3815_PGSF WMVD Pkg 2_Vol 4-2_03.03.2026_BXWA_Certified

ASPECT CONSULTING PROJECT NO. AS190583A-08  MAY 21, 2025 FINAL 27 operation of the SVE system continue until the mass discharge flow rate fell below 0.1 lbs/day (Landau, 1992). The SVE system operated on a near-continuous basis from startup in November 1991 through January 1993. From January 1993 through mid-1994, the SVE system was periodically shut down for 1- to 3-month periods and then restarted to operate on a pulsing basis. In mid-1994, laboratory analytical results indicated that the mass flow rate generated from the SVE system no longer warranted continued operation, and Landau initiated a compliance monitoring investigation of the tank area to assess whether the cleanup action had attained applicable cleanup standards. The June 1994 compliance monitoring investigation (Landau, 1994a) consisted of the following:  Advancing nine direct-push soil borings to a depth of approximately 9 feet bgs in areas adjacent to and within 100 feet west (downgradient) of the UST No. 29/67 excavation;  Collecting and analyzing for BTEX six soil samples from five borings located around and downgradient of the excavation;  Collecting and analyzing for BTEX grab groundwater samples from five downgradient borings; and  Collecting and submitting vapor samples from the SVE system for laboratory analysis of BTEX. In soil borings located immediately north of the excavation, detected concentrations of total xylenes ranged from 123 mg/kg in the vadose zone to 2,990 mg/kg in the saturated zone. In 1989, prior to operation of the SVE system, xylenes had been detected at 26,000 mg/kg in soil sample collected from the north excavation sidewall, and located adjacent to the 123 mg/kg sample, suggesting a substantial concentration decline in vadose zone soil at the excavation location. Within 10 feet west of the excavation, detected soil xylenes concentrations declined to less than 7 mg/kg. Xylenes were not detected in the soil sample collected approximately 50 feet west of the excavations. Concentrations of total xylenes detected in the grab groundwater samples declined with increasing downgradient distance. Xylenes were detected at a concentration of 30,560 µg/L in the groundwater sample collected about 35 feet west of the excavation’s western end. Approximately 60 feet west of the excavation, the detected groundwater xylenes concentration was 315 µg/L. In borings positioned 25 to 30 feet north and south of that location, xylenes were detected in groundwater at 5.1 µg/L and 1.5 µg/L, respectively. Approximately 90 feet west of the excavations, xylenes were not detected in the groundwater sample collected. Ethylbenzene concentrations in the groundwater samples were lower than detected xylenes concentrations. Low-level concentrations of benzene and/or toluene were also detected in the groundwater samples collected. Based on the collective data collected during the 1989 UST removal and in 1994, Landau (1994a) concluded the following: