Dynamic Steady State in Coastal Aquifers Is Driven by Multi‐Scale Cyclical Processes, Controlled by Aquifer Storativity

Abstract

Coastal aquifers supply freshwater to nearly half the global population, yet they are threatened by salinization. Salinities are typically estimated assuming steady‐state, neglecting the effect of cyclical forcings on average salinity distributions. Here, numerical modeling is used to test this assumption. Multi‐scale fluctuations in sea level (SL) are simulated, from tides to glacial cycles. Results show that high‐frequency fluctuations alter average salinities compared with the steady‐state distribution produced by average SL. Low‐frequency forcing generates discrepancies between present‐day salinities estimated with and without considering the cyclical forcing due to overshoot effects. This implies that salinities in coastal aquifers may be erroneously estimated when assuming steady‐state conditions, since present distributions are likely part of a dynamic steady state that includes forcing on multiple timescales. Further, typically neglected aquifer storage characteristics can strongly control average salinity distributions. This has important implications for managing vulnerable coastal groundwater resources and for calibration of hydrogeological models.

Morphologic, Atmospheric, and Oceanic Drivers Cause Multi‐Temporal Saltwater Intrusion on a Remote, Sand Island

“…However, the pronounced loss of the freshwater lens along the MS transects on South Beach (and potentially concomitant vertical lens thinning) suggest that erosion has been the major SWI driver along the MS transect. In agreement with past studies, we demonstrate that short‐term groundwater monitoring programs only present a snapshot of coastal groundwater dynamics and are not representative of long‐term continuous changes in subsurface salinity distributions (Grünenbaum et al., 2023; Heiss & Michael, 2014; Paldor, Frederiks, & Michael, 2022). Coastal aquifers are challenging to monitor directly, and long‐term monitoring is prohibitively expensive, forcing coastal water resource management to rely heavily on numerical models calibrated to few field observations (Werner et al., 2017).…”

“…Further, models that inappropriately assume a present‐day snapshot of coastal groundwater dynamics is representative of steady‐state initial conditions prior to future climatic forcing will produce invalid results. This emphasizes the importance of including the continuous response of coastal aquifers to multi‐scale forcing in alignment with the suggestions of Paldor, Frederiks, and Michael (2022). Our results highlight the importance of erosion and suggest that desktop reviews to assess past SWI should begin with assessing past erosion via analysis of aerial imagery or publicly available LiDAR data.…”

“…Groundwater flow and salinity patterns in coastal aquifers are complex as aquifer dynamics are influenced by hydraulic properties, vegetation, topography, ocean hydrodynamics, hydrometeorology, and coastline morphology across a range of temporal and spatial scales (Housego et al., 2021; Paldor, Frederiks, & Michael, 2022). Much of the scientific literature on SWI focuses on a single driver, such as groundwater pumping (Bear et al., 1999) or rising seas (e.g., Ketabchi et al., 2016), and either lateral or vertical intrusion pathways (Cantelon et al., 2022; Werner et al., 2013).…”

“…This threatens critical freshwater resources for island inhabitants and ecosystems (Werner et al., 2017). SWI dynamics in these islands occur across multi‐temporal scales, and their interplay makes identifying the individual effect of the different driving processes impossible (Paldor, Frederiks, & Michael, 2022). For example, morphology and bathymetry influence wave breaking, run‐up, and groundwater flow in the swash zone, resulting in complex feedbacks between morphodynamics, hydrodynamics, and SWI processes (Ataie‐Ashtiani et al., 2013; Paldor, Stark, et al., 2022; Y. Zhang et al., 2016).…”

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

“…The 10‐year mark was chosen because past studies have reported that aquifers reached potable conditions within about 10 years of a storm event (Anderson & Lauer, 2008; Yang et al., 2013). Importantly, recovery times that are longer than surge recurrence times may result in an accumulated salinization effect (Paldor et al., 2022). However, for the studied site and for small islands in general, it is likely that full recovery is reached on shorter time scales and therefore we neglected the effect of recurring storms.…”