Transport and biodegradation of Deepwater Horizon crude oil in Gulf of Mexico sediments
Markus Huettel, Joel Kostka, Florida State University
The Deep-C Project conducts a long-term, interdisciplinary study of deep-sea-to-coast connectivity in the northeastern Gulf of Mexico. The study investigates the environmental consequences of petroleum hydrocarbon release in the deep Gulf on living marine resources and ecosystem health. Deep-C examines the geomorphologic, hydrologic, and biogeochemical settings that influence the distribution and fate of the oil and dispersants released during the Deepwater Horizon (DwH) accident, and uses the resulting data for model studies that support improved responses to possible future incidents.
Transport and biodegradation of Deepwater Horizon crude oil in Floirda beach sands
(Funded by NSF RAPID Award OCE-1044939 and GOMRI)
This project,funded through a NSF RAPID award, investigates the transport and degradation of Deepwater Horizon oil in sandy Gulf beaches. Specifically, we investigate 1) how much and how deep oil is transported into the beaches, 2) how oil alters the filtration of water into the beach, 3) how oil alters oxygen penetration that accelerates oil degradation, 4) how oil contamination changes structure and function of the sand microbial community, 5) which factors regulate oil degradation in marine sands.
Nutrients (nitrogen and phosphorus components) can accelerate microbial degradation of buried crude oil. We will measure how oxygen and nutrient availability affect oil degradation rates.
The results of this research will provide quantitative data on the amount of oil that is transported into Gulf beaches and will allow estimates on how long it may take until this oil is degraded. The project will produce information regarding the environmental conditions under which the degradation takes place (i.e. aerobic or anaerobic). Because oil contains harmful components, an understanding of oil transport, storage and degradation in Gulf beaches is important for coastal management and local economies that depend on water quality, e.g. fisheries and tourism.
Oil and oil combined with dispersants (chemicals that disperse the oil into very small oil droplets) is carried by currents and winds to the Gulf shores, where it is washed up sandy beaches. Larger crude oil accumulations like pancake oil (flat accumulations of crude oil) and tar balls (weathered crude oil accumulation that have been formed into ball-shaped structures) are deposited on the beach, while liquid oil, dissolved oil components and small particles of dispersed oil can penetrate tens of centimeters into the permeable beach sand.
Water with oil that is washed up the beach filters into the porous sediment and carries some of the oil (small particles, low viscosity and dissolved components) with it into the sand. In addition, the water level drop between high and low tide causes also a water level drop within the beach sediment, and this pore water level drop can transport oil that penetrated into the beach in even deeper sediment layers.
The oil accumulations that are deposited on the beach surface can be removed relatively easily (e.g. by scraping off the surface layer of the sand), while the oil components that penetrated deep into the sand can only be removed by microbial degradation or massive sediment erosion. Oil degradation products may be washed out with the pore water flows.
Crude oil is a natural substance that constantly seeps out of Gulf of Mexico sediments (obviously in much smaller quantities than now caused by the drilling accident), and microbes have evolved that can degrade the oil. These microorganisms include bacteria and also microalgae that live in the water column and the sediments of the Gulf of Mexico.
Where oxygen is present, as in the water and the upper layers of the beach sand, the microbes decompose the oil aerobically, i.e. with the use of oxygen. This degradation process can be relatively rapid (days to months)
In contrast, the degradation under anaerobic conditions, (i.e. when no oxygen is available) is very slow (months to years).
That is one of the reasons why at the site of the Exxon Valdez oil spill in Alaska, oil still can be found deeply buried in the gravel beach sediments (http://www.afsc.noaa.gov/Quarterly/jas2001/feature_jas01.htm). In Alaska, microbial degradation is also slowed down by the cold temperatures. Oil here persisted in the beach sediments beyond a decade in surprising amounts and in toxic forms, was sufficiently bioavailable to induce chronic biological exposures, and had long-term impacts at the population level (Peterson et al. 2003). Significantly elevated embryo mortalities in pink salmon were observed from 1989 through 1993.
Thus, oil stored in deep layers of beach sediment presents a potential source of toxins to nearshore waters and groundwater.
For our investigations we collect near shore water and beach sediments. Water samples are collected in order to assess how much oil is contained in the water that washes up the beaches, and what kind of microorganisms are living in that water.
The sediment is collected to assess penetration of oil and sediment microbial community composition and activities. For the sediment sampling, we use plastic pipes of 5.5 cm inner diameter that we drive vertically into the beach sediment in the area where water washes up the beach. The sediment cores retrieved with the pipes are sectioned into 5cm depth intervals and these subsamples then are analyzed for oil content, oxygen consumption rate, and microbial community composition and activity. The oil is extracted from the sediment is analyzed using a gas chromatograph. A mass spectrometer connected to the gas chromatograph provides information on the composition of the oil components.
The microbial community in water and sediment samples are analyzed using a combination of culture-based and cultivation-independent molecular techniques. In the culture-based technique, the presence and abundance of oil-degrading microorganisms is tested by adding microorganisms extracted from the water and sediment samples to growth media that contain oil. The organisms that grow on these media then are quantified and characterized. Since the large majority of marine microorganisms cannot be cultured, molecular methods are used to provide measures of microbial community composition, diversity and abundance and changes caused by the oil. The bacterial populations is analyzed through the extraction and analysis of community genetic materials (DNA and RNA). Gene sequence data are used to produce a characterization of the most abundant microbial taxa with space and time. Select samples are subject to high-throughput gene sequencing (i.e. Roche 454 pyrosequencing), which provides comprehensive information on the composition of the microbial communities.
Our research shows how much and what oil components are transported into Gulf beach sands, how deep these oil components penetrate, and how rapidly they are degraded. We will identified the main microbial groups that degrade the oil and how the oil changes the microbial community in the sand and sediment biogeochemistry. We determined how sedimentary oil decreases oxygen availability in the beach sand, determinde the rates of oil degradation and how long toxic components persist in the sand. With this information we can make predictions on how long it may take until most of the oil in Gulf beaches is degraded. We will investigate how we could possibly accelerate the sedimentary oil degradation rates.
The sandy beaches of the Northeastern Gulf of Mexico are of high ecological and economical value as they are habitat, foraging area and breeding place of a large number species.
Tourism is Florida’s largest sector of the state economy with about 60 million visitors every year and a $57 billion impact. The pollution of the beaches by crude oil thus presents a threat to Florida’s economy.
Quantification of the uptake, transport and degradation of oil in sandy Gulf beaches is prerequisite for designing clean up and remediation procedures.
Previous research indicates that oil can penetrate and persist deep in sandy beaches, which may change microbial community activities and decrease the oil degradation rates.
It is critical that we have a thorough understanding of the fate of the oil in Gulf beaches as consequences may include release of oil components and its degradation products from such beaches extending over many years.
Further information on oil degradation and oil in marine sediments
Bernabeu, A. M. and others 2006. Beach morphodynamics forcements in oiled shorelines: Coupled physical and chemical processes during and after fuel burial. Marine Pollution Bulletin 52: 1156-1168.