Date of Graduation

5-2026

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Befus, Kevin

Committee Member

Holland, Edward

Second Committee Member

Ruhl-Whittle, Laura

Keywords

California; Groundwater; Hydrogeology; MODFLOW 6; Tidal Restoration

Abstract

Coastal wetlands represent a dynamic but vulnerable terrestrial-aquatic interface with future resilience controlled by historic hydrologic alterations, sea-level rise, and groundwater extraction. In diked coastal settings, poor tidal connectivity can degrade habitat from the impoundment of freshwater or hypersaline surface waters. Restoring tidal connectivity could foster marsh revitalization, but altering the surface hydrologic system could lead to unwanted salinity intrusion in shallow aquifers. Saline groundwater intrusion degrades freshwater quality and, while inevitable with sea-level rise, could be accelerated with tidal marsh restoration. To investigate this, I developed a steady-state groundwater flow model of Pelican Marsh, a diked, muted tidal marsh in Elkhorn Slough, Monterey Bay, California, where restoration efforts aim to reestablish tidal connectivity currently blocked by a railroad and undersized tide gates. I simulated current and higher sea levels of 0.25 m, 0.5 m, 0.75 m, and 1.0 m under tidally restored and unrestored scenarios, with sensitivity testing across hydraulic conductivities of 0.1–10 m/day and variable recharge, resulting in 90 simulations. From these simulations, I identified the extent and interface elevation of the saline groundwater wedge using the Ghyben-Herzberg relation. My results indicate that sea-level rise is the dominant control on inland migration of the saline interface, driving ~25 m of landward movement under 1.0 m of rise compared to ~5 m from tidal restoration, with hydraulic conductivity exerting the strongest control on intrusion extent across all scenarios. These findings suggest that sea-level rise poses a greater long-term risk to shallow groundwater quality in this system than tidal restoration, and that improved aquifer characterization and monitoring would strengthen future groundwater studies at Elkhorn Slough.

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