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

PANEL REPORT: MODELING AND SIMULATION OF GEOLOGIC SYSTEMSDeveloping coupled in silico biogeochemical modelsThe fate and transport of radionuclides and metals (Charlet and Polya 2006; Kretschmar andSchaefer 2005; Lloyd and Oremland 2006; Roden and Scheibe 2005; Steefel et al. 2005), thecorrosion of nuclear waste forms and packages (Bruno and Ewing 2006; Burns and Klingensmith2006; Ewing 2006; Grambow 2006), the performance of engineered barrier systems, the storageof CO 2 in deep aquifers (Bachu et al. 1994; Bruant et al. 2002; Elliott et al. 2001; Gaus et al.2005; Gunter et al. 1993; Johnson et al. 2004; Rochelle et al. 2004), and the global elemental andnutrient cycles (Berner 1995; Berner et al. 1983; Van Cappellen and Gaillard 1996) are allinfluenced by biogeochemical processes. In many cases, the rates of these processes are directlymediated by microbial activity. In other cases, new reactions may occur, or the extent ofreactions may be altered, relative to an analogous abiotic system through the participation ofcells or their byproducts in the reactions. Microbially mediated environmental processes rarelyresult from activity of a single group of organisms, but instead are the result of a diverse group oforganisms that may reside as biofilms in the subsurface. Competitive and/or symbioticcommunities may be present. Microbial metabolism and growth can directly or indirectly impactimportant geochemical processes, including electron-transfer, precipitation and dissolution, andsorption.Specific bacterial populations are known to catalyze the reactions of certain redox-couples, e.g.,the respiration of acetate with iron(III) as an electron acceptor. Moreover, there are feedbackmechanisms between the geochemical environment and bacteria that lead to gradual changes inmicrobial functions as the environment changes. For example, the bacterial community mayswitch from iron(III) reduction to sulfate reduction as iron(III) reaches limiting (but non-zero)concentrations in a porous medium (Chappelle and Lovley 1992). A variety of competitive andcooperative bacterial community processes generally lead to a shift in predominant species whenan evolution of geochemical conditions occurs. The transition of the bacterial community fromone electron acceptor to another is an important topic with regard to bioremediation or thenatural attenuation of redox-sensitive radionuclides, but also in marine sediments where steepredox gradients control the cycling of iron, manganese, and carbon (Thullner et al. 2005; VanCappellen and Gaillard 1996).Current microbial-biogeochemical models for reactive transport lack a suitable mechanistictreatment of these processes at the cellular level. This seriously hampers our ability to predictbehavior under a range of conditions and environments. The few existing models that couple theprocesses are largely empirical and do not account for effects of the local geochemicalenvironment (especially nutrient and solute fluxes) on metabolic rates, even though theseultimately govern the overall rates of subsurface reactions. For example, these simulatorstypically include “yield factors” in their kinetic rate laws that specify the partitioning of thenutrients between growth and metabolism as a constant. Or the switchover from the use of oneelectron acceptor to another (e.g., iron(III) to sulfate) is handled with empirical “inhibition”functions that do not capture the dynamic interaction between the bacteria and their environmentand between biotic and abiotic chemical cycles (Brun and Engesgaard 2002; Thullner et al.2005).Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 61