An Analysis of Passive Soil Vapor Extraction Wells

WSRC-TR-2000-00182
MetLab Treatability Study: June 2000 Update
Executive Summary
The passive soil vapor extraction (PSVE) system at the MetLab of the Savannah River
Site has been operating since May 1998. The results to date on the treatability study
indicate the technology is performing well. Well concentrations are decreasing and
contour maps of the vadose zone soil gas plume show a decrease in the extent of the
plume. In the 2 years of operation approximately 270 pounds of chlorinated organic
contaminants have been removed by natural barometric pumping of wells fitted with
BaroBall valves (low pressure check valves).
Additional characterization was completed in FY00 to evaluate the concentration of the
soil gas plume on the south and west sides of the well field. One well was installed and
two soil gas pushes were completed. The maximum soil gas concentrations observed
from both CPT soil gas pushes were 3.0 ppmv PCE, 3.5 ppmv TCE, and 4.5 ppmv
Freon 113. Additional wells were not installed due to the low concentrations. Based on
the new characterization data and the monitoring results from the existing wells, the
current well system adequately bounds the soil gas plume.
At the current fitted removal rates all but 5 of the wells will be below 5 ppmv by the end
of 2001. Projecting the concentration trends, all the wells will be below 5 ppmv by
1/2005. The initial source mass estimate is 170 lbs TCE and 250 lbs PCE. These
masses correspond to approximately 14 gallons of TCE and 18 gallons of PCE that will
be removed from the subsurface at the site.
Mass removal during this time frame is primarily attributed to residual contamination in
the coarse grained material and contaminant mass transfer from the finer grained
zones to coarse zones where the majority of the PSVE flow is achieved. Removal from
the fine grained sediments will be limited by the mass transfer from these zones to the
coarse grained zones.
The PSVE system has required minimal operating and maintenance costs and can be
expected to operate continuously for the life span of the remediation with little or no
intervention. All of the vadose zone extraction wells installed can also be used with an
active vapor extraction system.
The PSVE system is performing well in a cost-effective manner. It is recommended that
this system be allowed to continue operating to complete the remediation and to
continue monitoring activities to verify and monitor the anticipated contaminant removal
rates. The treatability study should be considered successfully completed and the
remediation should be considered in full operation.
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