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Work Plan For a Limited Subsurface Investigation — Former Chevron Service Station #9-5775
<br />November 11, 1999
<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 />The 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 form 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 the most recent groundwater monitoring and sampling event on September 20, 1999, wells MW -1,
<br />MW -3 and MW -4 had detectable concentrations of TPHg (up to 2,840 ppb), benzene (up to 576 ppb),
<br />MtBE (up to 7,420 ppb), and ETBE (up to 651 ppb). Well MW -2 was nondetect for TPHg, benzene,
<br />MtBE, and ETBE. Ethanol, tert-butyl alcohol, di -isopropyl ether (DIPE) or TAME were not detected in
<br />any of the wells. Groundwater flow direction was to the west at a gradient of approximately 0.007 ft/ft.
<br />Depth to water ranged from 56.66 to 57.41 feet below top of casing (btoc). Historically, groundwater in
<br />the immediate vicinity of the Chevron station has ranged from 56.33 to 69.35 feet btoc.
<br />SCOPE OF WORK
<br />To evaluate the extent of the dissolved hydrocarbons in the subsurface on-site and to the south and west
<br />of the site, GR proposes to drill four additional on-site borings and install three off-site and one on-site
<br />groundwater monitoring wells (Figures 2 and 3). Also, to prepare for remediation activities at the subject
<br />site, GR will install two on-site vapor extraction wells (Figures 2 and 3). Field Methods and Procedures
<br />are included in Appendix A. To complete the specified work, GR proposes the following six tasks:
<br />Task 1. Pre -field Activities
<br />GR will update the site safety plan and obtain the necessary well installation permit from
<br />SJCPHS and encroachment permits from the City of Lodi Department of Public Works for the
<br />well on South Lee Avenue, and from the California Department of Transportation for the two
<br />wells on West Kettleman Lane. USA will be notified at least 48 hours prior to initiating work. A
<br />private utility line locator will be contracted to clear all boring locations.
<br />346408.06
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