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Work Plan for Groundwater Monitoring Well Installation at the California Army National Guard
<br />Stockton Field Maintenance Shop, Stockton, California
<br />2.0 PREVIOUS ENVIRONMENTAL INVESTIGATIONS
<br />The area of environmental concern is the former location of the diesel and gasoline USTs
<br />(Figure 2). The former 10,000 -gallon gasoline and 5,000 -gallon diesel USTs, which were both
<br />single -walled steel tanks, were installed sometime in the early 1950s, when the FMS began site
<br />operations. As indicated above, the USTs were removed in 1996 (Versar, 2004). The
<br />subsections below describe the environmental investigations/activities that have occurred at the
<br />site.
<br />2.1 UST REMOVAL
<br />The USTs and associated piping were excavated and removed in 1996. Soil samples collected
<br />during the removal of the USTs indicated that TPH-g, TPH-d and BTEX contaminants were
<br />present in the soil. In March 2000, two soil borings were drilled and soil samples were collected
<br />at depths of 10, 15, and 20 feet bgs, and were analyzed for petroleum hydrocarbon
<br />constituents. Sampling results indicated TPH-g and TPH-d were present in soil at
<br />concentrations up to 25.4 mg/kg and 209 mg/kg, respectively (Versar, 2004).
<br />2.2 2004 SITE INVESTIGATION
<br />In February 2004, three additional borings were drilled to groundwater and a total of eight soil
<br />samples and three groundwater grab samples were collected and analyzed for TPH-g, TPH-d,
<br />BTEX and fuel oxygenates, and metals. Results associated with this investigation were
<br />presented in the Preliminary Assessment/Site Investigation Update Report, California Army
<br />National Guard Facility (Versar, 2004). The boreholes were all drilled within the vicinity of the
<br />USTs and pump island. Sampling results indicated the highest concentrations of TPH-d (260 •
<br />mglkg), ethylbenzene (18 mg/kg), and xylenes (50.4 mg/kg) were reported at 35 feet bgs in
<br />boring B31 (Figure 2), located approximately 75 feet south of the former diesel UST. The TPH-
<br />d concentration is above the 100 mg/kg Tri -Regional Board guideline (Regional Water Quality
<br />Control Board [RWQCB], 1990). The noted detections of ethylbenzene, xylenes, and other
<br />detected BTEX compounds were below preliminary remediation goals (PRGs) established by
<br />the United States Environmental Protection Agency (USEPA) Region IX. The maximum TPH-g
<br />concentration (47 mg/kg) was detected in boring B30 (Figure 2) at a depth of 40 feet bgs. All
<br />detected concentrations of benzene and toluene in soil were less than their respective
<br />residential PRGs. Analytical results indicated that soils beneath the area southeast and
<br />northeast of the previous diesel UST location have likely been impacted by leaks from the
<br />former diesel UST, and some contaminants in soil are present at concentrations exceeding Tri -
<br />Regional Board guidelines but less than residential PRGs.
<br />Groundwater grab samples were collected from the three boreholes and analyzed for TPH-g,
<br />TPH-d, BTEX and fuel oxygenates, and for lead by USEPA Methods SW8015G and D,
<br />SW8260B, and SW6010B, respectively. Groundwater analytical results indicated that the
<br />maximum concentrations of TPH-d (1,400 micrograms per liter [pg/Ll) and TPH-g (74,000 pg/L)
<br />in boring B30, east of the UST locations (Figure 2), exceeded taste and odor thresholds of 100
<br />pg/L and 5 pg/L, respectively. The highest concentrations of benzene (up to 24,000 pg/L) and
<br />ethylbenzene (up to 2,300 pg/L) were reported in boring B32 and exceeded the primary
<br />maximum contaminant levels (MCLs) of 1 pg/L for benzene and 300 pg/L for ethylbenzene. The
<br />highest concentrations of toluene (up to 6,000 pg/L) and xylenes (up to 4,900 pg/L) were
<br />reported in boring B30, and exceed the primary MCLs of 150 pg/L for toluene andl ,750 pg/L for
<br />xylenes (Versar, 2004). •
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