Soil Vapor Extraction and Groundwater Extraction, Used at Eight Service Stations in Maryland

Site Name:

Eight Service Stations

Location:

Maryland
(specific locations not provided)

Period of
Operation:

1990 – 1997
Specific period of operation varies by site

Cleanup
Type:

Full scale

Technology:
Soil Vapor Extraction and Groundwater Extraction (VE/GE)
- Remediation systems combined vapor extraction and groundwater extraction to treat smear-zone soils through dewatering and volatilization of contaminants
- SVE systems - number of vacuum extraction wells ranged from four to 17 at each site, with mean vacuums ranging from 22 to 41 inches of water, and air flow rates ranging from 4.6 to 18.5 scfm
- Groundwater extraction systems - number of groundwater extraction wells ranged from two to eight at each site, with the average pumping rate ranging from 0.11 to 0.67 gallons per minute

Cleanup Authority:
Not identified

Site Contact:
Tom Peargin
Chevron Research and Technology Company
100 Chevron Way
Richmond, CA 94802-0627
Telephone: (510) 242-5927
Fax: (510) 242-1380
E-mail: TRPE@chevron.com

Contaminants:
MTBE, BTEX
- Average MTBE concentration - 6,139 ug/L
- Average BTEX concentration - 5,511 ug/L

Waste Source:
Leaks from gasoline storage tanks

Type/Quantity of Media Treated:
Soil and Groundwater
- Four of the sites are located in the Coastal Plain Province and are characterized by alluvial sands, silts, and clay
- Four sites are located in the Piedmont Province and are characterized by saprolites (weathered bedrock) of schist and gneiss
- Hydraulic conductivities ranged from 3.4x 10-3 cm/sec to 7.1x 10-5 cm/sec.
- Pre-remediation depth to groundwater was <10 ft at one site, between 5-15 ft at a second site, and greater than 20 ft for the other 6 sites; water table varies from 2 to 13 ft
- Smear zone (located within or below the capillary fringe) is submerged most of the year; falling water table conditions (late summer and early fall) may increase NAPL saturation at the capillary fringe as a result of drainage from the newly formed vadose zone (lateral migration of NAPL)

Purpose/Significance of Application:
Use of Soil Vapor Extraction and Groundwater Extraction to treat soil and groundwater contaminated with MTBE and BTEX

Regulatory Requirements/Cleanup Goals:
According to Chevron, the objective of the VE/GE systems was smear-zone dewatering and remediation through volatilization. No specific cleanup goals were established for any of the sites.

Results:
Average MTBE concentrations were reduced from 6,139 ug/L to 791 ug/L; BTEX from 5,511 ug/L to 1,088 ug/L; and xylene from 8,539 ug/L to 2,859 ug/L
- VE system removed approximately 1,300 kg to 95,000 kg of contaminants per site; GE system removed approximately 12 kg to 3,200 kg of contaminants per site
- For those sites where system shutdown had occurred, the average post-reduction rebound was –2.87% for MTBE, -9.12% for benzene, and 22.63% for xylene; while there was variation in rebound by individual wells at each site and among sites, on average, MTBE and BTEX concentrations did not generally rebound at sites after shutdown, while on average, there was rebound in xylene concentrations.

Cost Factors:
No cost data were provided for these sites

Description:
At eight retail service stations in Maryland, Chevron Research and Technology Company used soil vapor extraction and groundwater extraction (VE/GE) to treat soil and groundwater contaminated with both MTBE and BTEX. A gasoline release had occurred at each site and had migrated to the water table and formed a smear zone (a volume of soil in the saturated aquifer that contains residual NAPL). The remediation systems combined vapor extraction and groundwater extraction to treat smear-zone soils through dewatering and volatilization of contaminants. The vacuum extraction systems at each site included vacuum extraction wells (ranging from 4 to 17 wells per site), one to two regenerative vacuum blowers used to generate an average air flow rate ranging from 4.6 to 18.5 scfm and a mean system vacuum ranging from 22 to 41 inches of water. The groundwater extraction system at each site included extraction wells (ranging from 2 to 8 wells per site) equipped with pneumatic total fluids pumps. The average pumping rate (water yield) ranged from 0.11 gpm to 0.67 gpm. The sites had an average remediation period of 3.2 years and an average monitoring period of 4.7 years.

The VE and GE systems removed between 1,330 and 95,000 kg and 12 to 3,200 kg of contaminants per site, respectively. Average contaminant concentrations at the sites were reduced to 791 ug/L for MTBE, 1,088 ug/L for BTEX, and 2,859 ug/L for xylene. For sites where the system had been shutdown, MTBE and BTEX concentrations did not generally rebound, whereas rebound was observed for xylene. According to the researchers, MTBE removal rates were found to be similar to those for BTEX, which was inconsistent with their assumptions of local chemical equilibrium for NAPL-liquid and NAPL-vapor partitioning. The researchers predicted removal rates for MTBE, based on several assumptions about local chemical equilibrium, were four times faster than benzene and 60 times faster than xylene. According to the researchers, while the reasons for this discrepancy were not known, it is likely that conventional groundwater remediation technologies will encounter similar mass removal limitations for MTBE and BTEX in all but ideal hydrogeologic settings.