ARCHIVED REPORTS XR0006404 - Laserfiche WebLink

Home Browse Search

SITE LOCATION Former Regal Station #603, is an operating EXXON service station located at 6425 Pacific Avenue at the northwest corner of Pacific Avenue and Douglas Road, Stockton, in San Joaquin County, California. The site lies in section 21; T2N; R6E; MDB&M at approximately 12 feet above mean sea level (MSL), see Figures 1, 2 and 3. The parcel is #31 and is recorded in Assessor's Book 97. GROUNDWATER MONITORING WITH OXYGENANTS ANALYSIS Depths to groundwater measurements were obtained from all monitor wells associated with this site (MW1, MW2, MW3, MW4, MW5, MW6, MW7, MW8, MW9, MW10, MW11, MW12 and MW13) on December 16, 1997. No vacuum influence was measured, the vapor extraction/sparging system had been non-operative for eight days, see Appendix A — Field Notes and Methods and Procedures. Figure 4 represents the groundwater gradient obtained from measurements obtained on December 16, 1997 and shows a mound of higher groundwater at MW-2 flowing outwardly in all directions. Measurements obtained at MW-12 suggest high groundwater flowing towards the site or from the east to the west, with a low water trough between MW-3 and MW-10. The most recent groundwater quarterly sampled all of the wells for analysis of Total Petroleum Hydrocarbons as gasoline (TPHg), Benzene, Toluene, Ethylbenzene, Xylenes and Methyl tertiary Butyl Ether (MBTEX) using EPA method 8020, see Table 1 and Figure 5. In addition all samples were also analyzed for the Oxygenants as directed by the December 9, 1997 Memo from San Joaquin County Environmental Health, see Appendix B and F. With the exception of MW11 which contained 30 ug/L of MTBE all remaining samples were below lower laboratory detection limits for the Oxygenants; Tertiary Butyl Alcohol (TBA), MTBE, Di-isopropyl ether (DIPE), Ethyl tertiary Butyl Ether (ETBE) and Tertiary Amyl Methyl Ether(TAME). During this sampling, field measurements were obtained from all of the wells for Dissolved Oxygen (DO), Nitrate (NOA Sulfate (SO4) and Ferrous Iron (Fez). Dissolved Oxygen measurements were also obtained from select wells (MW2, MW8, MW9, MW12 and MW13) prior to turning the vapor extraction sparging system off on December 8, 1997. Figure 6 represents DO measurements obtained while the vapor extraction — sparging system is operating and shows oxygenation of the entire plume area (compare figures 5 and 6). Figure 7 represents DO measurements approximately two weeks after the vapor extraction — sparging system was turned off. This figure shows a rapid depletion of DO to below field measurement levels except near sparging and vapor recovery wells MW and MW13, down gradient well MWI 1 and upgradient wells MW4 and MW7. Figures 8, 9 and 10 represent the other three field measurements for the electron receptors Nitrate, Sulfate and Ferrous Iron respectively. These three figures demonstrate that active bio-degradation is occurring at this site with reductions of NO3 and SO4 near MW2 and MW12 and elevated levels of Fee occurring in the same areas. The Fee plume shown in Figure 10 is currently this site's best indicator of the bio-degradation process. The Fez plume shows the area that is actively consuming 2 R603 Update##50 12/97