PNNL

Abstract

Here we describe a novel approach for, and present evidence of the effectiveness of, long-term, in situ bioimmobilization of toxic and soluble Cr(VI) in groundwater using a 13C-labeled slow-release polylactate. A field experiment was conducted at Hanford (Washington), the U.S. Department of Energy nuclear site. A combination of in situ hydrogeological, geophysical, geochemical, and microbiological measurements and analyses of water samples and sediments provided detailed and reliable description of in situ biogeochemical processes. The results of this experiment show that a single lactate injection into Cr(VI)-contaminated groundwater stimulates an average increase in biomass by up to 50 times, from—5·105 to 2.5·107 cells/mL. The results also show a depletion of terminal electron acceptors O2, NO3-, and SO42- , and an increase in Fe2+. The biogeochemical processes, including the reaction of ferrous ion with hexavalent Cr, cause the formation of nontoxic and insoluble trivalent Cr precipitates. Cr(VI) concentration in the lactate injection well has remained below the background concentration in downgradient monitoring wells and below the drinking water standards for 3 years. The current studies suggest that Cr(VI) biogeochemical immobilization under these conditions may be fairly resilient, the applicability and degree of resilience under different redox conditions at this and other contaminated sites is currently under study.