Date of Award
Summer 8-1-2024
Document Type
Thesis
Degree Name
Master of Science in Coastal Marine and Wetland Studies
Department
Coastal and Marine Systems Science
College
College of Science
First Advisor
Richard Viso
Second Advisor
Angelos Hannides
Third Advisor
Bret Jarrett
Additional Advisors
Shaowu Bao, Zhixiong Shen
Abstract
The freshwater resources of barrier islands are influenced by a variety of hydrogeologic factors, which create unique regions of groundwater composition. Factors including evapotranspiration, geologic constraints, tidal forcing, and meteoric inputs influence subterranean flow creating a groundwater salinity gradient throughout the island-marsh setting which directly influences the spatial distribution of dominant vegetation on the island. Rising sea levels and variation in local climate may alter the hydrologic balance on barrier islands, potentially shifting salinity gradients and ecological baselines on short timescales. The intrusion of saltwater into the barrier island freshwater system could result in the shifting of ecotones and potential destabilization of barrier sediments.
Over three years, multiple methods were adopted to characterize the groundwaters of Waties Island, a barrier island located in northeastern South Carolina. Three study transects located on the sound side of the island encompassed the ecological shift between maritime forest and salt marsh. Shallow groundwater monitoring wells were installed along the transects to observe mixing between fresh island-derived groundwaters and more saline groundwaters from the estuary. Water table elevation, temperature, and salinity measurements were recorded at 20-minute intervals over an 11-month period (n = 24,332). Modal analysis of the data revealed tidal and meteorological drivers of groundwater level across the transect. Twenty-nine time-series electrical resistivity (ER) surveys along the transition from forest to marsh revealed horizontal mixing of groundwater within highly permeable sediment layers. Geologic analyses of sediments along the transition elucidated a geologic framework with distinct zones of permeability bounded by impermeable muddy sediment layers. The primary movement and transport of porewaters from the island into the marsh platform was determined to be horizontal with sandy overwash and erosional fans serving as conduits. No tidal signal influence was observed within the surficial aquifer in the maritime forest, however at the forest-high marsh boundary, precipitation was the principal driver of subterranean flow with spring and storm tides periodically raising the water table and reversing flow towards the island. Within the low marsh, groundwater elevation and flow direction was directly linked to the tidal stage of Dunn Sound.
The results presented here highlight the dynamic nature of shallow hydrogeologic interactions between barrier islands and back-barrier environments with implications for shifting ecological baselines, understanding the subterranean estuary of biogeochemical reactions, and determining barrier island stability in an uncertain climatic future.
Recommended Citation
Wrobel, Dean Michael, "Characterizing Groundwater Flow Across the Barrier Island-High Marsh Interface" (2024). Electronic Theses and Dissertations. 196.
https://digitalcommons.coastal.edu/etd/196