1. CHAPTER 3: SOURCE AND BEHAVIOUR OF SUBSURFACE CONTAMINANTS

2. 3.1: Organic Geochemistry
Organic geochemistry deals with the behavior and reactions of organic chemicals in soils.
In this section we study the type and characteristics of organic contaminants commonly found at contaminated sites.

3. 3.2: Organic Contamination
Accidental spillage or improper disposal of products such as gasoline, diesel fuel, fuel oil, jet fuels, coal tars, motor oil and waste oil, and chlorinated solvents and degreasers has caused a variety of organic chemicals to enter into the subsurface.
Organic contaminants may exist in liquid or solid form in soils and may exhibit volatile or semi volatile characteristics.

4. 3.2: Organic Contamination
The most common types of organic contaminants found at contaminated sites include:
Hydrocarbons such as gasoline components (eg: benzene, toluene, ethylbenzene, and xylene).
Chlorinated hydrocarbons such as Perchloroethylene (PCE) and Trichloreothene (TCE).
Polycyclic aromatic hydrocarbons (PAHs) such as naphthalene, phenanthrene, and pyrene.
Polychlorinated biphenyls (PCBs) such as aroclor.
Pesticides such as aldrin and endrin

5. 3.2: Organic Contamination
These organic compounds are classified in different ways.
The organic compounds classified as hydrocarbons are commonly encountered at contaminated sites.
These hydrocarbon are classified as aliphatic hydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons.
A group of these hydrocarbons are often known as non-aqueous phase liquids (NAPL) have been encountered at numerous contaminated sites.

6. 3.2: Organic Contamination
This term is used because NAPL exist primarily as a separate, immiscible phase when in contact with water and or air.
Differences in the physical and chemical properties of water and NAPL result in the formation of a physical interface between the liquids that prevents the two fluids from mixing or solubilizing.

7. 3.3: Classification of NAPL
NAPL are classified into light non-aqueous phase liquid (LNAPL) and dense non-aqueous phase liquid (DNAPL).
LNAPL are that have densities less than that of the water.
DNAPL are that have densities greater than that of water.
Because of differences in density alone, the fate and migration of LNAPL and DNAPL in soils can be significantly different.

8. 3.3: Classification of NAPL
Typical NAPL (LNAPL and DNAPL) found at contaminated sites, along with their important properties, are presented in Table 6.3.
LNAPL consist primarily of petroleum products such as gasoline, kerosene, and diesel, which are all associated with spills and accidental releases during production, refinement, and distribution.

9. 3.3: Classification of NAPL

10. 3.3: Classification of NAPL
DNAPL consist primarily of chlorinated solvents, polycyclic aromatic hydrocarbons (PAH), and pentachlorobiphenyls (PCB) resulting from a wide variety of industrial activities such as degreasing, metal stripping, chemical manufacturing, pesticide manufacturing, wood-treating operations, and manufactured gas plant operations.

11. 3.4: Characteristics of NAPL
The most important characteristics that affect the behavior of NAPL in the subsurface are:
(1) density,
(2) viscosity,
(3) solubility,
(4) vapor pressure,
(5) volatility,
(6) interfacial tension,
(7) wettability,
(8) capillary pressure.

12. 3.4.1: Characteristic of NAPL (1) density
Density is defined as the mass of a substance per unit volume.
Density is also expressed as specific gravity (SG), which is the ratio of the mass of a given volume of substance at a specified temperature to the mass of the same volume of water at the same temperature.
LNAPL have SG less than 1 and will float on water.
DNAPL have SG greater than 1 and will sink in water.
Table 6.3 shows density values for selected LNAPL and DNAPL.

13. 3.4.1: Characteristic of NAPL (2) Viscosity
Viscosity is a fluid's resistance to flow.
Dynamic or absolute viscosity is expressed in units of mass per unit length per unit time [in centipoise (cP)].
Both density and viscosity of fluids generally decrease as temperature increases.
The permeability (K') of soils depends on density and viscosity of fluid and is given by the expression:
(1)

14. 3.4.1: Characteristic of NAPL (2) Viscosity
(1)
Where,
k is the intrinsic permeability, K the hydraulic conductivity, ρ the fluid mass density, g the acceleration due to gravity, and μ the dynamic (absolute) viscosity.
From the relationship (Eq1) it can be seen that as density increases, k increases; however, as viscosity increases, k decreases.

15. 3.4.1: Characteristic of NAPL (3) Solubility
Solubility is the equilibrium concentration of NAPL in water.
The solubility of NAPL varies considerably from infinitely miscible, for compounds such as ethanol and methanol, to extremely low solubility, for compounds such as polycyclic aromatic compounds (PAHs).

16. 3.4.1: Characteristic of NAPL (4) Vapor pressure
Vapor pressure is the pressure exerted by the vapor above a liquid, it is the partial pressure exerted by the free molecules of the NAPL.
Vapor pressure determines how readily vapors volatilize or evaporate from pure phase liquids.
Vapor pressure increases with increase in temperature.
Table 6.3 provides vapor pressure values for selected NAPL.

17. 3.4.1: Characteristic of NAPL (5) Volatility
Volatility is a measure of the transfer of the NAPL from the aqueous phase to the gaseous phase.
The Henry's law constant, defined as the vapor pressure divided by the aqueous solubility, is used to help evaluate the volatilization of a NAPL from the water.
Tables 6.3 provide Henry’s law constants for selected NAPL.

18. 3.4.1: Characteristic of NAPL (6) Interfacial tension
Interfacial tension is the surface energy at the interface that results from differences in the forces of molecular attraction within two immiscible fluids.
It is expressed in units of energy per unit area.
In general, the greater the interfacial tension, the greater the stability of the interface between the liquids.
Interfacial tension decreases with increasing temperature and may be affected by pH, surfactants, and dissolved gases.

19. 3.4.1: Characteristic of NAPL (6) Interfacial tension
When this force is present between a liquid and a gaseous phase, the same force is called surface tension.
Low interfacial tension between a NAPL and water allows the NAPL to exist mostly in pore spaces, thus facilitating greater migration in the subsurface.

20. 3.4.1: Characteristic of NAPL (7) Wettability
Wettability is generally defined as the tendency of one fluid to spread on or adhere to a solid surface (i.e.. preferentially coat) in the presence of another fluid with which it is immiscible.
This concept has been used to describe fluid distribution at the pore scale.
In multiphase system, the wetting fluid will preferentially coat (wet) the solid surfaces and tend to occupy smaller pore spaces.

21. 3.4.1: Characteristic of NAPL (7) Wettability
The non-wetting fluid will generally be restricted to the largest interconnected pore spaces.
Water is generally the wetting fluid displacing NAPL from pore spaces, while air is a non-wetting fluid that allows NAPL to adhere to soil surface.
This means that unsaturated (or dry) soils have more susceptibly than saturated soils for NAPL to adhere to a solid surface.

22. 3.4.1: Characteristic of NAPL (8) Capillary pressure
Capillary pressure is the pressure across the interface between the wetting and non-wetting phases and is often expressed as the height of an equivalent water column.
It determines the size of the pores in which an interface can exist.
Capillary pressure is a measure of the relative attraction of the molecules of a liquid (cohesion) to each other and for a solid surface (adhesion), represented by the tendency of the porous medium to attract the wetting fluid and repel the non-wetting fluid.

23. 3.4.1: Characteristic of NAPL (8) Capillary pressure
The capillary pressure of the largest pore space must be exceeded before the non-wetting fluid NAPL can enter the porous medium.
The minimum pressure required for the NAPL to enter the medium is termed the entry pressure.
In general, capillary pressure increases with decreasing pore size, decreasing initial moisture content, and increasing interfacial tension.

24. 3.4.1: Characteristic of NAPL (8) Capillary pressure
In general, when the NAPL pressure head is greater than the capillary pressure, the NAPL can enter into the soil pores, including the smaller pores.
When sufficient NAPL volume has been released and the pressure head of it exceeds the water capillary pressure at the capillary fringe (entry pressure), the NAPL will enter the groundwater.

25. 3.4.1: Characteristic of NAPL (8) Capillary pressure
If the NAPL pressure head is less than the water capillary pressure at the capillary fringe, NAPL tend to migrate laterally on the top of the water table.
Therefore, the extent of migration, and residual distribution of NAPL in the subsurface, are controlled by the capillary forces.

26. 3.5: Distribution of NAPL in Soil
NAPL are comprised of either a single organic compound or a complex mixture of several organic compounds.
The exact constituents of NAPL depend on the source of contamination and may include LNAPL, DNAPL, or a combination.

27. 3.5: Distribution of NAPL in Soil
NAPL may exist in any of four phases in soils:
(1) dissolved phase,
(2) adsorbed phase,
(3) gaseous phase, and
(4) free NAPL phase

28. 3.5: Distribution of NAPL in Soil
In unsaturated soils, all four phases exist, as depicted in Figure 1(a).
In saturated soils, NAPL are present in three phases: the dissolved, adsorbed, and free NAPL phases, as depicted in Figure 1(b).

29. 3.5: Distribution of NAPL in Soil
Figure 1 (a)
Figure 1 (b)

30. 3.5: Distribution of NAPL in Soil
Among these different phases, free NAPL phase is most significant.
The free NAPL phase may exist in soil pores as continuous slugs of NAPL near contamination source locations or as small individual NAPL blobs (or ganglia).
NAPL ganglia form due to capillary forces when continuous slugs of NAPL become discontinuous during migration.

31. 3.5: Distribution of NAPL in Soil
The amount of NAPL present in soils is often expressed by saturation (Si), which is the relative fraction of total pore space containing NAPL in a representative volume of soil:
(2)
where Vi, is the volume of NAPL in a general fluid i, and Vv, is the volume of voids in soil.

32. 3.5: Distribution of NAPL in Soil
When NAPL migrates through soils, some of it may be trapped in soil pores, due to capillary forces, and exist in the form of blobs.
The amount of this trapped NAPL is expressed as the residual saturation (Sr).
Sr is defined as the ratio of the volume of residual NAPL and the volume of pore space.

33. 3.5: Distribution of NAPL in Soil
Sr is generally higher in low permeability soils than in highly permeable soils.
Si and Sr, describe mobile and immobile amounts of NAPL, respectively, and are often used in assessing the potential for contaminant migration and the need for remediation.

34. 3.6: Processes controlling distribution of NAPL in soil
NAPL may change from one phase to another, depending on the following processes:
(1) volatilization,
(2) dissolution,
(3) adsorption, and
(4) biodegradation.

35. 3.6: Processes controlling distribution of NAPL in soil

36. 3.6: Processes controlling distribution of NAPL in soil

37. 3. 6: Processes controlling distribution of NAPL in soil
3.6: Processes controlling distribution of NAPL in soil (Volatilization)
Volatilization refers primarily to the partitioning of NAPL from dissolved and free phases into the gaseous phase.
Henry's law describes the partitioning of an organic compound between the aqueous and gaseous phases.
For a dilute solution (concentrations less than approximately 10-3 mol/L), the ideal gas vapor pressure of a volatile organic is proportional to its mole fraction in solution.

38. 3. 6: Processes controlling distribution of NAPL in soil
3.6: Processes controlling distribution of NAPL in soil (Volatilization)
Simply stated, the escaping tendency of the organic molecules from the dissolved phase to the gas phase is proportional to the dissolved organic concentrations.
This relationship assumes local equilibrium between dissolved and gaseous phases.
Given mathematically,
(3)
where Caq is the concentration in water, H is Henry's constant, and Pg is the vapor pressure of the organic component.

39. 3. 6: Processes controlling distribution of NAPL in soil
3.6: Processes controlling distribution of NAPL in soil (Volatilization)
For a more concentrated dissolved phase or free NAPL phase, which contains multiple organic compounds, Raoult's law describes the volatilization of an organic compound.
Raoult's law states that the vapor pressure over a solution is equal to the mole fraction of the solute times the vapor pressure of the pure phase liquid.

40. 3. 6: Processes controlling distribution of NAPL in soil
3.6: Processes controlling distribution of NAPL in soil (Volatilization)
This law is given mathematically as
(4)
where Pa is the vapor pressure of the NAPL mixture (atm), Pa0 the vapor pressure of the organic compound a as a pure-phase component (atm), and Xa the mole fraction of a hydrocarbon a in the NAPL mixture.
This relationship is valid only for equilibrium conditions.

41. 3. 6: Processes controlling distribution of NAPL in soil
3.6: Processes controlling distribution of NAPL in soil (Volatilization)
To represent mass-based concentrations, equation (4) is written using the ideal gas law (PV = nRT) as:
(5)
where Xa is the mole fraction, Pao is represented in atm, MW is the molecular weight in g/mol, R = (atm.L/mol.K), and T is the temperature in Kelvin.

42. Example:
A NAPL of pure TCE is released into the unsaturated zone. Calculate the theoretical concentration of TCE in the gaseous phase (in mg/l).

43. Example:
A NAPL consisting of benzene and TCE in mole fraction of 0.7 and 0.3, respectively, exists in the unsaturated zone. Calculate the theoretical concentration of benzene and TCE in the gaseous phase (in mg/l). Assume that adsorption and biodegradation are negligible.

44. 3.7: Processes controlling distribution of NAPL in soil (Dissolution)
Dissolution refers to the partitioning of free NAPL phase into the dissolved phase.
An NAPL in physical contact with soil solution or groundwater will dissolve (solubilize, partition) into the aqueous phase (dissolved phase).
The solubility of a single organic compound is the equilibrium concentration of that compound in water at a specified temperature and pressure.

45. 3.7: Processes controlling distribution of NAPL in soil (Dissolution)
In other words, the solubility represents the maximum possible concentration of that compound in water.
Ranging over several orders of magnitude, the solubility of organic compounds is affected by temperature, pH, solvents, dissolved organic matter and dissolved inorganic compounds.
For a multi components NAPL in contact with water, the equilibrium dissolved phase concentrations may be estimated using the solubility of the pure liquid in water and its mole fraction in the NAPL mixture.

46. 3.7: Processes controlling distribution of NAPL in soil (Dissolution)
The maximum concentration in such cases is referred to as the effective solubility.
This value is expressed mathematically as:
Where is the effective aqueous solubility of the compound i in the NAPL mixture, and Si the aqueous solubility represents the concentration that may occur at equilibrium under ideal conditions.

47. 3.7 : Processes controlling distribution of NAPL in soil (Dissolution)
However, this approach does not account for the tendency of certain solvents (alcohols) to increase the solubility of organic compounds.
Dissolution rates may also increase with higher groundwater velocity, higher NAPL saturation, increase contact between NAPL and water and increased the fraction of soluble components.
Many studies report that dissolution kinetics affects solubility as well and that the dissolution rate is limited under certain conditions such as high groundwater velocity.

48. 3.7 : Processes controlling distribution of NAPL in soil (Adsorption)
Adsorption refers to the partitioning of NAPL from various phases, primarily from the dissolved phase, into adsorbed phase.
On the other hand, desorption refers to the partitioning of adsorbed phase into, generally, dissolved phase.
Adsorption and sorption are used interchangeably to include NAPL accumulated at the interface with soil solids, as well as those partitioned into soil organic carbon.

49. 3.7 : Processes controlling distribution of NAPL in soil (Adsorption)
The extent of adsorption and desorption depends on parameters such as solubility, polarity, ionic charge, pH, redox potential, and the octanol water partition coefficient (Kow).

50. 3. 7 : Processes controlling distribution of NAPL in soil
3.7 : Processes controlling distribution of NAPL in soil (Biodegradation)
Many NAPL especially LNAPL, may undergo biological degradation.
The dissolved phase NAPL are particularly amenable to biological degradation if the right, naturally occurring microorganisms are present in the subsurface.
However, free-NAPL-phase biodegradation may not be conducive, because the free NAPL phase may hinder microbial activity.
The biodegradation process essentially involves oxidation-reduction reactions.

51. 3.8 : Chemical Analysis of NAPL in Soil
To understand the need for remediation and for design of a remediation system, we should know the constituents of NAPL and their concentration.
It may also be valuable to determine the distribution of NAPL in dissolved, gaseous, adsorbed, and free NAPL phases.
In addition, we should also know if the mobile and residual NAPL are present in the system.

52. 3.8: Chemical Analysis of NAPL in Soil
Accurate determination of each of the preceding factors under in-situ conditions is quite complex.
Commonly, soil, soil solution or groundwater, and soil gas samples are collected from the field and analyzed in the laboratory.
For the analysis of soils, an extraction is done first to recover all of the organics from the soil sample by dissolving them into solution.
The USEPA Method 3540C Soxhlet extraction procedure is the most commonly used extraction procedure for this purpose.

53. 3.8: Chemical Analysis of NAPL in Soil
The extract solution is then analyzed using GC, GC-MS, or HPLC, to identify individual compounds and their concentration.
The soil solution or groundwater may be analyzed using the samples directly, or they may require filtration and/or concentration prior to analysis using GC, GC-MS, or HPLC.
Soil gas analysis may also be performed by injecting the sample directly into the GC or GC-MS

54. 3.8: Chemical Analysis of NAPL in Soil
The chemical analyses above provide the constituents in NAPL and their total concentrations but do not distinguish directly between dissolved, adsorbed, gaseous, and free NAPL phases.
However using these data, an estimate of these phases may be made assuming that equilibrium conditions are valid.

55. 3.8: Chemical Analysis of NAPL in Soil
This procedure consists of the following steps:
1. Calculate the effective solubility of organic compound of interest .
2. Calculate concentration in gas phase using vapor pressures and mole fractions (Ca).
3. Estimate adsorbed concentration of organic chemicals.
4. The difference between the total measured concentration and the sum of the concentrations of dissolved, vapor, and adsorbed phases (step 1 until 3) provides the amount of free NAPL phase.

56. 3.9: Summary
Understanding the various aspects of geochemistry that deal with the behavior of chemicals in soils is essential for an understanding of the fate and transport of contaminants in subsurface environments.
Inorganic geochemistry deals with the study of inorganic contaminants, particularly toxic metals, while organic geochemistry includes the study of organic contaminants such as hydrocarbons.

57. 3.9: Summary
An understanding of the phase distribution of contaminants introduced into the subsurface is essential for the design of strategies that can effectively remediate contaminated soils.
Various interdependent geochemical processes control the distribution of contaminants in different phases

58. 3.9: Summary
Mathematical models or simple mass-balance analysis methods are often useful to assess the amount of contaminants in different phases under varying environmental conditions, such as pH, solution chemistry, and temperature.
Laboratory analysis methods to determine contaminant concentrations involve extraction procedures that must be selected carefully in order to attain accurate and useful information.