Date of Graduation

Summer 8-31-2025

Document Type

Thesis

Degree Name

Master of Science in Chemistry

College/School

College of Arts and Sciences

Department/Program

Chemistry

First Advisor

Amrita Bhattacharyya

Second Advisor

Amalia Kokkinaki

Third Advisor

Jennifer A. Tripp

Abstract

Methane (CH₄) emissions at terrestrial–aquatic interfaces (TAIs) are significant contributors to the global carbon budget, yet the mechanisms driving methanogenesis remain poorly understood. Limited data exist on how seasonal variations and redox processes influence CH₄ fluxes in these dynamic ecosystems. This study addressed two questions: (1) How are CH₄ fluxes in riverine floodplains and adjacent hillslopes impacted by seasonal variations? (2) What roles do metal redox processes (e.g., iron (Fe)) play in regulating methanogenesis?

In-situ CH₄ flux and solid-phase Fe speciation were quantified in East River, CO, under flooded and drained conditions. Groundwater chemistry, gas flux, and soil physical properties were integrated to characterize seasonal and spatial transitions.

CH₄ fluxes were significantly higher in floodplains than in hillslopes, especially under flooded conditions with increased soil moisture and reduced environments. Mean floodplain CH₄ fluxes increased from –0.0068 ± 0.011 nmol/m²/s (drained, Sept 2024) to 0.530 ± 0.112 nmol/m²/s (flooded, June 2024). Hillslope fluxes remained low, from –0.352 ± 0.018 nmol/m²/s (drained) to –0.173 ± 0.011 nmol/m²/s (flooded).

Fe(II) concentrations were substantially higher under flooded conditions in floodplains (53.31 ± 5.28 mg Fe/kg) than in hillslopes (34.84 ± 3.10 mg Fe/kg), indicating strong anoxia correlating with elevated CH₄ flux. However, Fe(II)–CH₄ correlations were inconsistent, suggesting interacting controls. Soil texture likely modulates water retention and anoxia, influencing methanogenesis.

Seasonal hydrologic changes exert dominant control over CH₄ flux dynamics. These findings improve understanding of how spatial and temporal gradients, particularly hydrology and redox dynamics, shape greenhouse gas emissions from TAIs.

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