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TWO-PHASE VACUUM EXTRACTION SYSTEM FOR GROUNDWATER REMEDIATION AT A GASOLINE SERVICE STATION
Mobil Oil Corporation, Blackwood, New Jersey
During the rebuilding of one of Mobil's gasoline service stations, it was discovered that the underground gasoline storage tanks had leaked. The tanks were removed and contaminated soil was excavated and transported offsite; however, investigations revealed that volatile organic compounds (VOCs) had impacted the groundwater. Two Phase Vacuum Extraction (TPVE) was applied to remediate residual soil contamination and capture and remediate the groundwater plume at the site. The project required obtaining local and state permits for construction of the system and for discharge of treated air and water streams. A catalytic oxidation unit was provided to treat extracted soil gases. A trailer mounted TPVE system was mobilized to the site. Application of TPVE by way of a single well resulted in the capture of the entire known ground water plume at the site (approximately 1 acre in extent) and the removal and destruction of as much as 300 pounds per day of hydrocarbons.
SOIL AND GROUNDWATER REMEDIATION GASOLINE STATION AND CONVENIENCE STORE
Munro Petroleum & Terminal Corporation, Biloxi, Mississippi
A gasoline leak from aging USTs had impacted soil and groundwater at the Central Beach gasoline station and convenience store site in Biloxi, Mississippi. Although the Mississippi Department of Environmental Quality had allocated funds for one year of TPVE operations, the remediation of the site was completed in just four months. An environmental assessment at this very active convenience store determined the concentration and location of contaminants in both the soil and groundwater. While most of contamination was limited to the immediate vicinity of the dispenser islands, some gasoline had migrated underneath the adjacent state highway.
Since heavy highway traffic prohibited soil cleanup by excavation, the use of TPVE was recommended to cleanup both soil and groundwater. The TPVE system was installed and started up in early August, 1993. The system extracted in excess of 7,000 gallons of groundwater per day. Mathematical modeling techniques showed that the zone of influence of the TPVE extended across the adjacent state highway. Approximately 2,500 pounds of hydrocarbons were removed from the site in four months. The cleanup progressed much faster than anticipated. The rate of extraction of hydrocarbons dropped off significantly by early December of 1993 and the analysis of groundwater samples collected then showed that the state mandated ground water cleanup objectives had been surpassed by a factor of more than ten.
MDEQ therefore required no further remediation. The project, initially budgeted at about $138,000 was completed for a total cost of less than $71,000 with no interruption of station operations.
EMERGENCY CLEANUP OF WATER SUPPLY
City of Rayville Physical Plant, Rayville, Louisiana
Routine sampling by the Louisiana Department of Environmental Quality (DEQ) detected benzene in a city water supply well. An underground gasoline tank system that had been formerly located just 70 feet from the well was the suspected source. A hydrogeological characterization of the site confirmed that gasoline from the UST had migrated towards the well.
DEQ required an emergency cleanup of the source area. With the concurrence of the DEQ, a Two-Phase Vacuum Extraction system was deployed to the site to extract contaminated groundwater and soil vapors. The removal of ground water was facilitated by the introduction of atmospheric air using the priming method invented by TSI employee Steven Morrow and described in U.S. Patent Number 5,076,360. More than 450,000 gallons of contaminated water and 1,000 pounds of hydrocarbons were removed in one month. Four subsequent rounds of sampling over a two year period have shown no recurrence of benzene in the well water.
The innovative use of the priming method and the successful restoration of the quality of the city water supply for very low cost (the remediation was completed for less than $30,000) led the Pennsylvania Consulting Engineers Council to recognize this project with an Honor Award for Engineering Excellence in 1994.
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 silty clay, clays, 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 areal extent of soil contamination is approximately 40,000 square feet, with concentrations of benzene, toluene, ethylbenzene, 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 beconsidered 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.
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