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Initial Screening Criteria Since intrinsic bioremediation demonstration is a somewhat emerging approach, a discussion of established screening criteria is provided. In EPA's Guide for Corrective Action Plan Reviewers, How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites (EPA, October 1994), the criteria for the initial screening of natural attenuation effectiveness are as follows: i • Current or projected groundwater uses may not be within 2 years of travel time from the site. • pH must be in the range of 6 to 8. • Constituents must be sufficiently biodegradable. At the estimated groundwater velocity of 5 to 10 feet per year at this site, the most the plume can be expected to move in 2 years is 20 feet. The only known groundwater wells within 3,000 feet of the site—Del Monte's production wells and two California Water Company wells (Stations 9 and 58)—are closed because of regional-contamination } On-site groundwater has a pH in the range of 7.2 to 8.6. The only pH measurements consistently above 8.0 were from monitoring wells in the deeper B Zone. Monitoring well field logs presented in Appendix A include pH data from monitoring well purging prior to collection of quarterly groundwater samples. It has been recognized for years that BTEX is .readily degraded by indigenous soil microorganisms under aerobic conditions. Recently, there has been increased recognition that BTEX can be biodegraded fairly readily under anoxic or anaerobic conditions. Therefore, the Disco site meets the initial screening criteria for intrinsic remediation. Technical Basis Naturally occurring contaminant attenuation processes include non-destructive processes, such as volatilization, hydrodynamic dispersion, and sorption, as well as destructive processes, such as chemical hydrolysis and biodegradation. Biodegradation is the critical attenuation mechanism in an intrinsic bioremediation demonstration. Contaminant biodegradation changes the chemistry of the groundwater (geochemistry). j Contaminant mass removal occurs as the contaminants are transformed by subsurface microorganisms into biomass, water, and other breakdown products. Other reactants are required in this contaminant biodegradation process. One class of required reactants is the inorganic chemicals that serve as electron acceptors (e.g., oxygen, nitrate, ferric iron, sulfate, and carbon dioxide). Another class of required elements are nutrients, including nitrogen and phosphorous. A feature of this biochemical process is the stoichiometric relationship between the contaminant mass and the electron acceptor. That is, to biodegrade a given quantity of ( organic contaminant, a corresponding quantity of electron acceptor is required. By SF010030983.DOC 4-4