A MULTICOMPONENT REACTIVE TRANSPORT MODEL OF IN SITU REDOX MANIPULATION FOR REMEDIATION OF CHROMIUM CONTAMINATED GROUNDWATER

In situ redox manipulation (ISRM) is a cost effective method to treat chromium contaminated groundwater. The injection of a strong chemical reductant into an aquifer with iron-bearing sediments reduces Fe(III) to Fe(II). Fe(II) then becomes a long-term, stationary source of reductant capable of reducing mobile Cr(VI) to its more immobile Cr(III) oxidation state. Several laboratory and field studies have confirmed this process using sodium dithionite as the reductant, and simple reactive transport models have been developed to help quantify different steps of the remediation process. Here, we present a novel and unique multicomponent reactive transport model capable of simulating the entire process of ISRM for chromium remediation using a sodium dithionite injectant. The model is built upon and utilizes existing functionality of the massively parallel subsurface flow and transport code PFLOTRAN (https://bitbucket.org/pflotran). It is capable of capturing many of the dominant oxidation-reduction pathways, along with important chemical and physical heterogeneities, that occur from the laboratory to field scale. We perform model-based analyses using MADS (https://github.com/madsjulia) to identify parameters most likely to influence remediation using an ISRM strategy. This research advances both our basic understanding and modeling capabilities of the multi-scale interactions of geochemistry and hydrology that occur during chromium remediation as well as our ability to quantify and reduce uncertainty related to contaminated groundwater sites and remediation strategies in a decision context. Results are presented for a synthetic problem, but the modeling approach is applicable to real-world problems such as the Los Alamos National Laboratory chromium contamination site, where the potential for in situ remediation is currently being evaluated.