Comparison of Field Groundwater Biostimulation Experiments Using Polylactate and Lactate Solutions at the Chromium-Contaminated Hanford 100-H Site

The primary contaminant of concern in groundwater at the DOE Hanford 100 Area (Washington State) is hexavalent chromium [Cr(VI)] in Hanford coarse-grained sediments. Three lactate injections were conducted in March, August, and October 2010 at the Hanford 100-H field site to assess the efficacy of in situ Cr(VI) bioreductive immobilization. Each time, 55 gal of lactate solution was injected into the Hanford aquifer. To characterize the biogeochemical regimes before and after electron donor injection, we implemented a comprehensive plan of groundwater sampling for microbial, geochemical, and isotopic analyses. These tests were performed to provide evidence of transformation of toxic and soluble Cr(VI) into less toxic and poorly soluble Cr(III) by bioimmobilization, and to quantify critical and interrelated microbial metabolic and geochemical mechanisms affecting chromium in situ reductive immobilization and the long-term sustainability of chromium bioremediation. The results of lactate injections were compared with data from two groundwater biostimulation tests that were conducted in 2004 and 2008 by injecting Hydrogen Release Compound (HRC^°), a slow-release glycerol polylactate, into the Hanford aquifer. In all HRC and lactate injection tests, ¹³C-labeled lactate was added to the injected solutions to track post-injection carbon pathways. Monitoring showed that despite a very low initial total microbial density (from <10⁴ to 10⁵ cells/mL), both HRC and lactate injections stimulated anaerobic microbial activity, which led to an increase in biomass to >10⁷ cells/mL (including sulfate- and nitrate-reducing bacteria), resulting in a significant decrease in soluble Cr(VI) concentrations to below the MCL. In all tests, lactate was consumed nearly completely within the first week, much faster than HRC. Modeling of biogeochemical and isotope fractionation processes with the reaction-transport code TOUGHREACT captured the biodegradation of lactate, fermentative production of acetate and propionate, the evolution of ¹³C in bicarbonate, and the rate of sulfate reduction. In contrast to the slow-release HRC injections, no long-term effects of biostimulation and Cr bioreduction were observed in groundwater after the lactate injections. The presentation will address these patterns of the geochemical, δ¹³C of DIC, and biomass changes in groundwater before and after the polylactate and lactate injections.