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Stantec • September 15, 2008 Page 7 of 9 Reference: Response to RWQCB July 30, 2008 Letter CONTINGENCY PLANS "The following contingency plans are proposed if trend analysis confirms that the plume has been cut-off and the results of the MNA study indicate that natural biodegradation will not occur at a sufficient rate to destroy remaining petroleum hydrocarbons and MTBE down- gradient of the proposed "A" and "B" zone ozone well remediation systems." A Zone Contingency Plan "If concentrations of petroleum hydrocarbons and MTBE have increased by one order of magnitude over historical concentrations for three consecutive quarters in remedial observation wells PS/MW-18 and AR/MW-21A, a second "A" zone ozone sparge array will be installed east of those wells to destroy the remaining etroleum hydrocarbons and MTBE. The ozone wells will be constructed as proposed in the CAP and will be connected to the ozone sparge remediation system for the initial ozone well array." "If concentrations of petroleum hydrocarbons and MTBE have increased to above water quality objectives for three consecutive quarters in trigger boundary wells 0W-1A and OW- 4A, a third "A" zone ozone sparge array will be installed east of those wells to destroy the remaining petroleum hydrocarbons and MTBE. The ozone wells will be constructed as proposed In the CAP and a separate ozone sparge remediation system will be constructed similar to the systems proposed in the CAP." Thus it was never envisioned that exceeding the trigger well criteria alone would require implementation of the contingency plan. It is noteworthy, that the criteria listed above is applicable to all of the established remedial observation wells and trigger wells, even though some of the wells were inadvertently not listed. The overall objective of the contingency plan was to guard against significant off-site plume migrations. It is important that a realistic cost-benefit perspective must be maintained. The resources required to implement the contingency plan should be warranted to achieve the benefit of keeping trace-level concentrations from the downgradient areas. The groundwater downgradient of the terminal is within the industrial Port of Stockton, which has numerous historical and current sources of contamination, is not a source of domestic or municipal water supply, and is already of degraded water quality. As described in more detail in SECOR's 2003 CAP, aside from the existence of off-site source of petroleum hydrocarbon groundwater impacts, background groundwater quality in the site area is generally poor, thereby limiting its beneficial use potential. Groundwater quality exceeds the California secondary MCLs for total dissolved solids, chloride, and sulfate of 500, 250, and 200 milligrams per liter (mg/L), respectively. Saltwater intrusion into the Sacramento-San Joaquin Delta region is commonly cited as the primary factor for the poor background groundwater quality. California and federal agencies are studying the saltwater intrusion issue. Because of the poor background groundwater quality, beneficial use of the groundwater in the site vicinity is already impaired. OZONE SPARGING SYSTEM UPGRADES I:\STTC-Stockton\Reports\Resp to RWQCB July 30,2008\Response to RWQCB July 30, 2008 Letter final.doc