Assessing Transformation of Trace Metals and Crude Oil in Mississippi and Louisiana Coastal Wetlands in Response to the Deepwater Horizon Oil Spill
Type of Degreethesis
Geology and Geography
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On April 20, 2010, the drilling rig Deepwater Horizon exploded in the Gulf of Mexico, resulting in the release of about 5 million barrels of crude oil into the environment. Coastal wetlands are particularly susceptible to oil contamination because they are composed largely of fine-grained sediments, which have a high capacity to adsorb oil and associated metals. Microbial activities may be enhanced by an increase in amounts of organic matter and subsequently influence the biogeochemical cycling of trace metals. This research assesses the levels of oil and trace metals, along with associated biogeochemical changes, in six coastal marshes in Mississippi and Louisiana. Total digestion analysis of wetland sediments shows higher concentrations of certain trace metals (e.g., Ni, Cu, Pb, Zn, Co, V, Ba, Hg, As) in heavily-oiled Louisiana sites (e.g., Nay Jimmy and Bayou Dulac) compared to those at less-affected and pristine sites. Due to chemical complexation among organic compounds and metals, crude oils often contain elevated levels (up to hundreds of mg/kg) of trace metals. At the heavily-oiled Louisiana sites (e.g., Bay Jimmy and Bayou Dulac), elevated levels of metals and total organic carbon were found in sediments down to depths of about 30 cm. The contamination is not limited to shallow sediments; oil, along with various associated metals, may be invading into deeper (pre-industrial) portions of the marsh sediments. Pore waters extracted from contaminated sediments are characterized by very high levels of reduced sulfur (up to 80 mg/kg). Microbial analysis of oiled sediments indicates that the influx of oil into the wetlands might have provided the additional substrate and carbon source for stimulating sulfate-reducing bacteria. Moreover, pore-water pH values show a general increasing trend (ranging from 6.6 to 8.0) with depth, possibly reflecting the combined effects of bacterial sulfate reduction and saltwater intrusion at depth. Despite high levels of trace metals in bulk sediments, concentrations of trace metals dissolved in pore waters are generally low. Pyrite-like iron-sulfides with distinct framboidal form are found in oiled sediments. Laser-ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) analyses show that sulfide solids contain various levels of trace metals, including As, Hg, Pb, Cu, Zn, V, and Zn. Sulfide solids likely serve as local sinks for chalcophile (“sulfur-loving”) trace metals under sulfate-reducing conditions. It appears that high organic matter content and bacterially-mediated sulfate reduction facilitate metal retention via sulfide formation in the oiled marsh sediments. In addition to metals retention in sulfide solids, geochemical analyses show that residual oils remain in these wetlands up to months or possibly longer. High TOC and DOC levels (up to about 29% and 330 mg/kg, respectively), and low 13C values of affected sediments, confirm the persistence of oil in these salt marshes. Gas-chromatograph mass-spectrometer (GC-MS) analyses of extracted organic contaminants show that only lighter compounds of the Macondo-1 crude oil are quickly degraded, while the heavier components tend to remain in the environment months after the spill. The long-term biogeochemical effects of the remnant oil in the wetlands remains unclear, as does the possibility of re-oxidation of wetland sediments and metal sulfides. A potential effect would be the disaggregation of the solid sulfides and subsequent mobilization of sorbed trace metals. This possibility warrants further monitoring of affected wetlands.