The International Information Center for Geotechnical Engineers

Soil Vapor Extrusion

Selected Case Studies

Camp LeJeune Military Reservation, Onslow County, North Carolina

Camp LeJeune is a United States Marine Corps military reservation located in Onslow County, North Carolina. The cleanup of contamination at the site is documented by the USEPA (1998) and is summarized here. From the time the camp was established in 1941 to the 1980s, a portion of the site, named Site 82, was used as a military disposal area. This disposal area is underlain primarily by fine- to medium-grained sands with traces of clay and silt. The ground water table is at a depth of approximately 18 feet below the ground surface. Throughout the site’s use as a disposal location, drums and debris were buried and left on the surface. The disposal resulted in contamination of both the groundwater and the soil with various VOCs. The main contaminants were Trichloroethene (TCE), Tetrachloroethene (PCE), and Benzene, with TCE being the most prevalent. Maximum concentrations of the contaminants were TCE: 1800 μg/kg, PCE 16.3 μg/kg, and benzene: μg/kg.

Owing to the soil conditions and nature of contamination at the site, soil vapor extraction was an appropriate remediation option. The system installed at the site consisted of eight vertical extraction wells, one horizontal injection well and 32 soil probe clusters. The extraction wells were installed to depths of approximately 15 to 16 feet. Upon extraction from the subsurface, vapors were passed through a vapor-liquid separator, after which the vapors were treated using activated carbon. The liquid effluent was transported to the groundwater treatment plant that was being used to treat the contaminated groundwater at the site. The system was operated from March 1995 to October 1995, with the startup and optimization process taking approximately 1 ½ weeks. Table 7 presents a breakdown of the costs involved with the project. The total cost of the project was nearly ½ million dollars and was ultimately a success. 

  

Figure 16a

Figure 16b

Table 7. Cost breakdown of SVE at Camp LeJeune (USEPA, 1998)

 

 The goals of the remediation project, in accordance with regulatory requirements, were to reach the following concentrations: TCE: 32.2 μg/kg, PCE: 10.5 μg/kg, and benzene: 5.4 μg/kg. All of these requirements were met and the SVE system was decommissioned. By the end of the project 17,500 yd3 of soil were successfully treated. One contributor to the success of the Camp LeJeune SVE operation was that it was performed in conjunction with an extensive ground water treatment operation. Much of the resources, as well as overhead, between these two projects could be shared. Camp LeJeune is an example of using traditional SVE as a cost-effective and relatively quick remediation technique. 

Former Rail Yard Locomotive Fueling Area, Upstate New York

A former locomotive fueling area (FLFA) had a locomotive operation that included rail car and engine repair and refueling of locomotives. This case study is thoroughly documented by Compston et al (2006) and is summarized here in order to illustrate a typical case of SVE in conjunction with other remediation technologies. Operation of the FLFA ceased in the 1980s and the refueling equipment removed from the site.  Petroleum products and light non-aqueous phase liquid (LNAPL) were found in the subsurface during the 1980s.  To remedy the affected soil and groundwater, a combination of air sparging (AS) and soil vapor extraction (SVE).  Air sparging will provide oxygen to the microorganisms capable of degrading the diesel fuel and volatilizing the petroleum compounds and soil vapor extraction will remove the volatilized diesel compounds from the subsurface.

The soil was characterized as fine-to-coarse sand and gravel with a layer of silt below at a depth of 16 to 18 feet below the ground surface (bgs).  The hydraulic conductivity of this soil was found to range from 10-3 cm/s to 10-1 cm/s and the effective porosity was 0.25.  In addition, the groundwater was located at a depth of between 8.5 and 13 feet bgs.  The average horizontal groundwater velocity was approximately 0.3 ft/day with a directional flow to the south towards the Susquehanna River located at a distance of approximately 2,500 feet away from the remediation site.

The amount of petroleum contaminated soils was found to cover an area of about 120,000 ft2, within the FLFA the contaminated soil went to a depth from 2.5 ft bgs to 14 ft bgs, and areas down gradient of the FLFA the soils were found to be limited by the smear zone.  The smear zone is the zone of vertical fluctuation of the water table, which ranges from 8.5 ft bgs to 13 ft bgs.  So the total volume of the petroleum contaminated soils ranges from 300,000 ft3 to 1,680,000 ft3

To treat the entire area, one treatment system was placed on the north side of the tracks and one on the south side.  Each of these plants included SVE and AS blowers, system controls, and activated carbon treatment for SVE off-gas, as well as eight SVE-AS wells.  On the south side of the tracks, there were also 20 biosparge (BS) wells that were installed to create a positive dissolved oxygen (DO) barrier from the migrating contaminants.  The extraction rates for SVE wells were 30 cfm, AS well were 10 cfm, whereas the extraction rates for the BS wells was only between 1-2 cfm.

VOC and SVOC analyses were performed semi-annually from two locations on the north system and two locations on the south system.  All four locations were soil samples taken from the area of influence of each respective area.  In October 2002 it was found that the concentration of VOCs found in the area of influence from the north system were approximately 22,000 μg/kg, and the south systems concentrations were 41,000 μg/kg and 23,000 μg/kg.  By June 2004, these concentrations had reduced to non-detectable concentrations on the north system, and non-detectable concentrations and 90 μg/kg, respectively, on the south system.  In addition, total SVOC concentrations on the north system had declined 99% and 14% while concentrations on the south system had declined 98% and 95%.  The lower percentage of decrease on the north system is due to a lower initial concentration of SVOC in that area.

For this specific remediation system, it was necessary to take groundwater samples from the wells within the area of influence in addition to down gradient of the treatment system.  Initially, the four wells contained groundwater where one or more of the regulatory VOC and SVOC compounds were in excess of the groundwater standards.  VOC concentrations decreased between 100% and 19%, where the average decline for the wells was 84%.  Furthermore, samples collected from the wells within the down gradient of the treatment area SVOC concentrations were found to have been reduced by 100 % and 52%, with the average reduction being 84%.  These decreases showed that after remediation of the site there were no VOCs or SVOCs in excess of the regulatory standards. 

During the course of the remediation process, the SVE system was monitored and extraction rates measured.  It was found that approximately 1,000 pounds of petroleum mass was removed over a 24 month period.  The highest percentage of the removal took place within the first seven months of operation (from May to November 2002), where the largest mass removal took place from July 2002 and September 2003.

One important process in this system was the injection of oxygen into the subsurface to accelerate the metabolism of the microbial population that are responsible for the breakdown of diesel fuel in the soil.  Between the months of May and October 2002, the bacterial population increased by two orders of magnitude in the saturated zone, three orders of magnitude in the unsaturated zone within the treatment area.  Down gradient of the treatment area, the unsaturated zone did not show a significant change in bacterial population.  Since then, the bacterial populations have declined, but that is probably due to the decrease in diesel fuel within the area.

The total project cost was estimated to be less than $350,000, which includes $235,000 being spent on the SVE-AS remedial program and $65,000 of that being spent on the treatment buildings.  If the remediation process was to be excavation and off-site disposal of the contaminated soil, the project cost would be estimated to be $3.7 million.  Therefore, choosing the SVE-AS system, significantly reduced the total project cost.  In addition, there was cost reduction due to the system being shut down during colder winter months.  This operational shut down was considered to be adequate given that microbial biodegradation is best in temperatures greater than 10°C. 

The remediation of the petroleum contaminated site was successful due to the use of air sparging and soil vapor extraction.  This success is largely due to the fact that there were multiple systems in place, including microbial degradation of the petroleum compounds, volatilization, and activated carbon capture.  To determine the overall success of the remediation process, continued monitoring of the site will take place to ensure that the regulated VOC and SVOC compounds have been removed to the necessary standards, or below.

 

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