Formatted Title

Hydraulic Emplacement of Zero-Valent Iron Coupled with In Situ Bioremediation for VOC Treatment in a Low-Permeability Aquifer 

Background/Objectives

Historical releases of tetrachloroethene and trichloroethene at an active aircraft parts manufacturing facility in central Kansas produced two distinct dissolved volatile organic compound (VOC) groundwater plumes in a shallow, low-permeability overburden aquifer. The remediation approach for these plumes utilized a combination of chemical reduction with zero-valent iron (ZVI) and in situ bioremediation (ISB), implemented in April and May 2022, with four post-injection monitoring events over the following two years.

Approach/Activities

The remedy involved source treatment of both plumes with a two-step process: (1) hydraulic emplacement of ZVI to create stacked subsurface propagations of higher permeability relative to the low-permeability silty clay overburden soils, and (2) the installation of injection wells with engineered connections to these features through which ISB amendments were delivered. The use of these enhanced injection wells was critical to the efficient injection and thorough distribution of ISB amendments in a tight clay formation. The injection locations were bioaugmented with KB-1^® microbial culture during both stages of the remedy. Specifically, this study presents the results of groundwater monitoring one year after the completion of the ZVI injections and ISB treatment. We show how hydraulic emplacement of ZVI and ISB remediation methodologies can be successfully used to reduce source area chlorinated solvent concentrations in challenging low permeability settings.

Results/Lessons Learned

Groundwater monitoring results from this study demonstrate that the novel injection design resulted in higher injection efficiency and more predictable subsurface amendment distribution. Post-treatment groundwater sampling results have shown significant VOC dechlorination trends in source area wells within the target treatment area and immediately downgradient. Although one source well in the primary plume resulted in total VOC concentrations similar to pre-treatment conditions, the VOCs detected at this location were predominantly end-product VOCs (cis-1,2,-dichloroethene [DCE] and vinyl chloride [VC]) along with ethane, indicating progress toward complete degradation. Moving further downgradient, the impacts of the ZVI and ISB treatment have yet to reach a number of the wells in the core of the plume, likely due to relatively slow groundwater velocities at the site. At these locations, VOC concentrations and redox conditions are similar to pre-injection results. Overall, the plumes continue to decrease in overall mass and show strong evidence of continued treatment, as evident through multiple indicators including reducing redox conditions (dissolved oxygen <1 milligram per liter [mg/L], and negative ORP values) and the conversion of parent VOCs to non-toxic terminal end products. Additional groundwater performance monitoring results from two years post-injection will also be presented.