Geochemical and Solute Transport Modeling of In Situ Biological Treatment of Chromium in Groundwater - 19334

Reactive transport modeling is an ideal tool for developing better site-specific understanding and predictive capability to design and optimize in situ remedial strategies. However, the widespread use of these models for in situ groundwater remediation applications is limited by increased model complexity. This paper discusses the challenges encountered and strategies developed while constructing site-relevant geochemical and solute transport models to describe an in situ groundwater treatment program for chromium. The treatment strategy chosen for the site includes injection of dissolved organic carbon to establish reducing conditions in conjunction with a groundwater recirculation/plume flushing program. The reducing conditions established result in the immobilization of hexavalent chromium as insoluble trivalent chromium hydroxides. Hydrogeochemical models were employed during design to describe and predict long-term treatment performance and secondary effects, such as the generation and attenuation of byproducts (including dissolved arsenic, manganese, and iron), water quality changes (including pH, alkalinity, and TDS), and potential permeability changes. A reactive transport model was developed using PHT3D, calibrated from bench-scale and pilot-scale testing and aquifer hydrologic characterization. A 'hybrid' geochemical/solute transport modeling approach was developed, resulting in substantial reduction in model complexity and computation time. These results present a unique strategy for striking the balance between model sophistication and model complexity - adequately capturing the complex hydrogeological and biogeochemical processes at work within the system - which can be readily applied to other in situ groundwater treatment implementations. (authors)