An oil spill is defined as the uncontrolled release of crude oil hydrocarbons into the environment (Ndimele et al. 2018). The magnitude of oil spills can range from a small amount of oil spilled during the refueling of a ship, to almost 5 million barrels of crude oil into the Gulf of Mexico by Deepwater Horizon (Silliman et al. 2012). When crude oil spills, it quickly rises to the water surface due to its lower density. There, it forms a layer a few millimeters thick, referred to as an oil slick (Kingston 2002). The volatile components, including most of the toxic components, quickly evaporate from the slick into the atmosphere (Kingston 2002). Winds and currents cause oil slicks to spread into an even thinner layer called an oil sheen and/or carry it towards shore (Piatt et al. 1990).
Three million metric tons of oil contaminants spill into the ocean annually (Azwell 2013). Common sources of these spills include tankers, pipelines, storage tanks, refineries, drilling rigs, wells, and platforms (Prendergast and Gschwend 2014). Although spill frequency and volume have declined since the 1970s, catastrophic oil spills remain a possibility. Two catastrophic events include the 2010 Deepwater Horizon disaster as well as the lesser-known Taylor Energy spill which has been leaking 300 to 700 barrels of oil per day since 2004 and may surpass Deepwater Horizon as the largest oil spill in US history should the leaking wells remain uncapped (Prendergast and Gschwend 2014; Fears 2018).
Oil spills can cause significant harm to humans and the environment. They can endanger public health through direct exposure and through contamination of drinking water, interfere with the livelihood of fishermen and disrupt the local economy, and severely harm organisms and ecosystems. The Exxon Valdez spill resulted in hundreds of millions of dollars in damages and killed hundreds of thousands of organisms including seabirds, otters, and whales (Peterson et al. 2003). Furthermore, the detrimental effects of oil spills can persist over time. Although environmental recovery at most spill sites takes only 2 to 10 years, a shoreline survey in Alaska found over 55,000 kg of relatively unweathered oil from the Exxon Valdez spill remained in subsurface sediments over a decade after the spill, continuing to increase mortality in local wildlife (Kingston 2002; Peterson et al. 2003).
In the US, the Coast Guard leads the spill response effort with the goal of containing, recovering, and dispersing as much oil as possible. Any oil that is not contained may find its way to shorelines, where humans and sensitive ecosystems may be subject to damage for years (Zengel et al. 2015). Oil spill cleanup techniques differ case by case, from highly invasive to passive approaches. Choosing the optimal remediation technique depends on a number of factors including oil properties, spill location, spill size, weather conditions, and local regulations and standards. There are at times circumstances in which taking no action may be less harmful than using any remediation technique. Spill response techniques differ depending on whether the oil is floating at sea or has reached the shoreline.
The remediation techniques used at sea are typically classified into three groups: mechanical (also known as physical), chemical, and bioremediation (Ventikos et al. 2004).
Mechanical methods are used to contain and recover oil that remains on the water surface without changing its properties. They include booms, skimmers, and sorbent materials (Ventikos et al. 2004). One advantage to using mechanical methods is that oil retains its properties. Therefore, it can still be refined and used in the future, reducing waste and potentially mitigating financial losses. However, mechanical methods require high capital investment, logistical support, and favorable weather and sea conditions (Ventikos et al. 2004).
Booms (shown in Figure 1) are physical barriers that enclose floating oil and prevent it from spreading. They must be used in relatively calm conditions because winds over 5.5 m/s, currents over 4 m/s, or waves taller than 1 m will pull oil under the boom, thus defeating its purpose (Hoang et al. 2018). Most booms fall into one of two categories - fence booms or curtain booms (Ndimele et al. 2018). Fence booms float vertically with about 60% of their height lying below the water surface (Hoang et al. 2018). A curtain boom consists of a 10- to 50-cm-long subsurface oil collection skirt hanging from a floating chamber that resembles a pool noodle (Ndimele et al. 2018). Fence booms are most reliable in high-current areas with smaller waves, while curtain booms are often used in offshore areas with larger waves. Despite their different applications, the operational limits for wind velocity, current velocity, and wave height are the same for each method (Ventikos et al. 2004). Additional considerations unrelated to performance may be factors in selecting a boom type. For instance, fence booms are easier to store compactly (Ndimele et al. 2018). This means that, assuming conditions are suitable for both types of booms, it may make sense to select fence booms over curtain booms if storage space is limited.
Skimmers are vessels that collect oil from the water surface and store it in onboard tanks (Ndimele et al. 2018). Nearly all skimmers can be separated into two broad categories based on their recovery method (Ventikos et al. 2004). Skimmers that recover oil through suction include vacuum, weir, vortex, and dynamically-inclined plane skimmers (Ventikos et al. 2004). Weir skimmers, the most widely available type of suction skimmer, rely on gravity to separate oil from water (Ventikos et al. 2004). A stationary weir skimmer positions a collection well just below the water’s surface to capture the maximum possible ratio of oil to water, then a pipe in the collection well sucks the mixture into a holding tank (Hoang et al. 2018). Skimmers that recover oil through adhesion are also known as oleophilic skimmers. Oleophilic skimmers cycle a material that adheres to oil through an oil slick, and oil is continuously scraped off or squeezed out of this material into a holding tank (Hoang et al. 2018). Weir skimmers require calm conditions and work best in debris-free waters. They may capture more water than oil in choppy conditions and are highly sensitive to debris clogs (Ventikos et al. 2004). Oleophilic skimmers outperform weir skimmers in rougher water conditions and debris- or ice-laden waters, and they accumulate a high ratio of oil to water (Ventikos et al. 2004; Li et al. 2016). However, they have a low oil recovery rate. The maximum oil recovery rate for oleophilic skimmers is 50 m³/hr versus 100 m³/hr for weir skimmers (Hoang et al. 2018). Typically, skimmers are used in conjunction with booms, as booms form a thicker oil slick with a smaller surface area, enhancing skimmer performance (Ndimele et al. 2018).
Sorbent materials recover oil through absorption or surface adhesion (Ventikos et al. 2004). The three types of sorbents are natural organic, natural inorganic, and synthetic (Hoang et al. 2018). Organic sorbents such as peat, feathers, and sawdust can soak up from 3 to 15 times their weight in oil (United States Environmental Protection Agency 1999). Inorganic sorbents like clay, wool, and volcanic ash can soak up from 4 to 20 times their weight (Ndimele et al. 2018). Synthetic sorbents, including polyethylene and polypropylene, are used most commonly since they are highly effective, absorbing 70 to 100 times their weight in oil (Ndimele et al. 2018). One unique advantage is their potential for reuse, although the cost of regeneration can sometimes exceed the cost of purchasing new sorbents (Li et al. 2016). Despite their advantages, synthetic sorbents can be very persistent in the environment as they are not biodegradable (Ndimele et al. 2018). Sorbents are only feasible for remediating small spills, usually in ports or close to shores, or cleaning up the final traces of larger spills (Ventikos et al. 2004; Li et al. 2016).
Chemical methods change the physical and chemical properties of the oil. They are generally cheaper and require less manpower than mechanical oil recovery. However, they alter oil properties, preventing the reuse of collected oil (Hoang et al. 2018). The three main chemical methods for marine oil spill remediation are burning, dispersants, and solidifiers (Li et al. 2016).
In situ burning of oil is often used in conjunction with booms. Fireproof booms will concentrate spilled oil into a smaller but thicker slick that is easily ignited (Prendergast and Gschwend 2014). Oil burns downward from the surface of the slick at a rate of approximately 3 mm/min, although this rate can vary from about 0.5 to 4 mm/min based on the properties of the spilled oil and the extent to which it has emulsified (Prendergast and Gschwend 2014; Li et al. 2016). There are several concerns associated with in situ burning of oil including secondary fires, destruction of vegetation and aquatic organisms near the site, and risk to human health from pollutants emitted during combustion (Ndimele et al. 2018). The pollutant that poses the greatest threat to human health is PM2.5. In situ burning should only be considered if weather conditions are calm and the distance to the nearest population is greater than y in Eq. 1, where y is the safety distance in m and x is the size of the burning slick in m² (Hoang et al. 2018).
y = 0.75x (1)
It is worth noting that oil properties can impact the amount of air pollutants released during combustion. For instance, diesel and kerosene generate more soot than other fuels (Li et al. 2016). In situ burning is most suitable for massive oil spills in remote locations, making it the preferred remediation technique in the Arctic (Ventikos et al. 2004).
Dispersants are typically sprayed onto an oil slick from an airplane and are most often used on spills with large surface areas (Prendergast and Gschwend 2014; Hoang et al. 2018). Each dispersant molecule contains oleophilic and hydrophilic parts which serve to weaken the surface tension at the oil/water interface (Ndimele et al. 2018). The dispersant causes small oil droplets to separate from the slick and enter the water column, where they are easily diluted or biodegraded (Ndimele et al. 2018). Some countries rely almost exclusively on dispersants because rough wind and water conditions make it impossible to collect spilled oil via mechanical methods (United States Environmental Protection Agency 1999). These natural conditions actually improve dispersant efficiency as they help the dispersant mix with water and oil at the surface (Ndimele et al. 2018). Another major advantage of dispersants is the rapid rate of treatment. A fixed-wing aircraft can treat over 160 hectares of oil per hour with dispersants, roughly three times the area that skimmers or in situ burning can treat in that time (Prendergast and Gschwend 2014). However, dispersants remain controversial and are often considered the final option, as some researchers argue that no treatment may cause less ecological damage than dispersants (Azwell 2013). The main concerns regarding the use of dispersants include the toxicity of the dispersants themselves and long-term persistence of synthetic dispersants in the environment (Li et al. 2016).
Solidifiers are dry particulate or semisolid materials which react with oil and convert it from a liquid to a buoyant rubber-like substance that can be easily removed from the surface of the water (Ndimele et al. 2018). Like dispersants, solidifiers can be used in rough conditions that would be unsuitable for mechanical methods (Hoang et al. 2018). Unlike dispersants, solidifiers are commonly used for small spills. Solidifiers are generally less efficient than dispersants and are incompatible with other oil spill countermeasures (Li et al. 2016).
Bioremediation refers to the use of additives to accelerate biodegradation, a natural process in which microorganisms break down complex compounds into simpler products to obtain energy and nutrients (United States Environmental Protection Agency 1999). Two bioremediation approaches used in the US include biostimulation and bioaugmentation.
In biostimulation, nutrients are added to stimulate the growth of oil-consuming microorganisms while in bioaugmentation, the microorganisms themselves are added (Ndimele et al. 2018). Bioremediation can be very effective - in the Exxon Valdez spill, about 30% of total hydrocarbons were degraded within a week of nitrogen fertilizer application to affected shorelines (Li et al. 2016). However, rates of oil biodegradation are impacted by volatile factors such as the bioavailability of nutrients, oil concentration, and temperature (Hoang et al. 2018). There are several advantages to bioremediation. First, it is cheap and sustainable (Ndimele et al. 2018). The only byproducts of bioremediation are carbon dioxide and oxygen, neither of which present a threat to environmental or human health in the amounts that are generated, making it less environmentally degrading than in situ burning (Hoang et al. 2018). Second, bioremediation is suitable in all weather conditions (Hoang et al. 2018). Finally, bioremediation complements the use of dispersants. Dispersants increase the available surface area for biodegradation and, thus, the rate of biodegradation (Li et al. 2016). Like chemical methods, bioremediation excludes the possibility of recovering oil for future use. Bioremediation is typically used as a polishing tool for spills once the majority of contaminants have been removed via other methods (Li et al. 2016).
During an oil spill, it is almost impossible to fully prevent oil from reaching a shoreline (United States Environmental Protection Agency 1999). Freshwater and marine shoreline areas provide important public and ecological resources that must be protected. Therefore, it is critical that responders have determined appropriate clean up methods before a spill occurs. The most critical considerations are the type and quantity of oil and the geology and sensitivity of a shoreline. Lighter oils evaporate quickly but tend to be more toxic, while heavier oils are usually toxic but tend to form a more difficult to clean oil-and-water mixture called mousse (United States Environmental Protection Agency 1999). NOAA has an Environmental Sensitivity Index (ESI) that ranks types of shorelines by their sensitivity to oil spills on a scale from 1 to 10. From least to most sensitive, they are: exposed rocky cliffs & seawalls, wave cut rocky platforms, fine to medium-grained sand beaches, coarse-grained sand beaches, mixed sand and gravel beaches, gravel beaches/rip-rap, exposed tidal flats, sheltered rocky shores/man-made structures, sheltered flats, and marshes/mangroves.
After the type of oil and shoreline has been identified, responders must choose the appropriate cleanup methods. Standard methods include shoreline flushing/washing, booms, manual removal, mechanical removal, natural processes, passive collection with sorbents, vacuums, in situ burning, and shoreline cleaners and biodegradation agents (NOAA 2015; United States Environmental Protection Agency 1999). This last method requires special permission in order to use. Most of the methods discussed in the Remediation Techniques at Sea section can also be used on the shore, including vacuums, sorbents, shoreline cleansers and biodegradation agents. Also, shoreline flushing/washing can be used to rinse oil from the shoreline back into the water, where it is collected using techniques aimed to collect oil at sea. The water pressure and temperature used during flushing are important factors to consider. Hot water and higher pressures can be more effective in removing oil stuck to hard surfaces, but when used on sensitive shorelines, can cause further harm to the environment (NOAA, 2019). Booms are also used to minimize the spread of spilled oil. Natural processes such as evaporation, oxidation, and biodegradation assist in removing oil from the shoreline (United States Environmental Protection Agency 1999)
The difficulty in removing oil from a shoreline largely depends on the type of shoreline. Sand shorelines present some of the simplest logistical conditions for shoreline treatment. Mechanical equipment can be used to remove, tile, and sift sands (Azwell 2013). The sand can then be chemically treated and placed back at the affected shorelines (Azwell 2013). In areas inaccessible to mechanical equipment, manual removal methods may be used, including clean up crews picking up oiled sand with shovels or other hand tools.
Marsh habitats pose a more significant challenge. Oil removal can reduce acute and chronic exposure of both resident and migratory species and has the potential to damage fragile soils and sensitive wetland biota (Azwell 2013). Physical disturbance and compaction of vegetation and soil from light foot traffic is enough to have detrimental long term effects on oiled marsh by driving oil into the sediment and destroying root structures (Pezeshki et al. 2000). In the case of the deepwater horizon spill, where a large amount of oil was spilled into sensitive marshlands, the primary response was natural attenuation, as oil was physically removed by wave action and tides or by natural degradation through microbial metabolism and photooxidation (Blum et al. 2014). Treatment options include low-pressure or ambient-temperature flushing, contained sorbents, manual removal, vacuuming, vegetation cutting, and natural recovery (DWH UC, 2010).
Azwell (2013) argues that any shoreline clean-up of a sensitive ecosystem’s remediation policy must address habitat restoration and a combination of three critical factors: oiling, erosion, and subsidence. Research has shown habitat restoration’s potential for the recovery of marshland. Baker (1971) shows that planting Spartina shoots directly into oil laden sediments has the potential to speed up recovery time. Similarly, Lin and Mendelssohn (1998) shows the potential that S. alterniflora has in recolonizing areas with high concentrations of weathered oil. Azwell argues that vermiremediation should also be considered as part of a shoreline remediation strategy. Vermiremediation is a noninvasive method that reduces risk of erosion or subsidence and speeds up natural recovery by using earthworms to mix and aerate soil and increase microbial activity and bioavailability (Azwell 2013).
Zengel et al. (2015) examined the long-term effectiveness and ecological effects of different types of treatment of heavily oiled test plots over the span of two years. They argue that treatment of an oiled marsh must strike a delicate balance between removing oil, speeding the degradation of remaining oil, protecting wildlife, fostering habitat recovery, and causing further ecological damage with treatment. The different types of treatment include manual treatment, mechanical treatment, natural recovery, and a referencial “set-aside” adjacent plot. Manual treatment included using hand tools to remove oil as well as possible while also exposing residual oil to the natural degradation process. Mechanical treatment included using mechanized tools with the same goals as manual treatment. Theoretically, this method should be more efficient and use less crew members. Natural recovery, the control, was the no treatment method. The different parameters used to compare were surface oil cover, tPAH percent, total vegetation cover, Spartina alterniflora cover, vegetation species composition, marsh periwinkle densities, crab burrow densities, and shoreline retreat. They came to the conclusion that both manual and mechanical treatments were effective at improving oil conditions and vegetation characteristics, yet the recovery process did not reach completion by the end of two years. Additionally, mechanical treatment had negative effects of mixing oil in the marsh and causing erosion. While manual treatments did not cause the same detrimental effects as mechanical treatment, marsh periwinkles were still reduced and vegetation recovery lagged. Planting was seen as an effective treatment option to accelerate vegetation recovery and reduce marsh erosion. Therefore, they recommend using manual treatment in combination with planting in heavily oiled areas (Zengel 2015). In areas with light oiling or non-persistent oiling, which constitutes a majority of oiled marshes, they recommend using natural recovery to not damage the marsh (Zengel 2015).
When the oil spill is severe enough, as was the case of the Deepwater Horizon, shoreline interventions are considered. Interventions are methods to prevent oil from getting to the shoreline. However, their use remains controversial as their efficacy and effect on the environment are uncertain (Azwell 2013). Three types of shoreline interventions are barrier sand berms, inlet restrictions, and freshwater diversions.
Barrier sand berms are used by moving dredged sediment seaward in an effort to mitigate inflow of oily seawater (Azwell 2013). While this may seem helpful when large amounts of oil are heading to the shoreline, dredging has the potential to kill off benthic biota. Also, sand berms are susceptible to erosion, especially during hurricane season. From a logistical standpoint, moving that much dirt can be too expensive to be considered feasible.
Inlet restrictions function by blocking inlets between barrier islands or at the mouth of estuaries with rocks and barges (Azwell 2013). Using this method can profoundly alter the physicochemistry and biota of inland waters and ecosystems (Azwell 2013). These alterations, such as changes in salinity, oxygen levels and turbidity of inland waters, can inland biota. Resulting restrictions can increase tidal flow velocity, which may cause the loss of adjacent shorelines.
Freshwater diversions work by allowing freshwater to flow through affected waterways. However, there is no evidence to suggest that freshwater diversions are effective in preventing or reducing the flow of oil into inland waters and ecosystems (Azwell 2013).
Since the Deepwater Horizon incident, there has been an increased focus in creating new and innovative technologies for oil spill containment and remediation. The Deepwater Horizon event was twenty times larger than the Exxon Valdez spill and proved that oil spill recovery and remediation technologies had not kept up with the magnitude of oil drilling (Azwell 2013). More than 2000 skimmers were deployed during the Deepwater Horizon incident but only captured 3% of the oil; of the 4.9 million barrels released in the event, roughly 2.9 million barrels found their way into the Gulf ecosystem (Azwell 2013).
One emerging technology is hydrophobic meshes, which act like a filter. Oil is able to go through the mesh while water is rejected (Prendergast and Gschwend 2014). Oil would need to be collected and removed from the interior of the mesh in order to perpetuate the process. Deng et al. (2013) found that the rate of recovery of oil was greater than the rate of spreading of relatively inviscid oils.
The Deepwater Horizon incident caused universal recognition that innovation was needed as part of the response effort (Azwell 2013). The Unified Command implemented the NOAA-led Alternative Response Technology Evaluation System (ARTES) to do so. 123,000 submissions were sent in from vendors, where 100 submissions were field tested and 25 were adopted. Some of the highlights include human hair sorbent boom, a whale skimmer, and the Costner Centrifuge (Blum et al. 2014).
The human hair sorbent would work by using human hair as a natural fiber in a sorbent boom (Blum et al. 2014). However, field tests showed that the boom did not float and, therefore, was not functional. The whale skimmer was a modified freighter that traveled through oil-water mixtures, where it picked up oil and returned water to the ocean. Reports showed that the carrier was inefficient and too slow to be considered a feasible option (Blum et al. 2014). A third invention to come out of this initiative was the Costner Centrifuge. The centrifuge came from the company Blue Planet Water Solutions, founded by actor-director-producer Kevin Costner. It was capable of separating oil and water mechanically at an efficient rate. Spinning the two fluids in the centrifuge could yield oil purity of 99.5% (Blum et al. 2014). Inventions like these show the potential that new and emerging technologies have in aiding the cleanup effort.
Our continued reliance on oil puts us at risk of environmentally harmful events like oil spills. Therefore, it is imperative to have reliable methods to recover and remediate as much oil as possible at sea before it gets to the shoreline. During the Deepwater Horizon incident, which was one of the greatest clean up responses, only 24% of the oil was recovered or remediated (Azwell 2013). Inevitably some oil will reach the shoreline, and, depending on the type of oil and type of shoreline, different methods must be considered to remove and remediate the oil without destroying the natural ecosystem. As oil exploration and extraction technology becomes more advanced, progress and investment into environmentally sensitive and effective removal cleanup technologies must keep up (Baker 1995).
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