Formatted Title
From Bench Test to Full-Scale Remedy: Combining ZVI with Bioremediation to Treat Mixed VOCs in a Low-Permeability Formation
Background/Objectives
Zero valent iron (ZVI) and electron donors have been co-delivered for more than 20 years, and specialty remediation products containing these materials are readily available. Controlled hydraulic fracturing methods can predictably deliver an engineered high-solids zero-valent iron (ZVI) slurry (e.g., up to 70% by weight ZVI) to low-permeability formations. Depending on the approach selected, combining a high-solids ZVI slurry with bioremediation amendments can require an additional injection step or cause issues with fracture form, resulting in unexpected surfacing of injectate. Advantages of combining ZVI and bioremediation amendments include: (1) treatment of a greater range of compounds of concern; (2) penetration of low permeability zones via chemical diffusion and other mechanisms; and (3) potential for polishing treatment near the end of the reactive life of the ZVI. Here we present a detailed case study demonstrating treatment of a mixture of chlorinated volatile organic compounds (VOCs) while utilizing two different methods for combining high-solids ZVI delivery with enhanced in situ bioremediation (EISB).
Approach/Activities
At a large active chemical facility located on the Gulf Coast, three areas of concern were impacted with a challenging mix of chlorinated ethenes, chloroform, and carbon tetrachloride above state regulatory standards. A comprehensive laboratory treatability study was performed to evaluate treatment of these compounds via in situ chemical reduction (ISCR) with micro-scale ZVI (mZVI), EISB, and a combination of ISCR and EISB. The treatability study led to the selection of a combined ISCR with ZVI and EISB approach, and a pilot test was designed to test this treatment approach in the field. The pilot test targeted treatment of a 110 square meter (m2) area at depths of 14 to 17 meters below ground surface (m bgs). Injections were performed at two locations using blank PVC casing installed with sonic drilling methods and grouted in place. Treatment of this zone required the application of 5.5 metric tons of mZVI delivered to 6 fractures followed by injection of 1,700 liters of emulsified vegetable oil (EVO) solution and KB-1® Plus microbial culture. The EVO injection was performed through the fracture-connected wells, but after the fracturing was completed.
Based on the results of the pilot, a full-scale remedy was implemented at the site. The full-scale remedy expanded treatment to all three of the areas of concern, targeting treatment of a total area of 2,000 m2 at depths of 14 to 17 m bgs. Injections were performed at 25 locations using direct push drilling methods. The full-scale remedy required the application of 139 metric tons of mZVI, 2.5 metric tons Provectus organic blend solid electron donor, 3,100 liters RNAS Newman Zone QR75TM liquid electron donor, and 94 liters of KB-1® Plus, all delivered to 94 fractures. This remedy incorporated injection of a blend of solid and liquid bioremediation amendments integrated into the mZVI slurry used for fracturing.
Results/Lessons Learned
Initial 7-month post-pilot test groundwater results showed promising treatment; however, results markedly improved after four years with total VOC concentrations decreasing by more than 90% at four of six performance monitoring wells. Implementing the full-scale remedy using direct push injections saved well installation cost and allowed additional flexibility during implementation. Integrating the bioremediation amendments into the mZVI slurry provided several advantages: (1) it reduced implementation costs by eliminating the need for an additional EISB injection event (2) it provided similar electron donor loading as the pilot; and (3) it allowed for the delivery of both mZVI and electron donor in a single step without compromising fracture form. Initial groundwater monitoring results from the full-scale remedy are promising, but additional data will be collected in late 2023 and will be presented.