Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Submarine groundwater discharge (SGD) is any flow of water along the continental margins from the seabed into the coastal ocean, and it represents an important source of nutrients and trace metals to the coastal ocean. The chemical composition of SGD is strongly influenced by biogeochemical reactions that take place within the subterranean estuary (STE), the subsurface mixing zone of fresh and saline waters. Understanding the reactions that take place within the shallow STE is critical to evaluating the composition of SGD, and therefore SGD-driven chemical fluxes. In this dissertation, I seek to determine the biogeochemical processes controlling the behavior of the redox-sensitive metals (RSMs) Mo, U, V, and Cr in a shallow subterranean estuary in Gloucester Point, VA (USA). These RSMs tend to form soluble oxyanions under oxidizing conditions but react to form more insoluble or particlereactive (i.e., more likely to adsorb to sediments) species under reducing conditions. In this STE, advection of water through the STE and the apparent respiration of organic matter drives the formation of a “classic” redox sequence typically observed in diffusion-dominated fine-grained sediments, with sequential zones with depth of high nitrate, dissolved Fe, and sulfide. While the general redox structure and RSM distributions in the STE remained consistent over time, concentrations and mixing behavior varied over the study period. Concentrations of DOC, humic carbon, and sulfide were higher in the summer, whereas Fe and Mn concentrations were higher in winter. This contrasting behavior may be due to sulfate and metal reducing bacteria responding differently to seasonally variable factors (such as temperature or substrate availability). Mo and U were supplied to the STE by surface water, and both showed nonconservative removal. Removal of Mo was correlated with sulfide concentrations, but unlike sulfide concentrations, did not show seasonal differences. This was likely due to sulfide concentrations consistently in excess of the 11 µM threshold required to quantitatively react with and remove dissolved Mo. However, U showed greater removal in the summer, possibly driven by greater activity of U-reducing microbes. Dissolved V concentrations co-varied with DOC (with both greater in summer), indicating that V is likely complexed with dissolved organic matter. In contrast, Cr was correlated with both humic carbon and dissolved Fe in different parts of the STE. Over half of total dissolved DOC and Fe occurred in the colloidal phase, demonstrating the importance of colloidal transport in the STE. The relative proportion of RSMs in the colloidal phase increased in the order Mo < U < V < Cr, with up to 75% of Cr existing in the colloidal size fractions, suggesting the importance of colloidal transport for RSMs. Incubation experiments conducted under aerobic and anaerobic conditions showed that RSM concentration change on the order of hundreds of nM can take place in hours to weeks, within water residence times in the shallow STE. Furthermore, removal and mobilization rates between redox zones with distinct microbial populations. The mechanistic approach used this work demonstrate how spatial and temporal variability of dissolved concentrations in the STE depend on redox zonation and microbemediated reactions. Findings from this work provide a basis for evaluating how changing environmental conditions may alter RSM fluxes.
© The Author
O'Connor, Alison E., "Biogeochemistry of Redox-Sensitive Elements in the Subterranean Estuary" (2016). Dissertations, Theses, and Masters Projects. Paper 1499449677.