Theme: 05. Fractured Rock and Complex Geology

Topic Area: 5c. Remediation Approaches in Fractured Rock and Karst Aquifers

Formatted Title:

Bedrock Vapor Extraction to Remove TCE: Santa Susana Field Laboratory, California

Background/Objectives:

NASA operated a rocket testing facility from 1948 to 2006 at the Santa Susana Field Laboratory (SSFL). Facility operations involved cleaning engines (over a short period in the early 1960s), which resulted in the release of trichloroethylene (TCE) to drainages at the site which eventually migrated to groundwater. A TCE recycling program was installed in 1961. In 2007 and 2010, NASA and other responsible parties signed a Consent Order for Corrective Action and Administrative Order of Consent with DTSC to address cleanup of soils and groundwater at SSFL. A groundwater RCRA facility investigation has been completed at SSFL and remedial technology pilot tests are being performed to support Corrective Measures Studies and Corrective Measures Design and Implementation, including Bedrock Vapor Extraction (BVE). The objective of a SSFL BVE pilot test is to explore the effectiveness of vapor extraction to remove TCE from fractured sandstone (the Chatsworth Formation sandstone is lightly to moderately faulted and fractured; depth to groundwater typically occurs 200 to 300 feet below ground surface). The pilot study will also help develop best practices for the potential application and rotation of BVE (as a mobile, solar-powered system) at several other locations across NASA SSFL.

Approach/Activities:

The BVE pilot study was conducted at one of the highest TCE concentration source areas in the NASA-administered area of SSFL, Alfa Test Stand 1. A corehole was advanced from 65 to 250 feet bgs, approximately 25 feet above the water table as the BVE well. An optical geophysical survey was conducted, revealing only one minor fracture, and a substantial coarse-sandstone bedrock matrix. This enabled the evaluation of bedrock matrix flow development over time. A 70-kW solar array with lithium iron phosphate (LFP) batteries was used to power a 15 HP positive displacement blower; vapor-phase granular activated carbon was used to treat emissions prior to permitted discharge. Multi-level vapor monitoring points and vapor piezometers were installed with transducers to track vacuum and sample ports to monitor vapor concentration responses. To preserve the viewshed from a candidate historic district near the well, a 500-foot manifold was installed to carry the BVE well vapors to the blower system behind a hill (where solar panels were located as well). After about 9 months of daytime-only operations (because of battery delivery delays), a nearby second well in a highly fractured zone was then connected and the system was operated continuously. This second well was connected to compare flow and the decline of extraction concentration to the initial bedrock matrix dominated well.

Results/Lessons Learned:

A sustainably powered, mobile BVE system was demonstrated as functional and reliable for TCE removal in fractured sandstone. Vacuum influence was seen by concentration changes as far as 130 feet away; compared to baseline, post-rebound concentrations in remote ports typically were reduced at least 95%. A total of over 1,000 pounds of TCE was removed in the first year of BVE operation. The original bedrock matrix well appeared to show higher pneumatic conductivity after 4 to 6 months of operation, believed due to the removal of moisture from the pore channels of the rock matrix; this well was able to produce 100 scfm at 10 inches Hg after initially not exceeding 50 scfm. The original bedrock matrix well quickly reached inlet concentrations of >15,000 ppm (97.3% TCE, 2.7% cis-1,2-DCE) which persisted for at least 8 months. The extraction from the fracture-dominated second well will provide further insight on the potential rate of vadose zone mass removal from the entire site. Information from this extraction study will be the foundation of forecasting time of BVE operation and assessment of contaminant rebound at NASA SSFL sites.

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Pete Zorba