Formatted Title
Lessons Learned from a Major Southern California Superfund Site
Background/Objectives
The Site is a former dichloro-diphenyl-trichloroethane (DDT) manufacturing facility in southern California which has more than 3 acres impacted by dense non-aqueous phase liquid (DNAPL), a portion of which is mobile (present at high enough saturation where the DNAPL can migrate under gravitational forces). The DNAPL is typically composed of 50% chlorobenzene and 50% total DDT by weight. The full-scale DNAPL remedy (targeting mobile DNAPL) combines thermal and soil vapor extraction (SVE) to render the DNAPL immobile, by removal of chlorobenzene, in the focused treatment area (FTA) which is approximately 29,000 square feet.
Approach/Activities
To evaluate the remedial strategy, a SVE and a thermal remediation pilot test utilizing electrical resistance heating (ERH) were conducted in unsaturated and saturated soils, respectively. Another aspect of the SVE pilot test was to evaluate the effectiveness of a passive vapor barrier wall (utilizing wells along the southern limits of the FTA which were open to the atmosphere to allow for the entrainment of ambient air, creating a vapor barrier) in preventing the migration of off-site contaminants during SVE operations. The pilot studies removed approximately 114,000 pounds of VOCs and the vapor barrier wall continues to successfully operate during ongoing SVE operations. Throughout these studies, the data density increased exponentially, leading to changes in the conceptual model and a more thorough understanding of the Site.
Results/Lessons Learned
Lessons learned through pilot testing affected the full-scale design and approach from design to implementation approach. Pilot testing allowed for confirmation of conceptual design and many aspects of the design were unchanged for full-scale implementation such as electrode design, contingent electrode wetting system, utilization of a steam-regenerated granular activated carbon (SRGAC) unit for vapor treatment, and liquid treatment and disposal to on-site groundwater treatment system. Materials of construction were confirmed compatible and removal characteristics of DDT and additional site compounds such as para-chlorobenzene sulfonic acid (pCBSA) were evaluated. Based on data collected during pilot testing, supplemental vapor recovery points were added surrounding the treatment volume, additional DNAPL storage was integrated into the full-scale design, and a two-phase approach to ERH operation is planned (splitting the treatment volume in two sections). The importance of mass removal from the vadose zone prior to heating was highlighted (to avoid overloading the treatment system during ERH operations) and SVE has been implemented prior to ERH. Performance of the SRGAC unit was analyzed and a second SRGAC unit was designed and installed for full-scale ERH implementation. Flexibility to seamlessly switch between multiple vapor extraction locations and the duplicate vapor treatment systems was designed to allow for maximum mass recovery. Feasibility studies suggested a “bottom-up” heating approach, which pilot test data indicated was not necessary and will not be implemented in full-scale implementation. Over 339,000 pounds total VOC have been removed by the pilot studies and ongoing SVE to date. Operation of the SRGAC in lieu of traditional carbon use and disposal methods have saved over 3 million pounds of carbon from disposal to date. Construction of the full-scale ERH system is expected to be complete in 2023, which includes 240 subsurface installations (approximately 4.5 miles of drilling completed) and phased thermal remediation is anticipated to be completed in 2024. SVE operations are expected to continue into 2025. The value of in situ thermal remediation pilot programs, key lessons learned from the Site’s pilot programs, and preliminary results from the operation of the full-scale systems will be presented at the conference.