Fluid and Pressure Dynamics in Natural and Engineered Coastal Aquifer Systems

Coastal aquifers are increasingly impacted by groundwater depletion, seawater intrusion, and land subsidence driven by long-term pumping. This dissertation uses 3D numerical modeling to evaluate how variable-density flow and geological heterogeneity influence pressure response, intrusion geometry, and deformation in stressed coastal systems. Three aquifer domains are examined: homogeneous aquifers, confined aquifers with continuous clay layers, and heterogeneous aquifers containing discontinuous clay layers (DCLs). Results show that geology strongly governs intrusion patterns. Homogeneous systems produce broad inland intrusion, continuous clays enhance vertical upconing, and DCLs create irregular and asymmetric intrusion zones. Because seawater is denser and less viscous than freshwater, saltwater cases exhibit larger and more persistent drawdowns, increasing modeled subsidence by 0.2 to 0.5 m after 100 years of pumping. The dissertation also evaluates Managed Aquifer Recharge (MAR) through analysis of the Sustainable Water Initiative for Tomorrow (SWIFT) pilot program in the Virginia Coastal Plain. The Potomac aquifer overlies crystalline basement rock, raising concern about downward pressure propagation in the context of injection-induced seismicity. Ensemble simulations reproducing the 2018 to 2022 pilot injections show that injection rates near 2 million gallons per day may generate pressure increases of approximately 40 kPa in the upper 200 m of the basement, although this response remains localized to within 2 km of the injector. Finally, models incorporating newly identified heterogeneity demonstrate that 20 m thick clay interbeds and laterally extensive DCLs significantly reduce pressure transmission to the basement, improving the stability and safety of MAR operations.