Formatted Title
Adaptive Injection Strategies for Zero-Valent Iron and In Situ Bioremediation in a Complex Geologic Setting: Lessons Learned and New Techniques
Background/Objectives
Remedial action with an adaptive approach was performed at a confidential site impacted with trichloroethene (TCE) and related volatile organic compounds (VOCs) in a complex geology comprised of quaternary sediment overlying fractured bedrock. Site-specific remediation goals (RGs) were to reduce concentrations of chemicals of concern (COCs) in two plumes using a combination of in situ chemical reduction (ISCR) treatment and in situ bioremediation (ISB) treatment.
Approach/Activities
The remedial design consisted of high and low doses of zero-valent iron (ZVI) suspended in bean guar gel with a loading of up to 7.2 pounds of ZVI per gallon of slurry with a total target ZVI mass per borehole based on the COC concentrations. Additionally, low-dose ZVI locations were augmented with a supplemental ISB injection consisting of Dehalococcoides (Dhc) culture suspended in anoxic water. Consecutive amendment slurries were injected top-down into shallow (5-15 feet below ground surface [bgs]) unconsolidated sediment via direct push technology (DPT) and bottom-up injections into deeper (15-30 feet bgs) fractured bedrock using pre-drilled boreholes and a straddle-packer assembly.
In anticipation of surfacing amendment, an adaptive ZVI target mass was designed so that any missed mass was reassigned to the nearest neighbor (next targeted interval, or nearest borehole location) to meet the metric for total ZVI mass per borehole.
Results/Lessons Learned
Because of adaptive design, we were able to achieve the target dose per borehole, however due to the significant site complexity and heterogeneous injection conditions, an evolving injection approach was required to adjust the methods, amendment dosing and volume with nearly every borehole, requiring real-time recalculations and rising decision-making in the field.
Where the geology allowed, target intervals were injected using DPT instead of the straddle packer assembly, which allowed for more exact initiation depths and more precise placement of amendment. Without the need of a pre-drilled borehole, DPT injections were completed faster, more easily, and at lower cost.
Several borehole locations were too rocky to advance the DPT rig but had borehole walls too unstable for the packer assembly, resulting in washouts, collapsed walls, and buried or lodged packer assemblies, leading to damage and partial or complete loss of downhole equipment. A lesson learned from this project was that unstable or unconsolidated lithology would require borehole stabilization.
Despite the significant challenges with the injections, over 316,700 lbs of ZVI and 22 liters of Dhc culture were successfully injected. Orders of magnitude reduction in chlorinated VOCs with concomitant production of biotic and abiotic reductive daughter products was achieved within one year of the remedial action.