Oil spill at Regugio Beach near Santa Barbara. Photo credit: Lara Cooper / Noozhawk.
Oil spills in coastal regions have devastating consequences for marine and coastal ecosystems. Oil slicks in particular can do much damage to coastlines, covering plants and animals with oil and threatening property as well as commercial operations along coastlines. The persistence of oil in seawater and the transformation it undergoes as a result of natural degradation processes is critical to understand for effective remediation efforts. There is evidence that the dissolved compounds originating from a slick can persist in the environment for prolonged periods and may be toxic to microorganisms and marine biota. Much research has focused on weathering of oil at the sea surface by different mechanisms: evaporation, dissolution, emulsification, aging and conversion to tar, degradation by microorganisms, and break up by chemical dispersants. Depending on the type of oil, dissolution into water-soluble constituents can be an important transformation mechanism. However, much still remains to be understood about the formation of dissolved constituents and their degradation, especially due to photochemical processes.
In May of 2015 there was an
onshore oil pipeline leak that released more than 20,000 gallons of heavy crude oil into the ocean at Refugio Beach off the coast of Santa Barbara, California. The spill occurred in the vicinity of natural oil seeps, which are an ever-present source of oil to the coastal environment. Understanding the differences between seep oil and spill oil, especially when spills occur in close proximity to natural seeps, is important for source tracking. In addition, obtaining a better understanding of the contrasting weathering processes for oil from different sources can aid in remediation efforts. In this study we sought to evaluate the processes of slick dissolution and photochemical degradation of the dissolved constituents in both industrial oil from the Refugio oil spill and natural seep oil.
Map showing the sites of the oil spill at Refugio and oil seep at El Capitan. Colored bar along the coastline indicates extent of shoreline oiling. Source: http://oil.piratelab.org.
We were able to acquire oil from the Refugio spill and collect oil from active seeps at Carpinteria Beach, about 12 miles southwest of Santa Barbara. We performed a set of experiments to simulate industrial oil and seep oil dissolution and photochemical oxidation in natural seawater. Our experiments were conducted in two phases to simulate natural conditions with sunlight and microbial processes (non-sterile, Phase 1) and to isolate the effects of only sunlight (sterile [microbe-free], Phase 2). Experiments were conducted using a solar simulator, which mimicked diurnal solar intensity patterns for the Santa Barbara area, and dark controls maintained at similar temperatures.
Active seep at site of sampling. Photo credit: Cari Campbell.
Oil contains many polycyclic and polyaromatic compounds that have the ability to fluoresce. Fluorescence spectroscopy is a rapid and proven technique to track various classes of dissolved organic matter (DOM) and is known to be a powerful tracer for oil compounds, including benzene, naphthalene, polycyclic aromatic hydrocarbons (PAHs) and other petroleum hydrocarbons. Fluorescent sensors have also been used extensively in-situ to track oil from coastal spills, such as the Deepwater Horizon Spill. Therefore, fluorescence was applied in this study to track the production and transformation of water-soluble constituents from both oil types over a period of 67 days. In both experimental phases, three-dimensional fluorescence spectra were acquired regularly from the water column below the slick over the course of the experiments. We also conducted additional analyses in the lab of
Dr. Eunha Hoh (SDSU, Public Health) using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC/TOF-MS) based non-targeted approach to characterize chemicals that were removed or produced as a result of photochemical processes. Samples from days 0, 20, and 67 were solid-phase extracted and analyzed with GC×GC/TOF-MS, and corresponding dissolved organic carbon (DOC) concentrations were measured.
Preliminary results show that when both microbial activity and photochemical processes are at work, substantial mineralization of organic compounds to CO2 occurs. DOC concentrations in irradiated samples containing microbes decreased substantially compared to irradiated samples in which microorganisms were not present. Furthermore, when microbial activity occurred in the absence of light, very little change in DOC concentrations, fluorescence peak intensities, or numbers of GCxGC-TOF/MS compounds was observed.
Graduate and undergraduate students preparing samples for solid phase extraction for GC×GC/TOF-MS analysis in the lab of Dr. Eunha Hoh. Photo credit: Natalie Mladenov.
Results also demonstrated that sunlight plays a large role in the dissolution of the oil slick. In all experiments, including those with and without microbial activity, dark controls had some initial dissolution, but remained mostly in slick form. By contrast, dissolution was much higher in irradiated samples.
Dissolution was substantially higher for industrial oil than for seep oil, even though the industrial oil is a heavy crude type, believed to have low dissolution rates. The fluorescence intensities of water-soluble organic compounds increased at a much higher rate for industrial oil than for seep oil. These results suggest that, under similar environmental conditions, industrial oil spills will produce far more dissolved constituents than seep oil.
Based on GCxGC-TOF/MS chromatograms, we were also able to identify distinct differences in the structural characteristics of dissolved compounds for the two oil types. We noted that industrial oil produced a larger amount of low molecular weight (LMW), high polarity (HP) compounds than seep oil. The total number of compounds identified in industrial oil using the GCxGC-TOF/MS approach stayed constant (or even increased slightly) during irradiation. By contrast, the number of compounds identified in seep oil decreased by ~40% due to photochemical processes. Seep oil contained a greater number of high molecular weight (HMW), low polarity (LP) compounds by day 20, in both light and dark conditions, which were either degraded completely by photochemical processes or transformed into smaller compounds after 67 days of irradiation. The greater distribution of HMW compounds in seep oil than in industrial oil may reflect the more weathered chemical quality of seep oil.
Non-targeted analysis of the GCxGC-TOF/MS data is ongoing and will allow us to identify specific compounds and probe for known toxic compounds and track their longevity. Ultimately, this research will allow us to better understand the differences between natural seep oil and industrial oil that enters the environment in accidental spills as well as the relative importance of photochemical processes for the degradation of both oil types. These findings can inform future modeling of the fate of dissolved constituents after oil spills.
Dr. Natalie Mladenov is an Associate Professor of Civil, Construction, and Environmental Engineering at San Diego State University. Kristen Snyder is a MS candidate in Environmental Engineering at San Diego State University. Dr. Eunha Hoh is a Professor of Environmental Health, Graduate School of Public Health at San Diego State University. COAST provided funding for this project: Rapid Response Funding Program Award# COAST-RR-2015-004, July 2015.