Oil spills in the ocean can have a devastating effect on marine and coastal ecosystems. In an article recently published in Reviews in Geophysics, Gustitus and Clement  describe the formation, fate and impact of oil-sediment residues, particularly in nearshore marine environments. The editor asked the authors to explain more about these oil-sediment residues, how they behave in the ocean and at the coast, and recent developments in this field of research.
What happens when crude oil leaks or spills into the ocean?
When oil is released into a marine environment it is subjected to multiple weathering processes including evaporation, dissolution, photo-oxidation and biodegradation. These processes work at various rates to degrade different chemical compounds in the oil. Crude oil is composed of light (low molecular weight) and heavy (high molecular weight) petroleum compounds, with light compounds being affected more rapidly by natural weathering processes.
Portions of the spilled oil that are resistant to weathering will persist in the environment for extended periods of time, especially when they become trapped close to the shoreline. Even when spills occur out in the open ocean, currents will often push the residual oil towards the shoreline, where it will present a hazard to humans and various coastal species. Oil that affects nearshore environments often interacts with suspended sediments in order to form various types of oil-sediment residues.
What are the different types of oil-sediment residues and how are they formed?
Oil that is not removed by ocean-scale natural weathering processes can form various types of residues including floating oil slicks (containing fresh or partially weathered oil), brownish emulsified mousse (containing partially or highly weathered oil), and rubbery tar balls (containing highly weathered oil). When floating oil approaches the shoreline in the form of either a slick or emulsified mousse, it may interact with sand or other sediments to form different sizes of macroscopic agglomerates or microscopic aggregates.
Macroscopic sediment-oil agglomerates (SOAs) are typically on the scale of several centimeters, and are typically formed with highly weathered mousse interacting with coarse or fine sediment. Microscopic oil-particle aggregates (OPAs) are typically under one millimeter in size, and are more likely to be formed when relatively fresh oil interacts with fine sediment. Sediment-oil mats (SOMs), which may reach up to several meters in length, can also be formed from the interaction of weathered mousse and sand and other types of sediments.
How do microscopic aggregates and macroscopic agglomerates behave differently in the nearshore marine environment?
Microscopic OPAs can be quickly dispersed throughout nearshore waters and therefore can reduce the amount of oil deposited on shorelines. Oil that is trapped within OPAs can be relatively easily degraded by natural remediation processes including biodegradation.
Conversely, SOAs and SOMs can remain trapped in nearshore environments for several years following a spill. For example, SOAs and SOMs related to the Deepwater Horizon rig oil spill in the Gulf of Mexico have been found along Alabama’s beaches in 2017, seven years after the initial spill.
Unlike OPAs, the formation of SOAs and SOMs is detrimental because these agglomerates hinder natural remediation by inhibiting several weathering processes that can degrade the residual oil. Although OPAs may also settle and accumulate in the benthic zone, there are generally few toxicity concerns since they are expected to degrade rather rapidly.
On the contrary, SOAs pose considerable risk to benthic wildlife since they are difficult to degrade and may contain several hazardous compounds including heavy PAHs. Due to these differences, oil remediation teams employ various engineering methods to promote the formation of OPAs, while they use physical methods remove SOAs and SOMs.
Why is it useful to consider microscopic aggregates and macroscopic agglomerates collectively?
Microscopic OPAs and macroscopic SOAs and SOMs have many differences, but at their core they are simply different forms of oil-sediment mixtures. Therefore, we believe that researchers who focus on either one of these research areas could benefit substantially by improving their understanding of the research done by the other group. For example, researchers focusing on microscopic aggregates have extensively investigated the effects of various environmental, oil, and sediment properties on OPA formation; in contrast, there is very little known about how these variables affect SOA and SOM formation.
Additionally, there are opportunities for both microscopic and macroscopic oil spill research communities to collaborate in order to develop improved remediation strategies.
For instance, the two groups could collaborate to develop more robust mathematical formulations that can be fully integrated with coastal dynamic models to predict the formation and transport patterns of different forms of oil-sediment residues.
What have been some of the challenges to a more holistic approach for managing oil-sediment residues?
Currently, the published literature on oil-sediment residues focuses solely on either microscopic aggregates or macroscopic agglomerates. Despite OPAs, SOAs, and SOMs all being composed of oil and sediment, they form under different conditions and tend to behave in vastly different ways. One of the challenges in understanding these processes is that enhancing OPA formation is something that is focused on in the immediate aftermath of a spill, while SOA and SOM remediation is not typically considered until after their formation becomes a problem, which can be several days or weeks after the initial spill.
In order to develop a holistic approach, researchers should start considering the short- and long-term potential for the formation of both microscopic and macroscopic oil-sediment residues from the outset. This could help optimize remediation activities, mitigate potential hazards, and minimize remediation costs.
What are some of the remaining questions in this field of research?
One primary question to consider is: how can we quantify the formation of microscopic and macroscopic residues following a major spill? The effects of many different variables on OPA formation have been documented reasonably well, at least under laboratory conditions; however, it is still difficult to assess the quantity of oil captured in these aggregates in a field situation.
The prevalence of SOAs following major spills is slightly easier to assess in the field since they can be collected and quantified, but their tendency to spread along shores and become buried in sediment means that total quantities cannot be determined with any level of accuracy. Locating and removing SOMs is by far one of the major problems faced by remediation engineers today. Additionally, the mechanisms behind SOA and SOM formation need to be integrated with sediment transport models and their results needs to be verified using controlled experimental datasets.
—T. Prabhakar Clement, Department of Civil, Construction and Environmental Engineering, University of Alabama; email: [email protected]