Megatidal controls on coastal groundwater and saltwater intrusion dynamics along agricultural dykelands

Abstract

Climate change is projected to substantially increase the global mean sea level, which will likely have adverse impacts on coastal communities worldwide. Atlantic Canada is particularly vulnerable to the effects of climate change due to its low-lying elevation and high rates of past and projected sea-level rise. Nova Scotia, Canada has over 240 kilometers of dykes that protect coastal areas from seawater inundation. Much of the dyked coast is impacted by mega-tidal conditions, which can have significant impacts on saltwater intrusion and groundwater flow dynamics along the coast. While the effects of surface flooding receive the most attention, coastal aquifers in dykelands are also vulnerable to salinization from higher maximum surface water elevations arising from sea-level rise, intensifying storms, or the highest tides. The goal of this study was to use both field methods and numerical modeling techniques to investigate saltwater intrusion dynamics in a mega-tidal dykeland setting to assess how the groundwater system responds to present forcing and to investigate how future climate change may drive further saltwater intrusion. This was accomplished by initiating a field campaign near the town of Wolfville, Nova Scotia. A climate station, wave buoy, and tidal station were deployed; shallow piezometers were drilled and instrumented; deep existing town wells were instrumented; and geophysics surveys were conducted. The field data highlight the strong connectivity between the Bay of Fundy and the adjacent aquifer given the observed pronounced tidal variations in groundwater levels and ground resistivity. These data were used to establish an environmental baseline to calibrate a present-day coupled numerical model of variable-density groundwater flow and salt transport. This model was then forced with climate scenarios including projections for sea-level rise and storm-induced dyke overtopping. Numerical model results indicate that storm surges pose the largest threat to the aquifer and shallow agricultural soils, particularly when the dyke is removed, and overtopping occurs. SLR results also decreased the freshwater volume in the aquifer by 32% for the worst-case scenario. These results suggest that dykeland management decision frameworks should include coastal groundwater to ensure a sustainable future for drinking water and irrigation resources.