Temporal Variations in Biogeochemical Processes that Influence Ground-Water Redox Zonation

By Jennifer T. McGuire, Erik W. Smith, David T. Long, David W. Hyndman, Sheridan K. Haack, Jonathan J. Kolak, Michael J. Klug, Michael A. Velbel, and Larry J. Forney

ABSTRACT

Hydrogeological, microbiological and geochemical parameters are being measured in ground water to determine and model processes affecting redox zonation. The ultimate objective of this project is to construct a three-dimensional, transient, reactive flow and transport model for the evolution of redox zonation. The study site is a shallow sandy aquifer contaminated with petroleum hydrocarbons and chlorinated solvents. This paper explores temporal variations in the biogeochemistry of ground water collected from the study site over a period of three years. Selected indicators of biogeochemical processes include dissolved hydrogen (H₂) and dissolved methane (CH₄). Measurements of these indicators have documented changes in redox zonation over short time scales (months) superimposed on more gradual changes that occurred over a three-year period. The patterns of water-table fluctuations and biogeochemical changes in ground water indicate that monthly changes in redox zonation are influenced by solute transport (recharge). At longer time scales, changes in microbial processes unrelated to solute transport may also influence redox zonation. Temporal changes in dissolved potassium concentrations support the concept that a change in microbial processes has occurred over three years. Resolving the possible controls on redox zonation on these two, distinct time scales is the focus of on-going studies.