|The effects of the 2010 BP Macondo-1 well oil spill on the geochemistry of sediments and water columns at ten Gulf salt-marsh sites were investigated, months after the spill ceased. The ten sampling sites include four heavily contaminated sites in Louisiana (Bay Jimmy North, Bay Jimmy South, Bayou Dulac, and Bay Batiste), three intermediately contaminated sites in Alabama (Walker Island), Mississippi (Point Aux Chenes Bay), and Louisiana (Rigolets), and three pristine sites in Alabama (Weeks Bay, Longs Bayou) and Mississippi (Bayou Heron). Five of the ten sites are discussed at length in this thesis; they include Bay Jimmy South, Rigolets, Walker Island, Longs Bayou, and Weeks Bay.
Results indicate immediate and potentially long-term impacts of MC-252 Macondo-1 spilled oil on sediments and pore-waters of coastal wetlands. High levels of total organic carbon (TOC) contents of oiled wetland sediments range from 10-28%, whereas pristine sites are generally < 3%. Furthermore, dissolved organic carbon (DOC) levels in pore-waters at oiled locations, which reach hundreds of mg/kg, are one to two orders of magnitude higher than those at pristine locations. GC-MS analysis of oil extracts clearly correlate source MC-252 crude to oil extracted from sediments down to 15 cm. GC-MS analysis also shows significant degradation of lighter compounds while heavier oils still persist in sediments.
Most probable number analysis of bacteria suggests that the oil and organic carbon that washed into the coastal wetlands have accelerated the growth and colonization of sulfate- and iron-reducing bacteria. Oiled sediments are characterized by very high sulfide concentrations (up to 80-100 mg/kg) and an abundance of sulfate-reducing bacteria. The influx of oil and its
biodegradation creates strong reducing conditions along with the increased microbial activity, which in turn facilitate the biological and chemical transformation of toxic trace metals. Highly elevated concentrations of certain trace metals and elements (Cu, Pb, Zn, Fe, Hg, As, V, Ni, and S) are found in heavily oiled sediments, likely resulting from MC-252 oil and its associated chemical constituents. Additionally, oiled wetlands are dominated by fine-grained mud and organic matter, which have a high capacity to adsorb metals. Interestingly, despite high levels of trace metals in bulk sediments, concentrations of trace metals dissolved in pore waters are generally low. Petrographic, SEM EDAX, and Laser Ablation-ICP/MS analyses indicate that many toxic metals (As, Hg, Pb, Zn, Cu, etc.) have been sequestered in biogenically produced metal sulfides at heavily oiled sites.
Results indicate that the increase of organic carbon (i.e., oil) at heavily contaminated sites causes an acceleration of sulfate- and iron-reducing bacteria colonization that promotes strongly reducing conditions. Under strongly reducing conditions, dissolved trace metals are sequestered by sulfate reducers through the formation of sulfide solids. Although heavier components of crude continue to exist in sediments, the biodegradation over the long term and possible oxidation of biogenically produced metal sulfides and the resultant re-mobilization of trace metals remains unclear. Continued evaluation of ecologically sensitive estuarine environments is needed to fully understand the long-term effects of the oil spill.