Toward Quantifying Kinetics of Biotic and Abiotic Metal Reudction with Electrical Geophysical Methods.

摘要

Although changes in bulk electrical conductivity in aquifers have been attributed to microbial activity, electrical conductivity has never been used to infer biogeochemical reaction rates quantitatively. To explore the use of electrical conductivity to measure reaction rates, we conducted iron oxide reduction experiments of increasing biological complexity. To quantify reaction rates, we proposed composite reactions that incorporated the stoichiometry of five different types of reactions: redox, acid-base, sorption, dissolution/precipitation, and biosynthesis. In batch and column experiments, such reaction stoichiometries inferred from a few chemical measurements allowed quantification of the Fe-oxide reduction rate based on changes in electrical conductivity. The relationship between electrical conductivity and fluid chemistry did not hold during the latter stages of the column experiment when electrical conductivity increased while fluid chemistry remained constant. Growth of an electrically conductive biofilm could explain this late stage electrical conductivity increase. The measured electrical conductivity increase is consistent with a model proposed by analogy from percolation theory that attributes the increased conductivity to growth of biofilms with conductivity of approx. 5.5 S/m in at least 3% of the column pore space. We followed up this work with reactive transport modeling to estimate the production of biomass, and linked that with a pore-scale electrical conductivity model to estimate changes in bulk electrical conductivity associated with a growing biomass. This work demonstrates that measurements of electrical conductivity and flow rate, combined with a few direct chemical measurements, can be used to quantify biogeochemical reaction rates in controlled laboratory situations and may be able to detect the presence of biofilms. This approach may help in designing future field experiments to interpret biogeochemical reactivity from conductivity measurements.