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Mr. Jeff Baker • -4- • 11 March 2010 Stockton Terminals Technical Committee Tesoro submitted the Pilot Study Work Plan Addendum (Addendum) on 19 February 2010. We will send comments on the Addendum under separate cover. Table 8 provides the results of the first two quarters of a four-quarter background sampling study for Tesoro's proposed well TS-4R hydrogen peroxide pilot test. Background groundwater monitoring is being conducted in four wells including TONT-3, TS-4R, TS/M-2, and TS-5 for total chromium, hexavalent chromium, vanadium, bromide, and bromate. Potential injection wells TS/RW-1 and TS-4R are being monitored for the presence of free phase product. No free phase product was observed during the fourth quarter. The background monitoring results show that low concentrations of all target compounds have been periodically detected in the background monitoring wells. Our comments are presented below. 1. The presence of MBAS in AR/MW-18C at 930 pg/L about 800 feet downgradient, and at a depth of 60 to 70 feet bgs, indicates that the lateral and vertical extent of this contaminant is undefined. The prevailing downward vertical gradients documented in the Annual Report and the high hydraulic conductivity value documented in Table 1 of the 9 July 1996 Bioassessment and Bioenhancement Investigation, Stockton Terminals Technical Committee Distribution Terminals show that the MBAS detected in AR/MW-18C could have migrated from November 2008 loading rack spill. In addition, this detection indicates that the OS may be incapable of preventing the offsite migration of this contaminant. To evaluated the lateral and vertical extent of MBAS, BP must now conduct quarterly MBAS monitoring in downgradient wells OW-9C, PS/MW-20C, OW-3CR, and OW-4C and report the results in future quarterly reports. If the first quarter 2010 monitoring has not yet taken place, these results should be reported in the first quarter report, which is due on 1 May 2010. 2. Our 8 October 2009 letter to BP concurred with BP's proposal to continue quarterly over-purge events in all on-site monitoring wells until sampling results show MBAS is less than or equal to 500 pg/L for four consecutive quarters. In addition, we requested that BP to conduct over-purging in off-site wells. The discovery of WAS in AR/MW-18C may also indicate that the interinn over-purge remedy may be ineffective at mitigating MBAS because this contaminant may not be limited to the immediate vicinity of the monitoring wells in which it has been detected. BP may continue to conduct the quarterly interim remedy until the additional monitoring requested in the previous comment results in the lateral delineation of the WAS plume. Then BP must begin to formulate a strategy for vertical delineation and identifying a permanent remedy. 3. By 1 May 2010, BP must submit the evaluation of alternate remedial technologies referenced in the 29 October 2009 Helium Tracer Study for Biosparging Feasibility Report. In light of the current groundwater monitoring data which show that WAS is widely distributed throughout the subsurface, the evaluation must also include a method for mitigating MBAS.