The explosion of the oil rig Deepwater Horizon in the Gulf of Mexico on April 20, 2010 has released ~3 million barrels of crude oil to the Gulf as of mid-June (The Economist, 2010). This oil has a range of deleterious effects on the aquatic and coastal ecosystems of the Gulf. One such effect may include alteration of biogeochemical cycling of heavy metals in the coastal wetlands. Cycling of mercury (Hg) and arsenic (As) both depend on microbial activity, particularly iron and sulfate reduction, which may be promoted by the influx of organic matter (i.e. oil). Coastal wetlands are particularly susceptible to heavy metal contamination and may therefore be especially vulnerable to altered heavy metal cycling as a result of the oil spill. This project would examine solids and pore waters from sediment cores in Weeks Bay, Alabama for changes in microbial activity, arsenic concentration and speciation, and mercury concentration and speciation over the next eight to twelve months. Results will be of broad interest to the fields of bioremediation, biogeochemistry, geomicrobiology, and environmental health. During the progress of the study, PIs will be cooperating with local scientists and government officials, and plan on presenting technical seminars and workshops in the Alabama Gulf shore region. Because ocean oil spills are a common environmental problem worldwide, the data gathered in the research should benefit many other affected regions. Research will expose students from Alabama to state of the art methods and to a timely research topic. The Weeks Bay field site will serve as an outdoor laboratory for Auburn University Water Education for Alabama Black Belt (WET) outreach activities and existing courses.
The drilling rig Deepwater Horizon exploded in the Gulf of Mexico on April 20, 2010, resulting in the release of approximately 5 million barrels of crude oil into the environment. Oil and various associated trace metals are known to have wide range of deleterious effects on the aquatic and ecosystems of coastal wetlands. Influx of oil could promote microbial growth and therefore can affect the biogeochemical cycling of trace metals such as iron, arsenic, and mercury. This research facilitates integrated assessment of the levels of oils and trace metals and their biogeochemical changes from ten salt marsh sites in Louisiana, Mississippi, and Alabama. These sampling sites range in their pollution levels from pristine to highly contaminated. Geochemical analyses show alarmingly high organic carbon loads in pore-waters and sediments at heavily contaminated sites months after the influx of oil ceased. Very high levels (10-28%) of total organic carbon (TOC) within the heavily oiled sediments (down to 30 cm) are clearly distinguished from those found in pristine wetland sediments (generally < 5%). Furthermore, dissolved organic carbon (DOC) levels of pore-waters extracted from oiled sediments, ranging up to hundreds of mg/kg, are on the order of one to two magnitudes higher than those at pristine and slightly contaminated sites. TOC and DOC data clearly indicate that not all the spilled oil rose to the water surface and washed on-shore. Plumes of partially degraded oil could be spreading at various levels of the water column and feeding the underlying sediments. Geochemical results from total digestion analysis of sediment samples show that concentrations of certain trace elements (e.g., Ni, Cu, Pb, Zn, Sr, Co, V, Ba, Hg, As) are higher in heavily oiled zones with respect to less-affected and pristine sites. Crude oils often contain elevated levels (up to hundreds of mg/kg) of trace metals due to chemical complexation among organic compounds and metals. At Louisiana heavily oiled sites (e.g., Bay Jimmy, Bayou Dulac, Bay Batiste) elevated levels of metals and total organic carbon extend beyond shallow (post-industrial) profiles to greater depths (down to 30 cm), suggesting that oil and various associated metals might have spread at all levels of the water column and invaded through the deeper sections of marshes sediments. Geochemical analyses of pore-waters show very high reduced sulfur levels (up to 80 mg/kg) but fairly low ferrous iron concentrations (< 0.02 mg/L) in heavily oiled sediments. Influx of oil in muddy sediments provided the initial substrate and carbon source for sulfate reducing bacteria. Actively bacterial sulfate reduction reactions that convert sulfate to sulfide can raise pH of porewaters. Despite high levels of trace metals in bulk sediments, concentrations of trace metals dissolved in pore-waters are generally low. Biogenic pyrite and other sulfides with distinct framboidal form are found in oiled sediments. These 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. The risks associated with re-mobilization of metals under changing redox and geochemical conditions underscore the necessity to monitor the fate and transformation of trace metals in affected salt marshes.
