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

Laboratory studies were conducted using salt marsh cores to determine the o xygen demand exerted by spilled crude oil and fertilizers added to stimulat e biodegradation. Measurements of oxygen uptake and CO2 efflux in flooded a nd non-flooded cores were conducted in addition to measurements of the magn itude and depth distribution of sulfate reduction rates using (SO42-)-S-35. Additions of crude oil and fertilizers increased sediment oxygen demand (S OD) by a factor of 1.5 to through a combination of enhanced aerobic respira tion and sulfate reduction. A strong dependency was observed between the ox ygen demand and the overlying dissolved oxygen concentration when crude oil and fertilizers were added. Oiled,Fertilized cores did nor exhibit an SOD higher than control cores when overlying dissolved oxygen levels dropped be low 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 2cm of marsh. Oxygen demand was apportioned betwee n sulfide oxidation and aerobic respiration. Appointment of SOD indicated t hat despite doubling of the sulfate reduction rate, most of the demand in n on-flooded cores (65%) is exerted by aerobic respiration with the majority of this demand exerted by hydrocarbon degrading organisms. When examined to gether, data indicate that significant biodegradation of crude oil in salt marshes occurs only when the tidal cycle exposes the surface of the salt ma rsh to the atmosphere. (C) 2000 Published by Elsevier Science Ltd. All righ ts reserved.