Formatted Title

Combined Soil Vapor Extraction and Enhanced In Situ Bioremediation to Reduce Mass Flux of Chlorinated Volatile Organic Compounds to Groundwater beneath a Highway and Residential Neighborhood

Background/Objectives

Leaking underground storage tanks (USTs) associated with a former industrial laundry and linen facility created a nearly half-mile long chlorinated volatile organic compound (CVOC) groundwater plume extending downgradient beneath an inaccessible highway (US 50) and residential neighborhood. Following extensive site investigations and bench- and pilot-scale testing, a combined enhanced in situ bioremediation (EISB) and soil vapor extraction (SVE) approach was selected to reduce CVOC concentrations in onsite soil and groundwater (i.e., source zone) and reduce the downgradient mass flux to offsite groundwater. Full-scale EISB injections were completed in early 2022 and SVE startup was completed in December 2022.

Approach/Activities

The full-scale EISB remedy targeted CVOC impacts in shallow groundwater (from approximately 30 to 40 feet below ground surface [ft bgs]) using four permeable reactive zones (PRZs) installed perpendicular to groundwater flow. Three PRZs were installed onsite to reduce source zone mass flux while a fourth offsite PRZ was installed downgradient of US 50 along a residential street to reduce mass flux from the upgradient plume. PRZs were constructed by injecting electron donor solution, zero valent iron, and Dehalococcoides culture through 47 temporary injection points installed via direct push technology (DPT) drilling techniques.

The SVE system was installed to remove soil vapors from the onsite source area via eight shallow SVE wells (screened from 12 and 21 ft bgs) and one deep well (screened from 25 to 30 ft bgs). Extracted soil vapors are treated with granular activated carbon and discharged to the atmosphere.

Results/Lessons Learned

Post-injection groundwater monitoring results indicated reducing conditions and complete CVOC dechlorination to ethene was achieved within onsite PRZs. CVOC groundwater concentrations within onsite PRZs decreased to below maximum contaminant levels (MCLs) three months after injection and remained below MCLs 12 months after injection. Reducing conditions and decreasing CVOC concentrations were initially measured in offsite groundwater at PRZ-4 six months after injection, however concentrations rebounded approximately 12 months after injection. Drilling refusal and high injection pressures encountered at PRZ-4 required a shift to sonic pre-drilling of all injection points. Post-injection geochemical groundwater results suggested ESIB reagent distribution was reduced and inconsistent relative to onsite PRZs. Groundwater performance monitoring is ongoing and additional offsite injections and PRZs may be installed to further reduce mass flux from the downgradient plume.

The SVE system has removed approximately 120 pounds of CVOCs in 10 months of operation.  System optimization is currently underway.  Approximately one year of continued SVE operation is anticipated.