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------------ Former Discovery Chevrolet March 20, 1998 Tracy.CaliforrLia, Problem Assessment Report 7AA Excavation and Aboveground Treatment r Excavation and aboveground treatment consists of various methods of aeration, soil vapor extraction, and biotreatment. Aeration involves thin-spreading of the excavated soils on an impermeable surface to allow removal of hydrocarbons through volatilization. Thin-spread soils are periodically turned to maximize surface exposure and aeration. Microbes naturally occurring in soils also convert both volatile and nonvolatile hydrocarbon components to primarily carbon dioxide (CO2) and water. Above ground soil vapor extraction involves the in-situ venting of hydrocarbons in soil through a vapor recovery line network installed within the stockpiled excavated soils. Hydrocarbons contained in the soil vapors may be removed by combustion, adsorption, or direct discharge to the atmosphere, depending on the concentrations. Biotreatment involves creating an environment in excavated soil that promotes decomposition of hydrocarbons by microbes. Fertilizer and oxygen are supplied to the excavated soils by cultivation, and forced ventilation enhances the G�:•:.:__� performance and propagation of the microbes. Following treatment, the soils may be disposed of through an appropriate landfill or used as backfill material. .2 rou dwat r The following groundwater remediation alternatives are discussed in this section: • Passive Remediation • Aboveground Treatment • In-Situ Bioremediation • Air Sparging 7 2.11 Passive Remediation Passive remediation involves monitoring of groundwater to determine whether natural biodegredation, volatilization, or dispersion of petroleum hydrocarbons is occurring. The effectiveness of passive remediation is indicated by a decrease in benzene concentrations in 3001-12.par 13 HORIZON ENVIRONMENTAL INC.