Electronic Theses and Dissertations

Identifier

6046

Date

2017

Date of Award

11-8-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Earth Sciences

Concentration

Environmental and Earth Sci

Committee Chair

Daniel Larsen

Committee Member

Roy Van Arsdale

Committee Member

Reza Pezeshki

Committee Member

George Swihart

Abstract

This study tests the hypothesis that Mississippi River islands significantly affect Nitrogen cycling in the Lower Mississippi River (LMR). The hypothesis was tested by characterizing stratigraphy, hydrogeology and water quality in and adjacent to two Mississippi River islands near Memphis, Tennessee; one vegetated island: Loosahatchie Bar (LB), and one non-vegetated island: Densford Bar (DB). Wells were installed to assess stratigraphy, hydrology, and water quality. Sediment was analyzed for sedimentological features, grain size distribution, total organic and inorganic carbon (TCs), and total nitrogen (TNs) content. Water samples and in situ analyses of dissolved oxygen (DO) and temperature were collected from wells and adjacent river locations. Lab analyses included conductivity, pH, alkalinity titration, and anion analysis. Additionally, samples were tested for dissolved organic carbon (DOC), total nitrogen (TN), and ammonia. Results of sediment analysis indicate that DB consists mostly of sand representing a high-energy, fluvial environment with minor lenses of fine material, whereas sediment at LB sites consists of alternating layers of fine sand and silty clay representing vertical accretion deposits from flood events in a low-energy fluvial setting. Conceptual models developed suggest DB behaves similar to a single, unconfined aquifer with perched zones, whereas LB acts as a series of confined to semi-confined aquifer units with both being dominated by radial, lateral drainage as river stage decreases. Peak TCs and TNs values were observed in samples with the highest organic content and percentages of fines, suggesting increased potential for nitrogen transformation and/or entrapment in fine, organic sediment. Results of chemical analyses indicate non-vegetated island well waters on the margin largely resemble the adjacent river water but differ from water samples interior to the island. Alkalinity, DOC, and DO relationships to NO3 and TN suggest that denitrification is the dominant process at LB, whereas the oxidizing areas of DB promote nitrification and the reduced environment of the interior are likely dominated by Dissimilatory Nitrate Reduction to Ammonia. These findings support the hypothesis that significant nitrogen cycling occurs in the subsurface of islands and suggest both types of islands play a significant role in cycling in the LMR.

Comments

Data is provided by the student.

Library Comment

dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.

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