SITE INFORMATION AND CORRESPONDENCE

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Beacon Station No. 3641 March 17, 2011 Stockton California No Further Action Required Request 4. The locations of the identified sensitive-receptor water supply wells are plotted on a map on Figure 15. Well details and distances from the site are summarized in Table 4. See Updated Site Conceptual Model, Sensitive Receptor Survey. 5. Scaled site maps with showing site features are shown on Figures 2 and 3. See Site Description. 6. The following wells have been installed at the site: 14 groundwater monitoring wells (MW-1 through MW-14); eight vapor wells (VW-1 through VW-8); and 1 air sparge well (SW-1). Monitoring well MW-8 was destroyed in April 2003. See Appendix C. 7. Approximately 1,100 cubic yards of soil were excavated and disposed off site during removal of the UST systems. See Site History. 8. Subsequent to assignment of NFAR status, all monitoring wells associated with this site shall be destroyed in a manner acceptable to San Joaquin County Environmental Health Department. 9. Depth-to-water and groundwater elevation data are summarized in the Groundwater Monitoring Data table. See Table 2. 10. Soil and groundwater concentration data are summarized in Soil Analytical Data and Groundwater Monitoring Data tables. See Tables I and 2. 11. Historical groundwater constituents of concern (COCs) TPHg, benzene, and MTBE show decreasing concentration trends over the project duration, especially through and after the remediation period. COC concentration trends are expected to continue through the post-remedial period. The most recent monitoring data indicate: TPHg and benzene concentrations are detected in one well, VW-3 at 2000 micrograms per liter(parts per billion or ppb) and 19 ppb, respectively; and a diffuse MTBE plume delineated by wells MW-I through MW-5, MW-10 and VW-3 with concentrations typically less than 5 ppb, and 32 ppb in well MW-10. See Updated Site Conceptual Model, Fate and Transport of Petroleum Hydrocarbons in Soil and Groundwater. 12. The lateral extent of TPHg and benzene concentrations in groundwater are limited to the immediate UST basin area. Concentrations of MTBE in groundwater extend off-site to the south. Fourth quarter 2010 isoconcentration contour maps for TPHg, benzene and MTBE in groundwater are shown on Figures 4, 5, and 6. The vertical extent of hydrocarbons are indicated on cross section on Figures 1 l through 14. See Updated Site Conceptual Model, Source, Nature and Extent of Soil and Groundwater Impacts. 13. Remedial methods utilized at the site were soil vapor extraction and air sparging. Soil vapor and air sparge influence was limited to the vicinity of the UST basin area. Vapor extraction and air sparging systems were operated from March 2005 to August 2009. See Updated Site Conceptual Model, Site History. NEAR Cvrutr Feb1 I Project No. 1641.26 2 HORIZON ENVIRONMENTAL INC.