SITE INFORMATION AND CORRESPONDENCE

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Mr. Sergio Morescalchi • -2 - • 13 July 2009 ARCO BP In general, the highest concentrations of dissolved hydrocarbons at the STTC site are observed in monitoring wells located on the BP property. BP has conducted an insitu pilot study and proposed follow-up studies in a continuing effort to mitigate the pollutants emanating from this property. A recent effort which proposed a nutrient injection pilot study was described in BP's 30 January 2009 Response Letter to the Regional Water Control Board BP. After we informed BP in our 8 April 2009 letter that , we could not concur with the proposed scope of the study, BP submitted a 6 May letter stating that efforts to conduct the nutrient injection study would be discontinued and an alternative proposal would be submitted on 19 June. Central Valley Water Board staff reviewed BP's 17 June 2009 Work Plan for Feasibility Study, BP Stockton Terminal No. 40T(Work Plan). The Work Plan proposes to conduct a one- to three-day biosparge feasibility study that will be restricted to the A-zone. BP is proposing to inject a 90110 mixture of ambient air and helium into two injection wells (IS-1 and IS-2) located close to monitoring wells MW-5A and MW-50 to evaluate communication between the injection points and the monitoring wells. The helium is a tracer. The groundwater in MW-5A and MW-50 contains elevated concentrations of dissolved petroleum hydrocarbons. During the study, pressure differentials and vapor phase carbon dioxide, helium, and volatile organic compound (VOC) concentrations will be measured in the field in MW-5A and MW-50. Groundwater samples will also be collected and submitted for laboratory analysis for dissolved oxygen (DO) and other groundwater parameters. If no evidence of increased DO and helium concentrations is detected in the groundwater monitoring wells, BP will conclude that communication between the injection and monitoring wells is inadequate. BP proposes to initiate the field activities upon receiving our concurrence. The study will be performed over a 12-week period. The results will be submitted to the Central Valley Water Board in a summary report. Our comments are presented below. 1. The first quarter MBAS monitoring results show that MBAS is present in A-, B-, and C- zone wells in the AGT area. However, Central Valley Water Board staff found no record of previous monitoring results for this compound making it difficult to currently determine whether the MBAS was introduced to the aquifers prior to or during the November 2008 foam release. Additional monitoring is needed to evaluate the spatial and temporal occurrence of this compound. We concur with the recommendations in the Letter, which are to perform MBAS monitoring in all BP wells through the 4th quarter of 2009. In the Second QMR, due on 1 August 2009, please also provide any accounts of past discharges, if any, of fire suppression foam. 2. The Letter adequately responds to the comments in our 17 March letter by (1) clarifying that a representative from the City of Stockton Office of Emergency Services (OES) was notified of the release and visited the site, and (2) providing replacement copies of the requested field data sheets.