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The Ohio State University

1. Wallace, Corey D. Surface Water-Groundwater Exchange and its Effect on Nitrogen Transformation in the Tidal Freshwater Zone.

Degree: PhD, Earth Sciences, 2019, The Ohio State University

In coastal rivers, tidal changes in streamflow and water table elevation influence river-groundwater exchange (hyporheic exchange), with important consequences for nitrogen transport and water chemistry. Excess nitrate, one of the most bioavailable forms of nitrogen, can have adverse environmental effects on surface waters and disrupt coastal ecosystems. Here, I use field observations and numerical models to quantify how hyporheic exchange and nitrogen transport in tidal rivers vary with river stage fluctuations and sediment properties. Using spectral analyses in the tidal freshwater zone (TFZ) of White Clay Creek (Delaware, USA), I link continuous redox measurements to hydrologic perturbations within the streambed and banks. Redox potential indicates the energetic favorability of reactions such as nitrification and denitrification, which in tidal systems are strongly coupled near the fluctuating water table. Storms perturb redox potential within both the bed and banks over timescales of days to weeks, while tides drive semi-diurnal oscillations in redox potential within the streambed. Tidal redox oscillations are greatest during the late summer when river stage fluctuations are large and microbial activity is likely high. Using numerical models, I next demonstrate that nitrate removal capacity initially increases towards the coast with increasing tidal range but then declines as sediment grain size and permeability decrease, which limits the supply of nitrate to the riparian aquifer. Nitrification is a significant source of nitrate to the variably-saturated zone, but new nitrate produced by nitrification within the shallow stream bank is removed via denitrification before discharge to the river. Nitrate is also removed from groundwater sources, but little nitrate is removed from surface water. Heterogeneity in permeability and organic matter influences the distribution of reactants and transport rates. Nitrification is more efficient along high permeability sand pathways, and denitrification is localized in silt lenses with slower flow and greater organic matter content. Together, these studies reveal that tidal rivers are a sink of nitrate to coastal waters, and management strategies for TFZs should aim to protect this ecosystem service. Advisors/Committee Members: Sawyer, Audrey (Advisor).

Subjects/Keywords: Hydrology; Geochemistry; Geology; tidal; river; freshwater; nitrogen transformation; nitrification; denitrification; nitrate; hyporheic exchange; surface water-groundwater exchange

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Wallace, C. D. (2019). Surface Water-Groundwater Exchange and its Effect on Nitrogen Transformation in the Tidal Freshwater Zone. (Doctoral Dissertation). The Ohio State University. Retrieved from http://rave.ohiolink.edu/etdc/view?acc_num=osu1563363127108385

Chicago Manual of Style (16th Edition):

Wallace, Corey D. “Surface Water-Groundwater Exchange and its Effect on Nitrogen Transformation in the Tidal Freshwater Zone.” 2019. Doctoral Dissertation, The Ohio State University. Accessed November 17, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1563363127108385.

MLA Handbook (7th Edition):

Wallace, Corey D. “Surface Water-Groundwater Exchange and its Effect on Nitrogen Transformation in the Tidal Freshwater Zone.” 2019. Web. 17 Nov 2019.

Vancouver:

Wallace CD. Surface Water-Groundwater Exchange and its Effect on Nitrogen Transformation in the Tidal Freshwater Zone. [Internet] [Doctoral dissertation]. The Ohio State University; 2019. [cited 2019 Nov 17]. Available from: http://rave.ohiolink.edu/etdc/view?acc_num=osu1563363127108385.

Council of Science Editors:

Wallace CD. Surface Water-Groundwater Exchange and its Effect on Nitrogen Transformation in the Tidal Freshwater Zone. [Doctoral Dissertation]. The Ohio State University; 2019. Available from: http://rave.ohiolink.edu/etdc/view?acc_num=osu1563363127108385

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