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the vapor samples and had a maximum concentration of 5.2 lig/m3 in SVP-1 at 15 fbg. Further
<br />information is available in Arcadis' March 26, 2014, Soil Vapor Investigation Report.
<br />July 2015 Soil Borings by Arcadis
<br />From July 27 through 31, 2015, Arcadis supervised the advancement of two soil boings, AB-1 and AB-2,
<br />to evaluate the downgradient extent of hydrocarbon impact to groundwater in the B' Zone. Each boring
<br />was advanced to a total depth of approximately 65 fbg. Soil samples were collected for laboratory
<br />analysis during boring advancement. Following completion of the soil borings, temporary casings were
<br />constructed to collect grab-groundwater samples. TPHg, BTEX, and MTBE were only detected in the soil
<br />sample from AB-1 at 57 fbg at concentrations of 650 mg/kg, 0.041 mg/kg, 0.022 mg/kg, 0.36 mg/kg,
<br />0.26 mg/kg, and 0.048 mg/kg, respectively. TPHg, BTEX, and MTBE were detected in groundwater
<br />samples from each boring. TPHg was detected at a maximum concentration of 4,800 µg/L in AB-1.
<br />BTEX and MTBE were detected at maximum concentrations of 2,400 µg/L, 180 µg/L, 240 µg/L, 210 µg/L,
<br />and 440 µg/L, respectively, in AB-2. Further information is available in Arcadis' September 29, 2015, Soil
<br />and Groundwater Investigation Report.
<br />November 2019 Monitoring Well Installation by GHD
<br />On November 15, 2019, GHD installed one groundwater monitoring well, MW-24, offsite to the east of the
<br />site to delineate the extent of dissolved hydrocarbons in the B' Zone. Well MW-24 was installed to 68 fbg
<br />and screened from 53 to 68 fbg. Well MW-23 was unable to be installed due to overhead and
<br />underground utility conflicts. Soil samples were collected at five foot intervals during well installation.
<br />TPHg was detected in only one soil sample at 65 fbg at a concentration of 0.43 mg/kg. Benzene was
<br />detected in two soil samples at a maximum concentration of 0.11 mg/kg at 65 fbg. Toluene,
<br />ethylbenzene, and total xylenes were each detected in one soil sample at concentrations of 0.0031 J
<br />(estimated value) mg/kg, 0.059 mg/kg, and 0.0036 J mg/kg, respectively, at 65 fbg. MTBE was detected
<br />in two soil samples at a maximum concentration of 0.028 mg/kg at 65 fbg. Further information of well
<br />installation activities is available in GHD's January 13, 2020, Well Installation Report.
<br />Remediation History
<br />April and June 1999 UST Removal and Over-excavation by IT Corporation
<br />From April 7 to June 2, 1999, all site infrastructure including the station canopy, three 10,000-gallon
<br />underground storage tanks (USTs), two gasoline and one diesel, dispenser islands, product piping, and
<br />unidentified fiberglass piping were removed from the site. UST removal consisted of over-excavating an
<br />area of approximately 35 feet by 40 feet by 15 feet deep. During over-excavation soil samples were
<br />collected from the bottom and sidewalls of the excavation. Groundwater was encountered during
<br />excavation and a grab-groundwater sample was collected. Upon completion of over-excavation,
<br />approximately 1,707 tons of soil were hauled offsite for disposal. Hydrocarbon impact in soil was
<br />predominantly detected in samples collected from the over-excavation of dispenser islands in the
<br />southern portion of the site. Total petroleum hydrocarbons as gasoline (TPHg) and TPH as diesel (TPHd)
<br />were detected at maximum concentrations of 3,000 milligrams per kilogram (mg/kg) in PBB-2 at 13 fbg
<br />and 6,300 mg/kg in T3-C at 15 fbg, respectively. Elevated dissolved hydrocarbon concentrations were
<br />detected in the grab-groundwater sample; TPHg, benzene, toluene, ethylbenzene, and total xylenes
<br />(BTEX) were detected at a concentrations of 36,500 microgams per liter (µg/L), 605 µg/L, 289 µg/L,
<br />GHD I Well Installation Work Plan Addendum I Attachment B111193145 (05)1 Page 5
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