SITE INFORMATION AND CORRESPONDENCE FILE 2

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1 1 Tosco-76/former BP#11162 1403 Country Club Blvd, Stockton. Page 2 of 3 To help verify that an MtBE degradation process is occurring as GRI has concluded, EHD will require groundwater samples from MW-3 and MW-7 to be analyzed for'full range' EPA Method 8260 with all peaks reported. If degradation is occurring as suspected by your consultant, other degradation constituents should be detected in the complete run of this analytical method. Groundwater samples from additional monitoring wells may be included for this analysis if other degradation analytes are detected in MW-3 and MW-7 so that TOSCO may delineate the extent of these chemicals. In addition, groundwater samples from selected wells should be analyzed for biological parameters to document the presence of selective degraders and suitable environmental conditions for their propagation. Dissolved oxygen concentrations should also be measured and reported. A proposal to install additional on and offsite, first and second hydrological unit monitoring wells, and ten ozone sparge points was included in the Updated CAP. The monitoring well installation portion of this proposal is necessary for the continued investigation required for this site. Installation of the ozone sparge points is not required by EHD at this time. EHD cannot approve a CAP for a site that has not completed delineation of the lateral and vertical extent of the contamination. The installation of ozone sparge points as a corrective action before the site is well characterized appears to EHD to be premature. However, to avoid delaying your attempt to reduce the contaminant mass by ozone sparging, EHD will approve the installation and operation of an ozone sparge system as an interim remedial action. Once the site has been vertically and laterally defined, the'interim' ozone sparge system may be continued as the chosen corrective action alternative, or be modified to adequately address the conditions identified by the vertical delineation investigation, or may be discontinued pending submittal of another acceptable remedial alternative choice. The highest concentration of MtBE in the deeper hydrological unit was obtained from the groundwater sample collected from the second boring for CPT-1 @ 75' bgs where MtBE was detected at 529 µg/I. MW-11 was placed in this area and screened in a silty sand layer from 69'to 79' bgs. The last quarterly monitoring report(QMR)received at EHD on behalf of TOSCO was for the sampling event held on February 8, 2002 and groundwater samples from MW-11 for this period contained MtBE at 310 µg/I. MW-11, the only deep monitoring well at this site, is located approximately 45 feet from MW-3/MW-7, which presumably are nearer the point of release. With concentrations of MtBE in groundwater at MW-11 that exceed the maximum contaminant level (MCL), EHD cannot consider the groundwater petroleum contamination plume to be vertically defined. Further investigation to determine the vertical extent of MtBE is required before the site may proceed into the final remediation phase (corrective action). Advice from the Central Valley Regional Water Quality Control Board (CVRWQCB)has indicated that only repeated sampling events that indicate MtBE concentrations are below the MCL will be accepted as support for vertical delineation. In addition to the vertical delineation of MtBE, EHD is concerned with the source and lateral delineation of TBA, MtBE, and TPI-Ig encountered in the MW-3/MW-7 area. MW-6, located 150 feet southwest of MW-3/MW-7, is the closest, up gradient, shallow monitoring well. MW-6 is located up gradient of the active USTs and groundwater samples collected on February 8, 2002 indicated petroleum constituents were not detected. MW-2 was removed from this area in February 1991 just prior to the upgrade of the USTs and never replaced. EHD recommends that an additional shallow monitoring well be installed just down gradient (northeast)of the existing USTs and southwest of the building. A monitoring well in this area would provide TOSCO with repeatable groundwater data up gradient of MW3/MW-7 and narrow the large gap between these wells and MW-6. Data from this new monitoring well may aid remediation system design, especially if this area is highly contaminated and provide for monitoring the ozone sparging area of influence. If data from this additional well indicates the groundwater is not impacted by petroleum,then the ozone sparge points proposed in the work plan for this area could be moved to areas where the remediation effort would be better served.