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� v L L_ cg A S S O C I A T E S I N C .11 Natural Attenuation with Groundwater Monitoring Natural attenuation, also known as passive bioremediation or intrinsic bioremediation, is a passive remedial approach that depends upon natural processes to degrade and dissipate petroleum constituents in soil and in groundwater Through biodegradation the actual reduction ' of petroleum constituent mass is realized Several detailed field studies have been performed examining indicators of intrinsic bioremediation and identify factors which significantly effect the rate and extent of bioremediation (Buscheck and others, 1993, McAllister and Chiang, 1994, Borden and others, ' 1995, Buscbeck and Alcantar, 1995) Through these studies and on-going research on the factors which control biodegradation, it is now understood that dissolved petroleum hydrocarbons in groundwater will biodegrade, without artificial enhancement, due to the presence of naturally ' occurring (indigenous) microorganisms Technical protocols for implementing and monitoring intrinsic bioremediation studies in groundwater have been developed by the US Air Force and Chevron Corporation ' Intrinsic bioremediation in brief, is the use of indigenous microorganisms to degrade contaminants which have been released into the subsurface The biodegradation of the contaminants is essentially an oxidation-reduction (redox) reaction where the hydrocarbon is oxidized (donates an electron) and an electron acceptor (i e oxygen) is reduced (accepts electrons) There are several compounds that can serve as electron acceptors including oxygen, initrate, iron oxides, sulfate, water, and carbon dioxide (Borden and others, 1995) Aerobic microorganisms use oxygen as the electron acceptors Anaerobic microorganisms use other ' compounds such as nitrate, iron oxides (ferric iron), and sulfate as electron acceptors This alternative would involve monitoring for geophysical parameters including dissolved oxygen, carbon dioxide, sulfate, alkalinity, ferrous iron, nitrate, and oxidation reduction potential 772 Groundwater Extraction and Treatment Groundwater extraction and treatment would entail removing the impacted groundwater from the aquifer, remediating the impacted groundwater if necesary, and then discharging the groundwater into the City of Ripon sewer system This method can be very expensive and requires the installation of a dedicated 4-inch diameter well equipped with a pump, installation of a groundwater treatment unit to remediate the impacted groundwater prior to discharge to the City of Ripon sewer system, and weekly or bi-weekly maintenance of the groundwater extraction ' system A pump test has already been performed to evaluate the effectiveness of groundwater extraction and treatment as a remedial alternative 773 In-Situ Air Sparging with Soil Vapor Extraction In-situ air sparging (AS) with soil vapor extraction (SVE) has proven effective in reducing volatile organic compounds (VOCs) and certain semi-volatile compounds In brief, air is w 1130991reportslsumweli doc 8