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Hydrocarbon concentration trends are the primary indicators of natural attenuation rates of hydrocarbons in ground water Secondary indicators such as dissolved oxygen (DO), oxidation- reduction potential (ORP), alkalinity, nitrate, sulfate and ferrous iron are also used to evaluate the existence of and/o the potential for natural attenuation Natural attenuation at a given site can be due to aerobic and anaerobic hydrocarbon degradation Most sites exhibit aerobic hydrocarbon degradation, and an inverse relationship is observed between hydrocarbon and DO concentrations More specifically, DO concentrations are typically reduced in the hydrocarbon source area compared to near the plume boundary For natural attenuation to occur by aerobic processes, a minimum of about 1 mg/l DO is required Under anaerobic processes, sulfates, nitrates and iron can act as electron receptors MTBE appears to degrade very slowly under aerobic subsurface conditions and has not been shown to be degradable under anaerobic conditions There is some evidence that suggests that degradation of MTBE is inhibited by the presence of other hydrocarbons such as BTEX Cost-Effectiveness Natural attenuation is a very cost effective alternative to reduce the level of hydrocarbons since it allows hydrocarbons to degrade naturally and does not require active remediation Due to the presence of BTEX compounds in addition to MTBE, natural attenuation may not be effective on MTBE in the short term Sampling for intrinsic bioremediation parameters typically costs about $100 per well when performed in conjunction with routine ground water monitoring The cost of additional ground water monitoring should be considered in the overall cost- effectiveness evaluation Recommendation Although the investigation and ground water monitoring to date have not shown that intrinsic bioremediation is occurring in the site subsurface, this alterative should be included in the implemented corrective action because it is a very cost effective method to reduce hydrocarbon concentrations Ground Water Oxygenation with Oxygen Releasing Compounds (ORCs) This is a relatively new remedial technique being implemented at numerous sites ORCs release DO into ground water to stimulate and accelerate naturally occurring aerobic hydrocarbon biodegradation ORCs are capable of elevating DO concentrations up to 40 mg/l, while DO concentrations from air infection techniques such as air sparging can typically only achieve up to about 10 mg/l Unlike air infection techniques, ORCs oxygenate ground water without the potential for causing hydrocarbon vapor migration ORCs are a solid magnesium peroxide compound that is activated by moisture ORCs release oxygen slowly to the ground water and are environmentally safe to use The byproducts of the ORC reaction with water are oxygen and magnesium hydroxide, which is essentially milk of magnesia ORCs can be 240-0783 21