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Interim Remedial Action Test Report,Former Unocal#0187,Stockton,CA
<br /> August 15,2001
<br /> downgradient of U-10 Each well was screened to correspond to separate water bearing zones from 20-35
<br /> feet bgs (U-ll), 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
<br /> benzene between 80 5 and 111 feet bgs of up to 0 0502 ppm Discrete groundwater samples collected
<br /> from the screened intervals in wells U-12 and U-13 contained detectable concentrations of TPHg of 390
<br /> and 800 ppb, respectively Benzene was detected from discrete screened interval groundwater samples
<br /> from wells U-10 (0 72 ppb), U-12(58 ppb), and U-13 (430 ppb) MtBE was detected indiscrete screen
<br /> interval groundwater samples from wells U-10 and U-12 at concentrations of 23 and 31 ppb,
<br /> respectively, by EPA Method 8260A Results of this subsurface investigation are presented in GR report
<br /> No 240042 05-1,Monitoring Well Installation Report,dated August 2, 2000
<br /> In October and November 2000, GR installed two additional on-site groundwater monitoring wells U-14
<br /> and U-15 near the western boundary of the subject site, and ten additional off-site groundwater
<br /> monitoring wells U-16 through U-25 from northeast to southeast of the subject site (Figure 4) In
<br /> preparation for remedial activities, two additional on-site vapor extraction wells UV-2 and UV-3 were
<br /> installed in the vicinity of the former gasoline USTs near the southeast corner of the site, and seven
<br /> sparge wells (SP-1, SP-2A, SP-2B, SP-3A, SP-313, SP-4A, and SP-4B) were installed near the eastern
<br /> boundary of the subject site
<br /> Wells U-14, U-16, U-20, U-22, and U-24 were drilled to a depth of 75 feet bgs, wells U-15, U-17, U-19,
<br /> U-21, U-23, and U-25 were drilled to a depth of 120 feet bgs, and well U-18 was drilled to a depth of 80
<br /> feet bgs Wells UV-2 and UV-3 were drilled to a depth of 25 feet bgs
<br /> MtBE was not detected in soil samples from the well borings TPHg were not detected in wells U-15, U-
<br /> 16, U-17, U-19, or U-25, and benzene was not detected in wells U-15, U-17, U-25, or UV-2 TPHg were
<br /> detected in wells U-21 (1 49 ppm at 515 feet bgs), U-23 (1 29 ppm at 81 feet bgs), UV-2(3 44 ppm at 26
<br /> feet bgs), and UV-3 (33 7 and 14 0 ppm at 21 and 26 feet bgs, respectively) Benzene was detected in
<br /> wells U-16 (0 0158 ppm at 56 5 feet bgs), U-19 (0 190 ppm at 119 5 feet bgs), U-21 (0 00506, 0 0296,
<br /> and 0 0453 ppm at 91, 1115, and 116 5 feet bgs, respectively), U-23 (0 0628 ppm at 81 feet bgs), and
<br /> UV-3 (0 0105 ppm at 26 feet bgs)
<br /> MtBE was not detected in groundwater samples from any well except for well U-22, where it was
<br /> detected at a concentration of 3 06 ppb by EPA Method 8260B TPHg or benzene were not detected in
<br /> wells U-1, U-2, U-4 through U-9, U-11, U-14 through U-18, U-22, U-24, or U-25, and TPHg were not
<br /> detected in wells U-20 or U-23 Petroleum hydrocarbons in the C6-C12 range (identified as TPHg)were
<br /> detected in wells U-3 (12,300 ppb), U-10 (383 ppb), U-12 (861 ppb), U-13 (5,120 ppb), U-19 (3,210
<br /> ppb), U-21 (2,530 ppb), NP-1-22 (16,700 ppb), NP-1-65 (38,490 ppb), NP-1-111 (210 ppb), NP-2-22
<br /> (431 ppb), NP-2-60 (13,300 ppb), and NP-2-116 (211 ppb) Benzene was detected in wells U-3 (515
<br /> ppb), U-10 (57 9 ppb), U-12 (99 9 ppb), U-13 (1,560 ppb), U-19 (810 ppb), U-20 (0 782 ppb), U-21 (706
<br /> ppb), U-23 (0 557 ppb), NP-1-22 (1,010 ppb), NP-1-65 (1,660 ppb), NP-1-111 (11 8 ppb), NP-2-22 (4 2
<br /> ppb), NP-2-60(4,350 ppb), and NP-2-116 (24 5 ppb) DIPE was detected in well U-12 at a concentration
<br /> of 3 58 ppb Results of this subsurface investigation are presented in GR Report No 240042 08-1,
<br /> Monitoring Well Installation Report, Vapor Extraction Pilot Test Report,And Interim Remedial Action
<br /> Plan,dated December 22, 2000
<br /> Quarterly monitoring and sampling of the groundwater monitoring wells began in August 1994 During
<br /> the most recent sampling event on May 14 through 16, 2001, TPHg were detected at concentrations
<br /> ranging from 175 to 8,700 ppb,and benzene concentrations ranged from 0 853 to 3,270 ppb The highest
<br /> 24004209-1 4
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