Suneel, V.; Vethamony, P.; Naik, B. G.; Kumar, K. V.; Sreenu, L.; Samiksha, S. V.; Tai, Y.; Sudheesh, K. “Source investigation of the Tar Balls Deposited along the Gujarat Coast, India, Using Chemical Fingerprinting and Transport Modeling Techniques. Env. Sci. Technol. 2014. DOI: 10.1021/es5032213
What are Tar Balls?
Tar balls are small globules of thick, sticky oil that can be found on some shorelines. They can occur as a result of offshore oil spills, though in some oil-rich marine environments, natural underwater oil seepage can lead to formation and shoreline deposition of tar balls. Because oil drilling often occurs in these same oil-rich environments, it can be difficult to determine whether tar ball deposition results from human activities or natural processes.
Tar ball oil is thick and sticky because it has undergone a process called weathering during transit from its source point. After oil enters the water, lighter components of the crude oil (oil released from underground reservoirs that has not been processed or refined in any way) are lost through a process called volatilization (conversion from the liquid to gas phase), and easily degradable components are eaten up by organisms. The more persistent components remain and eventually end up on the shore, often as tar balls. When they wash up, tar balls become hard and crusty on the outside but remain liquid on the inside, offering a nasty, sticky surprise to any beach goer who steps on one.
Tar balls became a highly visible issue in India due to the annual deposition of massive amounts of tar balls on the Goa Coast, which has been ascribed to passing crude oil tankers. The tar balls have hurt tourism by turning beaches unsightly and have threatened local species, including economically important fish species. In addition to being eyesores, tar balls threaten the health of coastal ecosystems, especially in areas where tar ball deposition is extreme. The polycyclic aromatic hydrocarbons (PAHs) that make up a significant fraction of crude oil are associated with elevated cancer risk and other detrimental health effects such as immunosuppression and birth defects.
In order to mitigate tar ball deposition, we need to know where the oil is coming from: Is there a specific offshore oil field to blame? If so, can changes in their operations stop tar ball accumulation on beaches? In this study, researchers used chemical analysis and hydrodynamic modeling to determine where tar balls on the Gujarat coast originated.
Methods & Results
Researchers collected twelve tar ball samples from four beaches, shown in blue in Figure 1. They also obtained samples of crude oils from suspected sources (shown in red in Figure 1) so that they could look for similarities in the chemical “fingerprints” of source oils and deposited tar balls. The suspected source samples were oil from the vessel MSC Chaitra, which collided with another vessel in August 2010, crude oil from the Cairn and Niko oil fields off the Gujarat coast, and crude from two different oil pipelines associated with offshore platforms operated by Bombay High. The researchers combined expertise in physical (hydrodynamic modeling) and chemical oceanography techniques (chemical fingerprinting) to determine which suspected source was the most likely culprit.
Hydrodynamic Modeling: Where Could Tar Balls Have Originated?
At the locations shown in black (P1 – P10) in Figure 1, virtual particles of the same approximate weight as the tar balls were released and transport of the particles was simulated using hydrodynamic modeling to determine whether tar balls released from each location were likely to end up at the sites where samples were collected. They simulated seasonal winds to reconstruct how the currents would have been moving in the four months preceding sample collection and then tracked the predicted trajectories of particles from each particle release location.
Modeled trajectories for particles traveling from the suspected source regions in June and July are shown in Figure 2. Trajectories during these months seemed to deliver particles more effectively to the study area than trajectories for April and May.
Chemical Fingerprinting: Which Crude Oil Best Matches the Tar Balls?
Chemical “fingerprinting” is the practice of measuring certain compounds that can tell researchers about the environment in which the sample originated and weathering processes it has undergone since release. Often, scientists use diagnostic ratios – relative concentrations of one compound to another compound – as unique markers of certain processes.
Here, researchers used a comprehensive battery of tests. They measured the ratios of certain PAHs, which can be used to get some idea of the processes an oil has undergone or where it originated. The ratios of PAHs in the samples suggested that the samples were petrogenic, meaning that the tar balls originated from crude oil, which the researchers had already expected. They also observed that the ratios of PAHs were similar for all of the tar balls collected, which hinted that all tar balls most likely shared a common source.
Secondly, the researchers measured another group of compounds called biomarkers, which are complex organic molecules that are difficult to degrade, often unique to the original type of environment and geological time that was the source of the oil. Biomarkers are the remnants of the ancient dead organisms that degraded over time to form the oil we have today. They found that oleanane was found in all tar balls and most source crudes. This compound usually suggests that an oil was formed from dead plant material in an ancient delta.
They plotted two different ratios of groups of pentacyclic triterpenes, the C29/C30 ratio (ratio of molecules in the group with 29 versus 30 carbon atoms) and the C31-C35/C30 ratio (ratio of molecules in the group with 31 to 35 versus 30 carbon atoms), to see where each tar ball sample and source crude fell on the plot – dots near to each other should be more likely to come from a common source (Figure 3). These procedures allowed the authors to effectively rule out four of the possible source crudes for which they had data: Middle East Crude Oil (MECO) (red X’s), South East Asian Crude Oil (SEACO) (pink triangles), Cairn Oil Field Crude Oil (CRN) (orange diamonds) and oil from the wrecked vessel MSC Chaitra (MSC) (green circle), which leaves the two Bombay High Crude Oils (BHM and BHH) (pink circles) as the most likely source of tar balls to the Gujarat coast.
Finally, the group used stable carbon isotope ratios to further confirm tar ball sources. The amount of “heavy” 13C to total C (12C + 13C) in various compounds that make up an oil sample is a unique characteristic of crude oil that will not change as the sample weathers. In Figure 4, the ratio of 13C to total C is plotted for organic molecules found in the oil samples. The molecules are organized based on the number of carbons they contain, meaning that the size of the molecules is increasing from C9 to C36. You can see that in most cases the molecules are becoming “isotopically lighter” (they contain less of the heavy 13C) as molecular size increases. This characteristic curve is expected to be similar for samples that come from the same source crude. The data supports the other lines of evidence, showing that oil from the vessel MSC Chaitra is the most distinct and is unlikely to be the source of the tar balls. Source crudes from Cairn (CRN) and Niko (NIK) oil fields are also divergent. The tar ball samples best match the Bombay High crudes (BHH and BHM).
This study showcases many of the powerful tools we can use to answer the essential question “Where does it come from?” Once we answer this question we can start planning how to stop it.
The researchers report that Bombay High offshore oil platforms are the most likely source of the tar balls being deposited along the Gujarat coast, but they also note that natural seepage from the area can’t be ruled out. If further studies can confirm the source as being spills from the oil sites, it may be possible to mitigate tar ball deposition through site repairs or improvements in operation.