Goal

The goal of the Biogeochemistry Element is to understand the fundamental biogeochemical reactions that will lead to long-term immobilization of metal and radionuclide contaminants in the subsurface. The focus is on reactions that govern the concentration, chemical speciation, and distribution of metals and radionuclides in both the aqueous and solid phases.

R&D Challenges

Contaminated subsurface environments are complex. Biogeochemical reactions in subsurface environments are influenced by a wide variety of factors, including the availability of electron donors and acceptors, the nature of the microbial community, the chemical species or form of the contaminants, the hydrology, and the nature of the environmental matrix. Often several competing redox reactions make the prediction of the substrates, products, and reaction kinetics difficult. Microbial processes directly and indirectly influence mobility of metals and radionuclides, (see Figure 3) including alteration of pH and redox conditions, complexation, and changes in partial pressure of gases, such as carbon dioxide. The biogeochemical reactions are further complicated by the sorption of contaminants to mineral surfaces and the presence of natural organic matter and co-contaminants. The research challenge is to identify and prioritize the key biogeochemical reactions needed to predict the rate and extent of reactions leading to the immobilization of radionuclides and metals for long-term stability. This fundamental knowledge will also help us to accelerate those processes through biostimulation.

R&D Initiatives: Current Status

Current research in this element focuses on three areas: 1) the relative importance of abiotic and biotic redox reactions, and the kinetics of those reactions; 2) the impact of reactive sediment surface chemistries, such as iron oxide crusts, on the mobility of radionuclides and metals; and 3) the influence of redox reactions on the mobility and stability of radionuclides and metals.

R&D Initiative: 3 Year Targets

Within three years, some of the key chemical pathways of redox reactions involved in metal and radionuclide transformations will be determined in laboratory-based experiments. The technical approaches will emphasize natural geological matrices. Field investigations on biogeochemical dynamics will have begun at the FRC and will include the influence of co-contaminants, such as nitrate, on biogeochemical processes leading to the immobilization of radionuclides and metals.

R&D Initiatives: 7—10 Year Targets

Within seven to ten years, experiments will focus on coupled systems (biological, geochemical, hydrological) to accelerate immobilization by biostimulation and on the long-term chemical stability of immobilized metals and radionuclides in subsurface environments. Information on biogeochemical processes at the FRC will be integrated with biological processes (from the Biotransformation Element) and hydrological processes (from the FRC site characterization) into numerical models.