Highvacuum Soil Vapor Extraction System
In September 1994, the NMSHTD retained a second consultant, Daniel B. Stephens & Associates, Inc. (DBS&A) to design and implement a corrective action system at the Artesia Patrol Yard. Sections 3.1 and 3.2 outline DBS&A's rationale for and design of the high-vacuum SVE system for the Artesia Yard. The techniques used to enhance the recovery of petroleum vapors from the vadose zone are described in Sections 3.3 and 3.4. These techniques extended the operation of the high-vacuum SVE system and allowed a more effective cleanup of the vadose zone in preparation for the current phase of bioventing.
3.1 Evaluation for High-Vacuum Soil Vapor Extraction
Although the Artesia Yard was not an ideal site for the use of SVE technology, it was the most reasonable alternative, given the depth of contamination and the estimated volume of product that had been released. Excavation to the contamination depth of 55 or 60 feet bgs was not attractive from an operational or economic standpoint. Direct recovery of PSH was a very slow process (hand bailing was tried for a period of time, with limited success), especially for the conservatively estimated 50,000 gallons of gasoline that had been released over the years at this site.
Accordingly, in preparation for designing the corrective action system, DBS&A carefully evaluated the existing SVE pilot test data. If the design parameters suggested by this test had been used, an inordinately large number of SVE wells (more than 100) would have been required for the site. Therefore, DBS&A elected to conduct a high-vacuum (75 to 200 inches wc) SVE pilot test prior to developing a design for the corrective action system.
DBS&A also defined the southern extent of perched groundwater with the installation of a dry borehole south of the site. Field measurements in the existing monitor wells showed that an extensive layer (up to 4 feet thick in the well bores) of PSH remained on the perched water table (Fig. 5).
AcuVac Remediation Inc. of Houston, Texas (AcuVac) conducted the high-vacuum (up to 205 inches wc) pilot tests. These tests yielded well flows of 12 to 18 cfm and extracted vapor concentrations in the tens of thousands of ppmv. The ROI was calculated using a rearranged equation from Johnson et al. (1990, Eq. 22) which is based on the vacuums measured at the extraction and the observation wells and the distance between them. The ROI can also be approximated using data from all observations wells. In this case, at a given imposed vacuum (and flow rate), the vacuum measurements (inches wc) in observation wells were plotted versus distance from the extraction well. The distance at which the imposed vacuum fell to between 0.25 and 0.30 inch wc was considered to be the ROI. This distance is larger for the higher vacuums used in these tests and resulted in an effective ROI in excess of 25 feet. These results suggested that a high-vacuum SVE system could successfully remove volatile contaminants from the fine-grained soils at the site. Using a conservative (but still larger than the original 8.6 feet) ROI of 20 feet, DBS&A prepared a design with 21 SVE wells to address the contamination at the site.
The practicality of using very high vacuums in the field is limited by the upwelling that can occur and eventually close off the screened interval that spans the contaminated soil. In addition, while a higher vacuum generally increases the ROI, it also increases the chances for short-circuiting to the surface, especially as the top of the SVE screen gets closer to the ground surface.
Continue reading here: High Vacuum SVE System Design and Installation
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