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v8E ( L, A S S O C I A T E S I N C , elevated inside the plume. This indicates that manganese is being used by the microorganisms as anaerobic electron acceptors. Aerobic biodegradation and denitrification and manganese production appear to be occurring inside the plume. This aerial distribution of electron acceptors and biodegradation byproducts across the plume coincides with the descriptions of well- documented intrinsic biodegradation sites published in the literature (Wiedemeier and others, 1995; Borden and others, 1995). Laboratory intrinsic bioremediation data are presented in Table 5 and relative levels and trends of geochemical indicators are presented in Table 6. The concentrations in Table 5 and the trends shown in Table 6 indicate the depletion of electron acceptors, nitrate, and sulfate within the hydrocarbon plume and the amplification of biodegradation byproduct manganese. Monitoring at the site would have to continue until concentrations of petroleum hydrocarbons reached acceptable levels. The advantages of intrinsic L bioremediation include: Readily available equipment and easy installation. • Implementation with minimal disturbance to aboveground activities. • Cost effective under optimal conditions. �. • Can be used under buildings and other inaccessible locations. 4_2 Dual-Phase Groundwater and Soil Vapor Extraction Dual-phase groundwater and soil vapor extraction has proven effective in reducing VOCs and certain semi-volatile compounds. Dual-phase extraction is an in-situ technology that uses a high i" vacuum system to remove both impacted groundwater and soil vapor. If necessary the effluent groundwater and vapor can be treated using a variety of treatment options. This method also stimulates biodegradation by increasing the supply of oxygen in the subsurface. The advantages of dual-phase groundwater and soil vapor extraction include: • Short treatment times; usually 6 months to 2 years under optimal conditions. • Readily available equipment and easy installation. . Implementation with minimal disturbance to aboveground activities. • Proven performance over a wide range of conditions. • Cost competitive. t,. Data provided by the vapor extraction pilot studies would be sufficient to properly design a full- scale portable dual-phase groundwater and soil vapor extraction system. Existing groundwater and vapor wells could be used for remediation purposes. ATC would still be required to complete a final remediation plan and obtain permits for air and groundwater discharge. 4.3 In-Situ Air Sparging with Soil Vapor Extraction In-situ air sparging with soil vapor extraction has proven effective in reducing VOCs and certain sena-volatile compounds. In brief, air is injected into the saturated zone. The air forms bubbles S:\Environmental162577LSCMCAPAdddoc 5