Brian Hitchens (California) will present on March 23 and his platform session is entitled "Full Scale In-Situ Reduction of Hexavalent Chromium in Vadose Zone Soils with H2S Gas,” and Brian will also chair the Bioremediation Session.
Remedial solutions for vadose zone hexavalent chromium (Cr6+) have typically been limited to dig and haul or soil flushing with liquid phase reductants in various forms. However, excavation is not always feasible and liquid phase amendments may pose significant concerns for mobilization of impacts to groundwater. In-situ gaseous reduction (ISGR) addresses these issues by reducing the Cr6+ impacts through the controlled injection of hydrogen sulfide (H2S) gas. This presentation will detail the design, field-scale implementation, and results of one of the first applications of an ISGR remedy for treatment of vadose zone Cr6+.
The project objective involved remediation of Cr6+ impacts in vadose zone soils beneath a former plating shop to residential screening levels such that the property could be sold without use restrictions. Initial characterization demonstrated that Cr6+ had penetrated to 45 feet below grade. Although well above groundwater (at 95 feet below grade), treatment of the Cr6+ through standard approaches were either infeasible due to the overlying structure, or not advisable due to the potential to mobilize Cr6+ to groundwater. After direct excavation of the accessible impacted soil in the upper 10-feet, an ISGR treatment area was established around the limited area with deeper impacts.
Bench scale treatability testing was performed with shallow site soil to determine the mass of H2S required to reduce the residual Cr6+ within the treatment area to Cr3+. An H2S injection/extraction skid was designed and built to inject an inline stream of H2S gas into a nested injection well at a concentration of approximately 500 ppmv, while simultaneously extracting from a ring of extraction wells to control and treat excess H2S gas. Post-remediation sampling of the soil within the treatment area after approximately eight weeks of treatment showed excellent Cr6+ reduction and a no further action determination was granted in September 2015.