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Commingled Plume#35 1%ftw� ,i � � � R �.✓ Chevron 9-4275/ARCO#2133 Site Conceptual Model September 1,2006 Page 8 4.0 CURRENT ASSESSMENT ACTIVITIES AND RESULTS Between February 10 and March 2, 2006, SECOR supervised Cascade Drilling of Rancho Cordova, California, in the drilling of three replacement monitoring wells at the Chevron site (MW 1 R, MW-2R, MW-3R), and four soil borings at the ARCO site (AS-8 through AS-11. Blasland, Bouck & Lee, Inc. (BBL) supervised the drilling of the remaining three replacement monitoring wells (MW-5R, MW-7R, and MW-9R) at Chevron 9-4275 on June 5, 2006. Table 6 presents the well construction and soil boring details. Boring logs for the recent assessment are presented in Appendix C. On July 31, 2006 the new monitoring wells were surveyed by Morrow Surveying, a California- licensed land surveyor. The wells were surveyed to establish horizontal position with respect to North American Datum 1986 (NAD 83) coordinates, and to establish elevation of top of casing with respect to mean sea level (msl). The survey data was formatted as Geo XY and Geo_Z files and uploaded to the State Geotracker database. Survey data is presented in Appendix D. 4.1 Chevron 9-4275 Soil Borings and Replacement Monitoring Wells Soil borings for monitoring wells MW-1 R, MW-2R, and MW-3R were advanced to depths of 60, 65, and 59 feet bgs, respectively, and soil and groundwater samples were collected. The borings were then tremie grouted from total depth to approximately 20 feet bgs. Well casings were then installed to 20 feet bgs for wells MW-1 R, MW-2R, and MW-3R. Each replacement monitoring well was screened between five and 20 feet bgs. The monitoring wells were constructed with 2-inch diameter schedule 40 poly vinyl chloride (PVC) casings with 15 feet of 0.020-inch slot screen material. Well completion details are presented in Table 6. All work was conducted in accordance with stipulations of permits obtained from the SJCEHD and the City of Stockton. SJCEHD and City of Stockton permits are included in Appendix E. Field and laboratory procedures are presented in Appendix F. A copy of the laboratory analytical data is included in Appendix B. Copies of the field notes are provided in Appendix G. Recent boring logs and well construction details are included in Appendix C. In June 2006, BBL installed replacement wells MW-5R, MW-7R, and MW-9R at the Chevron 9-4275 facility. The wells were drilled to 20 feet bgs and each well casing was screened between five and 20 feet bgs. Soil samples were not collected during the installation of these wells. Boring logs are provided in Appendix C. Soil samples were collected during the drilling of soil borings MW-1 R, MW-2R, and MW-3R and submitted for laboratory analysis. Minor concentrations of TPHg were detected in MWA R and MW-2R. The maximum TPHg concentrations in MWA R and MW-2R were 21 mg/kg (14.5 feet bgs) and 3.9 mg/kg (3.9 feet bgs), respectively. Low concentrations of TBA and MtBE were detected in soil samples collected from all three monitoring wells. No other compounds were detected and no detections were found in the samples collected at 60 foot bgs. Table 7 presents the soil analytical data for the Chevron 9-4275 facility. Two groundwater samples were collected from each of the soil borings. TPHg, benzene, and xylenes were only detected in the groundwater samples collected from MW-2R at 50 feet bgs at maximum concentrations of 560 pg/L, 46 pg/L, and 4 pg/L, respectively. MtBE, TBA, and 1,2-dichloroethane (1,2-DCA)were detected in groundwater samples collected from the borings. The maximum concentration of MtBE was detected in the sample collected from MW-3R at 51 feet bgs at 78 pg/L. TBA and 1,2-DCA were detected in samples collected from MW-2R at 94275-ARCO 2133 SCM 9-2006 8