Math Dissertation Defense Outline

The hyporheic zones of streams and rivers, consisting of the sediments beneath and immediately adjacent to the stream channel, are an important site of geochemical processing. Hyporheic processes include redox reactions, sorption and desorption, and mineral dissolution. Due to the difficulty of measuring the geochemical processes in the hyporheic zone in situ with meaningful spatial and temporal resolution, we conducted multiple column and large-scale flume experiments to model 1D and 2D hyporheic flow paths and observed important geochemical reactions, including the production and consumption of nitrous oxide (N2O). N2O is an important greenhouse gas, but the controls on its emissions from streams are poorly constrained. Based on reviews of existing literature, we describe the controlling factors for hyporheic N2O production and release, and also describe spatial and temporal trends in other geochemical processes occurring the hyporheic zone.

Based on the literature examining pathways for N2O production in soils and sediments, the current understanding of physical properties of hyporheic zone, and the existing studies of N2O emissions from streams and rivers, it appears that production of N2O via denitrification (and other pathways) occurs predominantly in the hyporheic zone, though production associated with suspended sediments may be significant in larger rivers or streams with high turbidity. Overall, lotic N2O emissions increase with nitrate and ammonia concentrations, and tend to be highest in the late spring and summer and below wastewater treatment plants. Observations and models combining hydromorphogical and chemical variables suggest that N2O emissions decrease downstream as sedimentary processes decrease relative to processes in the surface water. Downstream sites could have large N2O emissions, however, due to inputs of dissolved inorganic nitrogen.

Observations from column and flume experiments suggest that N2O emission from stream sediments requires subsurface residence times (and microbially mediated reduction rates) be sufficiently long (and fast reacting) to produce N2O by nitrate reduction, but also sufficiently short (or slow reacting) to limit N2O conversion to dinitrogen gas. We also confirm previous observations that elevated nitrate and declining organic carbon reactivity increase N2O production. These findings will aid in determining where and when streams will be a source of atmospheric N2O emissions.

Based on measurements of additional geochemical species collected during these flume experiments, observed spatial and temporal trends reflect microbiological processes, changing redox conditions, dissolution, sorption and desorption. In general, microbial respiration causes DO to decrease with residence time in the hyporheic zone, leading to aerobic and anaerobic zones, nitrate reduction, and a decreasing pH gradient. Most other species concentrations increase with residence time. Based on observations, we propose that increases in Ca, Mg, Si, Ba, and Sr with residence time are primarily due to silicate dissolution, and increases in Fe, Mn, Co, and As with distance along flow lines are due to reductive dissolution of metal oxides and desorption in the anoxic zone. Trends over elapsed time suggest higher flow velocities (as induced by steeper dune morphologies) lead to more rapid consumption of reactive carbon, larger oxic zones, and decreases in most species over elapsed time. This description of the trends of chemical species will inform future studies into the many geochemical functions of the hyporheic zone.

MS Thesis and PhD Dissertation Defense Regulations

The purpose of the defense is two-fold:  first, to give the student the opportunity to publicly present the results of his/her research; and second, to give the faculty, the students and, especially, the thesis committee the opportunity to critically assess and question the thesis/dissertation research.

It is suggested that an outline of the defense presentation be submitted in advance to the thesis/dissertation adviser(s) for approval, and also that the presentation be well rehearsed.

The following regulations apply to a defense.

  1. The student is responsible for ensuring that equipment needed for the presentation is available in the room and in good working order.
  2. The student will make a presentation between 30 and 50 minutes long.
  3. The student should wear "dress-up" attire (i.e., coat and tie, pants suit or dress).
  4. If large illustrations, such as fold-out maps, cannot be shown effectively on the screen, then they may be taped to the wall in a location where they are clearly visible to the audience.
  5. The defense consists of two parts: an initial, public ‘presentation’ on the research findings followed by a ‘defense’ of the research. The presentation is open to the public, and all faculty members, students, family and friends may attend. At the conclusion of the presentation and following a first round of questions, everyone will leave the room except the thesis committee and other interested faculty. The student will then defend his/her research and answer all questions put to him/her by the faculty. As a result of questions raised during the defense, additional revisions to the thesis/dissertation may be required.
  6. The advisory committee decides by a majority vote the outcome of the thesis defense. The student may pass, pass with revisions, be required to re-defend at a later date, or fail without further opportunity.

Graduate Program Information

Graduate Degree Requirements

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