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At the request of the EHD, WESTON conducted a second round of sampling on April 11, 1991 to confirm the results of the first sampling effort. The same sampling procedures were used during both sampling events. 2.3 Sample a Analysis Groundwater samples from the first sampling event were analyzed for BTEX by EPA Method 602 and for TPH-G by Method 8015 (modified) and for Total Lead by EPA Method 239.2. Groundwater samples from the second sampling event were analyzed only for BTEX and TPH-G. All samples were transported to the laboratory under chain-of- custody documentation. All analyses were performed by the WESTON Analytics Division laboratory which is certified by the State of California Department of Health Services for the required analyses. r, 2.4 Quality ControllQuali All field work, including monitoring well installation, soil sampling and groundwater sampling was performed under the supervision of a registered California geolog;st. For the purpose of groundwater sample quality assurance and quality control (QA/QC), WESTON prepared and analyzed a trip blank sample during the first sampling event. The laboratory-prepared sample of deionized water was transported to the site, handled like the other samples and then submitted for analysis. The sample was labeled "Trip Blank" and analyzed for BTEX compounds. During the second sampling event, QA/QC consisted of the collection and analysis of an equipment decontamination blank sample. This sample(labeled EB-1)was collected by pouring distilled water into the decontaminat- ed sample bailer and then pouring the water from the bailer into the sample bottles. Analysis was conducted for BTEX compounds. 2.5 , ite Geology The surface and shallow subsurface lithology in the area consists of mostly non-marine, alluvial, lake, playa and terrace deposits (Geologic Map of California, California Division of Mines and Geology, 1977). Sediments beneath the site consist mostly of clayey sift, silty clay, and silty sand as logged during the installation of the three monitoring well { borings. It should be noted that since MW-1 was drilled in the former location of an underground storage tank, the soil found in the upper zone of the borehole at MWA may not be native to the site due to back-filling of the tank pit. Lithologic logs of the monitoring wall borings are presented in the Appendix A, Field Report. t` - 5 -