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IN-SITU VACUUM EXTRACTION PROJECTS:
3. TWO-PHASE VACUUM EXTRACTION PILOT STUDY

Confidential Consumer Products Manufacturer, North Brunswick, New Jersey

In 1988, a leaking 4,000-gallon unleaded gasoline underground storage tank (UST) was removed from beneath an asphalt parking lot and the excavation was backfilled. Based on the results of a soil gas survey , another consultant installed a vapor extraction system. After approximately two years of operations, the consultant recommended that the single-phase system be expanded.

At that point, the site owner asked a third party to evaluate the proposed expansion. The third party collected soil samples to delineate the extent of residual soil contamination. It was determined that the parking lot was underlain by approximately 20 feet of fill material (sand and gravel) and at least 4 feet of clay and there was a two-foot thick perched water table at the sand-clay interface. The water contained benzene, toluene, ethylbenzene, and xylene (BTEX).

No water had been found during the original site investigation which had been conducted during a drought. Seven monitoring wells were installed to delineate the extent of the BTEX in the groundwater. The average dissolved concentration in the area was 18,000 micrograms per liter (ug/L). Although the formation was fairly permeable, the average initial yield of the monitoring wells was less than 0.5 gallons per minute (gpm) due to the low head available in the thin perched aquifer. BTEX concentrations in soil just above the water table exceeded the most stringent New Jersey standards over an area of approximately 6000 square feet. This zone had not been thoroughly cleaned by the previous soil vapor extraction.

New Jersey regulations applicable to the site require that residual hydrocarbons in both soil and groundwater be remediated. In June and July 1993, a Two-Phase Vacuum Extraction (TPVE) pilot study was conducted to demonstrate the ability of the TPVE process to accelerate removal of hydrocarbons from both media at the site. The TPVE system was operated for 30 days with an initial yield from the extraction well of 2.5 gpm. Significant dewatering was accomplished. After several days, the sustainable yield stabilized at the site hydraulic recharge rate of approximately 0.75 gpm. During the TPVE pilot study, 25,000 gallons of groundwater (approximately 1/3 of the total volume of water present in the area), and approximately 2.5 pounds of BTEX (approximately 1/3 of the total BTEX present in the groundwater before the pilot study) were removed from the site. In one month, 9.2 million cubic feet of air were moved through the soil column, removing approximately 125 pounds of non-methane hydrocarbons from the soil.

Measurements of induced vacuum and water table drawdown showed that the TPVE capture zones encompassed the entire known extent of hydrocarbon impact in the soil and groundwater.

A technology evaluation report is being prepared for this site. In addition to TPVE, other candidates for remediation at the site include natural attenuation, groundwater pumping supplemented by vapor extraction, bioremediation, and TPVE augmented by biological techniques.