Formatted Title

 Successful In Situ Treatment of Hexavalent Chromium in Saturated Clay Soils Using ISCR Enhanced Bioremediation

Background/Objectives

Hexavalent chromium was detected in an unconfined clay aquifer at a former plating facility in the Midwest, prompting remedial action.  The depth of contamination, along with the impermeable clay lithology, restricted the use of excavation and pump-and-treat methods to fully address these impacts.  Therefore, an in situ remediation treatment along with limited soil excavation was chosen to remediate the site.   A pilot test program was conducted for two treatment alternatives.  The first pilot test utilized in situ chemical reduction (ISCR) via zero valent iron (ZVI) slurry.  The second pilot test employed an organic electron donor in combination with a soluble divalent iron (DVI) compound to facilitate both anaerobic bioremediation and biogeochemical induced abiotic reactions. The pilot test results demonstrated that the ZVI-based approach achieved quicker reductions compared to the electron donor; however, the electron donor and DVI combination proved to be more effective overall.  The full-scale application effectively addressed a spectrum of contaminant concentrations, with some as high as 300 mg/L.  The full-scale remedy used the same organic electron donor but incorporated a superior ISCR reagent consisting of a colloidal suspension of sulfidated ZVI.  The switch to using sulfidated ZVI increased the reagent’s longevity. 

Approach/Activities

Two pilot test studies were performed to compare the use of an ISCR-only approach (ZVI) with a bio-enhanced ISCR approach (Electron Donor/DVI) for remediating the saturated clay soils and groundwater. The reagents were injected using direct push technology (DPT) methods.  Results demonstrated both approaches were adequate, but the combination of the electron donor and ZVI yielded better results.   In the full-scale implementation, the organic electron donor remained the same, but the ISCR reagent was replaced with a colloidal suspension of sulfidated ZVI to enhance ISCR durability and reactivity. Through 126 DPT injection points, approximately 18,000 gallons of the ISCR/donor solution was applied at the site over a nearly 9,000 square foot treatment area generally between 7 feet and 22 feet below ground surface into a saturated clay aquifer.  The post-application performance sampling began within a few months and continued for two years. 

Results/Lessons Learned

Results from the pre-remediation testing showed performance was significantly improved using a bio-enhanced ISCR approach with the combination of an organic electron donor and a sulfidated ZVI. Post-application results demonstrated >99% reduction of hexavalent chromium to below laboratory detection limits in groundwater for over two years resulting in a No Further Action (NFA) Letter to be issued for the site.  The results of these efforts verified the effectiveness of reductive technologies in treating elevated concentrations of hexavalent chromium (>300 mg/L) and demonstrated that a low-permeability aquifer can accommodate adequate fluids for in situ remediation.