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• Monitoring Well installation and VET Report,and IRAP-Former Unocal Station#0187,Stockton,CA
<br /> December 22 2000
<br /> groundwater samples from this boring Results of this subsurface investigation are presented in
<br /> ARCADIS Geraghty& Miller's Supplemental Site Characterization Report dated June 3, 1999
<br /> In May 2000, GR installed four groundwater monitoring wells downgradient of the site Well U-10 was
<br /> installed five feet north of existing nested piezometer well NP-2 to assess the reliability of chemical
<br /> analytical results from well NP-2-116, and wells U-11 through U-13 were installed in a group
<br /> downgradient of U-10 Each well was screened to correspond to separate water bearing zones from 20-35
<br /> feet bgs (U-11), 65-75 feet bgs (U-13), and 110 to 120 feet bgs (U-12) Soil samples were not collected
<br /> from wells U-10, U-11, and U-13 Soil samples from well U-12 had detectable concentrations of benzene
<br /> between 80 5 and 11 I feet bgs of up to 0 0502 ppm Discrete groundwater samples collected from the
<br /> screened intervals in wells U-12 and U-13 contained detectable concentrations of TPHg of 390 and 800
<br /> ppb, respectively Benzene was detected in groundwater samples from wells U-10 (0 72 ppb), U-12
<br /> (58 ppb), and U-13 (430 ppb) MtBE was detected in groundwater samples from wells U-10 and U-12 at
<br /> concentrations of 2 3 and 3 1 ppb, respectively, by EPA Method 8260A
<br /> Quarterly monitoring and sampling of thQ groundwater monitoring wells began in August 1994
<br /> Dissolved concentrations of TPHg have been detected in all wells except U-5, U-6, U-10, and U-11, and
<br /> benzene has been detected in all wells except-U-5,U-10, and U-11 MtBE has been detected is wells U-1,
<br /> U-3, U-4, and U-11 During the most recent sampling event on August 31 and September 1, 2000, TPHg
<br /> was detected at concentrations ranging from_L64 to 29,600 ppb, and benzene concentrations ranged from
<br /> 6 19 to 4,730 ppb The highest concentrations of TPHg and benzene were detected in wells NP-1-65 and
<br /> NP-2-60, respectively Quarterly sampling and monitoring of the wells is performed by GR Historical
<br /> groundwater chemical analytical data are summarized in the most recent GR report (Third Quarter 2000
<br /> Groundwater Monitoring and Sampling Report, dated October 27, 2000)
<br /> FIELD ACTIVITIES
<br /> To further evaluate the extent of petroleum hydrocarbons in soil and groundwater on-site and off-site in
<br /> the downgradient direction, two groundwater monitoring wells U-14 and U-15 were installed near the
<br /> western boundary of the subject site and ten groundwater monitoring wells U-16 through U-25 were
<br /> installed from northeast to southeast of the subject site (Figure 3) In preparation of remedial feasibility
<br /> testing, two vapor extraction wells W-2 and UV-3 were installed in the vicinity of the former gasoline
<br /> USTs, and eight sparge wells (SP-IA, SP-1B, SP-2A, SP-2B, SP-3A, SP-3B, SP-4A, and SP-413) were
<br /> installed near the eastern boundary of the subject site Field work was performed in accordance with
<br /> GR's Site Safety Plan #240042 08, dated September 28, 2000 GR Field Methods and Procedures are
<br /> included in Appendix A Underground Service Alert was notified prior to beginning site activities The
<br /> wells were installed by Cascade Drilling, Inc (C57 9717510) and V&W Drilling, Inc (C57 #720904)
<br /> under SJCPHS-EHD permits #24384, #24385, #24462, and #24463, and City of Stockton encroachment
<br /> permit#64848 Copies of the permits are included in Appendix B
<br /> The twelve groundwater monitoring wells, two vapor extraction wells, and seven sparge wells were
<br /> installed on October 30 through November 8,2000 Wells U-14, U-16,U-20,U-22,U-24, SP-113, SP-2B,
<br /> SP-313, and SP-4B were drilled to a depth of 75 feet bgs, wells U-15, U-17, U-19, U-21, U-23, U-25,
<br /> SP-IA, SP-2A, SP-3A, and SP-4A were drilled to a depth of 120 feet bgs, and well U-18 was drilled to a
<br /> depth of 80 feet bgs using a truck-mounted drill rig equipped with 8-inch-diameter hollow-stem augers
<br /> Wells UV-2 and UV-3 were drilled to a depth of 25 feet bgs using a truck-mounted drill rig equipped with
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