SITE INFORMATION AND CORRESPONDENCE

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By letter to the UST Cleanup Fund dated January 16, 1998, County staff expressed concern regarding Petitioner's failure to define the extent of affected soil and groundwater and to document the volume of affected soil excavated at the site. In March 1998, responding to the County's request for information demonstrating that natural attenuation was occurring at the site, Petitioner analyzed groundwater samples from site wells for selected inorganic constituents which are indicative of microbial metabolism of petroleum hydrocarbons. These analyses, in addition to showing that very active biodegradation was occurring within the dissolved phase hydrocarbon plume, also showed that in shallow groundwater, background concentrations of nitrate (about 70 ppm) and Total Dissolved Solids (1,000 to 1,200 ppm) exceeded State MCLS (45 ppm and 500 ppm respectively). In January 1999, responding to the County's demand that further delineation of affected soil and groundwater was required, Petitioner collected soil samples to a depth of 90 feet and installed two more monitor wells in the vicinity of the release. Analyses of the soil samples showed that soil affected with TPH-g and BTEX was present in the depth interval of 40 to 60 feet, that only trace concentrations (less than 0.02 ppm) of toluene and xylene were present at 70 feet, and that all gasoline constituents were not detectable in the samples from the 80 and 90 foot depths. Concentrations of MTBE were not detectable in any of the samples collected. The two new wells were'constructed to monitor groundwater at the depth intervals of 50 to 60 feet and 85 to 90 feet. These wells, in conjunction with other monitor wells, provided a means of assessing the vertical hydraulic gradient and the vertical extent of the dissolved phase hydrocarbon plume at the site. The initial sampling of groundwater from the two new wells indicated that groundwater at 50 to 60 feet and 85 to 90 below grade had benzene concentrations of 14,000 ppb and 210 ppb', respectively. The hydraulic gradient between the two zones monitored was about 0.02 upward. On May 4, 1999 the County issued a directive requiring Petitioner to submit a workplan to define the lateral and vertical extent of offsite, down-gradient soil and groundwater contamination, and to install wells onsite for feasibility studies of viable remedial alternatives. On.Tune 1999, Petitioner filed a petition pursuant to Health and Safety Code §25297.1 seeking review of the May 4, 1999 directive and the County's failure to close the site. Petitioner also ' Concentrations of dissolved phase petroleum hydrocarbons detected in groundwater from this deeper zone are an artifact of the well installation process, i.e.,contamination introduced at the time the well was being drilled and constructed. Subsequent sampling, in March and May 1999,have demonstrated a steady decline in the concentrations of all gasoline constituents. i.e., 150 ppb benzene in March,43 ppb in May. 7