Date of Award
8-2024
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Hydrogeology
Committee Chair/Advisor
Dr. Ronald Falta
Committee Member
Dr. Lawrence Murdoch
Committee Member
Dr. Brady Flinchum
Committee Member
Dr. Rong Yu
Abstract
Saltwater intrusion is a growing problem that is being exacerbated by climate change. Saltwater intrusion is the process by which saltwater moves towards freshwater supplies driven by saltwater’s greater density compared to freshwater. Saltwater intrusion can contribute to the salinization of freshwater resources, degradation of underground storage tanks, communication systems, and pipelines, as well as lead to groundwater flooding (Ketabchi et al., 2016). Numerical modeling can be used to simulate saltwater intrusion along coastal areas. One of the more computationally efficient modeling tools is the sharp interface approach, SWI2, that is a part of the MODFLOW 2005 suite (Werner et al., 2013 and Bakker et al., 2013). The SWI2 saltwater interface model allows for groundwater flow to be simulated in a single layer by assigning zones of specified salt concentration and using a vertically integrated groundwater head equation to predict the salt and freshwater interface without solving a separate solute transport equation. A new sharp interface model is being created for MODFLOW 6, known as SWI6 as part of an ongoing project with the U.S. Navy (Patterson, 2023). This model will be capable of working with MODFLOW 6 unstructured grids and utilize Newton Raphson linearization of nonlinear flow terms.
The long-term focus of this Navy project is to integrate the new sharp-interface approach, SWI6, with the Groundwater Modeling System (GMS) graphical user interface. Two example test sites have been selected, the Nav (MCAS) Beaufort, SC and Naval Base Kitsap (NBK) Keyport, WA. The objective is to determine the fresh and saltwater interface at these study sites under future sea-level rise conditions, as well as evaluate the effect rising sea-level has on the groundwater elevation. The SWI6 model is still under development, and therefore, only the SWI2 package was used to simulate these test sites. The goal is for the SWI6 saltwater interface model to produce numerical results similar to the SWI2 model, but with greater efficiency and flexibility.
The saltwater interface models were built by initially developing a case study model of the MCAS Beaufort and NBK Keyport sites. Geologic solids models representing MCAS Beaufort and NBK Keyport, and the surrounding areas were built based on published borehole data, digital elevation models (DEMs), and groundwater characterization reports. The hydrogeologic parameters of the sites were spatially variable and required parameter estimation and calibration to be performed to estimate hydraulic conductivity and recharge values. Calibration was performed by using observation wells with long-term water level averages as the target water level in that area. The hydraulic conductivity and recharge parameters were adjusted through a trial-and-error iterative process. The flow model was run repeatedly, with parameters being modified each time until the residual between the observed heads and the calculated heads fell within an acceptable range.
The calibrated flow models were then used to estimate the location of the fresh and saltwater interface using the SWI2 package. Two scenarios were analyzed under steady-state conditions, with one assuming the current sea-level and the other the rising sea-level predicted by the Department of Defense Regional Sea-level (DRSL) database (www.drsl.serdp-estcp.org/site/lookup). The rising sea-level prediction used the extreme sea-level rise scenario for the year 2100. The SWI2 results were analyzed by determining the fraction of saltwater in the aquifer, and results were compared with the fully coupled SEAWAT transport model (Langevin et al., 2003).
Results of the steady and rising sea-level simulations at MCAS Beaufort and NBK Keyport indicate that the fresh and saltwater interface are stable after a 7.5 ft and 7.6 ft rise in sea-level. The freshwater heads at both study sites were high enough to prevent any movement from the salt wedge under increasing sea-level. However, there is a significant increase in the groundwater elevation of Port Royal Island and the Keyport Peninsula following the sea-level rise, as well as groundwater flooding. These test cases will be used as case studies when the new SWI6 package becomes available.
Recommended Citation
Hiott, Caroline, "Evaluation of Future Sea-Level Impacts Using a Sharp Interface Saltwater Intrusion Model" (2024). All Theses. 4360.
https://open.clemson.edu/all_theses/4360