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5400-5606 Pacific Avenue - 2 - 23 December 2004 Stockton, San Joaqum County Owo-month reduction in daily mass removal from 19 02 pounds to 3 35 pounds of gasoline The Report states a comparative analysis shows that the average contaminant concentrations and plume size have decreased approximately 50%in 4 years, and the groundwater plume does not appear to be migrating further than the current monitoring well network Comment 2) I concur that the Report provides a satisfactory technical evaluation of the mass and distribution of contammants remaining in groundwater at the site The Report states on page 10 11 UNOCAL and the RWQCB are interested in accelerating the cleanup of this sites, therefore,potential system enhancements and technology changes were also considered In addition to continuing operation of the existing ASISVE system, additional enhancements to the AS system were considered including in situ chemical oxidation CISCO)for ozone, Fenton's reagent, hydrogen peroxide, or nutrients as suggested in ENSR's `Work Plan and Efforts to Cease Plume Movement'dated June 14, 2004 Prior to committing to a major technology shift, ENSR recommends that additional performance data on the AS/SVE system be collected The SVE system was restarted in September 2004 and, therefore, there is insufficient data to evaluate its current 12eZformance I concentrations rebound or do not appear to be approaching acceptable levels guickly enough ENSR will investigate enhancin the IAS s stem with ISC "(emphasis added) In contrast, the Report also states on page 10 and I 1 " The system appears to be well maintained, and operating as intended However, the co arative analysis indicated that system enhancements are needed to address the central mass of the plume "(emphasis added) Comment 3) The two underlined statements from the Report appear to contradict each other by saying a)there is insufficient data to evaluate performance of the AS/SVE system, and b) the comparative analysis shows system enhancements are needed In the 16 December 2004 phone conversation, I asked Mr Heningburg whether the Report signaled that ENSR would delay evaluating ISCO as proposed by ENSR on 14 June 2004 Mr Heningburg assured me that the Bench Scale Testing(BST)for Ozone ISCO is underway at the laboratory and would continue to completion, and that there is no plan to delay the BST Mr Hemngburg requested an extension to the schedule for the completion of the evaluation for Ozone, since a Pilot Study could not be finished by the 15 January 2004 deadline proposed by UNOCAL in a letter dated 15 September 2004 and approved in my letter of 4 October 2004 I agreed to extend the due dates for submitting a report recommending a proposal for enhancing the existing SVE/AS system, to help cease plume movement The new deadlines are as follows By 31 January 2005, provide a Report of the findings of the BST • By 15 February 2005, if the BST Report is favorable to Ozone, begm the Ozone Pilot Study + By 2 May 2005,provide a Report of the findings of the Ozone Pilot Study + By 15 June 2005, if the Pilot Study Report is favorable to Ozone, implement any additional work to complete enhancement to the existing SVE/AS system, to help cease plume movement The Repori states on p igc a x "A quarterly sparge and extraction well monitoring plan should be developed to more regularly collect data that benefits the performance of those wells in remediating the groundwater plume beneath the site EArSR should begin implementing this plan beginning with the f rst quarter of 2005, " Comment 4) By 31 January 2005,provide a detailed explanation and rationale for the proposed changes to the . Monitoring and Reporting Program of CAO NO 5-00-707