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29 April 2005 r - AGE-NCProject No..98-0440' _ Page•17 of-18 Oxygen depletion in the subsurface is a ' haracteristic`of biodegradation: The core of a contaminant ' plume is typically under anaerobic conditions and only the margins are aerobic. Therefore, even , though anaerobic biodegradation is much slower than aerobic,biodegradation, anaerobic processes s -may dominate,the degradation of contaminants:Grourid-watei movement will generally assist in the - transport of nutrients in'the saturated zone. . The objective of the MNA is to collect ground water samples through techniques which generally ' minimize undesired exposure to atmospheric conditions when and where allowable;and to monitor ' for parameters which signal'the natural attenuation of dissolved-hydrocarbons. Generally the ; geochemical parameters'selected for monitoring are based on the'.use of terminal electron:acceptors I consume hydrocarbons. The sampling for NINA analysis would be.conducted ' during,the'monitoring events to reduce overall,costs, to established a'trend of the geochemical parameters and demonstrate the presents of terminal electron acceptors metabolized by-products. Natural attenuation is normally used where .low, concentrations of hydrocarbons are present, following completion of another form of remediation or in an area where no domestic or,irrigation wells are threatened. 'Co'sts for natural attenuation are generally lower than most active.remediation options; however, costs can be highly variable depending on the duration and type of monitoring required.' Fate and Transport modeling may be used to demonstrate the.potential for hydrocarbons to impact. nearby ground water receptors. Quarterly,ground water monitoring may also be.used to demonstrate', ' astable and declining ground water plume. 8.0. _ 'RECOMMENDATIONS The- following recommendations for .remediation are based upon data collected during the investigatiori'of the property and predicted responses from the site conceptual'model. • .. ' .. ' „1 - , , i .. Y i , . try ' .8.1, REMEDIATION OF-HYDROCARBON-IMPACTED SOIL ; ' Vadose',zone soil impacted by,TPH-d, TPH-g; and BTEX extends vertically to the saturated zone ` under,the former:UST and.surrounding areas to an approximate depth of 17 feet bsg. Excavation' would require the removal of a moderate. volume of'overburden soil in-the former 'UST and ' surrounding areas.Figure 9 illustrates the approximate area of proposed excavation. Because'there- ' appears.to be a low-feasibility for bioremediation and`monitored natural attenuation,AGE concludes �. "Advanced GeoEnvireimental,Inc.