Oxygen demand and sulfate reduction in petroleum hydrocarbon contaminated salt marsh soils

Laboratory studies were conducted using salt marsh cores to determine the oxygen demand exerted by spilled crude oil and fertilizers added to stimulate biodegradation. Measurements of oxygen uptake and CO2 efflux in flooded and non-flooded cores were conducted in addition to measurements of the magnitude and depth distribution of sulfate reduction rates using 35SO2−4. Additions of crude oil and fertilizers increased sediment oxygen demand (SOD) by a factor of 1.5 to 5 through a combination of enhanced aerobic respiration and sulfate reduction. A strong dependency was observed between the oxygen demand and the overlying dissolved oxygen concentration when crude oil and fertilizers were added. Oiled/fertilized cores did not exhibit an SOD higher than control cores when overlying dissolved oxygen levels dropped below 5 mg/l. Crude oil stimulated sulfate reduction rates by a factor of two under flooded and non-flooded conditions. Highest sulfate reduction rates were observed in the top 2-cm of marsh. Oxygen demand was apportioned between sulfide oxidation and aerobic respiration. Apportionment of SOD indicated that despite doubling of the sulfate reduction rate, most of the demand in non-flooded cores (65%) is exerted by aerobic respiration with the majority of this demand exerted by hydrocarbon degrading organisms. When examined together, data indicate that significant biodegradation of crude oil in salt marshes occurs only when the tidal cycle exposes the surface of the salt marsh to the atmosphere.