SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

I By letter to UNOCAL 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 i 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, UNOCAL requested that its case be reviewed for closure. UNOCAL contended that petroleum hydrocarbon sources (USTs and piping) had been removed from the site, impacted soil had been excavated, S VE had removed additional residual petroleum hydrocarbons in soil, i 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, in a reversal of its January 1998 conclusions that the dissolved phase plume was defined and stable and that assessment of affected soil was complete, contended that UNOCAL had failed to define the extent of affected soil and groundwater. In March 1998, responding to the County's request for information demonstrating that natural attenuation was occurring at the site, UNOCAL analyzed groundwater samples from site wells for selected inorganic constituents which are indicative of p g 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), 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). In April 1998, UNOCAL met with County staff and RWQCB staff. The meeting was held to determine whether consensus could be reached between UNOCAL and the County as to whether enough data had been collected to provide the basis for site closure. UNOCAL was of the opinion that enough data were available to support a closure decision; the County contended that the extent of soil and groundwater impacts were not ` fully delineated and that additional investigation was needed. In January 1999, a pair of monitor wells were constructed at the site. The well pair was located about 20 feet down-gradient of the source of the release and constructed so as to monitor groundwater at depths of 50 to 60 feet and 85 to 90 feet. Analysis of soil samples collected while drilling showed that low concentrations of gasoline constituents were present at depths of 40, 50, and 60 feet, trace concentrations of toluene and xylene were present at 70 feet, and "non-detect" concentrations of all gasoline constituents at 80 and 90 feet. Based on the data developed as a consequence of the well pair installation and previously collected data, UNOCAL concluded that: 1) the site has been adequately characterized, 2) the site is self-remediating through the mechanisms of natural attenuation and intrinsic bioremediation, 3) the groundwater plume is stable, both vertically and horizontally, and f 3