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Subsurface Investigation Work Plan
<br /> SJC Records Retention Center
<br /> Page 3
<br /> approximately five feet bgs. Based upon lithology encountered in borings GP-1, GP-2, and MW-1, which
<br /> were drilled near the UST, soil beneath the Site consists of lean clay from the ground surface to
<br /> approximately 22/25 feet bgs, silty sand to poorly graded sand from approximately 22/25 feet bgs to
<br /> approximately 35 feet bgs, and lean clay from approximately 35 feet to approximately 41 feet bgs, the
<br /> maximum depth explored. The depth to groundwater in Site monitoring well MW-1, which has been
<br /> measured periodically since December 2013,has ranged from approximately 22.9 feet below the top of the
<br /> well casing (btoc) to 29.8 feet btoc. The implied direction of groundwater flow beneath the Site is east-
<br /> southeast to east-northeast.Figure 3 (Appendix A)presents a lithologic cross section and Figure 4 presents
<br /> a groundwater gradient map generated from data collected March 19,2018.
<br /> Groundwater samples collected from Site well MW-1 have routinely contained gasoline constituents,
<br /> including TPH-g,BTEX,and naphthalene.Total petroleum hydrocarbons in the diesel range(TPH-d)have
<br /> also been detected in Site well MW-1, but the laboratory reports have noted that the chromatogram of the
<br /> detected TPH-d is not typical of diesel.The maximum concentrations of TPH-g,benzene,and naphthalene
<br /> in well MW-1 were 29,000 micrograms per liter(µg/L), 440 pg/L, and 310 µg/L, respectively. The most
<br /> recent concentrations (March 2018)were 8,600 gg/L TPH-g, 0.31 pg/L(estimated)benzene, and 88 gg/L
<br /> naphthalene.No methyl tertiary-butyl ether(MTBE)has been detected in well MW-1.Appendix B contains
<br /> data tables that summarize historical data collected from the Site.
<br /> The adjoining property to the south (602 N. California St.) is the location of Stallworth Auto Sales
<br /> (Stallworth site), a former retail fueling facility with documented soil and groundwater contamination.
<br /> Groundwater gradient directions measured at the Stallworth site have been historically northeasterly to
<br /> easterly, and the depth to groundwater in the shallow wells has varied between approximately 23 and 35
<br /> feet bgs. The consultants for the Stallworth site have conducted extensive subsurface investigation at the
<br /> facility, and reported multiple hydrologic units (HUs) beneath the Stallworth site to a depth of
<br /> approximately 200 feet bgs. Soil beneath the Stallworth site to a depth of 40 feet bgs matches closely to
<br /> that of the subject Site. Appendix C presents figures from the Stallworth site' that identify subsurface
<br /> lithology and extent of contamination.The location of Site monitoring well MW-1 and the closed-in-place
<br /> UST has been added to the figures for reference.
<br /> Groundwater beneath the Stallworth site is impacted by petroleum hydrocarbons. Groundwater
<br /> contaminants include TPH-g, TPH-d, BTEX, and the fuel oxygenates 1,2-dichloroethane (1,2-DCA) and
<br /> tertiary-amyl methyl ether (TAME). Concentrations of TPH-g, benzene, and naphthalene in shallow
<br /> groundwater at the Stallworth site have been detected as high as 480,000 µg/L,50,000 µg/L,and 790 µg/L,
<br /> respectively. As of November 2017, maximum dissolved concentrations in shallow groundwater at the
<br /> Stallworth site are 75,000µg/L TPH-g,8,000µg/L TPH-d, 1.0 ltg/L benzene(well MW-IA was non-detect,
<br /> but with an elevated detection limit of <170 µg/L), and 790 gg/L naphthalene. Dissolved petroleum
<br /> hydrocarbon constituents have also been detected in deeper water-bearing zones beneath the Stallworth site.
<br /> However, consultants for the Stallworth site have concluded that the bulk of the dissolved phased
<br /> contaminants are located within the shallow water bearing zone(HU-1,approximately 20-40 feet bgs),and
<br /> HU-2 (approximately 60-70 feet bgs).
<br /> 4.0 PROPOSED SCOPE OF WORK FOR ADDITIONAL SUBSURFACE
<br /> INVESTIGATION
<br /> Condor proposes the following scope of work to address the additional investigation requested by the
<br /> Central Valley Water Board in their letter dated August 29,2017:
<br /> ' Figures and other Stallworth site data obtained via the State Water Resources Control Board's online data
<br /> management system,GeoTracker(Global ID#T0607700523).
<br /> +i� CONDOR
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