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Groundwater Monitoring Well Installation and Ozone Microsparge System Installation and Startup Report— ConocoPhillips(76)Service Station No. 1 1195,Lathrop,California May 20,2003 In February 2002, GR advanced five off-site geoprobe soil borings (B-1 through B-5) downgradient of the subject site. Soil and groundwater samples were collected at each of the geoprobe locations and analyzed for TPHg (by EPA Method 8015 modified), and benzene, toluene, ethylbenzene, and xylenes (BTEX) and MtBE by EPA Method 8021. In addition, all soil and groundwater samples were analyzed for Ethanol, TBA, MtBE, di-isopropyl ether (DIPE), ethyl tert-butyl ether (ETBE), 1,2-dichloroethane (1,2 DCA), TAME, and ethylene dibromide (EDB) by EPA Method 8260B. None of the hydrocarbon constituents were detected in soil. Four groundwater samples (B-1, B-2, B-4, and B-5) contained MtBE at concentrations ranging from 3.0 to 10,000 ppb. In May 2002, GR Installed five off-site groundwater monitoring wells (MW-13 through MW-17) at the locations shown on Figure 2. The well borings were drilled to depths ranging from 20 to 21 feet bgs. Soil samples were collected at approximate 5-foot intervals beginning at 5 feet bgs. One soil sample collected from boring MW-16, at 9.5 feet bgs contained MtBE at a concentration of 0.040 ppm. MtBE was detected in groundwater samples collected after the monitoring wells were developed in concentrations ranging from 28 to 3,900 ppb (GR, July 24, 2002). 5.0 FIELD WORK In order to further evaluate groundwater conditions downgradient of the site, GR installed four ' off-site groundwater monitoring wells. Field work was conducted in accordance with GR's Field Methods and Procedures (Appendix A), and the Site Safety Plan dated May 6, 2002. The monitoring wells were installed under drilling permit numbers SR0032087 and SR0032096, issued on December 5, 2002 by PHS-EHD. An encroachment permit was obtained from the City of Lathrop Public Works Department (permit No. 474) on December 19, 2002. Copies of the permits are included in Appendix B. Underground Service Alert (USA) was notified at least 48 hours prior to drilling at the site (confirmation No. 496962 and 497816). As a precautionary measure, a private subsurface utility locator was contracted to identify utilities near the proposed boring locations. The borings were hand excavated for the first five feet bgs to further insure that no utilities were disturbed. 5.1 Drilling Activities On December 9 through December 13, 2002, GR geologists observed Cascade Drilling (C-57 #717510) install four off-site groundwater monitoring wells and fifteen ozone sparge points (10 on- site and 5 off-site) at the locations shown on Figure 2. The borings were drilled to depths ranging from 30 feet bgs to approximately 37 feet bgs,using 8-inch diameter hollow-stem augers driven by a truck-mounted drill rig. Soil samples for lithologic description and chemical analysis, were collected from the monitoring well borings at approximately 5-foot intervals. Soil samples were collected from the sparge well borings for lithologic description only. Borings MW-18 through MW-21 were completed as monitoring wells with the installation of 2-inch diameter blank schedule ' 40 PVC well casing, and 0.02 inch machine slotted well screen, placed through the hollow-stem 140193.14-2 5