SITE INFORMATION AND CORRESPONDENCE

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Mr Jeff Baker 2 Stockton Terminals Technical`nmittee 10 July 2009 • COC concentrations generally occurred in A zone wells on the BP property. The highest concentrations of TPHg and TPHd were in AR/MW-3A and AR/MW-10A at 64,000 micrograms per liter (pg/L) and 220,000 pg/L, respectively. The highest concentrations of benzene and TBA were detected in B zone monitoring well AR/MW-8B at 11,.000 and 5,000 pg/L, respectively. The highest concentration of MTBE was observed in AR/MW-50 at 22,000 pg/L. However, the 9,000 pg/L of MTBE observed in AR/MW-8A represents a trend of increasing concentrations that started at about 20 pg/L in 2001. In the fall of 2004, the STTC began operation of two ozone sparge (OS) systems to clean up groundwater pollution migrating downgradient from the Site. The system is comprised of 57 A zone and 16 B zone wells connected to two 5 pounds-per-day (lbs/day) ozone generators. The OS system is divided into two portions. The west system is downgradient of Tesoro and the east system is downgradient of BP. System performance is evaluated using an array of six remedial observation wells, two midpoint wells, and four trigger wells. Between January and March 2009, the STTC estimates that the east system and west system had run times of 61 and 59 percent, respectively. The STTC attributes the excessive east system down-time to maintenance-related power interruptions. The west system down-time was attributed to excessive rainfall. The STTC estimates that collectively, 315 pounds of ozone were injected and 105 pounds of hydrocarbons were destroyed during this period. Ozone production equipment was upgraded during the first quarter of 2009 with the objective of increasing system run times. The STTC monitors the ozone injection system using 12 A- and B-zone wells. A contingency plan is triggered if the concentrations of BTEX, MTBE or TBA in any remedial observation wells exceed compliance limits. Table 12 shows that the TBA concentrations in remedial observation well AR/MW-18B over the last two quarters have been 13 and 31 pg/L. These two consecutive concentrations exceed the compliance limit of 6.6 pg/L. Pursuant to the terms described on Page 3 of the Revised Contingency Plan (Appendix K of the First QMR), three consecutive exceedences of an order of magnitude greater than the compliance limit trigger installation of a second ozone sparge array east of the remedial observation wells. Our comment is presented below. 1. The trend of increasing MTBE concentrations observed in AR/MW-8A shows that this compound is migrating southeast from AR/MW-50. However, there are no wells between the AR/MW-8A and the West OS system, located about 400 feet downgradient, to delineate and track the downgradient migration of MTBE and other petroleum hydrocarbons on to the property to the southeast. In the Second QMR due on 1 August 2009, please discuss any potential sources for this increasing concentration trend and provide any insight on whether mitigative measures are necessary.