THE EFFECT OF DISSIMILATORY IRON REDUCTION, NITRATE REDUCTION AND MICROBIAL GROWTH ON ELECTRICAL CONDUCTIVITY

Open Access
Author:
Regberg, Aaron Benjamin
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 21, 2011
Committee Members:
  • Susan Louise Brantley, Dissertation Advisor
  • Susan Louise Brantley, Committee Chair
  • Kamini Singha, Committee Chair
  • James David Kubicki, Committee Member
  • Ming Tien, Committee Member
Keywords:
  • Biofilms
  • Kinetics
  • Electrical Conductivity
  • Reactive Transport Modeling
  • Dissimilatory Iron Reduction
  • Nitrate Reduction
Abstract:
Changes in measured electrical conductivity are related to measured changes in aqueous concentrations of ions. In previous work anomalous increases in electrical conductivity measured in and around contaminated aquifers have been associated with biogeochemical activity. The respiration and growth of micro-organisms has been suggested as a possible cause of these conductivity signals. However, a quantitative link between the activity of micro-organisms and electrical conductivity has not been established. In this dissertation changes in the fluid electrical conductivity (σf) and σb are related to changes in chemical concentrations at multiple scales. Changes in σf can be interpreted as reaction rates if the stoichiometry of the reaction is understood. Results from batch experiments demonstrate that measured changes in σf over time document rates of abiotic and biotic reduction of goethite. In two column reactors packed with sediments from Oyster Virginia and fed with an input containing dissolved acetate, measured changes in σb initially document biogeochemical reaction rates of iron reduction or nitrate reduction. Eventually, σb becomes decoupled from changes in chemical concentrations and increases significantly. Fe(II) adsorption does not cause σb to become decoupled from aqueous chemistry. Instead, this decoupling is attributed to the growth and presence of an electrically conductive biofilm. Reactive transport modeling is used to constrain the amount of biomass produced in each reactor. A value for electrical conductivity is estimated that depends on the calculated growth in biomass and the assumed density of that biomass. For iron-reducing conditions biofilms, the model is consistent with values of biofilm conductivity > 2.2 S/m, and for nitrate reducing conditions, with conductivity > 300 S/m. These values are the first estimates for biofilm electrical conductivity based on column reactor experiments. The results of a field-scale tracer test involving nitrate reduction are also presented. The results suggest that electrical conductivity may be able to track biogeochemical activity if the tracers are large enough and conductive enough to be detectable. Finally, results of preliminary electrochemical experiments designed to directly measure the conductivity of biofilms are presented. Methods for improving the accuracy of these experiments are proposed.