Impacts of Fresh and Weathered Crude Oil from the 2010 Deepwater Horizon Spill to Redox Potential in Soil Cores from a Spartina Alterniflora Dominated Salt Marsh in Barataria Bay, La.

The Deepwater Horizon (DWH) oil spill in 2010 triggered extensive research on the impacts of crude oil on flora and fauna of the Gulf Coast. Little research has investigated the impact of the weathered status and location of spilled oil in the soil profile on oxidation-reduction (redox) conditions of wetland soils. Redox conditions in wetland soils provide an excellent indicator for defining oxygen levels which control biogeochemical functions linked to valuable ecosystem services. The goal of this study was to quantify the impact of crude oil on redox conditions in wetland soil cores collected from a Spartina alterniflora salt marsh in Barataria Bay, LA. The treatments tested were a non-oiled control, weathered crude oil at the soil profile surface, fresh crude oil at the soil profile surface, and a layer of buried weathered crude oil, 5-cm beneath the soil surface, matching the presence of buried oil found at a nearby site heavily oiled by the DWH spill. No significant differences in redox potentials were detected among treatments at any depth at the end of the 35-day flooded, anaerobic portion of the study.  After draining the cores, mean redox values for each set of treatments (n=3) were calculated for each sample time to determine the time necessary for the rhizosphere to reach the aerobic threshold (+300 mV). Mean redox values for the control cores reached +300 mV after 193 h.  Both fresh and weathered crude oil surface treatments reached +300 mV after 316 h.  The buried crude oil treatment reached the aerobic threshold after 418 h. These results suggest that the presence of crude oil and location of crude oil in the soil profile impact shifts in redox conditions which could alter biogeochemical processes over the long term and induce stress on wetland vegetation.