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Subsurface Investigation Work Plan SIC Records Retention Center Page 4 4.1 LATERAL AND VERTICAL EXTENT OF HYDROCARBON IMPACTS TO GROUNDWATER Given the extensive lithological data collected from the Stallworth site,the close proximity of the Stallworth and Records Retention Center sites, and the strong correlation between shallow soil and groundwater information collected at both sites,Condor concludes that sufficient data has been collected in the Site area to generate a Conceptual Site Model for the Site that assesses the nature,extent,and mobility of the release.No additional monitoring wells are warranted to evaluate the lateral and vertical extents of hydrocarbon impacts to groundwater,as discussed below. 4.1.1 Shallow Groundwater Investigation The locations of the closed-in-place UST and existing monitoring well MW-1, both which are positioned adjacent to the building,make it difficult to locate additional monitoring wells for further investigation.The implied gradient based upon the adjacent Stallworth site is toward the east-northeast, although groundwater gradient maps prepared by Condor that include groundwater elevations from both sites (MW-1 at 630 N. California and shallow wells MW-2, MW-3, MW-4, MW-5, and MW-6 at 602 N. California) suggest that groundwater at the subject site could flow toward the southeast. Due to the footprint of the existing Site building,the nearest location for a potential downgradient monitoring well in the northeast direction would be approximately 80 to 90 feet northeast of the former UST, on an adjacent property.The lateral extent of shallow groundwater contamination in the east-southeast direction has already been defined through subsurface investigation by the non-detect results of groundwater samples collected from Stallworth site borings P-521-1, P-521-2, and P-521-3, located between 8 and 100 feet east- southeast of the closed-in-place UST(please see Figures C-1 and C-6 in Appendix Q. Based upon the non-detect results in shallow groundwater samples collected approximately 80-100 feet east- southeast of the former UST, and the reduced concentrations of petroleum hydrocarbons in Site monitoring MW-1 during the last two sampling events (benzene concentrations ranging from <2.5 pg/L to 0.31 gg/L), additional investigation of the lateral extent of groundwater impact at the Site does not appear warranted. Current concentrations of TPH-G, BTEX (combined), and naphthalene in shallow groundwater at the Site (8,600 pg/L, 806 µg/L, and 88 ttg/L, respectively, in well MW-1)remain at least an order of magnitude less than at the Stallworth site (75,000 µg/L, 21,800 gg/L, and 790 gg/L, respectively, in well MW-IA). The length of the dissolved petroleum hydrocarbon plume in shallow groundwater at the Stallworth site is approximately 75-80 feet. Based upon the close proximity of the Stallworth site, the lithologic correlation, and Site dissolved gasoline constituents at least an order of magnitude less,we can infer that the areal extent of the dissolved hydrocarbon plume at the subject Site is smaller than the plume at the Stallworth site.Figure C-11 (Appendix C)presents a figure from the Stallworth site showing the extent of the plume;the location of Site well MW-1 has been added for reference. 4.1.2 Vertical Groundwater Investigation The consultants for the Stallworth site have conducted extensive subsurface investigations at the facility, including collecting groundwater samples from multiple HUs.The consultants have concluded that the bulk of the dissolved phased contaminants are located within the shallow water bearing zone (HU-1, approximately 20-40 feet bgs,and HU-2,approximately 60-70 feet bgs). The closed-in-place UST and Site monitoring well MW-1 are located within the Stallworth site's estimated dissolved hydrocarbon plume in HU-2,which emanates from the Stallworth site and extends northeast under the Records Retention Center building to the parking area and residential-use property east of the building (please see Figure C-12 in Appendix C). Several Stallworth site borings/monitoring wells have been installed in the presumed downgradient directions(northeast and southeast)from the closed-in-place UST at the subject Site.Boring B-8(62-65 feet bgs)is located approximately 120 feet northeast of Site well MW-1,boring B-7(65-68 feet �� CONDOR