Formatted Title

Case Study: Employing Soil Vapor Extraction in Unfavorable Soil Types

Background/Objectives

Former facility operations lead to releases of chlorinated solvents (primarily PCE) to subsurface soils and groundwater in the greater Seattle area. The soil stratigraphy across the site consists of an impermeable layer (Vashon Lodgment Till) with interbedded sand stringers that acted as a preferential pathway for chlorinated solvent migration. Thermal treatment was initially considered but was prohibitively expensive to implement. A pilot test was then conducted to assess the feasibility of dual phase extraction (DPE) and soil vapor extraction (SVE) technologies. SVE was selected as the remedial method as it generates less waste, was found to have comparable if not larger radius of influence (ROI) and incurs significantly less operational cost.

Approach/Activities

SVE was implemented in conjunction with air sparge (AS) to increase overall subsurface airflow and aid in removal of PCE from the groundwater. A mix of shallow (7 feet bgs) and deep (12 feet bgs) soil vapor extraction wells were installed to expand radius of influence (ROI) across the sand stringers and the presence of underground utilities. AS wells were installed to a maximum depth of 25 feet bgs to sparge the upper 10 feet of the water table and were installed between the SVE wells to maximize sparging in the SVE ROI.

A rotary lobe blower was selected as the vacuum to extract soil vapors based on favorable results from the pilot study and the cost-benefit when compared to using technologies with stronger vacuum capability, i.e., a liquid-ring vacuum pump. During operations, the vacuum applied to the SVE wells was balanced across the well network to preferentially from wells with higher volatile organic compound (VOC) concentrations.

Results/Lessons Learned

Despite the seemingly impermeable soil stratigraphy, an ROI of 30 feet was observed during operations and was consistent with those observed in the pilot study. After six months (1,800-pore volume exchanges) of continuous SVE and AS operations, a 97% decrease in PCE concentration was observed. In response, the AS system was set to cyclically operate (rather than continuous operation) and was implemented to allow groundwater levels to rise and fall through the vadose zone to increase PCE removal.

Twelve months after start of operations, the concentration of PCE became asymptotic and is consistent with pore volume exchange rates discussed in the US Army Corps Engineering Manual on SVE and Bioventing (EM 1110-1-4001). The system was further expanded to address downgradient groundwater concentrations and future results will be discussed as they are collected in the time leading up to the 2024 Battelle Chlorinated Conference. Lessons learned will be highlighted by discussion of:

• Detailed soil boring logs enabled consideration of SVE.
• Significant cost savings of SVE compared to that of thermal treatment, or DPE.
• Simplified permitting process compared to that of ISB or ISCO.