VAPOR EXTRACTION/VACUUM-ENHANCED GROUNDWATER RECOVERY:
A HIGH-VACUUM APPROACH
Sami A. Fam, Ph.D., P.E., L.S.P, Innovative Engineering Solutions, Inc., Norwood, MA 02062
Releases of organic solvents/petroleum products are a common cause of groundwater and unsaturated soil contamination. When these constituents are released to the subsurface, they can partition into four distinct yet interrelated phases; non-aqueous phase liquids (NAPLs) adsorbed to saturated or unsaturated soil particles, free-phase NAPLs, soluble constituents dissolved in the groundwater, and volatile constituents in the soil pore space of the vadose zone. The four phases of contamination are intimately related by equilibrium partitioning. Dynamic equilibrium of contaminant partitioning and mass balance approaches to contaminant quantification and treatment should be the cornerstone of remedial investigations and site remediation processes. The optimal plan for remediation will consist of an integrated strategy that addresses all four phases of contamination. The selected site remediation must not only address each constituent phase, but address each in the most efficient manner with respect to the rate of mass removal.
The combination of vapor extraction and groundwater pump-and-treat remediation has been successfully practiced as a remedial technique in medium and high permeability geologic formations. The success of this technique can be severely restricted in low permeability formations due to the diminished air and groundwater flows that can be achieved by standard remedial recovery/equipment. An example of these formation types are the silty-clay till formations of central Indiana, which will not allow air or groundwater flow at rates acceptable for rapid site restoration and can further hinder mass removal of constituents due to the presence of highly adsorptive silts and clays.
In order to overcome the air and groundwater flow restrictions of these low permeability formations, high vacuums created by liquid-ring pumps can be utilized. The application of these high vacuums (up to 25 inches of mercury as opposed to 3 to 6 inches of mercury for conventional vacuum blowers) creates a much greater driving force for air flow in the vadose zone and will also increase the rate of groundwater recovery. This increased air and groundwater flow will allow for greater constituent mass removal rates thus allowing for more rapid site closure.
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