ARCHIVED REPORTS_XR0006146 - Laserfiche WebLink

Home Browse Search

1f b. SUMMARY AND CONCLUSIONS In October 1990, EA proposed a groundwater extraction, treatment, and reinfection system for former Exxon RS 7-3942, and installed the required wells for this remedial approach Aquifer tests and injection/percolation tests were performed at the site to assess if a hydraulic barrier could be created which would prevent the migration of upgradient hydrocarbon-impacted groundwater from the Shell service station to the former Exxon site during groundwater extraction onsite (EA 1991a) Due to problems in securing the permit for reinfection of treated ' groundwater, this remedial option was not implemented t From 1992 to 2000, three remedial approaches were implemented to address petroleum hydrocarbons detected in soil and water The initial remedial approach was LPH removal using passive skimmers in wells RW3 and V1 The second remedial effort was an SVE/AS system, which operated from May 1994 to November 1996 and recovered petroleum hydrocarbon vapors from wells MW2, MW3, MW5, and MW6 (screened from 10 to 30 bgs), wells V1 and V2 (screened from 20 to 40 feet bgs), and well V4 (screened from 25 to 40 feet bgs) in conjunction with air sparging wells A total of approximately 14,500 pounds of petroleum hydrocarbons was removed from the vadose zone by using an SVE/AS system (EA 1997) The SVE influent concentration and the cumulative estimated pounds of TPH-g removed over time become asymptotic over time As presented in Figure 7 the groundwater elevations across the site had risen approximately 2 5 feet by October 1995, indicating that the decline 1n influent concentrations and the associated pounds removed is attributable to the success of the VES rather than rising groundwater conditions which can submerge the screened portion of the vapor extraction system The third remedial effort consisted of the installation of a vacuum vaporizer well system, known as UVB, for groundwater remediation at the site in November 1996 The installation of the UVB system consisted of overdrilling existing groundwater recovery well RW3 to install the dual- screen UVB well for groundwater extraction-injection, and the installation of adjacent monitoring wells UVB-MWl and UVB-MW2 to monitor system performance (EA 1997) Between 6 February 1997 and 22 September 2000, a total of 1,8902 pounds of petroleum hydrocarbons as gasoline (TPH-g) was removed from groundwater by the UVB system based on laboratory analytical results, and 8215 pounds based on field measurements (ETIC 2000b) Figure 8 presents the pounds removed based on both lab and field data over time The lower value of 821 5 pounds is viewed as more representative of the actual pounds removed by this remedial system as the field values are consistent and not dependent on a single high value, as are the laboratory data The operational measurements of the UVB system showed that it had reached the asymptotic stage of operation The WB system was therefore shut down; as referenced in ETIC's letter dated 29 October 2000 (ETIC 2000c) and approved by the SJCPHS in a letter received in December 2000 (Appendix A) Though passive skimming, vapor extraction, and groundwater remediation have been implemented at the site, there is still intermittent LPH detected in various wells LPH has been found in wells MW3, MW6, RW3, V1, V2, and V4 at various times and most recently at 0 02 feet in wells UVB-MW1, V4, and V2 in June 2001 However, all of these wells are onsite wells located within the former tank field or immediately downgradient of the former tank field This shows that the residual hydrocarbons in the saturated sods are not migrating 0 1PioJectS�739321MASTERIWP�RaOSOINtcxi2 doc 1 1