Basic Research Needs for Geosciences - Energetics Meetings and ...

PANEL REPORT: CHEMICAL MIGRATION PROCESSES IN GEOLOGIC MEDIAelectron donors for biologic electron transfer to radionuclides and other redox-sensitive elements.Hydrogen is a likely byproduct of radiolysis and metal corrosion in environments near buriednuclear wastes. Reduced carbon species are likely to be entrained in plumes rich in CO 2 that maymigrate in geological sequestration environments.Individual microorganisms are typically 2-5 μm in size. These may co-associate in films of likeorganisms (biofilms) or in complex associations of different species with interdependentfunctions (communities). These biologic features populate reactive mineral interfaces andtogether concentrate nutrients, organic carbon, and mineral-derived electron donors that provideenergy for metabolism and molecules for biosynthesis. Fluxes of metabolic products and theassembly of biostructures for geochemical functions create a unique and dynamic chemicalenvironment at the microbe-mineral interface that can dramatically alter inorganic molecular andmicroscopic structures and compositions of the associated mineral surface. Biogeochemicalprocesses have consequent large influences on chemical migration through their potential impacton all system variables and properties influencing solid-liquid distribution.A high priority research area is understanding the dynamics of the biogeochemistry of extremesubsurface environments associated with nuclear waste and CO 2 sequestration. Central to this isthe belief that perturbations in the indigenous microbial community by inoculation of surfacemicroorganisms (during drift, shaft, or well emplacement), followed by the development ofextreme conditions of temperature, radiation, water potential, and carbon dioxide concentration,will cause marked and unpredictable changes to the system’s biogeochemistry that must beunderstood. Basic research targeting the evolution of the subsurface microbiologic community inresponse to environmental perturbation, and the molecular and microscopic nature, rates, andproducts of biogeochemical reactions that occur at the microbe-mineral interface under extremeconditions, holds particular promise for both discovery science and problem application.Equilibrium and reaction rates in perturbed geochemical environmentsAny geochemical environment that has reached a steady or equilibrium state over a relativelylong time will respond to the emplacement of energy system byproducts through sudden orgradual reactions among minerals, pore waters, and the energy wastes. The reactions aretriggered by perturbations to the original in situ conditions such as changes in temperature andgas pressure, chemical gradients, and oxidation-reduction conditions, or imposition of intenseradiation fields. The reactions may be fast or slow, and may occur as single isolated events, becoupled, or have feedback effects on one another. They may have significant effects around theemplaced materials or at great distances. Matter and energy transfer between the geochemicalenvironment and emplaced waste is expected with either beneficial or detrimental effects. As anexample, the anticipated increased temperature near emplaced nuclear waste at the YuccaMountain repository site may cause complex brines to form in moisture-laden dust thataccumulates on the waste package surfaces (see sidebar on The hypothetical perturbedenvironment near emplaced nuclear waste in a geologic repository). These brine droplets willhave different reactivity and corrosive effects than natural groundwaters, and their presence maylead to the release of contaminants to the surroundings. Therefore, over a wide range ofscenarios, the thermodynamic properties of complex geological fluids and solids, and thereaction rates among phases and species, must be known to define the critical environmentalparameters that control migration or immobilization of wastes.Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 29