Saltwater Intrusion Into a Confined Island Aquifer Driven by Erosion, Changing Recharge, Sea‐Level Rise, and Coastal Flooding

Abstract

Aquifers on small islands are at risk of salinization due to low elevations and limited adaptive capacity, and present risks will be exacerbated by climate change. Most studies addressing small‐island saltwater intrusion (SWI) have focused on homogeneous sandy islands and one or two hydraulic disturbances. We herein investigate SWI dynamics in a layered, confined island aquifer in response to multiple environmental perturbations related to climate change, with two considered in tandem. Our field and modeling work is based on an island aquifer that provides the drinking water supply for an Indigenous community in Atlantic Canada. Observation well data and electrical resistivity profiles were used to calibrate a numerical model (HydroGeoSphere) of coupled groundwater flow and salt transport. The calibrated model was used to simulate the impacts of climate change including sea‐level rise (SLR), storm surge overtopping, changing aquifer recharge, and erosion. Simulated aquifer conditions were resilient to surges because the confining layer prevented deeper saltwater leaching. However, reduced recharge and erosion resulted in saltwater wedge migration of 170 and 110 m, respectively when considered individually, and up to 295 m (i.e., into the wellfield) when considered together. Despite the confining conditions, SLR resulted in wedge migration up to 55 m as the confining pressures were not sufficient to resist wedge movement. This is the first study to harness an integrated, surface‐subsurface hydrologic model to assess effects of coastal erosion and other hydroclimatic stressors on island aquifers, highlighting that climate change can drive extensive salinization of critical groundwater resources.

Interrelated Coastal Flooding, Erosion, and Groundwater Salinization on a Barrier Island During Hurricane Fiona

“…Paired monitoring data show that erosion and dune topography are the dominant drivers of SWI and flushing on Hog Island. While past work considered lateral SWI from perturbations in groundwater hydraulic heads and sea levels (Werner et al, 2013) and vertical SWI from decadal erosion and flooding Stanic et al, 2024), the present study demonstrates that coastal erosion can cause rapid vertical and lateral SWI over short timescales (<1 year) that persists if morphologic recovery does not occur. Islands come in many shapes and geologies, and the results of this study apply to relatively unfortified, low-lying islands with erodible sediments, particularly small island developing states, that often have insufficient resources to build extensive coastal defense structures (Nunn et al, 2021;Pelling & Uitto, 2011).…”

“…This island is composed of highly erodible sandstone, and despite low wave heights in Malpeque Bay due to the barrier islands (Figure 1b), the area of Lennox Island decreased by 18% between 1880 and 2010 driven by rising sea levels and winter wave action intensification (Dietz & Arnold, 2021;MacDonald, 2014). Erosion is an ongoing threat to the Lennox Island Mi'kmaq First Nation's fresh groundwater, infrastructure, archeological records, history, and culture and is a somber reminder of the critical role that barrier islands play in protecting sentinel coastal zones (Stanic et al, 2024). To preserve Mi'kmaq cultural land use traditions, ecological integrity, and ecosystem services tied to this barrier island chain, the Mi'kmaq of PEI and Parks Canada established Pituamkek ("At the long sand dune"), herein referred to as Hog Island, as a Canadian National Park Reserve (Parks Canada, 2021).…”

“…There is a need to understand how these processes along natural coastlines compare with those along developed coasts, where hardened structures may reduce flooding but consequently slow aquifer recovery (Lee et al, 2019). Future studies should also consider how erosion and SWI can interact with groundwater pumping and urbanized hydrologic regimes (Stanic et al, 2024).…”

“…Such changes can occur in response to groundwater pumping (Bryan et al, 2016), climate variations (precipitation and evapotranspiration; Briggs et al, 2021;Holding& Allen, 2015a), ocean levels (sea levels, tides, surge, waves; Anderson, 2002;Ataie-Ashtiani et al, 1999, and changes in island geomorphology (Chesnaux et al, 2021;Huizer et al, 2017;Schneider & Kruse, 2003). Further, climatic, oceanic, and morphologic drivers interact; for example, changes in morphology can impact the area for fresh recharge (Zhang et al, 2016) or the inundation of land with seawater (Stanic et al, 2024;Yu et al, 2016). However, most studies have assessed drivers in isolation, and few have considered the co-evolution of morphology and fresh groundwater resources.…”

“…Coastal geomorphologists have made significant progress in understanding shoreline movements in response to storms and interannual climate variability (Hesp & Walker, 2022;Houser et al, 2008;Mathew et al, 2010). Meanwhile, advances in groundwater monitoring and numerical modeling have allowed coastal hydrogeologists to investigate coastal groundwater dynamics, including the effects of coastal flooding (Elsayed & Oumeraci, 2018;Oberle et al, 2017;Post & Houben, 2017), gradual shoreline transgressions (Holt et al, 2019;Schneider & Kruse, 2006;Stanic et al, 2024), and anthropogenic nourishment and excavation (Briggs et al, 2021;Huizer et al, 2016) on aquifer salinity distributions. However, research assessing how rapid shoreline retreat during coastal storms impacts aquifer salinity remains limited as traditional monitoring infrastructure is often damaged or lost during storms, and inflexible groundwater model domains limit numerical simulations of these coupled surface-subsurface processes.…”