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2 3.1 In-Situ Vapor Extraction As with in-situ soil vapor extraction, this groundwater remediation alternative utilizes a series of vapor extraction wells to remove volatile hydrocarbons from the subsurface A vacuum is applied to the extraction wells, typically by means of a blower, and volatile constituents on and dissolved in groundwater may be effectively removed In-situ vapor extraction can be a time and cost-effective remedial technology in permeable soil, and if contaminants are volatile at ambient temperatures. I Vapor extraction systems have been shown to efficiently remove lighter-chain Petroleum hydrocarbons from groundwater 2.3.2 In-Situ Bioremediation This groundwater remediation alternative may involve the infection of nutrients, an appropriate bioculture, and air (or some combination of these) into the vadose zone, capillary fringe, and affected areas of the groundwater table. An appropriate monitoring program must be implemented concurrently with the treatment process to �! ' evaluate its effectiveness, the rate of biodegradation, and ensure the maintenance of optimum conditions for the biomass Prior to implementing an in-situ bioremediation alternative, key subsurface conditions and characteristics must be ascertained to ensure the adequacy of this treatment alternative Some of these include 11 The lateral and vertical heterogeneity and/or homogeneity of the subsurface material and/or soil types as defined by the Unified Soil Classification System 2) The horizontal and vertical permeabilities of subsurface soil 3) The types, concentrations, and distribution of hydrocarbons in the vadose and groundwater zone 4) The biological and chemical oxygen demand of the water-bearing zone ' and any potential bioculture ' 5) The pH of the affected soil mass and groundwater zone 6) The identification of any naturally occurring micro-organisms present in the subsurface which may potentially consume the contaminants in question Once these subsurface characteristics and conditions have been adequately evaluated, a determination must be made as to whether in-situ bioremediation is technically feasible For example, if permeability of soil represents a potential problem in ensuring the effective introduction of nutrients, bioculture, and/or air into the affected areas of the subsurface, then the feasibility of enhancing local permeabilities must be evaluated In some cases, these difficulties may be overcome by a series of appropriately located trenches If a preliminary assessment of the subsurface conditions indicates the lack of appropriate micro-organisms, then an appropriate artificial bioculture must be specified If the pH of soil and/or groundwater is untenable for the proposed bioculture, then a plan to alter the pH to appropriate levels must be developed