CA001 – Pneumatic Fracturing, SVE
Industrial Facility – Santa Clara, California
A common misconception about using Pneumatic Fracturing is that extensive, thick low permeability zones or lenses are required to justify the use of the process. However, the majority of sites where Pneumatic Fracturing was applied involves subsurface geology containing both high and low permeability zones.
Large industrial facility in Santa Clara, California
A semi-permeable layer of sandy silt and silty clay overlying a thick silty clay with very low permeability.
Primarily trichloroethylene (TCE) with other chlorinated solvents.
Fracture Depth Range
3.5-13.5 ft. below ground surface
Subsequent to fracturing, the rate of air flow increased by 3.5 times based on results of SVE extraction tests utilizing the entire borehole. The increase in permeability during the interval tests proved more dramatic, where it increased by 510 times in the low permeability clay zones.
The rate of TCE mass removal increased by 600% during extraction tests conducted on the entire borehole. The greatest increases in the rate of mass removal were found in clay zones, where the contaminants were removed at a rate up to 46,000 times greater than the pre-fracture conditions. Figure 1 summarizes the contaminant mass removal data for the interval extraction tests on a logarithmic scale.
Pre-fracture extraction tests indicated that the upper zones of the formation were somewhat permeable, while the clay zones beginning at a depth of 9.5 ft. displayed very low permeability, as shown in Figure 2. Application of Pneumatic Fracturing was able to increase the rate at which air could be extracted from both zones of the formation, with the most dramatic effects being observed in the clay zones. This resulted in a reduction in geologic heterogeneity’s or “uniformizing” the subsurface.
In 1996, based upon the pilot test data, a full-scale system was installed at the facility. The system consisted of 20 dual phase, single pump extraction wells to remove VOCs from silty clay soils and shallow groundwater at the site. Pneumatic Fracturing was utilized to enhance air flow through the subsurface soils. More than 40% of the VOC mass was removed from the vadose zone during the first month of operation. The system was operated for approximately 2 years.
Groundwater extraction provided greater mass removal rates than soil vapor extraction by the fifth month of operation. The use of Pneumatic Fracturing allowed soil vapor extraction to be effective in an area that is not otherwise well suited for in situ remediation.
The DPE system removed approximately 1,220 lbs. of VOC’s from the source area.
- VOC mass removed by soil vapor extraction > 782 lbs.
- Average source area VOC concentration in groundwater declined from over 12,000 ug/L to less than 800 ug/L
- During first month of operation, about 40% of the mass of VOCs removed was from the vadose zone
- By the fifth month, groundwater extraction was removing more VOC mass than SVE
- DPE system was shut down June through August 1998 to assess rebound
- VOC concentrations remained relatively constant during shut down and after restart
- 27 confirmed soil samples averaged 0.93 mg/L total VOCs.