44.1.1 How Often Do Spills Occur?

It can sometimes be misleading to compare oil spill statistics, however, because different methods are used to collect the data. In general, statistics on oil spills are difficult to obtain, and any data set should be viewed with caution. The spill volume or amount is the most difficult to determine or estimate. Sometimes, the exact character or physical properties of the oil lost are not known, and this leads to different estimations of the amount lost.

The number of spills reported also depends on the minimum size or volume of the spill. In both Canada and the United States, most oil spills reported are more than 4000 l (about 1000 gallons) [1]. In Canada, there are about 12 such oil spills every day from all sources, of which only about one is spilled into navigable waters. These 12 spills amount to about 40 tons of oil or petroleum product. In the United States, there are about 15 spills per day into navigable waters and an estimated 85 spills on land or into freshwater [2, 3].

Despite the large number of spills, only a small percentage of oil used in the world is actually spilled. Oil spills in Canada and the United States are summarized in Figures 44.1 and 44.2 in terms of the volume of oil spilled and the actual number of spills. In terms of oil spills, it can be seen from these figures that there are differences between the two countries. The pipeline source of spillage is highlighted in these figures.

In Canada and the United States, most spills take place on land and this accounts for a high volume of oil spilled. This means that the spill occurs on land, however, oil typically finds its way into a water body, be it a stream, lake, or local depression [4]. Pipeline spills account for the highest volume of oil spilled. In terms of the actual number of spills, most oil spills happen at petroleum production facilities, wells, production collection facilities, and battery sites. Pipeline spills dominate the volume spilled in both Canada and the United States; one such pipeline is shown in Figure 44.3. On water, the greatest volume of oil spilled comes from marine or refinery terminals, although the largest number of spills is from the same source as in the United States—vessels other than tankers, bulk carriers, or freighters [6]. It should be noted, however, that even though many spills occur on land that the receiver of these spills may be water bodies, rivers, or creeks.

Table 44.1 shows example summary statistics of pipeline spills for Canada from the Transportation Safety Board (TSB) of Canada [4]. These statistics are for the month of November 2013, and contain year-to-date statistics as well as historical averages. “Accidents” involve some form of accidental occurrence, whereas “incidents” involve other causes. As can be seen from Table 44.1, incidents exceed accidents by a factor of over 10. A larger view of Canadian accident statistics can be obtained on the TSB website [5].

44.1.2 Pipelines

There are many types and sizes of pipelines used for oil transport. Table 44.2 shows the categorization of these by use and size [7].

An important part of the consideration of the pipeline situation is the amount of oil transported by pipeline. Table 44.3 shows a summary of the 2013 monthly statistics on the quantity of oils transported by pipelines [8]. These data are from Statistics Canada. This is the amount of petroleum liquids transported by pipelines averaged over 1 month. Crude oil and pentanes plus include liquids that might be described as condensates. Petroleum products such as gasoline and diesel fuel are included with liquefied petroleum gases.

These tables and Figure 44.1 allow one to estimate the overall loss of crude oils and petroleum products by pipeline in Canada. Since on a typical average 40,000 tons of material are spilled and on average 17% of this is from pipelines, in a typical year about 2400 tons of crude oil and petroleum products are estimated to be spilled. Since there are about 4000 incidents per year, and about 14% of these are from pipelines, there are an estimated 560 incidents per year. The average pipeline spill is estimated to be about 4 tons. The amount spilled per transported amount is 3 × 10⁻⁶ or about 3 in a million. Given that there are about 300,000 km of substantive pipelines (from Table 44.3), the estimated probability per kilometer per year is about 0.002 or about 2 in 1000.

44.2.1 Oil Spill Contingency Plans

Contingency plans can be developed for a particular facility, such as a pipeline pump station, which would include organizations and resources from the immediate area, with escalating plans for spills of greater impact. Contingency plans for larger areas usually focus more on roles, resource availability, and responsibilities and provide the basis for cooperation between the appropriate response organizations rather than on specific response actions. Most contingency plans usually include

• a list of persons and agencies to be notified immediately after a spill occurs;
  
• an organization chart of response personnel and a list of their responsibilities, as well as a list of actions to be taken by them during the various phases of oil spill response;
  
• area-specific action plans;
  
• a communications network to ensure response efforts are coordinated;
  
• protection priorities for the affected areas;
  
• operational procedures for controlling and cleaning up the spill;
  
• reference material such as sensitivity maps and other technical data about the area;
  
• procedures for informing the public and keeping records;
  
• an inventory or database of the type and location of available equipment, supplies, and other resources; and
  
• scenarios for typical spills and decision trees for certain types of response actions, such as using different types of countermeasures.

To remain effective, response options detailed in contingency plans must be tested frequently. This testing is conducted by responding to a practice spill as though it is real. Such exercises not only maintain and increase the skills of the response personnel but also lead to improvements and fine tuning of the plan as weaknesses and gaps are identified in these complex plans.

44.2.2 Activation of Contingency Plans

The response actions defined in contingency plans, whether for spills at a single industrial facility or in an entire region, are separated into the following phases: alerting and reporting; evaluation and mobilization; containment and recovery; decontamination of equipment; disposal; and remediation or restoration. In practice, these phases often overlap rather than follow each other consecutively.

Most contingency plans also allow for a “tiered response,” which means that response steps and plans escalate as the incident becomes more serious. As the seriousness of an incident is often not known in the initial phases, one of the first priorities is to determine the magnitude of the spill and its potential impact.

44.2.3 Training

A high-quality training program is vital for a good oil spill response program. Response personnel at all levels require training in specific operations and in using equipment for containing and cleaning up spills. To minimize injury during response, general safety training is also crucial. In many countries, response personnel are required to have 12–40 h of safety training before they can perform field work. Ongoing training and refresher courses are also essential in order to maintain and upgrade skills.

44.2.4 Supporting Studies and Sensitivity Mapping

A contingency plan usually includes background information on the area covered by the plan. This consists of data collected from studies and surveys and often takes the form of a sensitivity map for the area. Sensitivity maps contain information on potentially sensitive physical and biological resources that could be affected by an oil spill. This includes concentrations of wildlife such as mammals, birds, and fish; human amenities, such as recreational beaches; natural features such as types of land.

44.2.5 Oil Spill Cooperatives

As most oil or pipeline companies that handle oil do not have staff dedicated to cleaning up oil spills, several companies in the same area often join forces to form cooperatives. By pooling resources and expertise, these oil spill cooperatives can then develop effective and financially viable response programs. The cooperative purchases and maintains containment, cleanup, and disposal equipment and provides training for its use.

In some spill situations, especially large spills, volunteers are an important part of the response effort. Volunteers are usually trained and their efforts coordinated with the main spill cleanup.

44.2.6 The Effectiveness of Cleanup

One topic of concern is the quantity of oil actually cleaned up during an oil spill. There is a public perception that most of the spills are not being cleaned up. More than 35 years ago, there was an estimate that only 10–30% of spills were actually cleaned up and removed. One must remember, however, that much of the oil may have evaporated and some lost to sedimentation or other unrecoverable processes. In the past decade, spills can be accounted for in much greater percentages, as high as 90%, once one accounts for losses such as sedimentation and evaporation. Further, there is a very large difference between small spills and very large catastrophic spills because it is much easier to remove a large portion of a small spill.

44.3.1 The Composition of Oil

Crude oils are mixtures of hydrocarbon compounds ranging from smaller, volatile compounds to very large compounds [6]. This mixture of compounds varies according to the geological formation of the area in which the oil is found and strongly influences the properties of the oil. For example, crude oils that consist primarily of large compounds are viscous and dense. Petroleum products such as gasoline or diesel fuel are mixtures of fewer compounds and thus their properties are more specific and less variable.

Oils also contain varying amounts of sulfur, nitrogen, oxygen, and sometimes mineral salts, as well as trace metals such as nickel, vanadium, and chromium.

The following are the oils used in this section to illustrate the fate, behavior, and cleanup of oil spills:

1. Gasoline: as used in automobiles;
   
2. Diesel fuel: as used in trucks, trains, and buses;
   
3. A light crude oil: as produced in great abundance in many countries;
   
4. A heavy crude oil: as produced in many countries, specific examples will be given here;
   
5. Bunker fuel: such as Bunker C, which is a heavy residual fuel remaining after the production of gasoline and diesel fuel in refineries and often used in heating plants;
   
6. Dilbit: a diluted bitumen, the diluent is typically a very light oil or condensate and
   
7. Bitumen: a heavy oil material typically from the oil sands operations in northern Alberta.

44.3.2 Properties of Oil

The properties of oil discussed here are viscosity, density, and specific gravity [9, 10]. These properties of the oils discussed in this chapter are listed in Table 44.4.

Viscosity is the resistance to flow in a liquid. The lower the viscosity, the more readily the liquid flows. For example, water has a low viscosity and flows readily, whereas peanut butter, with a high viscosity, flows poorly. The viscosity of the oil is largely determined by the amount of lighter and heavier fractions that it contains. Viscosity is affected by temperature, with a lower temperature giving a higher viscosity. Viscous oils do not spread rapidly, do not penetrate soil as readily, and affect the ability of pumps and skimmers to handle the oil.

Density is the weight of a given volume of oil and is typically expressed in grams per cubic centimeter (g/cm³). It is the property used by the petroleum industry to define light or heavy crude oils. Density is also important as it indicates whether a particular oil will float or sink in water. As the density of water is 1.0 g/cm³ at 15 °C and the density of most oils ranges from 0.7 to 0.99 g/cm³, most oils will float on water. As the density of seawater is 1.03 g/cm³, even heavier oils will usually float on it. Another measure of density is specific gravity, which is an oil’s relative density compared to that of water at 15 °C. It is the same value as density at the same temperature. Another gravity scale is that of the American Petroleum Institute (API). The API gravity is based on the density of pure water, which has an arbitrarily assigned API gravity value of 10°. Oils with progressively lower specific gravities have higher API gravities.

The following is the formula for calculating API gravity: API gravity = [141.5 ÷ (density at 15.5 °C)] − 131.5. Oils with high densities have low API gravities and vice versa.

44.4.1 An Overview of Weathering

Oil spilled on water undergoes changes in physical and chemical properties, which in combination are termed “weathering” [11, 12]. Weathering processes occur at very different rates but begin immediately after oil is spilled into the environment. Weathering rates are usually highest immediately after the spill. Both weathering processes and the rates at which they occur depend more on the type of oil than on environmental conditions. Most weathering processes are highly temperature dependent, however, and will often slow to insignificant rates as temperatures approach zero degrees.

The processes included in weathering are evaporation, emulsification, natural dispersion, dissolution, photooxidation, sedimentation, adhesion to materials, interaction with mineral fines, biodegradation, and the formation of tar balls. These processes are listed in order of importance in terms of their effect on the percentage of total mass balance, that is, the greatest loss from the slick in terms of percentage and what is known about the process. The ones that are most important to oil spill cleanup are evaporation and emulsification.

44.4.2 Evaporation

Evaporation is typically the most important weathering process. Over a period of several days, a light fuel such as gasoline evaporates completely at temperatures above freezing, whereas only a small percentage of heavier Bunker C oil evaporates. The rate at which an oil evaporates depends primarily on the oil’s composition. The more volatile components an oil or fuel contains, the greater the extent and rate of its evaporation. Many components of heavier oils will not evaporate at all, even over long periods of time and at high temperatures.

Oil and petroleum products evaporate in a slightly different manner than water, and the process is much less dependent on wind speed and surface area. Oil evaporation can be considerably slowed down, however, by the formation of a “crust” or “skin” on top of the oil. The skin or crust is formed when the smaller compounds in the oil are removed leaving some compounds, such as waxes and resins, at the surface. These then seal off the remainder of the oil and prevent evaporation. The rate of evaporation is very rapid immediately after a spill and then slows considerably. About 80% of evaporation occurs in the first 2 days after a spill. The properties of an oil can change significantly with the extent of evaporation. If about 40% of an oil evaporates, its viscosity could increase by as much as a thousand-fold. Its density could rise by as much as 20%.

An example of how evaporation is important relates to Dilbit. Dilbit rapidly loses the more volatile diluent portion and, within about a week or sooner of exposure to air, will return to the properties of the Bitumen from which it was created. This Bitumen typically has very different physical properties from the Dilbit.

44.4.3 Emulsification and Water Uptake

Water can enter oil through several processes [13]. Water can be present in oil in five ways. First, some oils contain about 1% water as soluble water. This water does not significantly change the physical or chemical properties of the oil. The second way is when water droplets are not held in the oil long enough to form an emulsion. These are called oils that do not form any type of water-in-oil mixtures or unstable emulsions. Unstable emulsions break down into water and oil within minutes or a few hours at most once the sea energy diminishes. Meso-stable emulsions represent the third way water can be present in oil. Meso-stable emulsions are formed when the small droplets of water are stabilized to a certain extent by a combination of the viscosity of the oil and the interfacial action of asphaltenes and resins. The fourth way that water exists in oil is in the form of stable emulsions. These form in a way similar to meso-stable emulsions except that the oil contains sufficient asphaltenes and resins to stabilize the mixture. The viscosity of stable emulsions is 800–1000 times higher than that of the starting oil, and the emulsion will remain stable for weeks and even months after formation. Stable emulsions are reddish-brown in color and appear to be nearly solid. The fifth way that oil can contain water is by viscosity entrainment. If the viscosity of the oil is such that water droplets can penetrate but will only slowly migrate downward, the oil can contain about 30–40% water as long as it is in an energetic body of water. Once the water calms or the oil is removed, the water slowly drains. Such water uptake is called “entrained water.”

44.4.4 Biodegradation

A large number of microorganisms are capable of degrading petroleum hydrocarbons. Hydrocarbons metabolized by microorganisms are generally converted to an oxidized compound, which may be further degraded, may be soluble, or may accumulate in the remaining oil [14–16]. The aquatic toxicity of the biodegradation products is sometimes greater than that of the parent compounds. Biodegradation occurs slowly and is not an important removal mechanism for oil spills.

44.4.5 Spreading

Oil spreads to a lesser extent and very slowly on land than on water (Figure 44.4). Oil spilled on or under ice spreads relatively rapidly but does not spread to as thin as on water. On any surface other than water, such as ice or land, a large amount of oil is retained in depressions, cracks, and other surface irregularities.

44.4.6 Movement of Oil Slicks on Water

In addition to their natural tendency to spread, oil slicks on water are moved along the water surface, primarily by surface currents and winds [17]. The slick generally moves at a rate that is 100% of the surface current and approximately 3% of the wind speed. If the wind is more than about 20 km/h, however, and the slick is on open water, wind predominates in determining the slick’s movement. Both the wind and surface current must be considered for most situations.

44.4.7 Sinking and Over Washing

If oil is denser than the surface water, it may sometimes actually sink. Some rare types of heavy crudes and Bunker C can reach these densities and sink [12]. Dense materials such as Bitumen can sink in freshwater but rarely in seawater. When oil does sink, it complicates cleanup operations as the oil can be recovered only with specialized underwater suction devices or special dredges. Sinking sometimes also occurs when sediment and oil interact. This results in a mixture that is denser than water.

44.4.8 Spill Modeling

Spill response personnel need to know the direction in which an oil spill is moving in order to protect sensitive resources and coastline [11, 12]. To assist them with this, computerized mathematical models have been developed to predict the trajectory or pathway and fate of oil.

44.5.1 Sampling and Laboratory Analysis

Taking a sample of oil and then transporting it to a laboratory for subsequent analysis is common practice. While there are many procedures for taking oil samples, it is always important to ensure that the oil is not tainted from contact with other materials and that the sample bottles are precleaned with solvents, such as hexane, that are suitable for the oil. The simplest and most common form of analysis is to measure how much oil is in water, soil, or sediment samples. Such analysis results in a value known as total petroleum hydrocarbons (TPH). A more sophisticated form of analysis is to use a gas chromatograph (GC). One type of detector used on a gas chromatogram is a mass spectrometer (MS). The method is usually called GC–MS and can be used to quantify and identify many components of oil [18].

44.5.2 Detection and Surveillance

44.6.1 Types of Booms and Their Construction

A boom is a floating mechanical barrier designed to stop or divert the movement of oil on water. Booms resemble a vertical curtain with portions extending above and below the water line. Booms are constructed in sections, usually 15 or 30 m long, with connectors installed on each end so that sections of the boom can be attached to each other, towed, or anchored. The three basic types of booms are fence and curtain booms, which are common, and shoreline seal booms.

44.6.2 Uses of Booms

Booms are used to enclose floating oil and prevent it from spreading, to protect biologically sensitive areas, to divert oil to areas where it can be recovered or treated, and to concentrate oil and maintain an adequate thickness so that skimmers can be used or other cleanup techniques, such as in-situ burning, can be applied. Booms are used primarily to contain oil, although they are also used to deflect oil. When used for containment, booms are often arranged in a U configuration. The U-shape is created by the current pushing against the center of the boom. The critical requirement is that the current in the apex of the “U” does not exceed 0.5 m/s or 1 knot, which is referred to as the critical velocity. This is the speed of the current flowing perpendicular to the boom, above which oil will be lost from the boom.

If used in areas where the currents are likely to exceed 0.5 m/s or 1 knot, such as in rivers and estuaries, booms are often used in the deflection mode. The boom is then deployed at various angles to the current. Figure 44.5 shows the use of diversionary booms in a fast-flowing river. The oil can then be deflected to areas where it can be collected or to less sensitive areas. If strong currents prevent the best positioning of the boom in relation to the current, several booms can be deployed in a cascading pattern to progressively move oil toward one side of the watercourse. This technique is effective in wide rivers or where strong currents may cause a single boom to fail. When booms are used for deflection, the forces of the current on the boom are usually so powerful that stronger booms are required, and they must be anchored along their entire length.

44.6.3 Boom Failures

A boom’s performance and its ability to contain oil are affected by water currents, waves, and winds [20–22]. Either alone or in combination, these forces often lead to boom failure and loss of oil. The most critical factor is the current speed relative to the boom. Failures will occur when this exceeds 0.35 m/s (0.7 knots).

44.6.4 Sorbent Booms and Barriers

Sorbent booms are specialized containment and recovery devices made of porous sorbent material such as woven or fabric polypropylene, which absorbs the oil while it is being contained [23]. Sorbent booms are used when the oil slick is relatively thin, that is, for the final “polishing” of an oil spill, to remove small traces of oil or sheen, or as a backup to other booms. Sorbent booms are often placed off a shoreline that is relatively unoiled or freshly cleaned to remove traces of oil that may recontaminate the shoreline. They are not absorbent enough to be used as a primary countermeasure technique for any significant amount of oil. Oil sorbent booms must also be removed from the water carefully to ensure that oil is not forced from them and the area recontaminated. Sorbent booms are often used on land spills to contain and remove light surface oiling.

44.7.1 Skimmers

Skimmers are mechanical devices designed to remove floating oil from a water surface. They vary greatly in size, application, and capacity, as well as in recovery efficiency [24, 25]. Skimmers are classified according to the area where they are used, for example, inshore, offshore, in shallow water, or in rivers, and by the viscosity of the oil they are intended to recover, that is heavy or light oil.

The effectiveness of a skimmer is rated according to the amount of oil that it recovers, as well as the amount of water picked up with the oil. Effectiveness depends on a variety of factors, including the type of oil spilled, the properties of the oil such as viscosity, the thickness of the slick, sea conditions, wind speed, ambient temperature, and the presence of ice or debris.

Most skimmers function best when the oil slick is relatively thick. The oil must, therefore, be collected in booms before skimmers can be used effectively (Figure 44.6). The skimmer is placed wherever the oil is most concentrated in order to recover as much oil as possible. Weather conditions at a spill site have a major effect on the efficiency of skimmers. Depending on the type of skimmer, most will not work effectively in waves >1 m or in currents exceeding 1 knot. Most skimmers do not operate effectively in waters with ice or debris, such as branches, seaweed, and floating waste. Some skimmers have screens around the intake to prevent debris or ice from entering, conveyors or similar devices to remove or deflect debris, and cutters to deal with seaweed. Very viscous oils, tar balls, or oiled debris can clog the intake or entrance of skimmers and make it impossible to pump oil from the skimmer’s recovery system.

Skimmers are also classified according to their basic operating principles: oleophilic surface skimmers; weir skimmers; suction skimmers, or vacuum devices; elevating skimmers; and submersion skimmers.

44.7.2 Sorbents

Sorbents are materials that recover oil through either absorption or adsorption [23, 26]. They play an important role in oil spill cleanup and are used in the following ways: to clean up the final traces of oil spills on water or land; as a backup to other containment means, such as sorbent booms; as a primary recovery means for very small spills; and as a passive means of cleanup. An example of such passive cleanup is when sorbent booms are anchored off lightly oiled shorelines to absorb any remaining oil released from the shore and prevent further contamination or reoiling of the shoreline.

Sorbents can be natural or synthetic materials. Natural sorbents are divided into organic materials, such as peat moss or wood products, and inorganic materials, such as vermiculite or clay. Sorbents are sometimes available in a loose form, which includes granules, powder, chunks, and cubes, often contained in bags, nets, or socks. Sorbents are typically available and formed into pads, rolls, blankets, and pillows. The use of synthetic sorbents in oil spill recovery has increased in the last few years. These sorbents are often used to wipe other oil spill recovery equipment, such as skimmers and booms, after a spill cleanup operation. Sheets of sorbent are often used for this purpose.

44.7.3 Manual Recovery

Small oil spills or those in remote areas are sometimes recovered by hand. Heavier oils are easier to manually remove than lighter oils. Spills on water close to shorelines are sometimes cleaned up with shovels, rakes, or by cutting the oiled vegetation. Hand bailers, which resemble a small bucket on the end of a handle, are sometimes used to recover oil from the water surface. Manual recovery is tedious and may involve dangers such as physical injury from falls.

44.8.1 Temporary Storage

When oil is recovered, sufficient storage space must be available for the recovered product. The recovered oil often contains large amounts of water and debris, which increases the amount of storage space required. Several types of specially built tanks are available to store recovered oil. Flexible portable tanks are common type of storage used for spills recovered on land and from rivers and lakes.

44.8.2 Pumps

Pumps play an important role in oil spill recovery. They are an integral part of most skimmers and are also used to transfer oil from skimmers to storage tanks. Pumps used for recovered oil differ from water pumps in that they must be capable of pumping very viscous oils and dealing with water, air, and debris.

44.8.3 Vacuum Systems

Vacuum systems consist of vacuum pumps and tanks mounted on a skid or truck. The vacuum pump creates a vacuum in the tank and the oil moves directly through a hose or pipe to the tank from the skimmer or the source of the oil. The oil does not go through the pump but moves directly from its source into the tank. Vacuum systems can cope with debris, viscous oils, and the intake of air or water.

44.8.4 Recovery from the Water Subsurface

Recovery of submerged oil on the bottom has been carried out in many different ways in the past [27]. Diver-directed pumping has been used often because divers are efficient in finding and contacting the oil when the oil coverage varies in size and thickness. The biggest problem is dealing with the large amount of recovered water and sediment along with viscous oils. Remotely operated vehicle pumping systems have largely been used to set up other equipment, but recently are proposed to cleanup oil. Several types of dredges have been used to recover oil from the bottom. Where the oil is solidified, environmental clamshell dredges have been used successfully. Hopper dredges have been proposed in the past, but the large volumes of water and sediment generated, compared to the amount of oil recovered, is a significant factor. Figure 44.7 shows the recovery of oil using a backhoe.

44.8.5 Separation

As all skimmers recover some water with the oil, a device to separate oil and water is usually required [28]. The oil must be separated from the recovery mixture for disposal, recycling, or direct reuse by a refinery. Sometimes, settling tanks or gravity separators are incorporated into skimmers, but separators are more often installed on recovery ships or barges. Portable storage tanks are often used as separators, with outlets installed on the bottom of the tanks so that water that has settled to the bottom of the tank can be drained off, leaving the oil in the tank. Vacuum trucks are also used to separate oil and water.

44.8.6 Decontamination

Equipment and vessels used during spills often become “contaminated” or covered with oil. Before transporting this equipment further, it is decontaminated. This typically involves removal to a lined area, a high-pressure wash, and treatment of the recovered water. Special areas are prepared for the decontamination of vessels, booms, or skimmers. Large vessels must of necessity, be decontaminated on the water and this involves circling the vessel with booms and recovering the oil released from the vessel. Often, lightly contaminated vessels are cleaned by hand using sorbent cloths. This procedure avoids the extra procedures of booming and oil recovery.

The primary tool for oil removal is high-pressure water. The water released from decontamination is treated as recovered oil would be. It is separated and the oil placed in recovered oil collection tanks.

A final note on this topic is that workers must also decontaminate their boots and clothing if covered with oil. Stations are often set up very close to embarkation points to avoid carrying contamination further.

44.8.7 Disposal

Disposing of the recovered oil and oiled debris is one of the most difficult aspects of an oil spill cleanup operation [29]. Any form of disposal is subject to a complex system of local, provincial or, state, and federal legislation. Unfortunately, most recovered oil consists of a wide range of contents and cannot be classified as simply liquid or solid waste. The recovered oil may contain water that is difficult to separate from the oil and many types of debris, including vegetation, sand, gravel, logs, branches, garbage, and pieces of containment booms. Incineration is a frequent means of disposal for recovered material. Approval must be obtained from government regulatory authorities. Emission guidelines for incinerators may preclude simply placing material into the incinerator.

44.10.1 Advantages

Burning has some advantages over other spill cleanup techniques, the most significant of which is its ability to be a final solution and its capacity to rapidly remove large amounts of oil. Burning can prevent oil from spreading to other areas and contaminating shorelines and biota.

Burning oil is a final, one-step solution. When oil is recovered mechanically, it must be transported, stored, and disposed of, which requires equipment, personnel, time, and money. Often, not enough of these resources are available when large spills occur. Burning generates a small amount of burn residue that can be recovered or further reduced through repeated burns.

It can be applied in remote areas where other methods cannot be used because of distances and lack of infrastructure. In some circumstances, such as when oil is mixed with or on ice, it may be the only available option for dealing with an oil spill. Finally, while the efficiency of a burn varies with a number of physical factors, removal efficiencies are generally greater than those for other response methods, such as skimming and the use of chemical dispersants. During several tests and actual burns, efficiency rates as high as 98% were achieved.

44.10.2 Disadvantages

The most obvious disadvantage of burning oil is the large black smoke plume. The concerns revolve around toxic emissions from the large black smoke plume. These emissions have been studied and can be dealt with. The second disadvantage is that the oil will not ignite and burn quantitatively unless it is thick enough. Most oils spread rapidly on water, and the slick quickly becomes too thin for burning to be feasible. Fire-resistant booms are used to concentrate the oil on water into thicker slicks so that the oil can be burned. Burning oil is sometimes not viewed as an appealing alternative to collecting the oil and processing it for reuse. Reprocessing facilities for this purpose, however, are not readily accessible in most parts of the world. Another factor that discourages the reuse of oil is that recovered oil often contains too many contaminants for reuse and is incinerated instead.

44.10.3 Ignition and What Will Burn

Early studies of in situ burning focused on ignition as being the key to the successful burning of oil on water [34]. It has since been found that ignition can be difficult, but only under certain circumstances. Figure 44.8 shows a land burn that is generally easier to ignite than oil on water [34]. Ignition may be difficult, however, at winds >20 m/s (40 knots). An important fact of in-situ burning is that oils can be readily ignited if they are at least 1–3 mm thick and will continue to burn down to slicks about 1/2–1 mm thick. Sufficient heat is required to vaporize material so the fire will continue to burn. In very thin slicks, most of the heat is lost to the water, and vaporization/combustion is not sustained.

Often, a primer, such as diesel fuel, may be needed to ignite heavy oils. This is also the case for oil that contains water, although oil that is completely emulsified with water is difficult to ignite. Several burns have been conducted in which some emulsion or high-water content in the oil did not affect the efficiency of the burn.

44.10.4 Burn Efficiency and Rates

Burn efficiency is measured as the percentage of starting oil removed compared to the amount of residue left. The amount of soot produced is usually ignored as it is a small amount and difficult to measure. Burn efficiency is largely a function of oil thickness. If a 2-mm thick slick is ignited and burns down to 1 mm, the maximum burn efficiency is 50%. If a 20-mm thick pool of oil is ignited, however, and burns down to 1 mm, the burn efficiency is about 95%. Recent research has shown that these efficiency values are affected by other factors such as the type of oil and the amount of water content. Higher efficiency is usually achieved when towing a fire-resistant boom as the oil is continually driven to the rear, burned, and leaving only a small amount of residue unburned at the end.

Most of the residue from burning oil is unburned oil, with some lighter or more volatile products removed. The residue is adhesive and, therefore, can be recovered manually. Residue from burning heavier oils and from very efficient burns may sometimes sink in water, although this rarely happens as the residue, when cooled, is only slightly denser than sea water.

Most oil pools burn at a rate of about 2–4 mm/min, which means that the depth of oil is reduced by 2–4 mm each minute. Table 44.5 shows the burning characteristics of several oils.

An optimal burn rate for diesel fuel and light crudes is about 5000 1 of oil per m² per day.

44.10.5 Use of Containment

As previously discussed, oil can be burned on water without using containment booms if the slick is thick enough (2–3 mm) to ignite. For most crude oils, however, this thickness is only maintained for a few hours, at most, after the spill occurs. Most oil on the open sea rapidly spreads to an equilibrium thickness, which is about 0.01–0.1 mm for light crude oils and about 0.05–0.5 mm for heavy crudes and residual oils. Such slicks are too thin to ignite and containment is required to concentrate the oil so it is thick enough to ignite and burn efficiently.

Fire-resistant booms are also used by spill responders to isolate the oil from the source of the spill. When considering burning as a spill cleanup technique, the integrity of the source of the spill and the possibility of further spillage is always a priority. If there is any possibility that the fire could flash back to the source of the spill, such as the pumping station, the oil is not ignited.

Fire-resistant booms are made of a variety of materials, including ceramic, stainless steel, and water-cooled fiberglass. Fire booms must withstand high temperatures, high heat flux as well as mechanical forces during an oil spill burn. In addition, it is expected that a particular fire-resistant boom should withstand a multi-hour burn and be able to be reused several times.

During the Deepwater Horizon spill in the Gulf of Mexico, more than 400 burns were carried out using fire-resistant booms [27].

Oil is sometimes contained by natural barriers such as land forms, shorelines, offshore sand bars, or ice. Several successful experiments and burns of actual spills have shown that ice acts as a natural boom so that in situ burning can be carried out successfully for spills in ice.

44.10.6 Emissions from Burning Oil

The possibility of releasing toxic emissions into the atmosphere or the water is a barrier to the widespread acceptance of burning oil as a spill countermeasure. Some atmospheric emissions of concern are particulate matter precipitating from the smoke plume, combustion gases, and unburned hydrocarbons. While soot particles consist primarily of carbon particles, they also contain a number of adsorbed chemicals. The residue left at the burn site is also a matter of concern. Possible water emissions include sinking or floating burn residue and soluble organic compounds.

Extensive studies have been conducted to measure and analyze all these components of emissions from oil spill burns [34]. The emphasis in sampling has been on air emissions at ground level as these are the primary human health concern and the regulated value. Most burns produce an abundance of particulate matter. The particulate matter at ground level is a health concern close to the fire and under the plume, although concentrations decline rapidly downwind from the fire. The greatest concern is the smaller or respirable particles that are 2.5 μm or less in size. Concentrations at ground level (1.5 m) can still be above normal health concern levels (9 μm³/m³) as close downwind as 500 m, such as from the amount of oil that could be contained in a typical burn.

Polyaromatic hydrocarbons (PAHs) are a primary concern in the emissions from burning oil, both in the soot particles and as a gaseous emission. All crude oils contain PAHs, varying from as much as 1% down to about 0.001%. Most of these PAHs are burned to fundamental gases except those left in the residue and the soot. In summary, PAHs are not a serious concern when assessing the impact of burning oil.

Current thinking on burning oil as an oil spill cleanup technique is that airborne emissions are not a serious health or environmental concern, especially at distances greater than a few kilometers from the fire. Studies have shown that emissions are low compared to other sources and generally result in concentrations of air contaminants that are below health concern levels 500 m downwind from the fire [34].

44.11.1 Behavior of Oil on Shorelines

The fate and behavior of oil on shorelines are influenced by many factors, some of which relate to the oil itself, some to characteristics of the margin or shoreline, and others to conditions when the oil is deposited on the shoreline [35, 36]. These factors include the type and amount of oil, the degree of weathering of the oil, both before it reaches the shoreline and while on the shoreline, the temperature, the water level when the oil washes onshore, the type of beach substrate, that is, its material composition, the type and sensitivity of biota on the beach, and the steepness of the shore.

The extent to which an oil penetrates and spreads, its adhesiveness, and how much the oil mixes with the type of material on the shoreline or land surface are all important factors in terms of cleanup. Cleanup is more difficult if the oil penetrates deeply into the shoreline. Penetration varies with the type of oil and the type of material on the shoreline. For example, oil does not penetrate much into fine beach material such as sand or clay, but will penetrate extensively on a shore consisting of coarse boulders. A very light oil such as diesel on a cobble beach can penetrate to about a meter under some conditions and is difficult to remove. On the contrary, a weathered crude deposited on fine sand can remain on the surface indefinitely and is removed fairly easily using mechanical equipment. The adhesiveness of the stranded oil varies with the type of oil and the degree of weathering. Most fresh oils are not highly adhesive, whereas weathered oils often are. Diesel and gasoline are relatively nonadhesive, crudes are generally moderately adhesive when fresh and more adhesive when weathered, and Bunker C is adhesive when fresh and highly adhesive when weathered. An oil that is not adhesive when it reaches the shore may get washed off, at least partially, on the next tidal cycle.

44.11.2 Types of Shorelines

The type of shoreline or similar land surface is crucial in determining the fate and effects of an oil spill as well as the cleanup methods to be used. In fact, the shoreline’s basic structure and the size of the material present are the most important factors in terms of oil spill cleanup. There are many types of shorelines, all of which are classified in terms of sensitivity to oil spills and ease of cleanup. The types of interest are bedrock, man-made solid structures, boulder beaches, pebble–cobble, mixed sand–gravel beaches, sand beaches, sand tidal flats, mud tidal flats, marshes, and peat and low-lying tundra. These types occur on both seashore and freshwater shores.

Marshes are important ecological habitats that often serve as nurseries for water and bird life in the area. Marshes range from fringing marshes, which are narrow areas beside a main water body, to wide marsh meadows. Marshes are rich in vegetation that traps oil. Light oils can penetrate into marsh sediments through animal burrows or cracks. Heavier oils tend to remain on the surface and smother plants or animals. Oiled marshes may take years or even decades to recover. Marshes are difficult to access and entering them by foot or by vehicle can cause more damage than the oil itself.

Peat and low-lying tundra are similar types of shorelines or landforms found in the Arctic regions. Although different, they have similar sensitivity and cleanup methodologies. Peat is a spongy, fibrous material formed by the incomplete decomposition of plant materials. Peat erodes from tundra cliffs and often accumulates in sheltered areas, as does oil. Oil does not penetrate wet peat, but dry peat can absorb large amounts of oil.

44.11.3 Shoreline Cleanup Assessment Technique (SCAT)

Priorities for shoreline cleanup are based on a sophisticated shoreline assessment procedure. A systematic evaluation of oiled shorelines can minimize damage to the most sensitive shorelines. When an oil spill occurs, site assessment surveys are usually conducted in direct support of spill response operations. These surveys rely heavily on previously obtained data, maps, and photographs. For example, the structure of the beach is usually already mapped and recorded as part of the sensitivity mapping exercise for the area.

The following are the objectives of site assessment surveys:

• to document the oiling conditions and the physico-ecological character of the oiled shoreline, using standardized procedures;
  
• to identify and describe human use and effects on the shoreline’s ecological and cultural resources;
  
• to identify constraints on cleanup operations;
  
• to verify existing information on environmental sensitivities or compare it to observations from the aerial survey; and
  
• to define the most appropriate cleanup techniques for each segment of the shoreline.

44.11.4 Cleanup Methods

Many methods are available for removing oil from shorelines or landforms. All of them are costly and take a long time to carry out. The selection of the appropriate cleanup technique is based on the type of substrate, the depth of oil in the sediments, the amount and type of oil and its present form/condition, the ability of the shoreline to support traffic, the environmental, human, and cultural sensitivity of the shoreline, and the prevailing ocean and weather conditions. The cleanup techniques suitable for use on the various types of landforms or shorelines are listed in Table 44.6.

The primary objective of cleanup operations is to minimize the effects of the stranded oil and accelerate the natural recovery of affected areas. Obviously, a cleanup technique should be safe and effective and not be so intrusive as to cause more damage than the oil itself. In general, cleanup techniques should not be used if they endanger human life or safety, leave toxic residue or, contaminate other shorelines or lower tidal areas, or kill plants and animals on the shoreline. In addition, excessive amounts of shoreline material should not be removed and the structure of the shoreline should not be changed so as to make it unstable. Finally, any technique that generates a lot of waste material should not be used. In the past, heavy equipment used on beaches resulted in thousands of tons of contaminated beach material, which then required disposal.

The length of time required to complete the cleanup is another important criterion when selecting a cleanup technique. The longer oil is on a beach, the harder it is to clean up. A method that removes most of the mobile oil rapidly is much better, in many circumstances, than a more thorough one that takes weeks to carry out. Time often dictates the cleanup method used.

44.11.5 Recommended Cleanup Methods

Some recommended shoreline cleanup methods are natural recovery, manual removal, flooding or washing, use of vacuums, mechanical removal, tilling and aeration, sediment reworking or surf washing, and the use of sorbents or chemical cleaning agents.

Sometimes, the best response to an oil spill on a shoreline may be to leave the oil and monitor the natural recovery of the affected area. This would be the case if more damage would be caused by cleanup than by leaving the environment to recover on its own. This option is suitable for small spills in sensitive environments and on a beach that will recover quickly on its own such as on exposed shorelines and with nonpersistent oils such as diesel fuel on impermeable beaches. This is not an appropriate response if important ecological or human resources are threatened by the long-term persistence of the oil.

Manual removal is the most common method of shoreline cleanup. Teams of workers pick up oil, oiled sediments, or oily debris with gloved hands, rakes, forks, trowels, shovels, sorbent materials, hand bailers, or poles. It may also include scraping or wiping with sorbent materials or sifting sand to remove tar balls. Material is usually collected directly into plastic bags, drums, or buckets for transfer. Figure 44.9 shows the manual removal of oil weeds on a lake shore.

Flooding or washing shorelines or landforms are common cleanup methods. Low-pressure washing with cool or lukewarm water causes little ecological damage and removes oil quickly. Warmer water removes more oil but causes more damage. High pressure and temperature cause severe ecological damage and recovery may take years. It is preferable to leave some oil on the shoreline than to remove more oil but kill all the biota with high pressure or temperature water.

Low-pressure cool or warm water washing uses water at pressures less than about 200 kPa (50 psi) and temperatures less than about 30 °C. Water is applied with hoses that do not focus the water excessively, avoiding the loss of plants and animals. Flooding is a process in which a large flow or deluge of water is released on the upper portion of the beach.

Low-pressure washing and flooding are often combined to ensure that oil is carried down the beach to the water, where it can be recovered with skimmers. Washing and flooding are best done on impermeable shoreline types and are not useful for shorelines with fine sediments such as sand or mud. These techniques are not used on shorelines where sensitive plant species are growing.

Several sizes of vacuum systems are useful for removing liquid oil that has pooled or collected in depressions on beaches and shorelines. Vacuum trucks used for collecting domestic waste are often used to remove large pools of recovered oil rather than for recovering oil directly on the spill site. For safety reasons, vacuum devices are not used for oils that are volatile.

Mechanical removal involves removing the surface oil and oiled debris with tractors, front-end loaders, scrapers, or larger equipment such as road graders and excavators. There are many types of specialized beach cleaning now available. These machines can screen out tar balls and return clean sand directly to the beaches. Front-end loaders and backhoes are used on a variety of beaches to move oiled materials and to expose buried material as well as to remove materials recovered manually from the beach. While mechanical devices remove oil quickly from shorelines, they also remove large amounts of other material and generate more waste than other techniques, unless specialized cleaners are used. Sand and sand–gravel shorelines are best suited to this technique as they can support mechanical equipment and are not usually damaged by the removal of material. Mechanical equipment should not be used on sensitive shorelines, shorelines with an abundance of plant and animal life, and shorelines that would become unstable if large amounts of material were removed.

Sorbents are used in several ways in beach cleanup. In a passive role, sorbents are left in place, on or near a beach, to absorb oil that is released from the beach by natural processes and prevent it from recontaminating other beaches or contacting wildlife. Sorbent booms as well as “pom-poms” designed for heavy oil can be staked on the beach or in the water on the beach face to catch oil released naturally. Loose sorbents such as peat moss and wood chips are generally not used because they may sink and migrate into nonoiled areas and are difficult to recover.

In-situ burning is useful if the water level is high and the burn residue is either removed or does not suppress future plant growth. Oil will not burn on a typical beach unless the oil is pooled or concentrated in sumps or trenches. In fact, burning is a useful restorative method for marshes if done in spring when the water level is high so that the heat does not affect the plant roots. Burning in late summer or early fall, however, can kill much of the plant life. An alternative is to flood the marsh using berms and pumps, which will raise up some of the oil for burning.

44.12.1 Behavior of Oil on Land

The spreading of oil across the surface and its movement downward through soil and rock is far more complicated and unpredictable on land than the spreading of oil on water. The movement of the oil varies for different types of oil and in different habitats and is influenced by conditions at the spill site, including the specific soil types and their arrangement, moisture conditions in the soil, the slope of the land, and the level and flow rate of the groundwater. Other factors, which vary in different habitats, are the presence of vegetation and its type and growth phase, the temperature, the presence of snow and ice, and the presence of micro-features, such as rock outcrops.

The basic types of soil to consider in relation to land oil spills are sand/gravel, loam, clay, and silt. Soil is defined as the loose unconsolidated material located near the surface, while rock is the hard consolidated material, that is, bedrock, usually found beneath the soil. Most soils consist of small fragments or grains that form openings or pores when compacted together. If these pores are sufficiently large and interconnected, the soil is said to be “permeable” and oil or water can pass through it. Gravel and sand are the most permeable type of soil. Materials such as clay, silt, or shale are termed impervious as they have extremely small, poorly interconnected pores and allow only limited passage of fluids. Soils also vary in terms of long-term retentivity. Loam tends to retain the most water or oil due to its high-organic content.

As most soils are a- heterogeneous mixture of these different types of soil, the degree of spreading and penetration of oil can vary considerably even in a single location. The types of soil are often arranged in layers, with loam on top and less permeable materials such as clay or bedrock underneath. If rock is fractured and contains fissures, oil can readily pass through it. The oil’s ability to permeate soils and its adhesion properties also vary significantly. Viscous oils, such as bunker fuel oil, often form a tarry mass when spilled and move slowly, particularly when the ambient temperature is low. Nonviscous products, such as gasoline, move in a manner similar to water in both summer and winter. For such light products, most spreading occurs immediately after a spill.

Crude oils have intermediate adhesion properties. In an area with typical agricultural loam, spilled crude oil usually saturates the upper 10–20 cm of soil and rarely penetrates more than 60 cm. Generally, the oil only penetrates to this depth if it has formed pools in dry depressions. If the depressions contain water, the oil may not penetrate at all.

44.12.2 Movement of Oil on Land Surfaces

Both the properties of the oil and the nature of the soil materials affect how rapidly the oil penetrates the soil and how much the oil adheres to the soil. For example, a low-viscosity oil penetrates rapidly into a dry porous soil such as coarse sand and therefore its rate of spreading over the surface is reduced.

When oil is spilled on land, it runs off the surface in the same direction and manner as water. The oil continues to move horizontally down-gradient until either blocked by an impermeable barrier or all the oil is absorbed by the soil. The oil will also sink into any depressions and penetrate into permeable soils.

The process whereby oil penetrates through permeable soils includes several steps. The bulk of the oil moves downward through permeable material under the influence of gravity until it is stopped by either the groundwater or an impermeable layer. It then moves down-gradient along the top of the impermeable layer or groundwater until it encounters another impermeable barrier or all the product is absorbed in the soil. Once in contact with the water-soluble material, the oil dissolves into and is transported away with the groundwater. Oils and fluids can flow along the top of the groundwater and reappear much later in springs or rivers. The descending oil is often referred to as a slug, of oil. As the slug moves through the soil, it leaves material behind that adheres to the soil. This depends on the adhesion properties of the spilled product and the nature of the soil. More of the adhered oil is moved downward by rainfall percolating through the soil. The rainwater carries dissolved components with it to the water table. The movement of the oil will be greatest where the water drainage is good.

44.12.3 Habitats/Ecosystems

As the effects of oil and its behavior vary in different habitats, cleanup techniques and priorities are tailored to the habitat in which the spill takes place. Returning the habitat as much and as quickly as possible to its original condition is always a high priority when cleaning up oil spills.

When spills occur in the urban environment, protecting human health and safety and quickly restoring land use are top priorities. Environmental considerations are generally not important as endangered species or ecosystems are not often found in the urban habitat. The urban environment usually includes a range of ecosystems, from natural forests to paved parking lots. Thus, a spill in an urban area often affects several ecosystems, each of which is treated individually.

The roadside environment is similar to the urban one in that restoring the use and surficial appearance of the land is given top priority. Roadside habitats are varied and include all the other ecosystems. The roadside habitat is different from the urban one, however, in that it is exposed to many emissions and is not generally viewed as a threatened or sensitive environment.

On agricultural land, the priority in cleaning up oil spills is to restore land use, for example, crop production. In this habitat, oil is more likely to penetrate deeply into the subsurface as plowing the fields creates macropores through which petroleum products and crude oils can rapidly penetrate. As oil penetrates deeper into dry agricultural land, the danger of groundwater contamination is greater than in other habitats.

On mineral soils, however, oil can make the soil nonwettable so that water runs off rather than soaking into the soil. This causes a soil water shortage, which can result in poor rehabilitation in the area. The opposite occurs in low-lying sites or poorly drained soils where water fills the macropores of the soil but is not absorbed into the soil itself because of the presence of the oil. This excludes air from the soil and the site becomes difficult to treat or cultivate and anaerobic conditions quickly develop.

Anaerobic conditions and restricted plant growth can also develop when oil on the surface weathers and forms an impermeable crust, which again reduces the air exchange. Recovery is affected by the amount of oil spilled in a given area. Lightly oiled soil recovers much faster than a heavier oiled area as the soil is not completely saturated, and both air and water can still penetrate. Residual oil in the soil can also slow recovery by inhibiting seed germination.

Dry grassland is similar to agricultural land in that the priority for cleanup is restoring the soil so that the crop, in this case grass, can continue to grow. The surface of the grassland is often less permeable than agricultural land. Once the surface is penetrated, however, the substrate may be permeable, and groundwater can be affected. Dry grassland recovers quickly from spills if the oil runs off or if the excess oil is removed without too much surface damage. The presence of dead vegetation is viewed as a symptom, not a problem. When excess oil is removed, replanting and fertilization can speed the recovery of an oiled grassland. As with agricultural land, oil on the surface of grassland can sometimes weather and form an impermeable crust, which reduces air exchange and causes anaerobic conditions.

Unlike most habitats, the forest has two distinct levels of vegetation: low-lying vegetation such as shrubs and grasses and trees. The low-lying vegetation is much more sensitive to oiling than trees but is much easier to replant and recovers much faster. Most species of trees are not seriously affected by light oil spills. If enough oil is spilled to affect the tree’s roots, most trees will be killed and the forest will not recover fully for decades. It is, therefore, very important to rapidly remove excess oil that has not yet been absorbed by the soil.

If a forest has mineral soil, the oil can make it nonwettable so that water runs off the soil rather than soaking in. In low-lying sites or forests with poorly drained soil, the opposite occurs. Water fills the macropores of the soil, but not the soil itself because of the presence of the oil. This excludes air from the soil and the site does not revegetate quickly. Oil on the surface of forest soils can weather and form an impermeable crust, which also reduces air exchange or restricts the growth of plants. Forest is far more difficult to access and treat than most other habitats.

Wetlands are the habitat most affected by oil spills because they are at the bottom of the gravity drainage scheme. Usually, oil cannot flow out of a wetland system, and oil from other areas flows into the system. Although there are variety of wetlands, oil tends to collect in all of them, creating anaerobic conditions, which slow oil degradation. Wetlands are also extremely sensitive to physical disturbance as many plants in this habitat propagate through root systems. If these root systems are damaged by the oil or the cleanup process, it takes years or even decades for the plants to grow back. Wetlands are the habitat of many species of birds and fish, as well as other aquatic resources. Wetlands are difficult to access and to clean up.

Taiga, which is characterized by coniferous trees and swampy land, generally forms the transition between northern forests and the tundra farther to the north. It is either underlain by permafrost or has a high water table. Many of the plants propagate through root systems and are highly sensitive to physical disturbance. Over a period of time, heavy loadings of oil will kill the coniferous trees. Oil on the surface of the taiga can weather and form an impermeable crust, which reduces the air exchange and restricts plant growth. Degradation of remaining oil is slow in this habitat, which takes a long time to recover. The presence of trees and the high moisture level make the taiga more difficult to access and clean up than most other habitats.

Tundra is a far northern habitat characterized by low plant growth and no trees. Tundra is underlain by permafrost, which is generally impermeable to oil. Vegetation on the tundra grows in tufts, which are generally grouped into polygons. Oil spilled on the surface drains into the spaces between the tufts and polygons and eventually kills the vegetation. Without the layer of vegetation, the permafrost melts and serious land damage results. Degradation of remaining oil on the tundra is very slow and could take decades.

In all the more sensitive habitats, which include the forest, the taiga, and the tundra, the priority for cleanup operations is to remove the excess oil as rapidly as possible and without causing physical damage.

44.12.4 Cleanup of Surface Spills

When dealing with oil spills on land, cleanup operations should begin as soon as possible. It is important to prevent the oil from spreading by containing it and to prevent further contamination by removing the source of the spill. It is also important to prevent the oil from penetrating the surface and possibly contaminating the groundwater.

Berms or dikes can be built to contain oil spills and prevent oil from spreading horizontally. Caution must be exerted, however, that the oil does not back up behind the berm and permeate the soil. Berms can be built with soil from the area, sandbags, or construction materials. Berms are removed after cleanup to restore the area’s natural drainage patterns. Sorbents can also be used to recover some of the oil and to prevent further spreading. The contaminated area can sometimes be flooded with water to slow penetration and possibly float oil to the surface, although care must be taken not to increase spreading and to ensure that water-soluble components of the oil are not carried down into the soil with the water. Shallow trenches can be dug as a method of containment, which is particularly effective if the water table is high and oil will not permeate the soil. Oil can either be recovered directly from the trenches or burned in the trenches. After the cleanup, trenches are filled in to restore natural water levels and drainage patterns.

There are a variety of methods for cleaning up surface oil spills on land, with the method used depending on the habitat in which the spill occurs. The various cleanup methods that can be used in the different habitats are shown in Table 44.7.

44.12.5 Natural Recovery

This is the process of leaving the spill site to recover on its own. This method is sometimes chosen for extremely sensitive habitats such as wetlands, taiga, and tundra and is always done after the excess oil has been removed from the site. In these cases, the excess oil that can be recovered is removed using techniques that do not disturb the surface or physically damage the environment. This is important as it can take years for wetlands or tundra to recover from vehicular traffic. In habitats such as wetlands and taiga, where the vegetation propagates through root systems, more damage can be done by the cleanup operation than by the oil.

44.12.6 Removal of Excess Oil

Any excess oil that can be recovered without causing physical damage to the environment is always removed from a spill site using techniques that do not disturb the surface. If excess oil on the surface is not removed quickly, the oil can penetrate into the soil, contaminating the groundwater and destroying vegetation.

Suction hoses, pumps, vacuum trucks, and certain skimmers and sorbents are generally effective in removing excess oil from the surface, especially from ditches or low areas. The use of sorbents can complicate cleanup operations, however, as contaminated sorbents must be disposed of appropriately.

Sorbents are best used to remove the final traces of oil from a water surface. Any removal of surface soil or vegetation also entails replanting.

Habitat	Removal of Excess Oil	Natural Recovery	Manual Oil Removal	Mechanical Oil/Surface Removal	Enhanced Biodegradation	In-situ Burning	Hydraulic Measures
Urban	✓		✓	✙	✙		✓
Roadside	✓	✙	✓	✙	✙	✙	✙
Agricultural land	✓	✙	✓	✙	✙	✙	✓
Grassland	✓	✙	✓	✙	✙	✙	✙
Forest	✓	✓	✓	✙	✙	❍	✓
Wetland	✓	✓	✓	✖	✙	✓	✙
Taiga	✓	✓	✓	✖	✙	✙	✙
Tundra	✓	✓	✓		✙	✙	✙

✓: acceptable or recommended; ✙: can be used under certain circumstances; ✖: should not be used; ❍: Only marginally applicable.

Manual removal of oil involves removing oil and often highly oiled soil and vegetation with shovels and other agricultural tools. This is always followed by fertilization, selective reseeding, or transplanting plugs of vegetation from nearby unaffected areas. This form of removal is labor intensive and can severely damage the surface, especially in sensitive environments.

Mechanical recovery equipment, such as bulldozers, scrapers, and front-end loaders, can cause severe and long-lasting damage to sensitive environments. It can be used in a limited capacity to clean oil from urban areas, roadsides, and possibly on agricultural land. The unselective removal of a large amount of soil leads to the problem of disposing of the contaminated material. Contaminated soil must be treated, washed, or contained before it can safely be disposed of in a landfill site. This could cost thousands of dollars per ton.

44.12.7 Other Cleanup Methods

Enhanced biodegradation is another possible method for cleaning up spills on land. Certain portions of oil are biodegradable, and the rate of biodegradation can sometimes be accelerated as much as 10-fold by the proper application of fertilizers.

The amount of degradation varies with the type of oil. Diesel fuel may largely evaporate and degrade on the land surface, whereas Bunker C will only slightly degrade. Under ideal conditions and using fertilizers to enhance degradation, however, it can still take decades for more than half of some oils to degrade, dependent on conditions and the type of oil. During this time, some of the oil will be removed by other processes such as evaporation or simply by movement

Scientists are now exploring the use of plants and their associated microorganisms for remediation, which is called phytoremediation. This is a low-cost process that is proving effective for a wide variety of contaminants, including petroleum hydrocarbons. It can be used in combination with other remediation technologies and may prove useful in the future for treating oiled soils or wetlands. It takes several years to remediate a site, and cleanup is limited to the depth of the soil within reach of the plant’s roots.

In-situ burning has been used for several years to deal with oil spills on land. This technique removes oil quickly and without disturbing the area extensively, although it does damage or kill shrubs and trees. The heat from burning can also destroy propagating root systems and change the soil’s properties. In addition, it can leave a hard crust of residual material that inhibits plant growth and changes natural water levels and drainage patterns.

These disadvantages can be overcome in some habitats. Some areas can be flooded before burning to minimize the effect of the heat and to remove oil by floating it out of the ground. Crust formation can be avoided by physically removing residue after a burn. On wetlands and in areas with high water levels, sorbents can be used to remove residues left after burning to ensure that they do not coat plants or soil after water levels fall. In marshes, burning is best done in spring when the water table is high.

Hydraulic measures, such as flooding and cold or warm water sprays, can be used to deal with land spills, although they are only effective in limited circumstances. Flooding an area where the oil is not strongly retained can cause the oil to rise to the water’s surface where it can subsequently be removed using skimmers or suction devices, or by burning. This is effective in areas where the water table is high, or the top layer of ground is underlain by impermeable material. Flooding may not work on soils that are high in organic material, however, as they strongly retain oil. Cold or warm water sprays can be used to clean oil from hard surfaces. Catchment basins and interceptor trenches are built to capture the released oil, which is then skimmed or pumped from the trenches.

A number of other techniques have been tried for cleaning up oil spills on land, with varying degrees of success. Tilling or aeration of soil is done to break up the crust surface and reaerate the soil. In areas where vegetation propagates by root systems, however, tilling kills all plants and destroys the potential for regrowth. Tilling oil into the soil can actually slow natural degradation because the soil can become anaerobic when it recompresses. Vegetation cutting is useful only if there is a risk that oil on vegetation could recontaminate other areas. Many plants cannot survive cutting, however, and growth is not reestablished in the area. To date, there are no effective chemical agents for cleaning up oil spills. Surfactant agents can actually increase oil penetration into the soil and could result in the more serious problem of groundwater contamination. Figure 44.10 shows that penetration of oil liquids occurs below the soil surface.

All cleanup methods include site restoration, which involves returning the site as closely as possible to the prespill conditions. The drainage pattern of the site is restored by removing dykes, dams, and berms, and filling in ditches or drains. It may be necessary to replace any soil that was removed and to revegetate the site by fertilizing, reseeding, or transplanting vegetation from nearby.

44.12.8 Cleanup of Subsurface Spills

Oil spills in the subsurface are much more complicated and expensive to clean up than those on the surface and the risk of groundwater contamination is greater. Spills in the subsurface can be difficult to locate and without knowledge of the geology of the area, it can be difficult to predict the horizontal and downward movement of the oil. The first step is typically to engage a hydrogeologist to map and assess both the subsurface and the oil location with respect to the soil geology.

In terms of countermeasures, the oil must be contained and its horizontal and downward movement stopped or slowed. Containment methods are difficult to implement and may cause physical damage to the site. Digging an interceptor trench can be effective in reducing horizontal spread. Such trenches are filled in after the cleanup operation to restore the natural drainage patterns of the land. Another method is to place walls around the spill source to stop its spreading. These can be slurry walls consisting of clay or cement mixtures that solidify and retain the oil, or solid sheets of steel or concrete can be positioned to retain the oil.

Once the subsurface spill is contained, there are a number of cleanup methods that can be used. The most appropriate method for a particular spill depends on the type of oil spilled and the type of soil at the site, as shown in Table 44.8.

Hydraulic measures for cleaning up subsurface spills include flooding, flushing, sumps, and subsurface drains. These methods are most effective in permeable soil and with nonadhesive oils. They all leave residual material in the soil that may be acceptable, depending on the land use. Flooding is the application of water either directly to the surface or to an interceptor trench in order to float out the oil. Flooding is effective only if the spilled oil has not already been absorbed into the soil, if sufficient water can be applied to perform the function, and if the oil is not accidentally moved into another area. Flushing involves the use of water to flush oil into a sump, recovery well, or interceptor trench. Placement of a sump or a deep hole is only effective for a light fuel in permeable soil above an impermeable layer of soil. A subsurface drain is a horizontal drain placed under the contamination, from which the fuel and often water are pumped out. Although effective in permeable soils, they are expensive and difficult to install.

Interceptor trenches are ditches or trenches dug downgradient from the spill, or in the direction in which the spill is flowing, to catch the flow of oil. They are placed just below the depth of the groundwater so that oil flowing on top of the groundwater will flow into the trench. Both water and oil are removed from the trench to ensure that flow will continue. Interceptor trenches are effective if the groundwater is very close to the surface and the soil above the groundwater is permeable.

Soil venting is done to remove vapors from permeable soil above a subsurface spill. This is effective for gasoline in warm climates and for portions of very light crude oils. Other oils do not have a high enough rate of evaporation to achieve a high recovery rate. Venting can be passive, in which vapors are released as a result of their own natural vapor pressure, or active, in which air is blown through the soil and/or drawn out with a vacuum pump. The fuel vapors are subsequently removed from the air to prevent air pollution. Soil venting is also done to enhance biodegradation.

Excavation is a commonly used technique for cleaning up subsurface spills, especially in urban areas where human safety is an issue. Vapors from gasoline can travel through the soil and explode if ignited. These vapors can also penetrate houses and buildings, forcing evacuation of the area. To prevent these situations, contaminated soils are often quickly excavated and treated or packaged for disposal in a landfill. Excavation may not always be possible, however, depending on the size of the spill and prevailing conditions at the site.

Recovery wells are frequently used in cleaning up subsurface spills. The well is drilled or dug to the depth of the water table so that oil flowing along the top of the water table will also enter the well. The water table is sometimes lowered, by pumping, to speed the recovery of the oil and to increase the area of the collection zone. The oil is recovered from the surface of the water by a pump or a specially designed skimmer.

Other methods are constantly being proposed or tried for cleaning up subsurface spills. One such method is biodegradation in-situ, although its effectiveness is very much restricted by the availability of oxygen in the soil and the degradability of the oil itself. An adaptation of the venting method has been used to try to solve the oxygen problem. So far, however, biodegradation methods have not been rapid enough to be an acceptable solution. Chemical agents have also been proposed for cleaning up subsurface spills, although most of them actually make the problem worse. For example, surfactants can release the oil from the soil but then render that same oil dispersible in the groundwater.

If the groundwater does become contaminated, it is pumped to the surface and treated to remove the dissolved components. Common treatment methods include reverse osmosis and carbon filtration. Groundwater treatment is expensive and generally involves a lengthy process before contamination levels are below acceptable standards.