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 FAQs

IN-SITU VACUUM EXTRACTION PROJECTS:
8. FREE PRODUCT GASOLINE RECOVERY

Cloyd's Corner Exxon Service Station Monroe, Louisiana

Inventory records and tank and line tightness tests indicated that the UST system at the Cloyd's Corner site had leaked at several locations between March, 1985 and April, 1990. In May 1990, an investigation was conducted to determine the horizontal and vertical extent of petroleum hydrocarbon migration in soil and groundwater. Subsurface conditions were found to be as follows:

• Shallow strata include relatively low permeability silt clay, clayey sand, and an aluminum residue waste material previously dumped at the site. Approximately 75 percent of the impacted areas consists of heterogeneous fill material.
• Groundwater levels across the site vary between three and six feet below ground surface (BGS).

The area extent of soil contamination is approximately 40,000 square feet, with concentrations of benzene, toluene, ethyl benzene, and xylene (BTEX) above 10 parts per million (ppm) predominately in the upper six feet of soil.

Three to five inches of free phase hydrocarbons were observed in two of the monitoring wells and discharging into an adjacent creek. It was estimated that the free phase plume extended over 4,275 square feet.

Dissolved hydrocarbons were present in groundwater over an approximate 31,000 square foot area.

As an interim measure, an interceptor trench was installed between the source of the release and the creek to provide a hydraulic barrier and to create a continuous zone of influence and capture zone for removal of free and dissolved phase hydrocarbons. The trench was approximately 125 feet long, 3 feet wide, 8 feet deep, and was filled with pea gravel. A pumping test conducted on a recovery well installed in the trench indicated that the sustainable pumping rate (from the trench) would be less than 3 gallons per minute (gpm). Using this pumping rate, cleanup costs and duration were estimated using results of a batch-flushing analysis (EPA/541/6-88/003: "Guidance on Remedial Actions of Contaminated Groundwater at Superfund Sites", Interim Final, December, 1988). Based on the significant costs and cleanup duration indicated by this model, installation of a two-phase vacuum extraction (TPVE) system was proposed to accelerate the remediation at the site.

LDEQ accepted the plan to conduct a TPVE pilot study at Cloyd's Corner. The results of the TPVE operation were as follows:

The application of TPVE resulted in the removal of all the free product from the subsurface at the site in less than 5 weeks. Free product has not been detected in any of the monitoring wells at the site since the system was shut down in June, 1993.

More than 3,100 pounds (1,600 pounds liquid plus 1,500 pounds vapor) of gasoline were removed from the subsurface.

The rate of groundwater withdrawal from the recovery trench and from the monitoring wells by TPVE was significantly greater than could be achieved by conventional pumping.

The entire known contaminant plume in groundwater was captured using TPVE applied to the interceptor trench.

While the ability of the process to remove free product and contaminated groundwater have been well demonstrated, it has not been demonstrated whether TPVE will or will not effectively cleanup soil at Cloyd's Corner.

LDEQ is conducting a risk assessment at the Cloyd's Corner Exxon Convenience Store. The removal of more than 3,000 pounds of gasoline during the TPVE pilot study has helped to reduce any risk to human health and the environment. If, nevertheless, the results of LDEQ's assessment indicate that there is an unacceptable risk due to residual gasoline components in the groundwater and that some additional active cleanup is necessary, TPVE applied to the interceptor trench should be considered based on its proven effectiveness.

If the results of the risk assessment indicate that there is an unacceptable risk due to residual gasoline components in the soil, TPVE applied to horizontal trenches constructed in the smear zone (the zone about 6 feet below the surface where hydrocarbons have been transported by fluctuations in water table elevation) is recommended based on the demonstrated ability of the process to dewater that zone and to remove hydrocarbons in the vapor phase.