Mobility Characteristics and Redox Influence In Controlling Degradation of Oil Hydrocarbons In Wetland Soils.

Little documented information is available on the fate of oil spill in marsh soils. Besides microbial degradation, the fate of petroleum hydrocarbons could be influenced by volatilization, photooxidation, chemical oxidation, bioaccumulation, adsorption, and adhesion to the soil matrix. Many of these have not been fully evaluated in Louisiana marsh wetlands. This study was conducted to evaluate mobility characteristics and bioavailable fractions of crude oil in wetland soils. In addition, effect of redox potential changes on oil degradation was also assessed in saline and freshwater marsh soils. The saline soil was collected off HWY 1 just east of Port Fourchon and represented soil/sediment from backside of Louisiana Barrier Islands. The fresh marsh soil (allemands muck) was collected from the Wildlife Reserve. The oil was collected from the near shore in Barrier Islands. Sediment slurry (1:6 solid to solution ratio) was spiked with weathered crude oil at 1% and incubated in microcosms in which redox potential was controlled through different proportions of air to N₂ gas. Three step sequential extraction with 0.01 M CaSO₄ – 2% sodium azide followed by dichloromethane (DCM) was used to determine leachable oil hydrocarbons after 2% of oil was reacted with soil/sediments. Hydroxypropyl-β-cyclodextrin (HP-β-CD) extraction procedure was used to assess the bioavailable fraction of petroleum hydrocarbons. The weathered crude oil used in this study was dominated by long chain hydrocarbons ranging from nC-10 to nC-35.  There was significant decrease of hydrocarbons especially carbon compounds below nC-17 after 50 day incubation under the oxidized condition as compared to the reduced condition. Petroleum hydrocarbons were more leachable in the saline marsh soil than the freshwater marsh soil. In addition, short chain (<C18) hydrocarbon compounds were more leachable in the freshwater marsh soil whereas both short and long chain (> C18) hydrocarbons are likely leachable in the saline marsh soil. These results have important implications in interpreting the bioavailability of petroleum hydrocarbons in wetland soils.