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. Groundwater Monitonng Well,CPT,and Ozone Microsparge System Installation Report Tosco(76) Service Station No 11192,Stockton,California May 9 2003 Groundwater samples collected from CPT-3, the deep downgradient boring, reveal that the two groundwater zones sampled at 74 and 100 feet bgs were free of hydrocarbon impact, except for a detection of 6 8 ppb of MtBE at 100 feet bgs Again, this detection is below the established MCL for MtBE The deepest sample from CPT-3, collected at 125 feet bgs, reportedly contained concentrations of MtBE at 290 ppb and TBA at 88 ppb This deeper zone is impacted with petroleum hydrocarbons, and requires lateral definition In GR's Updated Corrective Action Plan, dated June 26, 2002, four deep groundwater monitoring wells were proposed to be installed at the site, to a depth of 80 feet bgs The results of the recent CPT investigation show that groundwater in the 75 to 80 foot bgs range does not contain MtBE at concentrations above the established MCL Although the current onsite deep well MW-11 has been reported to contain elevated concentrations of MtBE, MtBE was not detect in CPT-3 at 74 feet bgs GR recommends that three of the previously proposed onsite deep monitoring wells be installed one onsite and two off-site (Figure 6) The one onsite deep monitoring well will be constructed with a conductor casing set to approximately 110 feet bgs, with screened intervals extending from 120-130 feet bgs, depending on actual field conditions encountered during drilling (Figure 7) The two off-site deep wells will be constructed similarly to the proposed onsite deep well except there will be no conductor casing used The actual depth of the proposed screen interval will be dependent on the encountered hthology in the field Groundwater monitoring of the three proposed deep wells will allow GR to determine the flow direction of the deeper aquifer, in order to determine if the previously proposed offsite deep monitoring wells are correctly positioned in the downgradient direction Groundwater samples collected from the proposed deep wells will assist with lateral delineation of the deep dissolved hydrocarbon plume GR also recommends that a hydropunch groundwater sample be collected at a depth of approximately 75 feet bgs from the proposed deep well boring in the vicinity of MW-1 and the proposed deep well boring in the vicinity of MW-8 The result of the hydropunch samples analysis will be used to verify the presence or absence of dissolved methanol in the groundwater 7.0 REFFRENCES Gettler-Ryan Inc, 2003, Groundwater Monitoring and Sampling Report, First Quarter - Event of January 3, 2003, Tosco (76) Service Station #11192, 1403 Country Club Boulevard, Stockton, California, dated February 6, 2003 2002, Corrective Action Plan Report, Tosco (76) Service Station No 11192, 1403 Country Club Boulevard, Stockton, California, dated March 13, 2002 140245 8 11