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SECTION 3.0 SUMMARY OF PREVIOUS INVESTIGATIONS TPH-g were detected above laboratory detection limits in four borings (MW-5, MW-3, S13-1, and VW-2) at depths greater than 40 feet bgs Concentrations of TPH-g were detected in well MW-5 at 45 feet bgs (17 mg/kg), well VW-1 at 65 feet bgs (I 3 mg/kg), well MW-9 at 41 feet bgs (31 mg/kg), and well VW-2 at 41, 46, 51 and 61 feet bgs (2 1, 550, 200, and 1 3 mg/kg, respectively) BTEX compounds were detected in soil samples with detectable concentrations of TPH-g The highest BTEX concentrations were detected in well VW-2 at 46 feet bgs (benzene = 0 78 mg/kg, toluene = 9 1 mg/kg, ethylbenzene = 13 mg/kg, and total xylenes = 210 mg/kg) • A limited vapor extraction pilot test was conducted by GEOCON in August 1996 (GEOCON, 1996b) to evaluate the feasibility of using soil vapor extraction (SVE) as a remedial alternative at the Stockton Yard The results of the SVE pilot test showed an initial TPH-g mass extraction rate of approximately 300 pounds per day (approximately 50 gallons per day) Assuming an extraction flowrate of 50 standard cubic feet per minute (scfm) per extraction well, an effective radius of influence of approximately 35 feet was estimated From their 1996 review of the cumulative soil and groundwater data at the site, GEOCON estimated 1,500 cubic yards of TPH-g impacted soil remain at the Stockton Yard site with the majority of the soil remaining beneath the gasoline and diesel tank excavations GEOCON estimated that a 100 scfin thermal oxidizer SVE system would need to be operated for 12 to 18 months to volatilize and capture the extractable TPH-g mass in soil 3.2 PREVIOUS GROUNDWATER INVESTIGATIONS HistoncaI groundwater analytical results are summarized in Table 3-3 A summary of the previous groundwater investigations is discussed. below • In 1992, Alton sampled three wells, MW-I through MW-3, which were installed as part of the preliminary site investigation The wells were sampled for TPH-g and BTEX TPH-g, benzene, toluene, and total xylenes were detected in the three monitoring wells, and the highest concentrations were detected in well MW-1 at 4 9 milligrams per liter (mg/L), 1,100 micrograms per liter(pg/L), 3,600 µg/L, and 4,000 µg/L, respectively (Alton, 1992) ' • Quarterly groundwater sampling of the 12 onsite groundwater monitoring wells and the two onsite vapor monitoring wells has been conducted since 1995 by GEOCON (1995 and 1996), PSI (1997 through April 1999), and Ninyo & Moore (June through December 1999) The ' historical groundwater results in Table 3-3 (through September 1999) show a decline in TPH- g. BTEX, and MTBE concentrations over time The maximum TPH-g concentration was ' detected in a groundwater sample collected from well VW-1 on April 23, 1996 after more than a 2-foot rise in the groundwater table In the groundwater sample collected from well VW-1, TPH-g, BTEX, and MTBE concentrations were detected at concentrations of 54 S fpao na1aao1000atsYrruzrnnuuarYatosa ooC 3_2 URS Greh7er Woodward Clyde