation is the decomposition of gasoline by microorganisms. The end products are water, carbon dioxide, and energy. Biotransformation is partial biodegradation. Gasoline compounds are partially degraded to simpler compounds that may be more or less soluble or toxic than the original compounds. Most of the biotic processes occur under aerobic conditions.

Abiotic chemical transformation is the reduction of chemical concentrations by degrading the chemicals into other products. The most important chemical transformations are hydrolysis and oxidation/reduction reactions.

Degradation is often the result of the combined effect of chemical transformation and biodegradation. For example, the oxidation/reduction of complex hydrocarbons can produce simple compounds such as peroxides, primary alcohols, and monocarbocylic acids. These compounds can then be further degraded by bacteria, leading to the formation of carbon dioxide, water, and new bacterial biomass.19,35

18.5.2.3 Movement of Gasoline Free Product

Most gasoline constituents are immiscible with water, and thus form free product of gasoline from water and usually float on groundwater.

The movement of free product is dependent on soil permeability and moisture. The released gasoline first infiltrates downward vertically, mainly governed by the gravity force, into and through the unsaturated zone, then reaches the water table. If there is an impermeable layer above the water table, the free product will be purged and may not reach the water table directly. In the unsaturated zone, gasoline can be retained by capillary forces and adsorbed onto soil particles. The capillary action in the unsaturated zone also enhances the extent of evaporation of both gasoline and groundwater.

As oil has a lighter specific gravity and lower viscosity than groundwater, the free product floats on the groundwater surface and moves at a faster rate than the groundwater. This horizontal movement is mainly governed by the hydraulic gradient. In the process, gasoline components are also partly adsorbed by the soil, evaporated into the soil void, and dissolved in the groundwater.

18.5.2.4 Movement of Gas-Phase Gasoline

Most gasoline constituents are volatile organics. Volatilization depends on the potential volatility of the compounds and on the soil and environmental conditions, which modify the vapor pressure of the chemicals. Factors affecting volatility are water content, clay content, surface area, temperature, surface wind speed, evaporation rate, and precipitation.

For vapor to move in the unsaturated zone, the soil formations must be sufficiently dry to permit the interconnection of air passages among the soil pores. Vapor concentration and vapor flow govern its movement. Vapor can move by diffusion from areas of higher concentration to areas of lower concentration and ultimately to the atmosphere. Therefore, the transportation of the vapor phase of gasoline components in the unsaturated zone can pose a significant health and safety threat because of inhalation and explosion potential.

Vapor can also move due to pressure gradient, as effected by a barometric-pumping-imposed pressure gradient, and due to density differences. If there is an impermeable layer above the rising vapors, such as a paved road, building, or a frozen ground surface, the vapors are able to move only by lateral underground travel; thus, migration can occur over relatively long distances.

The level of vapor movement in the unsaturated zone is much less important than transport in liquid form. However, this might not be true if the water content of the soil is very low or if there is a strong temperature gradient. The movement of vapor through the unsaturated zone is a function of temperature, humidity gradients, and molecular diffusion coefficients for water vapor in the soil.

18.5.2.5 Movement of Gasoline Solutes

Solubility causes gasoline compounds to be more mobile in association with the movement of groundwater. Dissolved gasoline compounds reach the saturated zone in several ways:

1. From groundwater flow that already has dissolved solute

2. From infiltrating water that has extracted solute from the soil or free product in its path due to the extraction of solute directly from soil adsorbates

3. From free product by the contacting groundwater

Dissolution of gasoline compounds to soil water is a function of each compound's solubility. A highly soluble gasoline substance often has a relatively low adsorption coefficient and also tends to be more readily degradable by microorganisms,19 as shown in Table 18.1.

The most soluble gasoline compound is methyl tertiary-butyl ether (MTBE) (43,000 mg/L). In addition, MTBE in solution has a cosolvent effect, causing some of the other compounds in gasoline to solubilize at higher concentrations than they normally would in clean water.

18.5.3 Multiphase Movement of Gasoline Compounds

Because gasoline is composed of some highly volatile and soluble hydrocarbon fractions, its components can move in the subsurface in three states: vapor, solute, and liquid. The form of its components in the soil are vapor, solute, free product, and adsorbate. The multiphase flow of gasoline is further complicated due to the various characteristics of the undersurface formation. The partition coefficients of the gasoline constituents in the gasoline free product, groundwater, soil particles, and soil gas determine the transformation of the gasoline forms.

The fate of gasoline in the subsurface is dependent on its interaction with soil and groundwater, volatilization, chemical reaction, biodegradability, and its movement, which in turn depends on the properties of both gasoline and the underground structure.

Soil moisture may greatly affect the movement of gasoline constituents. The adsorptive sites in a soil saturated with moisture are less available than those in a less saturated soil, so an unsaturated condition may promote adsorption of gasoline and retard the movement of gasoline away from a drier soil. Water makes gasoline less able to "wet" the soil, thus promoting the movement of gasoline as long as the pore space of soil is not fully occupied with water.

The extent of soil adsorption and suction forces varies depending on soil components. For example, clay has a much greater adsorption capacity and suction force than sand. The depth of gasoline penetrating the subsurface depends on the volume release, and the adsorption capacity and permeability of the soil. Gravitational force causes downward vertical migration. Suction can cause both vertical movement and horizontal movement. A higher suction force may cause a wider dispersion of gasoline away from the contaminated area.

In different soil zones, the effect of the forces is different, so the movement of gasoline should be considered separately in each zone. Based on the above discussion, the reader should be able to determine the fate and movement of a gasoline compound in different soil zones. The following gives a brief summary:

1. In the saturated zone, the most important phase of gasoline is its free product above the groundwater, then the gasoline as adsorbate in the soil; the gasoline as solute in the ground-water is less important.

2. In the upper unsaturated zone (above the capillary fringe), multiphase movement and transformation are typical. Vapor-phase gasoline becomes more important; gasoline adsorption by soil, dissolution in pore water, and free product in the pore space can also be significant.

3. In the capillary fringe, movement by suction occurs in all directions. Transport in the capillary fringe is also governed by multiphase flow. The increased water content in the capillary zone affects the rates of volatilization and dissolution. As soil water content increases, volatilization and vapor transport generally decrease and dissolution and solute transport generally increase. Free product migration occurs on top of the water table; the free product continues to spread and is held by capillary forces in the soil matrix. When the free product is exhausted, migration stops and residual saturation is reached.

Note that the heterogeneity of underground conditions would favor the flow along the path of least resistance, which is another factor controlling flow besides control by the hydraulic or concentration gradients.

18.6 MANAGEMENT OF TANKS AND THE ENVIRONMENT

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