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Work Plan For Monitoring Well Installation — Former Chevron Service Station #9-5775, Lodi, CA
<br />Anril 7. 2000
<br />piping. Very low to non-detectable concentrations of petroleum hydrocarbons were detected in all the
<br />soil samples from beneath the waste oil USTs, the product lines, the hydraulic hoist, and five of the six
<br />samples from the gasoline UST pit. The sample from 14.5 feet below ground surface (bgs) beneath the
<br />eastern end of the middle gasoline UST contained 14,000 parts per million (ppm) of Total Petroleum
<br />Hydrocarbons as gasoline (TPHg), 84 ppm of benzene and 56 ppm of methyl tert-butyl ether (MtBE).
<br />Based on field screening data, the area beneath the eastern end of the middle gasoline UST was excavated
<br />from 14.5 to 16 feet bgs, and a second sample was collected. This sample contained 880 ppm of TPHg
<br />and 11 ppm of MtBE. Benzene was not detected in this sample.
<br />Tile area beneath the eastern end of the middle gasoline UST was overexcavated from 16 to 22.5 feet bgs
<br />on August 31, 1998. Soil samples were collected from the base and the eastern wall of this excavated
<br />area. The base sample contained 2,300 ppm of TPHg, 4.1 ppm of benzene and 330 ppm of MtBE. The
<br />wall sample, collected at 18.5 feet bgs, contained 290 ppm of TPHg, 0.35 ppm of benzene and 22 ppm of
<br />MtBE.
<br />On January 25 through 27, 1999, five soil borings were drilled at the subject site, with four of the borings
<br />completed as monitoring wells. The four monitoring wells (MW -1 through MW -4) were completed to 65
<br />feet bgs. Soil samples collected from the borings contained detectable concentrations of TPHg (up to
<br />1,200 ppm), benzene (up to 0.81 ppm), and MtBE (up to 110 ppm). Ethyl tert-butyl ether (ETBE) (up to
<br />1.3 ppm) and tert-amyl methyl ether (TAME) (0.34 ppm) were also detected in soil samples. Groundwater
<br />samples contained concentrations of TPHg (up to 11,000 parts per billion, or ppb), benzene (up to 6,800
<br />ppb), MtBE (up to 62,000 ppb), ETBE (2,400 ppb), and TAME (780 ppb).
<br />In October 1999, GR continuously monitored the groundwater elevations at the site over a three day
<br />period (GR report 346408.05, Offsite Source Survey, dated November 2, 1999). The results of this study
<br />show that groundwater elevation fluctuates in daily cycles corresponding to the operation of nearby City
<br />of Lodi Municipal Well #12, located 600 feet southwest of the site. Based on this, Chevron initiated
<br />further subsurface assessment work to evaluated the extent of petroleum hydrocarbons between the site
<br />and the municipal well, and further define the the on-site source around the former USTs.
<br />Five additional groundwater monitoring wells (MW -5 through MW -9), four vapor extraction wells (VE -1
<br />through VE -4), and one groundwater extraction well (EX -1) were installed in December 1999 and
<br />February 2000. Two additional soil borings (SB -2 and SB -3) were installed in December 1999. Soil
<br />samples collected from the borings contained detectable concentrations of TPHg (up to 1,740 ppm),
<br />benzene (up to 446 ppm), and MtBE (up to 100 ppm). The highest concentrations were from well borings
<br />EX -1 and VE -1 located in the area of the former USTs. Water samples from the groundwater monitoring
<br />and extraction wells had detectable concentrations of TPHg (up to 51,000 ppb), benzene (up to 39,000
<br />ppb), MtBE (up to 720,000 ppb), ETBE (up to 18,000 ppb), and TAME (up to 6,000), with the highest
<br />concentrations in well EX -1 and diminishing significantly to mostly non -detect levels away from the
<br />former UST area.
<br />Groundwater Monitoring and Sampling
<br />In a recent groundwater monitoring and sampling event on December 3, 1999, wells MW -1, MW -3 and
<br />MW -4 had detectable concentrations of TPHg (up to 3,130 ppb), benzene (up to 996 ppb), MtBE (up to
<br />14,400 ppb), and ETBE (up to 1,120 ppb). Wells MW -1 and MW -4 had detectable concentrations of
<br />346408.09
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