Theme: 01. Remediation Technology Innovations

Topic Area: 01c. Permeable Reactive Barriers: Best Practices and Lessons Learned

Formatted Title:

Lessons Learned during Column Testing for PRB Design

Background/Objectives:

A permeable reactive barrier (PRB) constructed from zero-valent iron (ZVI) is the selected remedy for containing chlorinated volatile organic compounds in groundwater at this superfund site in USEPA Region 1. This presentation shares two unique aspects of this project:

Column testing: Column testing revealed that sand initially intended for use in the PRB decomposed under the strong geochemical conditions created by ZVI which affected the reactivity of ZVI over time – this was a surprising result since sand is frequently assumed to be inert.
Space and depth challenges guided the project towards a PRB constructed as a series of overlapping caisson. Construction of the PRB is initiating in early spring 2024. An update on this design approach and PRB construction will be provided.

Approach/Activities:

Column testing using Site groundwater, Connolly ZVI and sand from a local quarry was performed to determine a Site-specific reaction rate for PRB design. The testing unexpectedly showed decreasing reactivity of the ZVI over time which could not be explained from geochemistry of groundwater. Subsequent column and minerology testing revealed that feldspar in locally-sourced sand was decomposing in the presence of ZVI and causing accelerated passivation. To our knowledge, this is the first study identifying the role sand mineralogy plays in design of a PRB.

The PRB extends to approximately 60 feet below the ground surface and is located in a portion of the Site with limited working space. A PRB constructed from overlapping caissons was designed to accommodate these constraints. This presentation reviews the PRB design approach, final design and provides an update on the initial three months of PRB construction.

Results/Lessons Learned:

Unexpected finding during column testing revealed that PRB design should not assume sand is an inert medium, and that column testing should be performed with Site groundwater and the sand specific for construction.

Installing PRBs using caisson drilling is well suited for certain situations. The design approach which minimize wasting ZVI when overlapping caissons will be presented as well as observations from the initial months of construction.

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Carl Elder