Formatted Title
Groundwater Remedy Transition from Enhanced In Situ Biotreatment to Long-Term Monitoring following TCE Source Removal in Fractured Sedimentary Bedrock and Overburden: Lessons Learned
Background/Objectives
The site is underlain by bedrock consisting of the Passaic Formation of the Brunswick Group, which forms a steeply sloping bedrock ridge along the western edge of the site. The site operated as a sheet-fed printing business from 1969 to 1988. Wastewater generated from the business operations was discharged into a septic located on the site. The discharge led to trichloroethene (TCE) impacts at the overburden and shallow bedrock. Bedrock underlying the site is strictly sedimentary comprising of interbedded siltstone and sandstone. A pilot test was conducted in August 2012 to evaluate the anaerobic enhanced in situ biotreatment of TCE in the overburden and shallow bedrock. Based on the pilot test results, a full-scale injection was designed and completed in September 2017 to treat and eliminate the TCE source. Based on the first full-scale injections results, a second full-scale injection was designed and completed in January 2022 with a reformulation of the reagents to more effectively treat TCE degradation byproducts formed in the first full-scale injections and transition to long-term monitoring of residual impacts.
Approach/Activities
The in situ pilot test was conducted by injecting an organic carbon electron donor 3-D MicroemulsionTM (3DMeTM) to enhance TCE dechlorination. The injection parameters (such as bedrock capacity to accept volumes and radius of influence) were collected to design full-scale treatment. Up to three full-scale injections were projected to treat TCE source and to allow transition to long-term monitoring of the remaining plume. The first full-scale application injected EHC® Liquid electron donor to provide both organic carbon (ELS® Microemulsion) and organo-ferrous iron (EHC Liquid Mix) to promote both biotic and abiotic dechlorination. Based on the first full-scale application results, the second full-scale application was performed with micro zero-valent (mZVI) substituted for the organo-ferrous iron component. This modification provided more reducing reagent for the abiotic dechlorination of TCE daughter products cis-1,2-dichloroethene (cis-1,2-DCE) and vinyl chloride (VC). Dechlorinating bacteria Dehalococcoides sp. were also injected in the first and second full-scale applications.
Results/Lessons Learned
The pilot 3DMe™ injections enhanced native dechlorination in the shallow bedrock and overburden groundwater. This is based on the decrease in TCE concentrations and the steady increase in the TCE daughter products cis-1,2-DCE and VC. However, TCE concentrations rebounded two years after injection, likely from back-diffusion. The first full-scale injections with EHC® Liquid significantly reduced TCE concentrations and cis-1,2-DCE, VC, ethene, and ethane formation was measured in most bedrock and overburden monitoring wells, indicating that complete TCE dechlorination to innocuous by-products was occurring. TCE concentrations again rebounded two years after injection but to a substantially lower degree compared to the pilot test. The second full-scale application with ELS and mZVI reduced TCE groundwater concentrations to below regulatory standards with no rebound measured 1.5 years after injection. Groundwater cis-1,2-DCE concentrations were also reduced to below regulatory standard in some monitoring wells. The projected third full-scale application was not required, and regulators approved transition to long-term monitoring to address remaining cis-1,2-DCE and VC in groundwater.