A Geochemical and Hydrological Assessment of Oil and Related Compounds from the 2010 Deepwater Horizon Oil Spill in Gulf Coastal Saltmarshes
Type of Degreethesis
Geology and Geography
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In April of 2010, the Macondo-1 wellhead ruptured in the Gulf of Mexico, releasing an estimated 4.9 million barrels of crude oil. Ten saltmarsh sites along the northern Gulf coast were investigated to determine the fate, transport, and transformation of oil and metal contaminants associated with the spill. Sites ranged from heavily contaminated (Bay Jimmy North, Bay Jimmy South, Bayou Dulac, and Batiste), moderately contaminated (Walker Island, Point Aux Chenes Bay, and Rigolets), and pristine (Weeks Bay, Longs Bayou, and Bayou Heron). Weathered oil samples at heavily-contaminated sites were fingerprinted for two targeted biomarkers with special mass-to-charge ratios (m/z). Fragmentograms of m/z 217 and 218 reveal a strong correlation between oiled sediments and original MC-252 crude oil. Tarballs collected from the Alabama coast in September 2012, 28 months after the spill also were fingerprinted to the 2010 Deepwater Horizon spill by fragmentograms of m/z 191, 217, and 218. Isolated plumes of MC-252 spilled oil thus still exist on the Gulf of Mexico seafloor, and it is probable that these oil plumes will continue to wash into coastal regions over time with the aid of storm surges or hurricane activity. Fragmentograms of the tarballs and weathered oil samples both indicate the enrichment of heavier hydrocarbon compounds due to preferential weathering and biodegradation of lighter compounds. MC-252 crude oil was identified in Gulf coastal sediments at depths down to 15 cm. Saltwater-freshwater dynamics may have played a role in this intrusion of crude oil. Physical tank experiments and numerical modeling both support the viability of two proposed intrusion mechanics: intrusion with a saltwater wedge and downward intrusion with onlapping seawater that infiltrates the sediment surface. Following the spill in 2010, BP and NOAA applied 2.1 million gallons of COREXIT 9500A to disperse oil slicks throughout the water column. Gas and liquid chromatography-mass spectrometric (GC-MS and LC-MS) analyses of COREXIT 9500A revealed several compounds: light hydrocarbons, dioctyl sodium sulfosuccinate (DOSS), palmitic acid, and a sulfonic acid base to a series of polyethylene glycols (PEGs). Among these, DOCC will serve as the most effective conservative biomarker to trace the occurrence and geochemical evolution of the dispersant in natural environments. Inductively coupled plasma-mass spectrometry (ICP-MS) reveals generally low concentrations (a few to hundreds of ppb) of various trace metals in pore waters, despite high levels of those trace metals in sediments. Elevated total sulfur concentrations at contaminated sites and at shallower depths suggest a large influx of sulfur along with crude oil. Low dissolved iron and very high levels of sulfide indicate that bacterial sulfate reduction has fixed most reduced Fe through the formation of iron sulfide solids, such as pyrite. High levels of arsenic and manganese in both sediments and pore water at contaminated sites indicate the influx of these metals with crude oil but suggest the possible sequestration of arsenic into iron sulfide solids. A series of geochemical models illustrate the probable evolution of oil-inundated saltmarshes. An activity model indicates that pH and H2S conditions at oiled saltmarshes occur within the stability field of pyrite under reducing conditions (pE = -3). In a system affected by crude oil influx, pyrite is shown to precipitate and hematite to dissolve as conditions transition from oxidizing to sulfate reducing (increasingly negative Eh). Sequestration of metals into pyrite and adsorption to hydrous ferric oxides (HFOs) in coastal marsh sediments may temporarily mitigate groundwater contamination concerns, but geochemical models show that a rise in pH from 7 to 9 would result in the desorption of over 500 µg/kg arsenic into pore waters. Furthermore, competing ions dissolved in seawater may replace arsenic on HFO sorption sites, prompting additional increases of arsenic in solution.