Theme: 01. Remediation Technology Innovations

Topic Area: 01a. Abiotic and In Situ Biogeochemical Processes: Applications and Lessons Learned

Formatted Title:

In Situ Biological and Chemical Reduction of Chlorinated Compounds in a Complex Setting

Background/Objectives:

Background/Objectives. The Superfund site is located in western Harris County, Texas, just outside the city limits of Houston. The site is a former dry cleaner facility located in a small shopping center in an area of mixed commercial and residential uses. The site contaminants of concern (COCs) include tetrachloroethene (PCE), trichloroethene (TCE), and the daughter compounds (i.e., 1,2-dichloroethene [DCE] and vinyl chloride [VC]). Release of chlorinated solvents due to previous improper practices has contaminated subsurface soil and groundwater, causing more than 100 private water wells in the neighborhood to become impacted and therefore plugged and abandoned. The COC mass is primarily concentrated in the area immediately under the former dry cleaner facility. Vertical impact expands from the shallow soil (0-25 feet [ft] below ground surface [bgs]), shallow water-bearing zone (WBZ) (25 to 35 ft bgs), to an interval of the unsaturated/dewatered Chicot aquifer (35 to 110 ft bgs) overlaying a deep WBZ in the Chicot aquifer. The COCs underneath the shopping center have posed significant indoor air risks to the businesses in the area. To address the immediate risks and reduce the contaminant source, a sub-slab depressurization system was installed to reduce the indoor air COCs, and soil vapor extraction (SVE) systems were installed in the shallow and deep unsaturated soils using horizontal wells under the shopping center. The shallow WBZ requires a treatment to reduce its contribution to the contamination of the deep unsaturated soil and the deep WBZ. In situ chemical reduction (ISCR) and bioremediation (ISB) was selected to treat the shallow WBZ because of its compatibility with the other technologies used at the site, and less interference with the area businesses.

Approach/Activities:

Approach/Activities. An initial full-scale ISCR/ISB was conducted in January 2016. Direct push technology (DPT) was used to inject EHC-L^® with pH buffer and inoculum dehalococcoides (DHC) in a grid system with a spacing of approximately 15 ft in the source area. EHC-L^® is composed primarily of a controlled-release organic carbon substrate and an organo-ferrous compound to promote both biotic and abiotic dechlorination reactions. The injection targeted the shallow WBZ in depths between 27 ft to 54 ft depending on vertical position of the impacted sand bodies. A total of 6,489 gallons of EHC-L^®, 3,344 pounds of EHC-L^® Dry Mix, 4,798 pounds of potassium bicarbonate, 108 liters of DHC, and 40,948 gallons of water were injected into 63 locations. A hot spot injection was conducted in March 2018 to treat rebounds of the COCs in two areas. A total of 825 gallons of EHC-L^®, 375 pounds of EHC‑L^® Dry Mix, 550 pounds of potassium bicarbonate, and 3,600 gallons of water were injected into 10 DPT locations. In September 2023, another hot spot injection was conducted. A total of 850 gallons of EHC-L^®, 419 pounds of EHC-L^® Dry Mix, 747 pounds of potassium bicarbonate, 23 liters of DHC, and 7,700 gallons of water were injected into 17 DPT locations. During the injection event, the shallow SVE system was shut down to prevent accidental extraction of the injected amendments through the SVE system.

Results/Lessons Learned:

Results/Lessons Learned. Groundwater samples were collected prior to and quarterly after injection events. Concentrations of PCE, TCE, and cis-1,2-DCE have decreased in a range from 78% to 99% since the initial injection 7 years ago. Significant PCE and TCE reduction was observed in some wells with the increase in concentrations of cis-1,2-DCE, VC, total organic carbon, arsenic, and iron and the decrease in nitrate, which indicated anaerobic microbial activities were stimulated by the amendments injected. Minimal or no accumulation of biodegradation intermediates was observed in some other wells with increased concentrations of ethene, ethane and methane, which reflected in part abiotic destruction of the COCs. Biological and abiotic processes may present a strong interrelationship at the site. The combination of the two processes has promoted long lasting reduction of PCE and TCE. This presentation will discuss future activities to better evaluate and understand abiotic and biotic degradations.

Beth Liu

Affiliation

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(Group 2, Poster Board #191) In Situ Biological and Chemical Reduction of Chlorinated Compounds in a Complex Setting