Electrical conductivity as an indicator of iron reduction rates in abiotic and biotic systems

Although changes in bulk electrical conductivity (sigma(b)) in aquifers have been attributed to microbial activity, sigma(b) 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 propose composite reactions that incorporate the stoichiometry of five different types of reactions: redox, acid-base, sorption, dissolution/precipitation, and biosynthesis. In batch experiments and the early stages of a column experiment, 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 sigma(b) increased while fluid chemistry remained constant. Growth of an electrically conductive biofilm could possibly explain this late stage sigma(b) increase. The measured sigma(b) increase is consistent with a model proposed by analogy from percolation theory that attributes the increased conductivity to growth of biofilms with conductivity of similar to 5.5 S m(-1) in at least 3% of the column pore space. This work demonstrates that measurements of sigma(b) 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.