BIOGEOCHEMICAL IMPRINT OF DEEPWATER HORIZON OIL SPILL ON GULF COASTAL WETLAND SEDIMENTS AND POREWATER

This research investigates the fate and transformation of oil and trace metals at ten selected Gulf salt marsh sites in Louisiana, Mississippi, and Alabama, months after the 2010 BP Deepwater Horizon oil spill. The geochemical characteristics of sediments and pore-waters were studied intensively every 3 cm down to a depth of 30 cm. The sampling sites range in their contamination levels from pristine to completely inundated, allowing us to assess the geochemical and microbial impacts of the oil spill. In general, the total organic carbon contents of contaminated pore-water and sediments were found to be one to two orders of magnitude higher than those of pristine sites. The DOC levels of surface water at heavily oiled sites fall in the same range as those of surface water and pore-water at unaffected sites. These results clearly indicate that elevated organic carbon levels could persist in wetland sediments and pore-water even after a significant portion of oil in the surface water has been evaporated, dispersed, or degraded by microorganisms months after the spill. Abundance of sulfate reducing bacteria and elevated sulfide concentrations in pore-waters extracted from heavily oiled Louisiana sites suggest that anaerobic sulfate reduction may be enhanced by the influx of oil and organic matter. High organic matter content and bacterially-mediated sulfate reduction facilitate the formation of metal sulfides and retention of trace metals. GC-MS analysis shows significant degradation of lighter compounds while heavier oils persist in sediments. Organic materials recovered from oiled Louisiana wetlands match the chemical fingerprint of oil from the Deepwater Horizon wellhead. Stable carbon isotopic measurements show that the spilled BP oil has a unique carbon isotope signature that is significantly different from those of Louisiana salt marshes dominated by C4 plants (Spartina sp.). Such a large carbon isotopic difference between the marsh vegetation and the oil provides an excellent opportunity to examine the source and movement of spilled oils in coastal marshes. Depth profiles of DOC and TOC clearly indicated that the oil contamination is not limited to surface sediments. New research efforts are currently underway to model the oil intrusion dynamics and accompanying biogeochemical processes in coastal wetlands.