Date of Award

Spring 2023

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Timothy J. Shaw

Abstract

Ground, pore, and surface water exchange has been extensively studied in beach and near shore environments but has been long neglected in saltmarsh systems due to their complexity. Saltmarsh systems are of particular importance as they are the interface between fresh and saltwater. Saltmarshes impact carbon exchange, release and sequestration, and they process particulate matter. Understanding the characteristics of a marsh system such as the nutrient and particulate loads in conjunction with the flux of water exchange through the marsh provides a deeper understanding of these systems and can impact our understanding of the global water and carbon cycles, as well as local impacts such as pollution management. Flux in a marsh system can be impacted by many different and simultaneous physical drivers. This work is a study of the Folly Marsh in coastal South Carolina in the context of expanding our knowledge of water exchange in salt marshes.

Chapter 2 introduces and verifies the abilities of a technique to measure the flux of the coastal marsh porewater and submarine groundwater exchange. This technique uses radium daughter and thorium parent isotopes and their known regeneration behavior to calculate a disequilibrium of these isotopes as related to water flux in coastal sediments. This method integrates the heterogeneous nature of sediments on a localized scale. This method is evaluated for method, sampling, and handling error as well as sediment characteristics such as grain size. This method was found to have a sensitivity of 20 L/m2 /day.

Chapter 3 presents work for quantifying submarine groundwater discharge (SGD) and differentiating it from porewater exchange (PEX) which requires special consideration. A particular consideration is PEX involves recycling water in the tidal prism, while SGD is typically considered only an outward flow from the submarine to the surface. This chapter presents four mass balances and models are used to estimate SGD in Folly marsh South Carolina. A basic mass balance in conjunction with PEX estimates from previous sediment work and detailed bathymetry data indicate that SGD is a significant contributor to the overall flux. Three sampling campaigns are undertaken to measure the radium quartet as transects across the marsh. A three-end member mass balance model of 226Ra finds a return flow factor b for three transects, with flushing times ranging from 0.6 – 2 days. The apparent age of water in the system was also calculated using the decay of 224Ra relative to 228Ra. These estimates for the upper marsh agree with the flushing times at ~1.8 days. The apparent age was found to increase along the transect towards the older ocean member which is indicative of mixing. Finally, a model that considers decay, mixing, and PEX is used to calculate SGD. In this model the major export of activity is mixing with the ocean endmember, and the major imports are PEX and SGD. SGD flux was found to be a greater than PEX by as much as a factor of 2.

Rights

© 2023, Madeline R. Bodine

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