Passive Soil Vapor Extraction - GSI Environmental Inc.

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SRNL-STI-2009-00571Rev. 1abFigure 2.8: Schematic of Advective and Diffusive Flow of Soil Gas during a. Early andb. Late Stages of Soil Vapor Extraction.During a rebound test, the operating SVE system (barometric or MicroBlower) is shut-offto allow the different soil layers to equilibrate. Over time, the concentration of VOCs inthe more permeable layers should “rebound” at a rate that is dictated by diffusionprocesses within the low permeability soils. This maximum or equilibrium concentrationachieved in the extraction well during the rebound test can be used to qualitatively16 Enhanced Attenuation TechnologiesPassive Soil Vapor Extraction
SRNL-STI-2009-00571Rev. 1predict the residual mass of contaminants remaining in the source zone. For moredetails on how to conduct a rebound test, refer to the USACE SVE design guide (2002).Site example 1 (shown in grey box) describes a method used at the MAPSL site (Riha,2005c) to evaluate long-term performance effectiveness of a barometric strategy. Thissame strategy can be used to calculate performance objectives for a future PSVEstrategy as well.SITE EXAMPLE 1: COMPARISON OF MASS REMOVAL RATES TO MASS TRANSFERRATES IN THE TARGET ZONE AT THE MAPSL SITE, SRS, SCSite: In 2005, a rebound test was conducted using five inactive ASVE wells at the M-AreaAbandoned Process Sewer Line (MAPSL) at the Savannah River Site, SC. These wellswere screened in or very close to three fine-grained zones containing residual VOCs. Datacollection included periodic analysis of soil gas for PCE and TCE concentrations.In addition, two nearby inactive ASVE wells, not included in the rebound test, were fittedwith the Baroball TM assembly and measured for continuous flow rates and soil gasconcentrations.Performance Metric: In order to reduce the migration of contaminants to groundwater, aPSVE system should be capable of extracting VOC mass at a rate that equals or exceedsthe rate of mass transfer of contaminants from contaminated soils within the target zone tosoil gas phase.Data Analysis: The mass transfer rate of VOCs in the soils surrounding each well wasobtained by calculating the slope on a linear fit applied to the initial concentrationsmeasured at each well during the rebound test. These estimates were then scaled to obtainthe VOC mass loading to the entire source zone area.The average VOC mass removal rates were calculated using soil gas flow rates and VOCconcentrations in the soil gas extracted from the two inactive ASVE wells fitted with theBaroball TM assembly. These estimates were then scaled to obtain total VOC mass removalin the source zone area.Results:· PCE: Mass loading of PCE from the source area was calculated to be approximately 0.024kg- PCE/day (8.8 kg-PCE/yr) whereas the measured mass removal rates by barometricpumping was approximately 0.03 lbs-PCE/day (11.6 kg-PCE/yr)· TCE: Mass loading of TCE from the source area was approximately 0.022 kg-TCE/day(8.0 kg-TCE/yr). In contrast, the actual mass removal rates by barometric pumping wasmeasured to be approximately 0.027 lbs-TCE/day (9.95 kg-TCE/yr).Site Example 1 Conclusion: The PSVE technology was shown to be effectively removingmass from the source zone at rates greater than the rates of dissolution or diffusion andtherefore, the technology was considered protective of the groundwater at the Site.17 Enhanced Attenuation TechnologiesPassive Soil Vapor Extraction
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