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SITE CLOSURE REQUEST REPORT Former Exxon Service Station No.7-3330 • 3128 West Benjamin Holt Drive Stockton,California Delta Project No. D093-810 Page 14 in these processes include aerobic (oxygen enriched) and anaerobic (oxygen deficient) biodegradation, dispersion, volatilization, and adsorption. Of these processes, biodegradation is related to the natural reduction of the petroleum hydrocarbon constituent mass. There are differences between aerobic and anaerobic biodegradation. Aerobic biodegradation consumes oxygen which is used as the catalyst for the chemical breakdown of petroleum hydrocarbon constituents to carbon dioxide and water. Aerobic biodegradation may occur when dissolved oxygen(DO) is present at concentrations generally exceeding 1 part per million (ppm). In petroleum hydrocarbon affected i environments, the rate at which oxygen is replenished to the subsurface matrix by diffusion is dependent on soil porosity, permeability, and carbon percentage and may be inhibited by high concentrations of petroleum hydrocarbon constituents. Anaerobic biodegradation occurs when the subsurface matrix is oxygen deficient (DO levels generally • less than 1 ppm). In the event of oxygen deficiency, the microorganisms will utilize iron, sulfates, or nitrates as the mechanism for petroleum hydrocarbon breakdown. Anaerobic biodegradation typically is 10 to several hundred times less effective than aerobic biodegradation (EPA 510-B-94-003, 1994). To evaluate the aerobic/anaerobic conditions at the subject site, Delta collected DO measurements from monitoring wells MW-3 through MW-6 on May 5, 1995, utilizing a down-hole direct-reading DO meter. Dissolved oxygen measurements collected in monitoring wells MW-3, MW-4, MW-5 and MW-5 reported concentrations of 7.6, 1.9, 1.3, and 5.9 ppm, respectively. Each of the DO readings collected from the four wells exceeded 1 ppm, indicating a sufficient supply of oxygen for aerobic biodegradation of any remaining petroleum hydrocarbon constituents. 7.0 RATIONALE FOR CLOSURE Based on information collected to date, soil containing residual petroleum hydrocarbons is present in limited areas at the site. Concentrations of TPH as gasoline and BTEX have been below the laboratory's detection limits in all ground water monitoring wells for four consecutive quarters. During this time, . ground water has fluctuated approximately four feet and has been at a site historical high level. The total RPT001.810