COASTAL FLOODING, EROSION, AND SEA-LEVEL RISE THREATEN SMALL-ISLAND GROUNDWATER RESOURCES IN A CHANGING CLIMATE

Abstract

Dense human populations and productive ecosystems on small islands disproportionately rely on groundwater to meet their freshwater needs because fresh surface water is limited. The ocean controls groundwater level and salinity dynamics via an ocean-aquifer hydraulic connection, and saltwater intrusion (SWI) from seawater flooding and sea-level rise (SLR) will increasingly threaten potable groundwater in a changing climate. Thus, understanding SWI processes is critical to evaluate fresh groundwater vulnerability. This dissertation reviews SWI from seawater flooding to identify knowledge gaps and management challenges. Few studies have characterized complex SWI processes over multiple timescales or their feedbacks with coastal zone dynamics. Using field data and numerical models, subsequent chapters of this dissertation investigate knowledge gaps related to island groundwater dynamics and salinization over multiple timescales in response to oceanic, morphologic, hydrologic, and climatic drivers. Monitoring of Hog Island, Prince Edward Island and Sable Island, Nova Scotia, captures morphologic change with drone-based LiDAR and groundwater level and salinity dynamics with electromagnetic geophysics and monitoring wells. Data from four major hurricanes reveals that groundwater levels recover quickly, but salinization is persistent as flood effects outpace freshening from meteoric recharge. Data from Hog Island, demonstrate that rapid erosion during seawater flooding increases SWI and shifts coastal boundaries inland such that flooding from frequent smaller events (tides and seasonal waves) prolongs recovery. Comparing new data to historical data from the 1970s on Sable Island shows a feedback between frequent and rapid seawater flooding, erosion, and SWI events that drive long-term fresh groundwater loss that lags long-term morphologic change. Simulations of groundwater levels, pond areas, and SWI on Sable Island during a seawater flood and projected SLR reveal SWI along the coast, but a paradoxical increase in inland fresh surface water via groundwater rise and pond expansion. These studies provide new insights into the spatiotemporal dynamics of island freshwater resources and demonstrate the importance of understanding multi-temporal interconnections between coastal flooding/SLR, morphodynamics, SWI, and groundwater-surface water exchanges to address coastal groundwater vulnerability in a changing climate.