Publication Date

5-2018

Date of Final Oral Examination (Defense)

3-14-2018

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Geosciences

Department

Geosciences

Major Advisor

Shawn Benner, Ph.D.

Advisor

Kevin Feris, Ph.D.

Advisor

Daniele Tonina, Ph.D.

Advisor

Jennifer Pierce, Ph.D.

Abstract

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. Due to the difficulty of measuring these 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 a significant greenhouse gas, but the controls on its emissions from streams are poorly constrained. 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, including those relevant to pollutant remediation.

Based on the literature examining pathways for N2O production in soils and sediments, the current understanding of physical properties of the 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 downstream of 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 nitrogen 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 experiments, spatial and temporal trends reflect microbiological processes, changing redox conditions, dissolution, sorption and desorption. In general, microbial respiration causes dissolved oxygen 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. 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 bedform 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.

DOI

10.18122/td/1415/boisestate

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