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air stripping, aeration, adsorption, absorption, ultraviolet oxidization, chemical oxidization, or biological. The removal of groundwater by pumping creates a depression in the groundwater potentiometric surface that draws in groundwater from the surrounding area. This depression (radius of influence) can contain the petroleum hydrocarbon plume and prevent further migration of the plume downgradient toward sensitive receptors such as drinking water wells. 3.1.3 In-situ Bioremediation Remediation of organic contaminants dissolved in groundwater can be accomplished through bioremediation. Biodegradation of organic compounds occurs naturally in the subsurface due to the activity of microorganisms. Aerobic and anaerobic bacteria break down the organic compounds into simple inorganic compounds such as carbon dioxide and water. Natural biodegradation is often limited due to the lack of nutrients and oxygen in the subsurface. Bioremediation is the process by which natural biodegradation is accelerated by the addition of oxygen and nutrients. Injection wells are installed into the water bearing zone to inject air and nutrients to aid the biodegradation process and accelerate the rate of biodegradation reactions. 3.1.4 Containment Barriers Containment of the groundwater can be accomplished by installing either a low permeability barrier to prevent the flow of groundwater or a reactive barrier to chemically control migration of the contaminants. In shallow groundwater conditions (<20 feet), a slurry wall can be installed using a backhoe or excavator and filling the trenches with a bentonite/cement slurry. In deeper groundwater areas, the low permeability material is installed using a large diameter auger rig with overlapping drilling locations to eliminate gaps in the barrier. The borings created by the drilling rig are filled with the bentonite cement slurry. A reactive barrier is constructed using a material that chemically reacts with the contaminates to reduce or oxidize them to inert compounds. 3.2 Evaluation of Remedial Alternatives In this section, the remedial alternatives. presented in Section 3.1 are evaluated with respect to constraints of implementation at an operational service station, relative advantages and disadvantages, and potential costs. The rationale for implementation of the proposed alternative is also provided. M 3-2 E53r1E:1U5A100651REPORT510065P201.DOC-98usg:I Rev.0,1/26199 20A41-007.001