Unraveling Biogeochemical Pathways Challenge
Chromium contamination in groundwater is widespread within the DOE complex, including the subsurface sediments and groundwater at Hanford, Idaho National Laboratory, Savannah River, Pantex Plant, and Lawrence Livermore National Laboratory sites (DOE Groundwater Data Base, 2003), and a recently discovered chromium plume at Los Alamos National Laboratory. The total volume of DOE Cr-contaminated groundwater is estimated to be 1.3x1010 gal, and the maximum mass of Cr is estimated to be 2.8x105 kg (Hazen et al., 2008).
Map of Cr concentration in groundwater
at Hanford 100H and 100D areas, both located
near the Columbia River (from PNNL, 2007).
Chromium exists in the environment primarily in one of two oxidation states , Cr (III) or Cr (VI). Cr (III) is usually found as (hydr)oxides that are sparingly soluble, and therefore immobile and relatively nontoxic. In contrast, Cr(VI) exists as the chromate oxynation (CrO42), which is highly soluble, mobile, toxic, carcinogenic, and mutagenic. Reduction of Cr(VI) has been reported in a wide variety of aerobic facultative, and anaerobic bacterial strains. However, it is also possible that biogenic products like hydrogen sulfide (from sulfate-reducing bacteria) and Fe(II) (from iron-reducing bacteria could participate in indirect (i.e. abiotic) Cr(VI) reduction. Once reduced to Cr(III), the reoxidation of Cr(III) to Cr(VI) by molecular O2 and other common oxidants is kinetically limited, in spite of the fact that chromate is the thermodynamically stable form of Cr in oxidized environments. This kinetic limitation on oxidation is crucial from a remediation perspective, because it means that, unlike less kinetically hindered elements like U, it is feasible to maintain a vast majority of Cr in the reduced Cr(III) state even under transient or predominantly aerobic conditions.
Gaining a predictive understanding of the biogeochemical pathways associated with chromium reduction is particularly relevant to the Hanford 100 Area, where the volume of Cr-contaminated groundwater is estimated to be 1.5x109 gal, and the maximum mass of Cr is estimated to be 1x104 kg. A plume of hexavalent chromium , Cr(VI), was discovered in the groundwater near Columbia river shoreline to the west of Hanford 100D area in the early 1990s. Hanford's goal is to decrease the Cr concentration to 20 ppb in the monitoring wells located 40 ft from the river. To contribute towards this goal, LBNL led a biostimulation experiment in the Hanford 100H area in 2004. The test site used for that study (and for current studies) is located along the Cr(VI)-contaminated groundwater pathway from the Hanford 100D site to the Columbia river (Faybishenko et al, 2008). The biostimulation was successful at reducing Cr(VI) concentrations below MCL near the HRC injection well for more than 3 years. Although the LBNL experiment at the Hanford 100H site indicated that Cr reduction via biostimulation was feasible, many questions regarding the mechanisms and sustainability of the approach remain unresolved.
Although the LBNL biostimulation at the 100H site demonstrated that Cr(VI) reduction is attainable, many questions about the mechanisms and sustainability of the treatment remain; these questions motivate the research in this challenge. In order to implement sustained chromium reductive immobilization in the Hanford 100H aquifer, it is necessary to develop a conceptual model that integrates flow and transport mechanisms with the effects of complex and interrelated microbial metabolic and geochemical processes, including direct (enzymatic) and indirect (secondary abiotic) Cr(VI) reduction, as well as re-oxidation under aerobic or anaerobic conditions. This challenge will combine a variety of state-of-the art approaches, including a new systems biology technique to assess whole-community gene expression, and the novel use of biomolecular, spectroscopic and isotopic signatures in reactive transport models to study key processes at scales encompassing the molecular scale, the pore scale, and the local field scale.
- Hypothesis 1 - Microbial processes mediate both direct (enzymatic) and indirect Cr(VI) reduction at Hanford 100H, but indirect pathways dominate sustained reduction. Furthermore, sulfate reduction is the electron-accepting process ultimately driving sustained Cr(VI) reduction at Hanford 100H.
- Hypothesis 2 – The rate and extent of Cr(III) (re)oxidation will be controlled by the abundance and mineral form of Mn (III/IV) oxides in the sediment.
- Hypothesis 3 - Fermentative/acetogenic metabolism will promote retention of organic carbon in the aquifer more than respiratory metabolism.
- Hypothesis 4 - Hydrologic, geochemical, and microbiological interactions between the high-permeability sediments of the Hanford Formation with the underlying, low-permeability sediments of the Ringold Formation result in a redox-stratified aquifer system that has a strong influence on Cr mobility.
Through research conducted in this challenge, we expect to greatly improve our understanding of the complex interactions between microbiological and geochemical processes mediating chromium reduction and re-oxidation under conditions representative of those in the Hanford 100H subsurface environment. Specific examples of expected outcomes are given below.
- Develop a next-generation sequencing for meta-transcriptome-based gene expression microarray for the Hanford 100H site and assess performance relative to a pre-defined functional gene array
- For several major metabolic processes relevant to the Hanford 100H site, assess quantitative relationships between metabolic activity and biomolecular signatures
- Incorporate biomolecular signatures into multicomponent reactive transport model for the Hanford 100H site
- Assess the ability of isotopic techniques to identify and quantify the effects of key redox reactions during field tests of Cr reduction through electron donor addition
- Incorporate isotope systematics into a multicomponent reactive transport model of the Hanford 100H site; calibrate and verify the model using the results of laboratory-scale column experiments and field push-pull tests
- Quantify role of Hanford-Ringold interface in redox stratification and Cr immobilization
- Through integration of data derived from various characterization methods with process-level reactive transport modeling, demonstrate an improved understanding of the relative importance of pathways within the biogeochemical reaction network that mediate Cr reduction and re-oxidation at the field scale
- Use model and experiments to document the long-term stability of Cr reductive immobilization at the Hanford 100H site, including relative susceptibility to oxidation.
- Using hydrobiogeochemical knowledge and insights gained at 100H, work with EM activities at 100D to further understanding of Cr biogeochemistry across the Hanford 100 Area.
- Harry Beller
- Eoin Brodie
- Terry Hazen
- Ruyan Han
- John Christensen
- Mark Conrad
- Shaun Brown
- Boris Faybishenko
- Peter Nico
- Charuleka Varadharajan
- Eric Sonnenthal
- Carl Steefel
- Li Yang
- Sergi Molins-Rafa
- Yoshiko Fujita (INL)
- Phil Long (PNNL)
This challenge will leverage on activities conducted by the Arkin/Hazen Genomics:GtL project, which is using a systems biology approach to investigating the responses of model bacteria to environmental stressors (e.g., nitrate, oxygen) during push-pull tests at the Hanford 100H site.