Theme: 01. Remediation Technology Innovations

Topic Area: 01m. In Situ Technologies: Lessons Learned

Formatted Title:

Large-Scale, In Situ Reductive Dechlorination of Groundwater Impacted with Commingled TCE and Hexavalent Chromium in Israel

Background/Objectives:

The site covers approximately 6 acres and was used for industrial purposes from 1949 to 1998. Subsurface investigations conducted between 2002 and 2008 identified the presence of chlorinated volatile organic compounds (cVOCs) in groundwater including trichloroethene (TCE) concentrations as high as 97,300 micrograms per liter (µg/L), and tetrachloroethene (PCE) concentrations as high as 5,380 µg/L. Groundwater level fluctuations of approximately 3 to 4 meters that occurred during the last decade have led to substantial migration of cVOCs in the vicinity of the site and to a significant decrease in cVOC concentrations within the site. Hexavalent chromium has also been detected, at concentrations as high as 59,100 µg/L. This impacted groundwater covered an area of approximately 1.5 acres to a depth of approximately 100 feet below ground surface (bgs) and is present within calcite-cemented sandstone interbedded with marine and continental silty clay lenses. Remedial objectives include groundwater protection and minimizing human health risks to future receptors due to ingestion of groundwater and inhalation of soil vapor, in order to facilitate redevelopment of the property. Substantial heterogeneities within the treatment zone posed significant challenges to effective distribution of injected amendments.

Approach/Activities:

Groundwater remediation using enhanced in situ anaerobic bioremediation (ISB) paired with in situ chemical reduction (ISCR) was implemented in early 2023 to address the cVOC and hexavalent chromium at the contamination sources in the shallow portion of the aquifer within the site. The full-scale injection design was based on estimated hydrogen equivalents for complete electron acceptor consumption. A total of 42,500 pounds of organic carbon substrate and 660 pounds of a soluble iron substrate was then injected through 57 injection wells that are screened from approximately 55 to 80 feet bgs in March/April 2023. A total of 280,000 gallons of fluid was injected, targeting approximately 75% of the available aquifer pore space. Once onset of anaerobic conditions was confirmed, 171 liters of dechlorinating culture were injected through the 57 injection wells in May 2023.

Results/Lessons Learned:

The results of post-injection groundwater monitoring revealed effective electron donor distribution based on total organic carbon concentrations, establishment of reducing conditions based on nitrate, ferrous iron, sulfate, and methane data, and substantial reductive dechlorination based on evaluation of cVOC molar concentrations and molar fractions. The following reductions in arithmetic mean TCE and hexavalent chromium concentrations have been observed in the wells, 3 months post-injection:

TCE decreased by 84%, from 1,610 µg/L to 251 µg/L;
Hexavalent chromium decreased by 98%, from 4,030 µg/L to 82 µg/L.

Based on these encouraging findings, the owner of the site and the water authority have requested preparation of an injection design to implement ISB/ISCR to treat hexavalent chromium and cVOC‑impacted groundwater within the deep portion of the aquifer of interest (to depths of approximately 100 feet bgs). This presentation will provide discussions concerning the basis of the shallow groundwater treatment injection design, interpretation of and lessons learned from forthcoming (late 2023 through early 2024) post-injection groundwater data, and rationales regarding proposed deep groundwater treatment injection approaches and next steps.

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J. Mark Nielsen