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• Monitoring Well Installation and VET Report—Former Unocal Station#2859,Stockton,CA
<br /> July 30,2001
<br /> In 1988,two 10,000-gallon gasoline USTs and one 550-gallon waste oil UST were removed At that time
<br /> approximately 1,350 cubic yards of impacted soil was excavated and removed from the site After
<br /> excavation, soil samples collected from the base of the tank pit contained TPHg and benzene at
<br /> concentrations up to 13 and 0 29 ppm, respectively
<br /> Between June 1988 and February 1992, seven monitoring wells (MW-1 through MW-7), three vapor
<br /> extraction wells (VW-1 through VW-3), and one soil boring were installed by Applied Geosystems and
<br /> RESNA The highest TPHg and benzene concentrations reported were from an area around the northern
<br /> product island location at concentrations up to 17,300 and 28 ppm, respectively
<br /> A soil vapor extraction test was conducted by RESNA in 1992 The test showed that a wellhead flow rate
<br /> of 43 cubic feet per minute and an estimated radius of influence of approximately 25 feet could be
<br /> achieved
<br /> In July 1994, Pacific Environmental Group, Inc (PEG) installed monitoring wells MW-8 and MW-9
<br /> Groundwater samples from the wells were non-detect for benzene, but TPHg were detected in wells
<br /> MW-8 and MW-9 at concentrations of 93 and 110 parts per billion(ppb), respectively
<br /> PEG performed a six-month on-stte vapor extraction pilot study in 1996 During that study
<br /> approximately 2,745 pounds (450 gallons) of TPHg and 12 pounds (1 6 gallons) of benzene were
<br /> removed
<br /> ARCADIS Geraghty& Miller, Inc installed wells MW-10 and NP-1 in January 1999 Soil samples from
<br /> well NP-1 showed impact beneath the site between 40 and 60 feet below ground surface (bgs) The
<br /> highest TPHg and benzene concentrations were from a sample at 60 feet bgs at 290 and 5 8 ppm,
<br /> respectively Discrete grab groundwater samples from well boring NP-I had concentrations of TPHg and
<br /> benzene in samples collected between 50 and 70 feet bgs The highest concentrations of TPHg and
<br /> benzene detected were 200,000(50 feet bgs)and 21,000(60 feet bgs)ppb, respectively
<br /> Groundwater Monitoring and Sampling
<br /> Quarterly groundwater sampling started in June 1988 with wells MW-1 through MW-3 Wells MW-4
<br /> and MW-5 were added in August 1989, MW-6 and MW-7 in March 1992, MW-8 and MW-9 in
<br /> November 1994, and MW-10 and NP-1 in February 1999 Separate phase hydrocarbons have not been
<br /> observed in the wells Since monitoring of the wells began in the first quarter of 1993, groundwater
<br /> beneath the site has risen more than 20 feet In the most recent monitoring and sampling event on June 5,
<br /> 2001, depth to water in the wells ranged from 23 04 to 25 09 feet below top of casing. Groundwater flow
<br /> during this event was to the northeast at an approximate gradient 0 002 in Zone A (Figure 2), and flow
<br /> was to the southeast at an approximate gradient of 0 027 in Zone B (Figure 3) The groundwater flow
<br /> direction and gradient in Zone A is consistent with historical data
<br /> During the June 5, 2001 event, TPHg range hydrocarbons were detected in wells MW-2 (62,000 ppb),
<br /> MW-10 (130,000 ppb), MW-11 (31,000 ppb), MW-12 (3,100 ppb), MW-13 (87 ppb), MW-15 (3,100
<br /> is ppb), and MW-17 (75 ppb) Benzene was detected in wells MW-2 (3,200 ppb), MW-10 (3,900 ppb),
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