Crude oil could be dissolved, oxidized, evaporated, degraded (including microbial and photochemical degradation), and fractionated into hydrocarbon fractions in the marine environment, changing its original optical, chemical and isotopic signatures such as fluorescence excitation-emission characteristics and spectral slope as well as molecular size distribution. However, the quantitative changes in hydrocarbon speciation and thus the chemical and optical properties of seawater during the degradation of oil and dispersant are poorly understood, which makes interpretation of field data challenging. Thus, laboratory studies are needed to provide highly complimentary data in addressing the fate, transport, and toxicity, as well as the impacts and consequences of dispersant and oil in the water column of the Gulf of Mexico.
We propose to evaluate systematically the dynamic changes in molecular size distribution, fluorescence excitation-emission matrices (EEM) spectra, stable C isotopic composition, and other optical properties during the degradation of oil and dispersant, including microbial degradation and photochemical degradation, through controlled laboratory experiments. Our working hypothesis is that the crude oil from the Deepwater Horizon incident and the addition of dispersant should significantly alter the optical properties and size distribution of natural organic matter in the water column of the Gulf of Mexico. The dispersant and oil should be selectively degraded, through microbial and photochemical processes, resulting in a significant physicochemical fractionation and shift in optical properties and molecular size distribution of the residual dispersant and oil in marine environments.
Degradation experiments, including photochemical degradation and microbial degradation of oil, dispersant and oil-dispersant mixture, will be conducted in the laboratory under different treatments. Variation in optical properties, organic molecular size distribution, concentration of carbon species and nutrients, and stable isotopic composition will be quantified during the degradation experiments using UV-vis spectroscopy, 3D fluorescence spectroscopy, flow field-flow fractionation techniques, and isotopic ratio-mass spectrometry.
Data collected from our proposed laboratory experiments will be used to:
1. Estimate the degradation rate based on changes in absorbance and fluorescence intensities
2. To determine the interrelationship between degradation of oil and dispersant and fluorescence EEM spectra, optical properties, nutrients and stable isotopic composition
3. To determine whether there is a significant fractionation in organic composition (aromaticity and molecular size), fluorescence excitation-emission characteristics, and stable carbon isotopic composition
Our proposed research is highly relevant to the NGI themes, NOAA strategic goals, and BP Gulf of Mexico Research Initiative themes. This study will also provide relevant new data to advance our understanding of the fate, transport mechanisms, degradation pathways, composition change and effects on the ecosystem as a whole of the oil spill. Results from these laboratory experiments will be very useful in interpreting field observations and help with field sampling strategies in oil spill research. The proposed work will integrate the PIs research with education, professional training and teaching through the participation of graduate and undergraduate students and collaboration with scientists at USM and other universities/institutions.