Utilizing hydrocarbon biodegradation capability of microorganisms in soil and groundwater is a costeffective approach to minimize hydrocarbon impact to the environment. Beyond the normal operational control to minimize leaks and spills, using soil around facility as a containment zone is effective for a sustainable growth. Petroleum hydrocarbons are naturally occurring organic compounds that are readily degraded by microorganisms found in soil and groundwater. However, hydrocarbons will accumulate and spread when the degradation capacity of soil and groundwater is exceeded, causing environmental damages and leaving a long-term liability. A biodegradation equilibrium can be re-established through proper stimulation of the microbial environment in the soil and groundwater. It has been shown that, given sufficient time, bioremediation is the most cost-effective method to clean up hydrocarbon-impacted soil and groundwater. Time constrain is normally not an issue for an on-going oil and gas operation. Therefore, bio-stimulation can be a very cost-effective approach to control and minimize hydrocarbon impact to the environment.
It is known since the early days of oil industry that most of petroleum hydrocarbons are biodegradable in the environment. Studies conducted in the past 20 years have shown that even the most recalcitrant components of petroleum crude mixtures, such as polycyclic aromatic hydrocarbons (PAHs), can be degraded by the microorganisms found in most soil environment 1,2. During the last 10 years of rapid evolution in environmental remediation, several variations of bioremediation, which include bioventing, biosparging, and natural attenuation, have emerged as among the most economical approaches to cleanup hydrocarbon impacted soil and groundwater3.
It is also known that, petroleum hydrocarbons can persist and accumulate in soil and groundwater, when a limited natural biodegradation capacity of the soil and groundwater is exceeded by the rate of hydrocarbon buildup. If left uncontrolled, it can potentially pose as a threat to the environment and natural resources. Therefore, it is important to understand how hydrocarbon biodegradation process is affected by the various conditions in soil and groundwater, as well as ways to stimulated biodegradation.
Hydrocarbon biodegradation basically is the oxidation of hydrocarbons through respiration of microorganisms. Biodegradation occurs when hydrocarbons come in contact with an oxidizer, also known as electron acceptor, in the presence of active microorganisms, or a biomass. Under aerobic conditions, oxygen (O2) is the preferred oxidizer or electron acceptor. Under anaerobic conditions, nitrate (NO3-), ferric iron (Fe3+), or sulfate (SO42-) are alternative electron acceptors 4–6.
Because of abundance of oxygen in the air (over 20%), hydrocarbons in shallow surface soil are readily degraded aerobically by the microorganisms in the soil. Under such condition, the rate of hydrocarbon biodegradation is often limited by microbial activities or biomass, which is dictated by a given soil condition (moisture content, temperature, pH, and nutrients). Therefore, biodegradation can be enhanced with proper amendment.
Hydrocarbon Biodegradation in the deeper subsurface is more often dictated by the limited availability of electron acceptors. Because of the low solubility of oxygen in water (10 ppm) and relative abundance of alternative electron acceptors in most groundwater, most hydrocarbon plumes in groundwater are found to be under anaerobic conditions.