SITE INFORMATION AND CORRESPONDENCE

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DRAFT Petitioner then concluded that the site was suitable for closure as a low-risk groundwater case and that further site assessment or corrective action was not necessary. By letter to petitioner dated March 3, 1997,County staff concluded that the plume of dissolved phase petroleum hydrocarbons was about 130 feet long and 60 feet wide, that the average benzene concentration within the limits of the plume was 32 ppb, and that the plume was stable and declining. However, the County also concluded that site closure at that time was not appropriate without additional information that demonstrated that natural attenuation was occurring and an explanation that natural attenuation would restore the beneficial use of groundwater. By letter to the UST Cleanup Fund Manager dated June 13, 1997, petitioner requested that its case be reviewed for closure. Petitioner contended that petroleum hydrocarbon sources (USTs and piping)had been removed from the site, impacted soil had been excavated, SVE had removed additional residual petroleum hydrocarbons in soil, free product has never been observed, MTBE has never been detected in groundwater, the nearest public water supply well is approximately 800 feet from the site, and the County had determined that assessment of the extent of affected soil and groundwater is complete. 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. Theseanalyses, 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), iron (as high as 3.4 ppm), and Total Dissolved Solids (1,000 to 1,200 ppm) exceeded State MCLs (45 ppm, 0.3 ppm, and 500 ppm respectively).