2.1 Dispersion

For the purposes of this text, the term dispersion encompasses all phenomena which give rise to the proliferation of substances through the man-made and natural environment. Thus, the physical transport of oil droplets into the water column is referred to as dispersion. This is often a result of water surface turbulence, but also may result from the application of chemical agents (dispersants). These droplets may remain in the water column or coalesce with other droplets and gain enough buoyancy to resurface. Dispersed oil tends to biodegrade and dissolve more rapidly than floating slicks because of high surface area relative to volume. Most of this process occurs from about half an hour to half a day after the spill.

Emissions of many chemicals of concern occur into the air initially, from where they are dispersed into other media. In fact, air is one of the main carriers of chemical carcinogens to humans (Corvalán and Kjellström, 1996). Many chemicals emitted into the air, for instance from combustion processes, tend to become associated with particulate matter. Removal from the air occurs through a range of complex processes involving photo-degradation, and particle sedimentation and/or precipitation (known respectively as dry and wet deposition). Volatile organic chemicals and semivolatile organic chemicals may undergo several cycles of evaporation and precipitation, which can also make chemicals more accessible to photochemical or biodegradation.

The dispersion of chemicals released into the environment has been the focus of much attention because of the realization that the dispersion behavior of organic chemicals can be markedly different when different chemicals are considered. Accidents that involve organic chemicals give rise to a new class of problems in dispersion prediction for the following reasons: (1) the material is, in almost all cases, stored as a liquid, which may appear as a gas after a spill, (2) the modes of release can vary widely and geometry of the source can take many forms and the initial momentum of the spill may be significant, and (3) in some cases, a chemical transformation also takes place as a result of reaction with water vapor in the ambient atmosphere.

In addition, the physical properties of the organic chemical(s) usually result in interactions with the surrounding ecosystem, especially if the chemical is a reactive liquid and has the potential of interacting with the air, water, or soil. This reactivity will influence the dispersibility of the chemical. Moreover, if the release occurs over a short time-scale, compared to the steady-state releases characteristic of many chemical releases problems, this can give rise to the complication of predicting dispersion for time-varying releases and to uncertainty in individual predictions resulting from variability about the behavior of a mixture. Also, the dispersing chemical, which is typically denser that air, may form a low-level cloud that is sensitive to the effects of man-made and natural obstructions and of topography.

Wind (Aeolian) transport (relocation by wind) can also occur and is particularly relevant when dust from organic carbonaceous solids (such as coke dust) and catalyst dust are considered. Dust becomes airborne when winds traversing arid land with little vegetation cover pick up small particles such as catalyst dust, coke dust, and other refinery debris and send them skyward after which the movement of pollutants in the atmosphere is caused by transport, dispersion, and deposition. Dispersion results from local turbulence, that is, motions that last less than the time used to average the transport. Deposition processes, including precipitation, scavenging, and sedimentation, cause downward movement of pollutants in the atmosphere, which ultimately remove the pollutants to the ground surface.

2.2 Dissolution

The solubility (dissolution) characteristics of organic molecules in water are complex and very much dependent upon the structure and properties of the organic chemicals(s). Solubility is the property of organic chemicals which might be a gas, a liquid, or a solid (the solute) that dissolves in a solvent which might also be a gas, a liquid, or a solid organic chemical (the solvent). The solubility of a gas, a liquid, or a solid organic chemical depends on the physical and chemical properties of the solute and solvent as well as on temperature, pressure, and the pH of the solution. The extent of the solubility of a substance in a specific solvent is measured as the saturation concentration, where adding more solute does not increase the concentration of the solution and begins to precipitate the excess amount of solute. In terms of environmental organic chemistry, the solvent is typically a liquid, which can be a pure organic chemical (such as hexane or toluene) or a mixture of organic chemicals (such as naphtha or kerosene).

The extent of solubility of the solute ranges widely, from infinitely soluble (without limit) (i.e., the solute is fully miscible with the solvent, such as ethanol in water) to poorly soluble (such as some polynuclear aromatic systems in water)—the term insoluble is often applied to poorly or very poorly soluble compounds and (in the world of organic chemistry) a common threshold to describe an organic chemical as insoluble is a solubility < 0.1 g per 100 mL of solvent. However, solubility of an organic compound in a solvent should not be confused with the ability to of a solvent to dissolve a solute, because the solution might also occur because of a chemical reaction (reactive solubility).

Solubility of a solute in a solvent applies not only to environmental organic chemistry but to areas of chemistry, such as (alphabetically): biochemistry, geochemistry, inorganic chemistry, physical chemistry, and organic chemistry. In all cases, solubility depends on the physical conditions (temperature, pressure, and concentration of the solute) and the enthalpy and entropy directly relating to the solvents and solutes concerned. By far the most common solvent in chemistry is water which is a solvent for most ionic compounds as well as a wide range of organic chemicals. This is a crucial factor in the chemical phenomena known as acidity and alkalinity as well as in much of the area known as environmental organic chemistry.

In addition, the term dissolved organic carbon (DOC) is used as a broad classification for organic molecules of varied origin and composition within aquatic systems (the aquasphere) and the dissolved fraction of organic carbon is an operational classification. The source of the DOC in marine systems and in freshwater systems depends on the body of water. In general, organic chemical compounds are a result of decomposition processes from dead organic matter such as plants or marine organisms. When water contacts highly organic soils, these components can drain into rivers and lakes as DOC. Whatever the source of the DOC, it is also extremely important in the transport of metals in aquatic systems—certain metals can form extremely strong organo-metallic complexes with DOC which enhances the solubility of the metal in aqueous systems while also reducing the bioavailability of the metal.

In terms of organic chemicals that are not naturally occurring, knowledge of the structure and properties can be used to examine relationships between the solubility properties of an organic chemical and its structure, and vice versa. In fact, structure dictates function which means that by knowing the structure of an organic chemical it may be (but not always) possible to predict the properties of the chemical such as its solubility, acidity or basicity, stability, and reactivity. In the context of environmental distribution of a chemical, predicting the solubility of an organic molecule is a useful component of knowledge.

At the molecular level, solubility is controlled by the energy balance of the intermolecular forces between solute-solute, solvent-solvent, and solute-solvent molecules. But without getting too fay into a study of such forces, the simple, very useful, and practical empirical rule that is quite reliable which is like dissolves like which is based on the polarity of the organic chemical systems insofar as polar organic chemicals typically dissolve in polar solvents (such as water, alcohols) and nonpolar organic chemicals typically dissolve in nonpolar solvents (such as nonpolar hydrocarbon solvents). The polarity of organic molecules is determined by the presence of polar bonds due to the presence of electronegative atoms (such as nitrogen and oxygen) in polar functional groups such as amine derivatives (RNH₂) and alcohol derivatives (ROH) (Chapter 2). Furthermore, since the polarity of an organic chemical is related to the presence of a functional group, the solubility characteristics of an unknown organic contaminant can provide evidence (as well as evidence form other analytical techniques) (Chapter 5) for the presence (or absence) of an organic functional group:

Most organic molecules are relatively nonpolar and are usually soluble in organic solvents (such as diethyl ether, dichloromethane, chloroform, and hexane) but not in polar solvents (such as water). If the organic chemical is soluble in water, this denotes that the chemical is polar and therefore soluble in water which denotes a high ratio of polar group(s) to the nonpolar hydrocarbon chain, such as a low molecular weight organic chemical that contains a hydroxy function (OH), or an amino function (NH₂) or a carboxylic acid function (CO₂H) group, or a higher molecular weight organic chemical that contains two or more functional (polar) groups. The presence of an acidic carboxylic acid or basic amino group in a water-soluble compound can be detected by measurement of the pH of the solution—a low pH (pH ≤ 7) indicates the presence of an acidic function while a high pH (pH ≥ 7) indicates the presence of a basic function. Thus, organic chemicals that are insoluble in water can become soluble in an aqueous environment if they form an ionic species when treated with an acid or a base—the ionic form (for example the sedum salt of a carbocyclic acid, RCO₂⁻ Na⁺) is much more polar than the carboxylic acid.

The solubility of carboxylic acid derivatives and phenol derivatives in sodium aqueous hydroxide is due to the formation of the polar (ionic) carboxylate ions (CO₂⁻) or the polar (ionic) phenoxide (C₆H₅O⁻) ions since they are much stronger acids than water, and therefore the acid-base equilibria lie far to the right of the equation:

${\mathrm{R}\mathrm{C}\mathrm{O}}_2\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}\leftrightarrow {{\mathrm{R}\mathrm{C}\mathrm{O}}_2}^{-}+{\mathrm{H}}_2\mathrm{O}$

$\mathrm{ArOH}+{\mathrm{O}\mathrm{H}}^{-}\leftrightarrow {\mathrm{A}\mathrm{r}\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}$

Carboxylic acid derivatives, but not phenol derivatives, are also stronger acids than carbonic acid (H₂CO₃), and are therefore also soluble in aqueous sodium bicarbonate (NaHCO₃ solution):

${\mathrm{R}\mathrm{C}\mathrm{O}}_2\mathrm{H}+{{\mathrm{H}\mathrm{C}\mathrm{O}}_3}^{-}\leftrightarrow {{\mathrm{R}\mathrm{C}\mathrm{O}}_2}^{-}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{C}\mathrm{O}}_2$

The solubility of amine derivatives in dilute aqueous acid is in accordance with the amine derivatives being stronger bases than water, and the amine derivatives are converted to the polar ammonium ion by protonation (through reaction with a proton):

${\mathrm{R}\mathrm{N}\mathrm{H}}_2+{\mathrm{H}}_3{\mathrm{O}}^{+}\leftrightarrow {{\mathrm{R}\mathrm{N}\mathrm{H}}_3}^{+}+{\mathrm{H}}_2\mathrm{O}$

Amine derivatives are the only common class of organic compounds which are protonated in dilute aqueous acid.

Thus, dissolution is the solubilization of organic chemicals in water. Many of the acutely toxic components of oils such as benzene, toluene, and xylene will also dissolve into water but only to a minute extent. Nevertheless, when the volume of water (as in a lake or in running water such as a river) is large the transportation of the benzene, toluene, ethylbenzene, and the xylene isomers (BTEX) through dissolution can be substantial. This process also occurs quickly after a discharge of the chemical(s) but tends to be less important than evaporation. For example, in a typical discharge into the aquasphere, generally < 5–10% v/v of the benzene is lost to dissolution while > 90–95% v/v can be lost to evaporation. The polynuclear aromatic compounds (Chapter 2) offer a different scenario.

For alkylated polynuclear aromatic compounds, solubility is inversely proportional to the number of rings and extent of alkylation (i.e., the number of alkyl moieties attached to the polynuclear aromatic ring system). The dissolution process is thought to be much more important in rivers because natural containment within the river system (i.e., the water, the character of the river banks, and the river bed) may prevent spreading, thereby reducing the surface area of the chemical slick (the chemicals on the surface of the water) and thus retard evaporation. However, river turbulence must not be ignored since turbulence increases the potential for mixing and dissolution.

Groundwater that is contaminated with organic chemicals tends to be enriched in aromatic derivatives relative to other organic chemicals constituents. Relatively insoluble hydrocarbons may be entrained in water through adsorption on to clay (kaolinite) particles suspended in the water or as an agglomeration of oil droplets (microemulsion) in the water. In cases where groundwater contains only dissolved hydrocarbons, it may not be possible to identify the original organic chemicals because only a portion of the free product will be present in the dissolved phase. If a hydrocarbon mixture has been spilled into the aquasphere, initially the whole mixture floats on top of the groundwater but any constituents with a tendency to water-solubility will be extracted from the mixture. Groundwater containing entrained hydrocarbons will have a gas chromatographic fingerprint that is a combination of the total mixture plus enhanced amounts of the soluble aromatics minus losses to the water. Generally, dissolved aromatics may be found quite far from the origin of a spill but entrained hydrocarbons may be found in water close to the organic chemical source. Oxygenates, such as methyl-t-butyl ether, are even more water soluble than aromatics and are highly mobile in the environment.

2.3 Emulsification

An emulsion is a mixture of two or more liquids that are usually immiscible. Examples include crude oil and water which can form an oil-in-water emulsion, wherein the oil is the dispersed phase, and water is the dispersion medium. In addition, crude oil and water can also form a water-in-oil emulsion, wherein water is the dispersed phase and crude oil is the external phase. Whether an emulsion of oil and water exists as a water-in-oil emulsion or an oil-in-water emulsion depends on the volume fraction of both phases and the type of emulsifier (surfactant) present. Multiple emulsions are also possible, including a water-in-oil-in-water emulsion and an oil-in-water-in-oil emulsion. Emulsions, being liquids, do not exhibit a static internal structure—the droplets dispersed in the liquid matrix (the dispersion medium) are usually assumed to be statistically distributed.

The stability of an emulsion refers to the ability of the emulsion to resist change in its properties over time. There are three types of instability in emulsions: (1) flocculation, (2) creaming, and (3) coalescence. Flocculation occurs when there is an attractive force between the droplets, so they form flocs. Coalescence occurs when droplets bump into each other and combine to form a larger droplet, so the average droplet size increases over time. Creaming occurs when the droplets rise to the top of the emulsion under the influence of buoyancy. Use of a surface-active agent (surfactant) can increase the stability of an emulsion so that the size of the droplets does not change significantly with time and the emulsion is defined as stable. Most emulsions contain droplets with a mean diameter of more than around 1 μm (1 micron, 1 × 10^− 6 m) however mini-emulsions and nano-emulsions can be formed with droplet sizes in the 100–500 nm (nanometer, 1 × 10^− 9 m) range, and with proper formulation, highly stable microemulsion can be prepared having droplets as small as a few nanometers.

Crude oil and crude oil products as well as many other organic chemicals can form water-in-oil emulsions (where water is incorporated into oil) or in the form of a mousse as weathering occurs. This process is significant because, for example, the apparent volume of the organic chemical may increase dramatically, and the emulsification will slow the other weathering processes, especially evaporation of the volatile organic chemicals.

Emulsification in the aquasphere (especially in marine environment) depends primarily on the composition of the crude oil (or the crude oil product) and the turbulent regime of the water mass. The most stable emulsions such as water-in-oil contain from 30% to 80% v/v water and usually appear after strong storms in the zones of spills of heavy oil or the higher boiling crude oil product that have an increased content of nonvolatile fractions (especially asphaltene constituents) (Speight, 2014). These types of emulsions can exist in the marine environment for over several months (100 days or more) in the form of (what is colloquially called) a chocolate mousse (which is not recommended for human consumption!). The stability of these emulsions usually increases (demulsification) with decreasing temperature—the reverse emulsions, such as oil-in-water emulsion (where droplets of oil are suspended in water) are much less stable because surface-tension forces quickly decrease the dispersion of oil.

This demulsification process can be slowed with the help of emulsifiers which are surface-active substances with strong hydrophilic properties that are used to eliminate the prolonged effects of spills of crude oil and crude oil products. Emulsifiers help to stabilize oil emulsions and promote dispersing oil to form microscopic (invisible) droplets which accelerates the decomposition of oil products in the water.

2.4 Evaporation

Evaporation (the opposite of condensation) occurs when a liquid organic chemical becomes a gas (or vapor), sublimation is the phenomenon that occurs when a solid becomes a gas without the conversion of the solid to liquid and thence to a gas.

$\mathrm{Evaporation}:\mathrm{Liquid}\to \mathrm{gas}$

$\mathrm{Sublimation}:\mathrm{Solid}\to \mathrm{gas}$

Both phenomena are important aspects of the environmental organic chemical cycle since both processes involve disappearance of organic contaminant for the water or land but the contaminant does appear in the atmosphere. Of the two processes, the evaporation is the most common process since organic chemicals that go through the sublimation processes are not as obvious or as common as chemicals that evaporate.

Many factors affect the evaporation process. For example, if the air is already saturated with other chemicals (or the humidity is high) there is typically little chance of a liquid evaporating as quickly as when the air is not saturated (or humidity is low). In addition, the air pressure also affects the evaporation process since, under high air pressure, an organic chemical is much more difficult to evaporate. Temperature also affects the ability of an organic chemical to evaporate.

Evaporative processes are very important in the weathering of volatile organic chemical products, and may be the dominant weathering process for naphtha, gasoline, and other mixtures that contain low-boiling organic chemicals. Automotive gasoline, aviation gasoline, and some grades of jet fuel (e.g., JP-4) contain 20–99% highly volatile constituents (i.e., constituents with less than nine carbon atoms). The evaporative processes typically begin immediately after a volatile organic chemical is discharged into the environment. For spilled crude oil, the amount lost to evaporation can typically range from approximately 20% to 60% v/v. Some low-boiling petroleum-derived products such as one-ring and two-ring aromatic hydrocarbons and/or low molecular weight alkane derivatives (having < 15 carbon atoms) may evaporate entirely. In fact, a substantial fraction of higher molecular weight organic chemicals may also evaporate.

The primary factors that control evaporation are the composition of the oil, slick thickness, temperature and solar radiation, wind speed, and wave height. While evaporation rates increase with temperature, this process is not restricted to warm climates. For the Exxon Valdez incident, which occurred in cold conditions (Mar. 1989), it has been estimated that appreciable evaporation occurred even before all the oil escaped from the ship, and that evaporation ultimately accounted for 20% v/v of the crude oil. Most of this process occurs within the first few days after the spill. However, it is not unusual for the evaporation process to be active simultaneously with other processes to remove any volatile organic chemicals.

2.5 Leaching

Leaching is a natural process by which water-soluble substances (such as water-soluble organic chemicals or hydrophilic organic chemicals) are washed out from soil or waste disposal areas (such as a landfill). These leached out chemicals (leachates) can cause pollution of surface waters (ponds, lakes, rivers, and the sea) and subsurface water (groundwater aquifers). Thus, leaching is the process by which (in the current context) organic contaminants are released from the solid phase into the water phase under the influence of dissolution, desorption, or complexation processes as affected by acidity or alkalinity (the pH value). The process itself is universal, as any material exposed to contact with water will leach components from its surface or its interior depending on the porosity of the material under consideration. Leaching often occurs naturally with soil contaminants such as organic chemicals with the result that the chemicals end up in potable waters.

Many organic chemicals occur in a mixture of different components in a solid (such as the carbonaceous deposits on petroleum coke). In order to separate the desired solute constituent or remove an undesirable solute component from the solid phase, the solid is brought into contact with a liquid during which time the solid and liquid are in contact and the solute or solutes can diffuse from the solid into the solvent, resulting in separation of the components originally in the solid (leaching, liquid-solid leaching). In addition, leaching may also be referred to as extraction because the solute is being extracted from the solid or the proceeds may also be referred to as washing because a contaminant is removed from a solid with water.

Leaching is affected by (1) soil texture, (2) structure, and (3) water content of the soil. For example, in terms of soil texture, the proportions of sand, silt, and clay affect the movement of water through the soil. Coarse-textured soil containing more sand particles have large pores and is highly permeable which allow the water to move rapidly through the soil and, in fact, organic chemicals (such as pesticides) carried by water through coarse-textured soil are more likely to reach and contaminate groundwater. On the other hand, clay-textured soils have low permeability and tend to retain more water and adsorb more organic chemicals from the water. This slows the downward movement of chemicals, helps increase the chance of degradation and adsorption to soil particles, and reduces the chance of groundwater contamination.

In terms of soil structure, loosely packed soil particles allow speedy movement of water through the soil while tightly compacted soil holds water back and does not allow the water to move freely through it. Plant roots penetrate soil, creating excellent water channels when they die and rot away. These openings and channels may permit relatively rapid water movement even through clay-containing soil. On the other hand, the amount of water already in the soil has a direct bearing on whether rain or irrigation results in the recharging of groundwater and possible leaching of organic chemicals into an aquifer. Soluble chemicals are more likely to reach groundwater when the water content of the soil approaches or is at saturation. Saturation is typical in the spring when rain and snowmelt occurs but when soil is dry the added water just fills the pores in the soil near the soil surface, making it unlikely that the water will reach the groundwater supply.

Leaching processes introduce hydrocarbon into the water phase by solubility and entrainment. Leaching processes of organic chemical products in soils can have a variety of potential scenarios. Part of the aromatic fraction of a spill of organic chemicals on to soil may partition into water that has been in contact with the contamination.

2.6 Sedimentation or Adsorption

Sedimentation occurs when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier, which is typically the basement of a waterway. Settling is due to the motion of the particles through the fluid in response to the forces acting on them, which in an ecosystem, can be due to gravity. The term sedimentation is often used as the opposite of erosion, i.e., the terminal end of sediment transport. Settling occurs when suspended particles fall through the liquid, whereas sedimentation is the termination of the settling process.

Sedimentation can be generally classified into three different types that are all applicable to organic chemicals. Type 1 sedimentation is characterized by particles that settle discretely at a constant settling velocity and typically these particles settle as individual particles and do not flocculate or stick to others during settling. Type 2 sedimentation is characterized by particles that flocculate during sedimentation and because of this the particle size is constantly changing and therefore their settling velocity keeps changing. Type 3 sedimentation (also known as zone sedimentation) involves particles that are at a high concentration (e.g., > 1000 mg L^− 1) such that the particles tend to settle as a mass and a distinct clear zone and sludge zone are present. Zone settling occurs in active sludge sedimentation and sedimentation of sludge thickeners.

Sediments typically consist of a few weight percent of organic matter and the balance being shared by various mineral constituents. The organic carbon environment acts as an attractor or accumulator of hydrophobic compounds, such as polychlorobiphenyl derivatives (PCBs). However, organic carbon in sediment comes in different forms that may have very different sorption capacities for hydrophobic organic compounds. In addition to natural sources such as vegetative debris, decayed remains of plants and animals, and humic matter, sediment organic carbon is also derived from particles of coal, coke, charcoal, and soot that are known to have extremely high sorption capacities.

Many organic chemicals (especially crude oil and crude oil-derived products) are less dense than water or are buoyant in water. However, in areas with high levels of suspended sediment, organic chemicals may be transported to the river, lake, or ocean floor through the process of sedimentation. The organic chemicals may adsorb on to sediments and sink—most of this process occurs from about 2 to 7 days after the spill. Furthermore, hydrophobic organic compounds, such as polynuclear aromatic hydrocarbons (PNAs) and PCBs, bind strongly to sediments and can serve as a long-term source of contaminants in water bodies and biota long after the original source has been removed. The inherent heterogeneity of most sediments makes it difficult to describe in terms of bulk sediment physicochemical parameters such as total organic carbon content, surface area, and particle size distribution. Therefore, management of sediments and the control of sediment contaminants are among the most challenging and complex problems faced by the environmental scientists and the environmental engineers and will become increasingly more so as other organic contaminants enter the environment in greater quantities.

In terms of the sedimentation of organic chemicals (Crompton, 2012), some of the chemicals may be adsorbed on the suspended material (especially if the suspended material is clay or another highly adsorptive mineral) and deposited to the bottom. This mainly happens in shallow waters where particulate matter is abundant and water is subjected to intense mixing, usually through turbulence. Simultaneously, the process of biosedimentation can also occur when plankton and other organisms absorb the organic chemical. The suspended forms of the organic chemicals undergo intense chemical and biological (microbial in particular) decomposition in the water. However, this situation radically changes when the suspended organic chemical reaches the lake bed, river bed, or sea bed and the decomposition rate of the organic chemical(s) buried on the bottom abruptly drops. The oxidation processes slow down, especially under anaerobic conditions in the bottom environment and the organic chemical(s) accumulated inside the sediments can be preserved for many months and even years.

2.7 Spreading

The movement of organic chemical contaminants through the subsurface is complex and is difficult to predict since different types of contaminants react differently with soils, sediments, and other geologic materials and commonly travel along different flow paths and at different velocity. Contaminants released at the source areas (typically on the surface) infiltrate into the subsurface and migrate downward by gravity through the vadose zone (the zone that extends from the top of the ground surface to the water table). When low-permeability soil units are encountered, the contaminants can also spread laterally along the permeability contrast.

Most organic chemical contaminants are introduced into the subsurface by percolation through soil strata and the interactions between the soil and a chemical contaminant are important for assessing the fate and transport of the contaminant in the subsurface, especially in the groundwater system. Contaminants that are highly soluble, such as salts of carboxylic acid derivatives can move readily from surface soil to saturated materials below the water table and often occurs during and after rainfall events. Those contaminants that are not highly soluble may have considerably longer residence times in the surface strata (the soil zone). Some contaminants adsorb readily onto soil particles and slowly dissolve during precipitation events, resulting in migration into groundwater—this is typical of the mode of transport for chemicals such as trichloroethylene (CCl₂CHCl). Liquids spilled onto surface soils can migrate downward or can evaporate, which limits their potential for reaching the water table. Once below the water table, organic chemical contaminants are also subject to dispersion (mechanical mixing with uncontaminated water) and diffusion (dilution by concentration gradients).

Many organic contaminants (such as crude oil and crude oil-derived products) begin to spread immediately after entry into the environment. The viscosity of the oil, its pour point, and the ambient temperature will determine how rapidly the oil will spread, but light oils typically spread more rapidly than heavy oils. The rate of spreading and ultimate thickness of the oil slick will affect the rates of the other weathering processes. For example, discharges that occur in geographically contained areas (such as a pond or slow-moving stream) will evaporate more slowly than if the oil is allowed to spread.

4.1 Chemical and Biochemical Properties

One strategy to remediate such polluted environments, especially the subsurface, is to make use of the degradative capacity of bacteria. It is often sufficient to supply the subsurface with nutrients containing elements such as nitrogen and phosphorus. However, anaerobic processes have advantages such as low biomass production and good electron acceptor availability, and they are sometimes the only possible solution for cleanup and protection of the environment.

Whereas hydrocarbons are oxidized and completely mineralized under anaerobic conditions in the presence of electron acceptors such as nitrate (NO₃), iron (Fe), sulfate (SO₄), and carbon dioxide (CO₂), chlorinated organic compounds and nitroaromatic compounds are reductively transformed to products that are not always benign. For the often persistent polychlorinated compounds, reductive dechlorination leads to harmless products or to compounds that are aerobically degradable. The nitroaromatic compounds (ArNO₂) are first reductively transformed to the corresponding amines (ArNH₂) and can subsequently be bound to the humic fraction of the soil in an aerobic process—humic constituents of soil are the major organic constituents of soil that are produced by biodegradation of dead organic matter.

Such new findings and developments give hope that in the near future contaminated aquifers can efficiently be remediated, a prerequisite for a sustainable use of the precious subsurface drinking water resources.

4.2 Partitioning

The increasing awareness of chemicals in the environment, their disposition, and their ultimate fate has created the need to find reliable mechanisms to assess the environmental behavior and effects of new chemicals. Whether or not a chemical will pose a hazard to the environment will depend on the concentration levels it will reach in various ecosystems and whether or not those concentrations are toxic to the floral and faunal species within the ecosystem. It is, therefore, important to determine expected environmental distribution patterns of chemicals in order to identify which areas will be of primary environmental concern (McCall et al., 1983).

In order to assess the behavior of an organic chemical in the environment, typical physical and chemical properties play a role but a property that is not often considered in the partitioning of the organic chemical in an ecosystem is a function of the chemical and physical structure of the chemical. This leads to a determination of the way in which the chemical or the mixture of chemicals are distributed among the different environmental phases (McCall et al., 1983). These phases may include air, water, organic matter, mineral solids, and even organisms.

More specifically, the focus of partitioning studies is on the equilibrium distribution of an organic chemical that is established between the phases the associated issue problem of calculating the distribution of a compound between the different phases (partitioning equilibrium). There are many situations in which it is correct to assume that phase transfer processes are fast compared to the other processes, such as chemical transformation of the organic contaminant to other (benign or hazardous) chemicals (Chapter 7) that play a role in determining the fate of contaminants. In such cases, it is appropriate to describe the distribution of the chemical as a change of phase by the equilibrium approach using the valid assumption that an equilibrium condition will be reached with the chemical passing to one phase or remaining distributed between different phases. By way of explanation, an example of phase equilibrium is the partitioning of a contaminant (such as carboxylic acid or a carboxylic acid derivative—that is, the sodium salt of the acid) between the pore water and other water in the bed of a sediment and solids in sediment beds. Thus, from the equilibrium partition coefficient it is possible to calculate the rate of transfer of an organic chemical contaminant across interfaces and the rate at which such transfer can be anticipated to occur.

However, before too much excitement is generated at the thought of the answers that a study of partitioning will provide, it must be recognized that there are many situations where an equilibrium of the chemical between the phases is not reached. However, some observers (justifiably) find the information useful insofar as the data are used (correctly or incorrectly) to characterize what the equilibrium distribution of the chemical would be if sufficient time (often difficult to define) is allowed. However, in such cases a quantitative description of the potential partitioning equilibrium can be employed to estimate the possible direction of the transport of the organic chemical contaminant from one environmental ecosystem to another. If this is the case, it may be possible to evaluate whether or not the chemical component(s) of a contaminant (such as a solvent or naphtha) will continue to dissolve in the groundwater and/or volatilize into the overlying soil. Both options are viable, but (hopefully) the data will provide the most likely option to occur, given the position of the chemical within the underground chemical and geological system. Thus, the goal of any partitioning study of the ability of an organic contaminant is to gain insight into the role played by the chemical (and physical) structure of a contaminant in determining the fate of the contaminant in the environment (McCall et al., 1983).

However, when dealing with phase partitioning parameters, it is necessary to develop an understanding of the intramolecular interactions and the intermolecular interactions (Chapter 5) between the organic chemical(s) and the specific molecular environment in which the chemical is spilled or disposed. Thus, there is the necessity to understand the means by which structural groups function within the contaminants and the means by which the functional groups are related to chemical and physical behavior (Chapters 2 and 5). Therefore, there must be an effort to understand (1) the interactions arising from contacts of functional groups within a molecule, i.e., the intramolecular interactions, (2) the influence of these intramolecular interactions on the existence of the molecule in the environment, (3) the interactions arising from contacts of functional groups with the functional groups of another molecule, i.e., the intermolecular interactions, and (4) the influence of these intermolecular interactions on the existence of the molecule in the environment. However, partition coefficients are not the only means of estimating the transfer of organic chemicals between phases. It is also valuable to correlate the partition coefficients on the basis of solubility of the chemical(s) which are convenient and readily understood measurable expressions of single-phase activity coefficients (Cole and Mackay, 2000).

Thus, this section presents an examination of the pertinent compound properties and environmental factors that are needed for quantifying such partitioning.

4.3 Vapor Pressure

The vapor pressure (or equilibrium vapor pressure) of an organic compound is the pressure exerted by a vapor in thermodynamic equilibrium with the condensed phase (liquid or solid) at a given temperature in a closed system (Boethling and Mackay, 2000; Mackay et al., 2006). The equilibrium vapor pressure is an indication of the evaporation rate of the liquid and relates to the tendency of particles to escape from the liquid (or a solid). A substance with a high vapor pressure at normal temperatures is often referred to as volatile. As the temperature of a liquid increases, the kinetic energy of its molecules also increases and the number of molecules passing from the liquid phase to the vapor phase also increases, thereby increasing the vapor pressure.

Vapor pressure controls the volatility of a chemical from soil, and along with its water solubility, determines evaporation from water (Boethling and Mackay, 2000). Predicting the volatility of a chemical in soil systems is important for estimating chemical partitioning in the subsurface between sorbed phase, dissolved phase, and gas phase. Vapor pressure is an important parameter in estimating vapor transport of an organic chemical in air. It is often useful to determine and other properties of a chemical by assuming that it is a liquid or supercooled liquid at a temperature less than the melting point (Mackay et al., 2006). At very low environmental concentrations such as in liquid solutions or on aerosol particles, pure chemical behavior relates to the liquid rather than the solid state.

The vapor pressure of any substance increases nonlinearly with temperature according to the Clausius-Clapeyron relation, which equation allows an expression of the pressure, P, the enthalpy of vaporization, DH_vap, and the temperature, T, as:

$P=Aexp\left(-\mathrm{D}{H}_{\mathrm{v}\mathrm{a}\mathrm{p}}/RT\right)$

In this equation, R (= 8.3145 J mol^− 1 K^− 1) and A are the gas constant and unknown constant, respectively. If P₁ and P₂ are the pressures at two temperatures T₁ and T₂, the equation has the form:

$ln{P}_1/P_2=\mathrm{D}{H}_{\mathrm{v}\mathrm{a}\mathrm{p}}/R\left(1/T_1-1/T_2\right)$

The Clausius-Clapeyron equation allows an estimate of the vapor pressure at another temperature, if the vapor pressure is known at some temperature, and if the enthalpy of vaporization is known.

The atmospheric pressure boiling point (the normal boiling point) of a liquid is the temperature at which the vapor pressure equals the ambient atmospheric pressure. With any incremental increase in that temperature, the vapor pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form vapor bubbles inside the bulk of the substance. Bubble formation deeper in the liquid requires a higher pressure, and therefore higher temperature, because the fluid pressure increases above the atmospheric pressure as the depth increases.

4.4 Volatility

Volatility of an organic compound or of the organic components of a mixture is an important loss mechanism in the overall materials balance. The key environmental factors affecting volatilization are the reaction constant (surface transfer rate of dissolved oxygen per mixed depth of the water body), wind speed, and the mixed depth of the water. Furthermore, when evaluating the volatilization of complex mixtures vs. single chemicals, only physicochemical properties can be affected differently by mixtures. Thus, only by altering either aqueous solubility or vapor pressure of a chemical by interactions with other chemicals can volatilization rates be altered. Also, if the composition of a given mixture is known as well as the solubility of each constituent (from knowledge of the physiochemical properties of each constituent), it is possible to anticipate that rate of change for volatilization due to chemical interactions.

Organic chemicals, whether they are produced by the chemical industry or by the pharmaceutical industry or the petroleum industry, can be considered as environmentally transportable materials, the character of which is determined by several chemical and physical properties (i.e., solubility, vapor pressure, and propensity to bind with soil and organic particles). These properties are the basis of measures of leachability and volatility of individual hydrocarbons. Thus, the transport or organic chemicals either as individual chemicals or as mixtures (such as the various crude oil-derived products) can be considered by equivalent carbon number to be grouped into thirteen different fractions. The analytical fractions are then set to match these transport fractions, using specific n-alkane derivatives to mark the analytical results for aliphatic compounds and selected aromatic compounds to delineate hydrocarbons containing benzene rings.

Although organic chemicals grouped by transport fraction generally have similar toxicological properties, this is not always the case. For example, benzene is a carcinogen but many alkyl-substituted benzenes do not fall under this classification. However, it is more appropriate to group benzene with compounds that have similar environmental transport properties than to group it with other carcinogens such as benzo(a)pyrene that have very different environmental transport properties. Nevertheless, consultation of any reference work that lists the properties of chemicals will show the properties and hazardous nature of the types of chemicals that are found in petroleum. In addition, petroleum is used to make petroleum products, which can contaminate the environment.

The range of chemicals produced by the organic chemicals industry and by the petroleum refining industry is so vast that summarizing the properties and/or the toxicity or general hazard of crude oil in general or even for a specific crude oil is a difficult task. However, petroleum and some petroleum products, because of the hydrocarbon content, are at least theoretically biodegradable but large-scale spills can overwhelm the ability of the ecosystem to break the oil down. The toxicological implications from petroleum occur primarily from exposure to or biological metabolism of aromatic structures. These implications change as an oil spill ages or is weathered.

4.5 Water Solubility

The water solubility of an organic compound is the solubility of the organic compound in water (typically in grams per liter) at a designated temperature and pressure. At the molecular level, solubility is controlled by the energy balance of intermolecular forces between solute-solute, solvent-solvent, and solute-solvent molecules. Intermolecular forces vary in strength from very weak induced dipole to the much stronger dipole-dipole forces (such as hydrogen bonding). Most organic molecules are relatively nonpolar and are usually soluble in organic solvents (e.g., diethyl ether, dichloromethane, chloroform, organic chemicals ether, hexanes, etc.) but not in polar solvents such as water. However, some organic molecules are more polar and therefore soluble in water, is generally indicative of a high ratio of polar group(s) to the nonpolar hydrocarbon chain, i.e., a low molecular weight compound containing a hydroxyl group (OH) or an amino group (NH₂) or a carboxylic acid group (CO₂H group) or a higher molecular weight compound containing several polar groups. The presence of an acidic carboxylic acid group or a basic amino group in a water-soluble compound can be detected by the low or high pH, respectively, of the solution.

Water solubility is one of the most important properties for evaluating the fate and direct measure of chemical since chemicals with high water solubility will partition readily and rapidly into the aqueous phase and will often remain in solution and be available for degradation. Chemicals that are sparingly soluble in water will often dissolve slowly into solution and partition more readily into other phases including air, solids, and the surface of solid particles including soil (Boethling and Mackay, 2000). Water solubility is used to determine the maximum theoretical concentration of a chemical in the soil pore water and is also important for estimating the air-water partitioning coefficient (K_aw), used to determine the partitioning behavior of an organic chemical in the soil.

There is a simple, very useful, and practical empirical rule that is quite reliable—like dissolves like—and it is based on the polarity of the systems insofar as polar molecules dissolve in polar solvents (such as water, alcohols) and nonpolar molecules dissolve in nonpolar solvents (such as liquid hydrocarbons).

The solubility of organic chemicals is an important property when assessing toxicity since the water solubility of an organic chemical determines the routes of exposure that are possible. In fact, an easy way of assessment of the water solubility of organic chemicals is that the solubility of an organic chemical is approximately inversely proportional to molecular weight—lower molecular weight hydrocarbon derivatives (excluding the presence of polar functional groups) are typically more soluble in water than the higher molecular weight compounds. Thus, lower molecular weight hydrocarbons (specifically, the C₄ to C₈ alkanes, including the aromatic compounds) are relatively soluble, up to approximately 2000 ppm, while the higher molecular weight hydrocarbon derivatives are much less soluble in water. Typically, the most soluble components are also the most toxic but whether this toxicity is due to the chemical and structural aspects of the lower molecular weight derivatives or whether it is noticed more frequently because of the relative ease of transportation is subject to debate.

Finally, each functional group has a particular set of chemical properties that allow it to be identified. Some of these properties can be demonstrated by observing solubility behavior, while others can be seen in chemical reactions that are accompanied by color changes, precipitate formation, or other visible effects. The identification and characterization of the organic chemicals is an important aspect of environmental organic chemistry. Although it is often possible to establish the structure of spectroscopic test methods (Chapter 5), the spectroscopic data should be supplemented with additional information such as (1) the physical state and (2) the relevant properties such as melting point, boiling point, solubility, odor, elemental analysis, and confirmatory tests for the presence of functional groups. The later test methods usually involve simple laboratory tests for solubility from which conclusions can be drawn which, when combined with other test data, can provide valuable information about the nature of the sample, but whether or not this can be applied to complex mixtures is another issue. Nevertheless, the solubility of an organic compound in water, dilute acid, or dilute base can provide useful information about the presence or absence of certain functional groups. For example, (1) solubility in water, (2) solubility in sodium hydroxide, and (3) solubility in hydrochloric acid (Fig. 6.1).

Most organic compounds are insoluble in water, except for low molecular weight amines and oxygen-containing compounds. Low molecular weight compounds are generally limited to those with fewer than five carbon atoms. Carboxylic acids (RCO₂H) with fewer than five carbon atoms in the molecule are soluble in water and form solutions that give an acidic response (pH < 7) when tested with litmus paper. Amines (RNH₂) with fewer than five carbon atoms in the molecule are also soluble in water, and their amine solution gives a basic response (pH > 7) when tested with litmus paper. Ketones, aldehydes, and alcohols with fewer than five carbon atoms are soluble in water and form neutral solutions (pH 7).

In addition, solubility in sodium hydroxide solution (usually 6 M NaOH, 6 molar NaOH) is a positive identification test for acids and acid derivatives. A carboxylic acid that is insoluble in pure water will be soluble in base due to the formation of the sodium salt of the acid as the acid is neutralized by the base. Also, solubility in hydrochloric acid (usually 6 M HCl, 6 molar HCl) is a positive identification test for bases. Amines that are insoluble in pure water will be soluble in hydrochloric acid due to the formation of an ammonium chloride-type salt (RNH₃⁺ Cl⁻).

4.6 Total Petroleum Hydrocarbons

Within the environmental arena, there is often reference to total petroleum hydrocarbons (TPHs) which is used because; given the wide variety of chemicals in petroleum and in petroleum products, it is not practical to consider each constituent separately. After an incident, it is more usual to measure the amount of TPHs at the site. The term TPHs is used environmentally to describe the family of several hundred chemical compounds that originally come from petroleum (Speight, 2001, 2014, 2015). Chemicals that may be included in the TPHs are hexane, heptane (and higher molecular weight homologs), benzene, toluene, xylene isomers (and the higher molecular weight homologs), and naphthalene as well as the constituents of other petroleum products such as gasoline and diesel fuel. It is likely that samples of the TPHs collected at a specific site will contain only some, or a mixture, of these chemicals but samples from different sites cannot be (should not be) expected to be the same in terms of composition and content of chemicals because of the variations in the composition of the starting crude oil feedstocks.

Petroleum products, themselves, are the source of many components, but do not adequately define TPHs. However, the composition of petroleum products assists in understanding the hydrocarbon derivatives that become environmental contaminants, but any ultimate exposure is determined also by how the product changes with use, by the nature of the release, and by the hydrocarbon's environmental fate. When petroleum products are released into the environment, changes occur that significantly affect their potential effects. Physical, chemical, and biological processes change the location and concentration of hydrocarbons at any particular site.

Hydrocarbons (in this context, petroleum-derived hydrocarbons) may enter the environment through accidents, from industrial releases, or as by-products from commercial or private uses such as direct release into water through spills or leaks. When release into water occurs, some of the hydrocarbons float on the water and form surface films while others may sink and form bottom sediments. Bacteria and microorganisms in the water have the potential to break down some of the hydrocarbons over varying periods of time that are dependent on the ambient conditions. On the other hand, hydrocarbon derivatives that are spilled on to the soil may remain for a long time, subject to the molecular size and volatility.

In addition, the amount of TPHs is the measurable amount of petroleum-based hydrocarbon in an environmental medium, whether it is air, water, or land. It is, thus, dependent on analysis of the medium in which it is found and since it is a measured, gross quantity without identification of its constituents, the TPHs’ data still represent a mixture. Thus, the data derived from measurement of the petroleum hydrocarbons in a particular environment is not a direct indicator of risk to humans or to the environment. The data may even be the results from one of several analytical methods, some of which have been used for decades and others developed in the past several years.

Analysis for TPHs (EPA Method 418.1, 2016) provides a one number value of the petroleum hydrocarbons in a given environmental medium. It does not, however, provide information on the composition (i.e., individual constituents) of the hydrocarbon mixture. The amount of hydrocarbon contaminants measured by this method depends on the ability of the solvent used to extract the hydrocarbon from the environmental media and the absorption of infrared light (infrared spectroscopy) by the hydrocarbons in the solvent extract. The method is not specific to hydrocarbon derivatives and does not always indicate petroleum contamination since humic acid, a nonpetroleum material and a constituent of many soils, can be detected by this method. Another analytical method commonly used for TPHs (EPA Method 8015 Modified, 2016) gives the concentration of purgeable and extractable hydrocarbons. These are sometimes referred to as gasoline range organics and diesel range organics because the boiling point ranges of the hydrocarbon in each roughly correspond to those of gasoline (C₆ to C_10–12) and diesel fuel (C_8–12 to C_24–26), respectively. Purgeable hydrocarbons are measured by purge-and-trap gas chromatography analysis using a flame ionization detector, while the extractable hydrocarbons are extracted and concentrated prior to analysis. The results are most frequently reported as single numbers for purgeable and extractable hydrocarbon derivatives. Another method (based on EPA Method 8015 Modified, 2016) gives a measure of the aromatic and aliphatic content of the hydrocarbon in each of several carbon number ranges (fractions). An important feature of the analytical methods for the TPHs is the use of an equivalent carbon number index. This index represents equivalent boiling points for hydrocarbons and is the physical characteristic that is the basis for separating petroleum (and other) components in chemical analysis and for identifying the ratios of specific hydrocarbon derivatives in the sample from which a source might be deduced.