Relative Roles of Stream-Subsurface Hydrologic Exchange and Microbial and Chemical Transformations in Controlling Heavy Metal Transport

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9523881 Conklin The specific aims of this project are to determine the constraints on heavy-metal transport in a shallow groundwater aquifer with low dissolved oxygen that intercepts oxic surface water in a perennial stream. Heterogeneous chemical and biological reactions are enhanced in shallow groundwater flow paths that are recharged from the stream (hyporheic zone), due to mixing with oxic streamwater and increased contact with geochemically and biologically active sediment coatings. Our central hypothesis is that the secondary hydrologic residence time in the hyporheic zone and the increased contact of the streamwater with the sediment are adequate to permit microbial/chemical transformations of heavy metals to be the dominant removal mechanism for a coupled stream-aquifer system. We plan to identify the most important hydrologic, biological and chemical factors that enhance oxidation of dissolved metals in the hyporheic zone. The proposed work spans point measurements (batch experiments) to field measurement at the kilometer-scale. The field situation that we will study in a contaminated alluvial basin in Arizona (Pinal Creek) is typical of heavy-metal contaminated stream-shallow groundwater systems. Previous mass balance work at the field site indicated that interplay between hydrologic and microbial/chemical processes in the hyporheic zone at Pinal Creek significantly reduced basin-wide exports of dissolved manganese (the principal metal contaminant at this study site) to downstream areas. In this study we will use a combination of measurements in the laboratory (batch, column, and sandbox), along with field experimental data and modeling analysis to determine the relative importance of physical and chemical processes. Specifically, we will identify the dominant mechanism of Mn(II) oxidation in the hyporheic zone and determine the relative roles of biological and chemical factors, and hydrologic mass transfer limitations. We will adapt and extend the Transient Storage Mod el for application as a reactive transport model for stream-shallow groundwater systems. Our multi-scale study will allow us to determine the role the hyporheic-zone processes have on the mass balance of manganese in the entire contaminated alluvial drainage basin. This system provides a unique opportunity to separate hyporheic zone processes from basin-wide processes; the parameters determined from this study will be used to constrain a coupled stream-shallow groundwater transport model with applications in other drainage basins.