Chapter 6 - Creation of a Subsurface Permeable Reactive Barrier Using in Situ Redox Manipulation

Abstract

This chapter presents an in situ redox manipulation method (ISRM) for creating a permeable reactive barrier in the subsurface, and conducts its field-scale demonstration at a chromate-contaminated site on the U.S. Department of Energy's Hanford site in southeastern Washington State. The ISRM treatment zone is created by reducing the ferric iron [Fe(III) ] phases naturally present in the aquifer sediments to ferrous iron phases [mainly adsorbed Fe(II)] with a chemical reducing agent using an injection/withdrawal (push and pull) emplacement strategy. Sodium dithionite (Na2S2O4) is injected into the aquifer, providing a residence time sufficient to react the sediment, and any remaining unreacted reagent and reaction products are withdrawn from the aquifer. Standard groundwater wells are used, allowing treatment of contaminants too deep below the ground surface for conventional trench-and-fill technologies. Once in place, redox-sensitive contaminants migrating through this manipulated zone are destroyed or immobilized. A multiscale approach is utilized during development of the technology with experiments ranging from bench-scale laboratory tests to emplacement of a full field-scale treatability test barrier. Each successively larger scale experiment permits isolation of effects related to each scale of experiment and enabled better design and regulatory approval of the next larger scale experiment. Performance data obtained from field-scale demonstrations of the ISRM technology indicate that the method is effective for the remediation of Cr(VI)-contaminated groundwater. No secondary effects are identified during laboratory- or field-scale testing of the technology that would preclude deployment of the ISRM technology for full-scale remediation of Cr (VI)-contaminated groundwater.