Theme: 01. Remediation Technology Innovations

Topic Area: 01a. Abiotic and In Situ Biogeochemical Processes: Applications and Lessons Learned

Formatted Title:

What to Expect When You are Not Expecting: Challenges, Solutions, and Results of a Limited Bedrock Injection

Background/Objectives:

This former central Pennsylvania manufacturing facility, listed on the National Registry of Historic Places and now used for apartments, a school, and offices, was impacted by chlorinated solvents from several potential source areas. Bedrock across the site is shallow and DNAPL likely drained through overburden into the fractured rock. A groundwater pump and treat system has been providing hydraulic control since 1995. The project objectives were to evaluate site status, update the conceptual site model (CSM), and interface with regulators to develop an achievable path forward and reduce off-site migration. Primary constituents of concern (COCs) present in the bedrock groundwater include trichloroethylene (TCE) and 1,1,1-trichloroethane (1,1,1-TCA).

Approach/Activities:

Extensive pump testing evaluated the effectiveness of the existing pump and treat system to address the deep impacted groundwater (~100 feet below grade) and other potential remedial alternatives. Sub-slab depressurization and vapor mitigation systems were installed to reduce potential residential vapor intrusion exposure. Expanded site investigation (additional bedrock wells, geophysical studies, packer and tracer testing) updated the CSM.

To expedite cleanup, in 2015 GES began treating groundwater within the fractured bedrock on the central portion of the site using a biogeochemical transformation process including an anaerobic bioremediation process coupled with abiotic dechlorination reactions. The injected amendment was EHC® Liquid Reagent (a mixture of lecithin [organic carbon] and ferrous iron salt) and ferrous sulfate to promote the formation of iron sulfides and the biogeochemical process. Due to very low porosity, small fracture apertures of the bedrock, and based on prior testing data (pump, packer, and bromide tracer tests) only limited volumes of amendment are able to be injected. To date, four rounds of injection have been performed (1,480 gallons total). An initial pilot test was conducted via injection into one well and later expanded to two more wells. A third and fourth injection event were later completed in two other areas of the site. Each injection event was modified based on the results and monitoring of the prior injections, and as a response to lessons learned as the project progressed.

Results/Lessons Learned:

The combined abiotic and biotic approach has resulted in greater than 99% reduction of TCE and TCA in the initial injection area. Initial TCE concentrations ranged from 800 to 88,000 micrograms per liter (µg/L) and are currently non-detect to 340 µg/L within the initial treatment area. Across all areas significant reduction of CVOCs has been achieved (i.e., reductions ranging from 55% to greater than 99% for TCE and 1,1,1-TCA in the injection and nearby monitoring wells, compared to pre-injection concentrations). Minimal rebound has occurred in the majority of the injection areas, indicating that there is likely not a continuing source or back-diffusion from the bedrock matrices into groundwater occurring.

The formation of reactive iron precipitates is a desired part of the anaerobic reduction process; however, excessive formation in the first three injection wells resulted in accumulation of a thick layer of precipitates in the injection wells after the first two injections. To combat this, well redevelopment is periodically conducted to remove the excess precipitates allowing the wells to be accessed for monitoring purposes, and to promote a hydraulic connection with the surrounding aquifer. In addition, during the third injection into two additional wells, the injected amendment was diluted further. This modification has successfully prevented excess formation of precipitates in the wells. Elevated methane was also observed in several wells during post-injection monitoring. To address methane, the third and fourth injections also included an anti-methanogen reagent to control growth. Bioaugmentation was initially planned; however, based on COC reductions and increased levels of microbial populations which were naturally induced by the reductive conditions, it was deemed unnecessary.

A fourth injection was completed near one of the three extraction wells for the groundwater extraction system, closer to the western property boundary. We will also highlight the benefits of using dedicated injection wells over packers in existing wells, as both approaches were employed. This case study presentation will illustrate how a phased remedy, and adaptive approach, using a combined approach of biological and abiotic reduction processes proved to be successful at reducing COC concentrations despite many challenges including severely limited injection volumes.

Stephanie Grillo

Affiliation

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What to Expect When You are Not Expecting: Challenges, Solutions, and Results of a Limited Bedrock Injection