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Stantec <br /> Response to RWQCB Letter Dated March 11, 2010 <br /> Stockton Terminals Technical Committee <br /> April 30, 2010 <br /> MBAS has been added to the analysis for samples collected from downgradient wells OW-9C, <br /> PS/MW-20C, OW-3CR, and OW-4C. Because the first quarter 2010 sampling had already <br /> taken place, these MBAS analysis will begin with the second quarter 2010 and results will be <br /> included in the second quarter 2010 report. <br /> Stantec is currently monitoring the frequency and distribution of reported MBAS at the site. <br /> Based on data collected to date, it is unclear whether the loading rack spill could be the only <br /> contributing factor to the MBAS introduced to the subsurface in the area. Further evaluation of <br /> reported MBAS concentrations at the site, assessment of potential bias in the analytical method <br /> used for reporting MBAS, and the migration potential of MBAS at this site is ongoing. <br /> 2. Our 8 October 2009 letter to BP concurred with BP's proposal to continue <br /> quarterly over-purge events in all on-site monitoring wells until sampling results show <br /> MBAS is less than or equal to 500 pg1L for four consecutive quarters. In addition, we <br /> requested that BP to conduct over-purging in off-site wells. The discovery of MBAS in <br /> AR/MW-18C may also indicate that the interim over-purge remedy may be ineffective at <br /> mitigating MBAS because this contaminant may not be limited to the immediate <br /> vicinity of the monitoring wells in which it has been detected. BP may continue to <br /> conduct the quarterly interim remedy until the additional monitoring requested in the <br /> previous comment results in the lateral delineation of the MBAS plume. Then BP must <br /> begin to formulate a strategy for vertical delineation and identifying a permanent <br /> remedy. <br /> MBAS data and the overpurging technique for remediating MBAS impacts caused by the fire <br /> suppressant release at the site will continue to be evaluated. In addition, alternate remedial <br /> techniques are being explored. See the response to comment 3 below. <br /> 3. By 1 May 2010, BP must submit the evaluation of alternate remedial technologies <br /> referenced in the 29 October 2009 Helium Tracer Study for Biosparging Feasibility <br /> Report. In light of the current groundwater monitoring data which show that MBAS is <br /> widely distributed throughout the subsurface, the evaluation must also include a <br /> method for mitigating MBAS. <br /> On behalf of ARC, Stantec has been evaluating remedial technologies for the site as proposed <br /> in the report entitled Helium Tracer Study for Biosparging Feasibility, prepared by Stantec and <br /> dated October 29, 2009. However, because the site is an active petroleum terminal, typical <br /> remedial technologies cannot be considered due to inherent risks to human safety. In addition, <br /> soil and groundwater conditions at the site limit the number of applicable technologies that could <br /> be used. In the aforementioned report, Stantec proposed the possibility of using a passive <br /> diffuser technology as a safe and effective way of reducing petroleum concentrations at the site. <br /> Tesoro is currently moving forward with the installation of such technology in the adjacent <br /> property. ARC will follow that pilot study closely to assess the applicability of passive diffuser <br /> technology at the BP Terminal site. Concurrently, Stantec will continue to monitor the MBAS <br /> plume and the need to implement further remedial efforts in this heavy industrial area. <br /> Response to RWQCB letter dated 11 march 2010.FINAL.doc 2 <br />