105 Coastal Hydrogeology in an Age of Rising Seas (Posters)
As sea levels rise, understanding hydrogeology is crucial in coastal areas. Seawater intrusion can lead to loss of potable or agricultural water supplies, and water table rise due to increasing sea level can lead to increased flooding and affect infrastructure.
Monday, 20 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1
Forecasting groundwater responses of a coastal marsh to tidal restoration and sea-level rise in the Elkhorn Slough National Estuarine Research Reserve, California
Paper number: 105-3
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM
Authors
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Kevin BefusUniversity of Arkansas - Geosciences, Fayetteville, AR
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Monique FountainElkhorn Slough National Estuarine Research Reserve, Monterey Bay, CA
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 lead to degraded habitat, potentially from the impoundment of freshwater or hypersaline surface waters, depending on the local hydrology and climate. Restoring tidal connectivity to such areas could foster marsh revitalization and net accretion with sea-level rise. However, altering the salinity of the surface hydrologic system could lead to unwanted salinity intrusion in areas with shallow aquifers. Saline groundwater intrusion into coastal aquifers degrades freshwater quality and, while inevitable with sea-level rise, could be accelerated with tidal marsh restoration projects (e.g., removing levees). To investigate this, we developed a groundwater flow model of Pelican Marsh, a diked, muted, tidally influenced salt marsh located in Elkhorn Slough, Monterey Bay, California. Restoration efforts for the marsh aim to reestablish natural tidal connectivity that is currently blocked by a railroad and a series of undersized and failing tide gates. We developed the model using current and higher sea levels of 0.5 m, 0.75 m, and 1.0 m under tidally restored and unrestored scenarios. From these simulations, we identified the extent and interface elevation of the saline groundwater wedge and evaluated how the tidal restoration could affect groundwater levels and domestic well salinities. Our results indicate that tidal reconnection of the marsh could lead to a small inland movement of the saline groundwater wedge and rising sea levels result in a small landward migration of the saline groundwater. Comparing the restoration with no restoration scenarios showed very little change in the interface position or extent, but the surficial hydrologic feedback of the restoration was represented simply in the models. Our findings suggest that the groundwater response to coastal wetland tidal restoration occurs but may have minimal influence on shallow domestic wells farther inland. However, it is important to quantify such long-term potential hazards to water quality in vulnerable coastal aquifers.
Geological Society of America Abstracts with Programs. Vol. 57, No. 6
doi: 10.1130/abs/2025AM-413026
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