The Environmental Chemical Sciences (ECS) program of the Division of Chemistry will support the collaborative RAPID research project of Prof. Thomas Bianchi of Texas A & M University, Prof. Robert Cook of Louisiana State University and Prof. Michael Perdue of Georgia Institute of Technology. The collaborative team will investigate the impact of oil contamination from the Deep Horizon Disaster in the Gulf of Mexico on the composition of dissolved organic matter (DOM) in the Louisiana coastal marshes. The study aims to prove that changes in the component signature of DOM collected from the Louisiana coastal marshes will be indicative of the impact of the Deep Horizon oil spill on geochemical processes in the Louisiana marshes. The team of investigators will utilize boats from the Louisiana Marine Universities Consortium (LUMCON), located in Terrebonne, LA to collect water samples from marshes along Terrebonne Bay. The samples will be filtered to collect the DOM from the water samples. Advanced analytical methods will be used to analyze the DOM content.
The study is crucial to elucidate the impact of the Deep Horizon oil spill on the ecosystems along the Louisiana coast. Better understanding of this impact is imperative to development of strategies to restore the marshes to their condition prior to the Deep Horizon oil spill disaster.
The coastal marshes of Barataria Bay, Louisiana normally contain relatively high concentrations of dissolved organic matter (DOM) that is a mixture of terrestrially derived DOM, which is transported to the coastal marshes by local rivers and streams, and ocean-derived DOM, which is generated through biological activity in coastal seawater. The Deepwater Horizon MC252 oil spill in April-July, 2010 delivered substantial quantities of light south Louisiana crude oil into this environment, which formed oil films on the surface of the marsh waters and fouled and eventually killed a substantial amount of local marsh vegetation. Photodegradation and biodegradation of the oil are expected to transform a portion of the oil into water-soluble compounds that become part of the local DOM. Similarly, local marsh vegetation that was killed by the crude oil would also be expected to contribute some water-soluble compounds to the DOM of coastal marsh waters. It was thus anticipated that the oil spill would lead to transient differences between DOM in oil-impacted marsh waters and DOM in relatively pristine marsh waters. Because DOM regulates the chemical speciation of both toxic and micronutrient trace metal cations, modifies photosynthesis by limiting the penetration of sunlight into coastal marsh waters, and serves as a food source for heterotrophic microorganisms in those waters, it was expected that all of those processes might be perturbed by changes in local DOM that resulted from the oil spill. This study consists of a comparison of the chemical and spectroscopic properties of DOM from relatively pristine and oil-impacted marshes in Barataria Bay. It was anticipated that evidence of oil-derived DOM would be found in the oil-impacted waters. The technical approach consisted of (1) collection of a relatively large number of small samples (a few mL) that were carefully analyzed for optical properties of chromophores and fluorophores and for taxonomic characteristics of microbial assemblages at each sampling site and (2) large samples (500 liters), from which DOM was isolated using the coupled reverse osmosis/electrodialysis (RO/ED) method and then characterized by sophisticated analytical methods. The small samples were collected in July, 2010, approximately when the oil well was successfully capped, and again in September, 2010, several months after the last known leakage of oil from the Deepwater Horizon MC252 well. The large samples were collected only in July, 2010. The concentrations of chromophores and fluorophores in relatively pristine and oil-impacted sampling sites vary both spatially and temporally, but, when the measurements were scaled to take into account the differences in concentration (i.e., corrected for dilution), it was found that the underlying mixtures of chromophores and fluorophores in the pristine and oil-impacted sampling sites are nearly indistinguishable. Very careful analysis did reveal that the terrestrially derived fraction of DOM was greater in July than in September. This observation, in conjunction with a much stronger east-west gradient of salinity within Barataria Bay in July, supports the hypothesis that most observed differences in water quality between oil-impacted sampling sites on the east side of Barataria Bay and relatively pristine sites on the west side of the bay are due to massive releases of freshwater from the Mississippi River via the Davis Freshwater Diversion Project to Barataria Bay, not to the oil spill. That large-scale engineering effort appears to have provided some protection to Barataria Bay, successfully mitigating the impact of the Deepwater Horizon oil spill. The 500-liter samples that were collected in July, 2010 were processed by our research group at Georgia Tech to obtain substantial amounts of freeze-dried organic matter. The samples have been distributed to the three participating research groups. Dr. Robert Cook’s research group at Louisiana State University has measured 13C nuclear magnetic resonance (NMR) spectra to detect subtle differences in the types of organic carbon in DOM. The samples have also been analyzed for lignin content by Dr. Tom Bianchi’s research group at Texas A&M University to better quantify the role of terrestrially derived DOM in Barataria Bay during this period of time. Our research group has acquired Fourier transform ion cyclotron resonance (FTICR) mass spectra that enable the assignment of molecular formulae to thousands of organic compounds in each sample. We have also conducted pH titrations to quantify the concentrations and acidic strengths of carboxylic acids in each of the isolated samples. The resulting data sets are still being interpreted and may yet reveal subtle differences between DOM from relatively pristine and oil-impacted sampling locations.
