Passive Soil Vapor Extraction - GSI Environmental Inc.

SRNL-STI-2009-00571Rev. 12.5 Site Transition to MNAThe ultimate goal for any PSVE site is to eventually transition to MNA and/or siteclosure. Determining specific criteria that can predict when this transition can be madeis essential and should be identified early-on in the site decision-making process by sitestakeholders. Depending on site conditions and identified remediation drivers, a sitecould have requirements other than generic MCLs to meet the “no further action” level.This allows for development of innovative site-specific metrics that can be used tosupport this transition. See Site Example 2 (grey box) for description of one of the sitetransition goals developed for the MAPSL site at the SRS, SC (Riha and Whiteside,2008).SITE EXAMPLE 2: SITE TRANSITION CRITERIAAT THE MAPSL SITE, SAVANNAH, SCAt the MAPSL site, a risk-based site specific target level (SSTL) was proposed as thetransition point from a PSVE to an MNA strategy. Simple partitioning equations were usedto determine the maximum soil vapor concentration of each contaminant that could exist inequilibrium with impacted pore water in a source zone without posing any additional risk togroundwater due to leaching.First, a pore water concentration protective of the groundwater was calculated by dividingthe lowest concentration measured in on-site wells by a safety factor of 10. At the site, thelowest concentration of PCE and TCE measured was 0.6 mg-PCE/L and 1.6 mg-TCE/L.Based on a safety factor of 10, the pore water concentration considered protective of thegroundwater was assumed to be 0.06 mg-PCE/L and 0.16 mg-TCE/L.Next, the soil vapor concentration in equilibrium with the pore water was calculated underequilibrium conditions using Henry’s Law. This soil vapor concentration was considered tobe protective of the groundwater. At the MAPSL site, the soil vapor SSTL for PCE andTCE was calculated to be 4.7 ppmv-PCE and 9.3 ppmv-TCE and was therefore, proposedas a potential end-point for the PSVE strategy.18 Enhanced Attenuation TechnologiesPassive Soil Vapor Extraction