Date of Award
8-2026
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Hydrogeology
Committee Chair/Advisor
Dr. Ronald Falta
Committee Member
Dr. Lawrence Murdoch
Committee Member
Scott Brame
Abstract
Rising sea levels caused by global warming exacerbate saltwater intrusion, the inland movement of seawater into coastal aquifers. These processes increase susceptibility to freshwater contamination, surface flooding, and infrastructure damage. Numerical models are utilized to assess hazards to coastal aquifers on a regional scale.
The U.S. Navy is currently developing a robust, reduced-order groundwater modeling platform, SWI6, that uses the sharp interface approach to approximate the location of the freshwater-saltwater interface to define the extent of saltwater intrusion within an aquifer. Featuring compatibility with MODFLOW 6 unstructured grid formats and the Newton-Raphson formulation for better solution stability, SWI6 improves upon existing versions of the Seawater Intrusion (SWI) Package with the additional capability of computing steady-state solutions using the single-fluid freshwater simplification. In this approach, the saltwater domain is assumed to equilibrate instantly to freshwater movement so that only the freshwater equations are solved through application of the Ghyben-Herzberg approximation. Implemented in an updated version of Aquaveo’s Groundwater Modeling System (GMS 10.9), a tool in the SWI6 package automatically integrates site-specific future sea-level predictions into groundwater flow model boundary conditions.
The objective of this thesis is to demonstrate successful implementation of the SWI6 Package into GMS, then assess future sea-level rise impacts associated with saltwater intrusion at three Department of War coastal installations.
This research undertakes initial implementation of the SWI6 Package in two phases. Primarily, numerical solutions produced with SWI6 are verified against Strack’s 1976 analytical solutions for combined shallow interface flow within a coastal aquifer cross section under both confined and unconfined conditions. Following reproduction of these steady-state results, a transient MODFLOW 6 simulation with SWI6 is conducted for a hypothetical three-dimensional coastal aquifer that incorporates the effects of sea-level rise. In addition to establishing a simulation workflow, the resulting solution predicts groundwater rise, increased flooding, and advancement of the saltwater wedge, all of which are expected with an increase in sea level.
The SWI6 Package is then utilized to assess saltwater intrusion at three Department of War installations through 2100: Marine Corps Air Station (MCAS) Beaufort, SC, Naval Base Kitsap (NBK) Keyport, WA, and U.S. Army Garrison (USAG) Kwajalein Atoll, Republic of the Marshall Islands. Groundwater flow models of MCAS Beaufort and NBK Keyport are adapted from Hiott’s (2024) simulations conducted with MODFLOW-2005 using SWI2. Treating the previous simulations as a benchmark to compare the SWI Packages, the Department of Defense Regional Sea Level (DRSL) Database future sea-level rise predictions of 7.5 and 7.6 ft are used for each respective site. The third site, USAG Kwajalein Atoll, is constructed from a case study model depicting the regional hydrogeologic framework and sources the estimated Intermediate sea-level rise magnitude of 3.4 ft from the National Oceanic and Atmospheric Administration (NOAA) Sea Level Rise Viewer.
The simulations indicate that flooding is the most prevalent hazard resulting from an increase in sea level at MCAS Beaufort and NBK Keyport whereas flooding at USAG Kwajalein Atoll is confined to the reef flats. Saltwater intrusion depicted with SWI6 is typically more extensive than Hiott’s (2024) solutions produced with SWI2. With the SWI6 GMS tool, the boundary conditions automatically update with sea level to simulate gradual inundation and groundwater discharge to the surface, comparatively limiting the freshwater heads that offset saltwater intrusion. In each simulation, saltwater intrudes into the principal aquifer and is generally confined to the areal vicinity of the shoreline by recharge to the system. In multi-layer groundwater models, such as USAG Kwajalein Atoll and MCAS Beaufort, sea-level rise also reduces the depth of localized freshwater lenses beneath island features.
Ultimately, the groundwater flow models created with SWI6 demonstrate that the installations are vulnerable to an increase in mean sea level. Numerical simulations using the SWI6 Package are more accessible than alternative approaches used to model saltwater intrusion and provide suitable assessments of future sea-level rise hazards across diverse coastal settings.
Recommended Citation
Defalco, Alexandra G., "Implementation of a Practical Sharp Interface Saltwater Intrusion Model to Assess Future Sea-Level Impacts" (2026). All Theses. 4844.
https://open.clemson.edu/all_theses/4844
Included in
Geology Commons, Hydrology Commons, Water Resource Management Commons, Water Resources Engineering Commons