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21 March 2005 AGE-NC Project No. 95-0103 Page 2 of 4 a significant portion of contaminated soil,leaving a greatly reduced vadose zone addressable by SVE technology and apparently rendering IAS ineffective due to a thicker saturated zone and diminished ability to recover sparged air.Areas in the eastern portion of the site and at the northwest corner of the site had not demonstrated a sustained reduction of dissolved hydrocarbons. Additional contamination was detected at the southwest corner of the site and along the western edge of the site, not previously remediated. Subsequently a more effective and extensive remedial system was required. GROUND WATER EXTRACTION REMEDIATION In accordance with EHD/CVRWQCB directives and approval,AGE conducted three separate ground water extraction pumping tests in March 2001.Analysis of the ground water extraction data indicted that ex-situ ground water treatment was feasible;the radius of influence generated by a pumping rate of three gallons per minute is interpreted to be adequate to control further off-site migration of contaminants toward the west.Under EHD directives and approval,a ground water pump-and-treat system began continuous operation at the site in April 2002. The operation of the system has produced 3.6 million gallons of impacted ground water and treated cumulative total of 6.46 lbs of benzene and 178.7 lbs of total petroleum hydrocarbons as gasoline (TPH-g).Presently,at most impacted ground water monitoring wells,the contaminant concentrations have been reduced to approximately 1%benzene and 7%TPH-g of the pre-remediation levels. Additional site history and findings from previous investigations and monitoring events at the site are detailed in the Corrective Action Plan (CAP)and the Quarterly Reports prepared by AGE. MONITORED NATURAL ATTENUATION Natural attenuation, and/or passive bioremediation, is a remedial alternative that relies on natural processes to degrade hydrocarbons. Biodegradation is the most important natural attenuation mechanism.Nutrients required for biodegradation should be evaluated to quantify their present in the subsurface.However,where high concentrations of hydrocarbons are present,nutrients may be consumed before hydrocarbon remediation is complete. Oxygen depletion in the subsurface is a characteristic 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 may dominate the degradation of contaminants. Ground water movement will generally assist in the transport of nutrients into the saturated zone.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. The duration for natural attenuation can be many years. Advanced GeoEmironmental,Inc.