ARCHIVED REPORTS_XR0012358 - Laserfiche WebLink

Home Browse Search

Monitoring Well and Sparge Points Installation Report—Former Unocal Station#2859,Stockton,CA August 19,2002 borings KB-3 and K13-5 at approximately 20 feet below ground surface (bgs), at up to 239 and 370 parts per million (ppm), respectively, and benzene was detected in the two borings at up to 11 2 and 6 58 ppm, respectively In 1988, two 10,000-gallon gasoline USTs and one 550-gallon waste oil UST were removed Soil samples collected from the base of the tank pit contained TPHg and benzene at concentrations up to 13 and 0 29 ppm, respectively Between June 1988 and February 1992, seven monitoring wells (MW-1 through MW-7), three vapor extraction wells (VW-1 through VW-3), and one soil boring were installed by Applied Geosystems and RESNA The highest TPHg and benzene concentrations reported were from an area around the northern product island location, at concentrations up to 17,300 and 28 ppm, respectively A soil vapor extraction test was conducted by RESNA in 1992 The test showed that a wellhead flow rate of 43 cubic feet per minute and an estimated radius of influence of approximately 25 feet could be achieved In July 1994, Pacific Environmental Group, Inc (PEG) installed monitoring wells MW-8 and MW-9 Groundwater samples from the wells were non-detect for benzene, but TPHg were detected in wells MW-8 and MW-9 at concentrations of 93 and I 10 parts per billion (ppb), respectively PEG performed a six-month on-site vapor extraction pilot study in 1996 During that study, approximately 2,745 pounds (450 gallons) of TPHg and 12 pounds (1 6 gallons) of benzene were removed ARCADIS Geraghty & Miller, Inc installed wells MW-10 and NP-1 in January 1999 Soil samples from well NP-1 showed impact beneath the site between 40 and 60 feet below ground surface (bgs) The highest TPHg and benzene concentrations were from a sample at 60 feet bgs at 290 and 5 8 ppm, respectively Discrete grab groundwater samples from well boring NP-1 had concentrations of TPHg and benzene in samples collected between 50 and 70 feet bgs The highest concentrations of TPHg and benzene detected were 200,000 (50 feet bgs) and 21,000(60 feet bgs) ppb, respectively In May 2001, GR installed nine groundwater monitoring wells (MW-11 through MW-19) on and downgradient of the site TPHg, benzene, or methyl tert-butyl ether (MtBE) were not detected in soil samples from the well borings MtBE was not detected in groundwater samples from the wells TPHg and benzene were reported in groundwater samples from wells installed on and immediately downgradient of the site at concentrations up to 31,000 and 720 ppb,respectively Groundwater Monitoring and Sam lin Quarterly groundwater sampling started in June 1988 Separate phase hydrocarbons have not been observed in the wells Since monitoring of the wells began in the first quarter of 1993, groundwater beneath the site has risen more than 20 feet In the most recent monitoring and sampling event on May 13 and 14, 2002, depth to water in the wells ranged from 23 94 to 25 87 feet below top of casing Groundwater flow during this event was variable at gradients ranging from 0 002 to 0 02 24007206-1 2