WORK PLANS - Laserfiche WebLink

Home Browse Search

} r! Man Ms. Mary Meays September 29, 1995 (9347-3011) A total of twenty onsite and one offsite soil borings were drilled and sampled at the site between September 1990 and November 1991. ',The highest concentrations of total petroleum hydrocarbons as gasoline (TPH-G) were detected!at 5,100 and 14,000 parts per million (ppm) in the soil samples collected from borings SB-3 and MW-2, respectively. Benzene was detected at up to 0.9 ppm in the soil sample collected fromE'boring SB-3. The bulk of the adsorbed-phase petroleum hydrocarbon mass in the subsurface soil was detected in the soil samples collected from the borings in the immediate vicinity of 'the former UST cluster. The subsurface soils beneath the former UST cluster and eastern dispenser island are penetrated by vapor extraction wells connected to the VES (RESNA, 1992a). Nine additional borings were drilled and sampled at the site in September 1992 after the VES had operated for approximately one.year. TPH-G was detected at up to 4,600 ppm only in borings CB8 and CB9. Benzene was not detected above reported laboratory detection limits in any of the samples collected and analyzed from; the nine additional borings (RESNA, 1992a). Pertinent excerpts from the investigative report are included as Attachment A. Results of this preliminary investigation indicated that the VES was effective in reducing the volume of petroleum hydrocarbons in the subsurface soil during the limited duration of VES operation. F� Monitoring wells were first installed on the site in September 1990. The average depth to stabilized ground water at the site has decreased from approximately 36 feet below surface (bs) in August 1992 to approximately 29 feet bs in May 1995. The vadose zone soils, soils above the saturated zone and capillary fringe, are primarily composed of clay with some silt and sand. However, the soil in and beneath the area of the former UST cluster appear to be primarily sand with silt and clay which is conducive to remediation using a VES (RESNA, 1991b). The airflow to the VESs has averaged in excess of 128 standard cubic feet per minute. The ground water gradient is flat, averaging about 0.01 foot per foot in a easterly- northeasterly flow direction. The downgradient extent of the dissolved-phase petroleum hydrocarbons in ground water has been defined. No benzene has been detected above the reported laboratory detection limits in MW-4, the farthest downgradient well, since February 1994. TPH-G was detected at up to 0.31 ppm irilthe water samples collected from wells'RW-2, MW-3, and MW-4, while benzene was detected at 0.0025 and 0.0098 ppm in the water samples collected from wells MW-3 and RW-2, respectively, during the May 1995 sampling event. No TPH-G or benzene was detected above the reported detection limits in the water samples collected from wells MW-1, MW-2, MW-5, an&RW-1. Vapor extraction was initiated at the site in late January 1992 using venting wells VEW-1 through VEW-4 and an internal combustion (IC);unit. An estimated 70 pounds of TPH-G were removed from the site by the IC engine. (RESNA, refer to 04/02/92 letter to APCD). A thermal oxidation unit replaced the IC unit in March 1992. The thermal oxidation unit was then replaced by an activated vapor-phase carbon unit in July.` 1992 (RESNA, 1992b). An estimated 1,545 pounds of TPH-G and benzene were removed from the site by the thermal oxidation and carbon units between April 2, 1992 and September 24, 1992 (RESNA, 1992c). €k. I{ 93473011.ARP 3 = r; �k