ARCHIVED REPORTS_SECOND SEMIANNUAL 2002 GROUNDWATER MONITORING

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GEOMATRIX bioremediation of the dissolved petroleum hydrocarbons in the saturated zone beneath the site. The indicator parameter results obtained this quarter generally are consistent with those previously measured at the site (Table 2). 4.0 2002 ANNUAL PROGRESS SUMMARY The extent of BTEX in groundwater beneath the site is limited to the vicinity of wells MW-6, MW-7, and MW-12, with low BTEX concentrations in MW-8. Historical BTEX concentrations of groundwater samples collected from these wells are shown on Figure 5. These graphs demonstrate that concentrations of dissolved petroleum hydrocarbon compounds ' in groundwater beneath the site have been decreasing with time. BTEX concentrations in the groundwater samples collected from these wells in 2002 are one to five orders of magnitude less than the initial concentrations measured in 1991 (for MW-6, MW-7, and MW-8) and 1994 ' (for MW-12). Based on the site data, the RWQCB (RWQCB, 2002) requested that ChevronTexaco continue to assess the progress of NA at the site by conducting semiannual groundwater monitoring in accordance with the revised MRP (RWQCB, 1998) and measuring NA parameters. 5.0 SUMMARY Twelve years of groundwater monitoring data indicate a stable and shrinking plume at the site (Figure 5) due to past active remedial actions and ongoing NA. An HHRA(Geomatrix, 1998) approved by the DTSC (DTSC, 1999) indicates that the presence of petroleum hydrocarbons at the site do not pose an unacceptable risk to human health. An evaluation(Geomatrix, 2000a) indicates that site conditions conform to the RWQCB low risk criteria. Monitoring data demonstrate evidence of ongoing intrinsic bioremediation of petroleum hydrocarbons in the subsurface and that the beneficial use of site groundwater will eventually be restored in accordance with State Resolutions via NA. 1 1 1APR03ECM000s\2463\WCUMENTQ463 12-02 tptdw 5