Date of Award
Doctor of Philosophy in Biological and Environmental Sciences
Environmental and Earth Sciences
Coastal groundwater plays a vital role in the sustainability of coastal ecosystems and the freshwater supply for over one billion people worldwide. Understanding the movement and interaction of saltwater and freshwater in coastal aquifers is crucial for effective water resource management. The interface between these density-contrast waters is highly dynamic and sensitive to natural drivers, such as sea level rise, storm surges, and drought, as well as anthropogenic factors, including groundwater over-pumping. In recent years, the drivers of coastal groundwater dynamics have been increasingly well understood and quantified. However, there is a need for data-model fusion, including integrating different forms of data, to address the challenges posed by the impacts of climate change and human activities.
Non-invasive geophysical techniques have emerged as effective tools for monitoring hydrological processes in coastal aquifers. For instance, the time-lapse electrical resistivity survey method, combined with groundwater monitoring, provides insight into saltwater-freshwater interaction in heterogeneous aquifers. I used this method along the southern coast of Rhode Island and developed baseline saltwater intrusion maps and demonstrated its applicability for rapidly estimating fresh groundwater discharge.
Bedrock topography delineation is essential for shallow groundwater mapping and can be achieved using non-invasive geophysical techniques, such as the Horizontal to Vertical Spectral Ratio (HVSR) seismic method. I developed a high-resolution bedrock topography map for the southern coast of Rhode Island using historical borehole lithological data and the HVSR technique. I validated the HVSR-based bedrock topography mapping with the Electrical Resistivity Imaging technique. These non-invasive geophysical techniques provide a cost-effective and time-efficient approach to monitoring hydrological processes in coastal aquifers.
While the impacts of sea-level rise, storm surges, and over-pumping have been extensively studied, the impact of droughts on coastal aquifers remains largely under-investigated. I present a new approach for evaluating the fate of a freshwater lens during drought conditions by combining in-situ observations, geophysical measurements, and numerical modeling. Using a density-driven flow model and time-lapse electrical resistivity imaging, I examined the response of a shallow unconfined aquifer on a barrier island during the 2020 Northeastern United States drought. My results indicate an 11% reduction in the freshwater lens due to reduced recharge during the drought, returning to its normal position over the 2021 spring season. The impacts of drought on coastal aquifers are likely to become more significant with the predicted increase in the frequency and severity of droughts due to climate change. Therefore, my findings highlight the vulnerability of shallow unconfined coastal aquifers to droughts, emphasizing the importance of further in-depth studies.
In summary, the understanding of coastal groundwater dynamics is critical for sustainable water resource management in the face of potential climate change impacts. Non-invasive geophysical techniques, such as the time-lapse electrical resistivity survey method and the HVSR seismic method, have been demonstrated as effective tools for monitoring hydrological processes in coastal aquifers. In addition, there is a need for more monitoring networks and multi-disciplinary collaborations to address the challenges posed by the impacts of climate change and human activities on coastal aquifers.
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Panthi, Jeeban, "GROUNDWATER DYNAMICS IN AN UNCONFINED COASTAL AQUIFER: GEOPHYSICAL INVESTIGATIONS AND MODELING" (2023). Open Access Dissertations. Paper 1529.