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Coastal aquifers sit at the interface between land and sea, forming a network that stores, transpires, and transmits freshwater to communities, farms, and ecosystems. When sea level rises, the natural balance between freshwater and seawater shifts, allowing saltier water to push inland and upward. The result is a moving boundary between fresh groundwater and the saline intrusions from the ocean. This intrusion not only degrades drinking water quality but also reduces the available freshwater resource in coastal aquifers. The process occurs gradually as hydrostatic pressures redistribute, yet localized disruptions through pumping and recharge changes can accelerate salinization in vulnerable pockets.

The intrusion mechanism hinges on density differences between freshwater and saline water, creating a distinct interface called the fresh-salt boundary. In a freshwater lens, less dense freshwater sits atop denser seawater, maintaining a stable gradient. As sea level rises, the hydraulic head along coastlines increases seawater pressure, pushing the interface landward and landward-like inland. Over time, repeated droughts or heavy rainfall events modify recharge rates, altering the thickness of the freshwater lens. Human activities, such as excessive groundwater pumping, can exacerbate the intrusion by pulling water from the lens and creating downward migration of the salinity front, threatening wells and agricultural zones.

In many coastal regions, the freshwater lens is a dynamic, layered phenomenon. The upper portion sustains domestic wells and irrigation systems, while deeper zones may feed industrial supplies or regional streams. As salinity rises, the density contrast at depth can intensify, leading to more abrupt vertical transitions in chemical composition. This layering can protect some zones from rapid salinization, yet it also concentrates salt in selective intervals that are difficult to remediate. Understanding the vertical distribution of salinity helps water managers tailor well placement and design freshwater treatment systems that minimize disruption during saltfront movement.

Climate-driven sea level rise does not act alone; it interacts with land use, geology, and hydrological timing. Surficial recharge from rainfall and snowmelt can either bolster the freshwater lens or fail to replenish it during droughts. Coastal sediments with high permeability may transmit saline water quickly, while clay-rich layers slow groundwater movement and create elongated response times. Modeling these processes requires integrating sea level projections, local aquifer geometry, recharge cycles, and pumping regimes. By simulating different scenarios, scientists and planners can anticipate vulnerable wells, design protective barriers, and guide sustainable extraction practices that balance supply and water quality.

Saline intrusion alters the chemistry of groundwater, affecting not only taste but also mineral content and corrodibility of infrastructure. As salts invade the freshwater lens, chloride, sulfate, and sodium concentrations rise, potentially exceeding safe drinking water standards. In agricultural areas, high salinity can stress crops, reduce yields, and necessitate investment in desalinization or blended water strategies. The economic implications extend to utility operations, where treatment costs increase and maintenance needs grow. Monitoring networks that track electrical conductivity, temperature, and specific conductance become essential tools for early warning, allowing communities to adjust pumping schedules before wells reach unacceptable salinity levels.

Even small changes in rainfall patterns can amplify salinization risk. Intense rainfall events increase recharge and diluting fresh water, but rapid groundwater discharge during dry spells concentrates salts near the plume edges. In coastal aquifers near estuaries or tidal channels, tidal cycles can drive oscillations in salinity that propagate inland with each flood tide. Over time, these oscillations can erode the integrity of freshwater pockets and cause long-term shifts in the subsurface boundary. Adaptive management, including groundwater banking or managed aquifer recharge with fresh water, can help mitigate declines in freshwater availability while preserving water quality.

The geographic variability of coastal aquifers means responses to sea level rise are not uniform. In dunes and beach ridge systems, thinner, more permeable sands permit faster inland movement of saline fronts, while thicker, clay-rich sediments slow progression. Karstic coastlines, with conduits allowing rapid groundwater flow, present unique challenges as salinity pulses can bypass some barriers. In continental shelf regions, saltwater may intrude via submarine groundwater discharge, adding a hidden dimension to salinization that affects coastal ecosystems and microbial activity. Recognizing these heterogeneities helps tailor mitigation measures, from aquifer storage projects to targeted pumping adjustments.

Collaboration across disciplines strengthens the understanding of saline intrusion. Geologists, hydrogeologists, oceanographers, and engineers map subsurface properties, ocean boundary conditions, and pumping regimes to build robust scenarios. Field observations, including monitoring wells, electrical resistivity surveys, and tracer tests, reveal how salinity moves through different sediments. Remote sensing and GIS tools enable large-scale visualization of changing interfaces, while citizen science programs provide ground-level observations of well performance and water taste. Together, these approaches produce actionable knowledge for communities facing rising seas and shifting groundwater supplies.

One practical response is to optimize pumping strategies to prevent rapid drawdown of the freshwater lens. By aligning extraction with recharge periods and diversifying water sources, utilities can limit freshwater-saltwater interface movement. Where feasible, managed aquifer recharge using clean water allows the aquifer to store freshwater during wet periods and resist salinization during dry spells. In addition, installing barrier wells or constructing subsurface dams can redirect saline fronts away from critical wells. Such interventions require careful design to avoid unintended consequences, such as concentrating salinity in other parts of the aquifer system.

Rehabilitating degraded aquifers calls for adaptive governance and long-term planning. Policymakers should incorporate sea level rise projections into water resource models, setting safety margins for well placement and aquifer storage. Community engagement ensures that residents understand the risks and participate in decisions about water use, land management, and infrastructure investments. Financial mechanisms, including demand management programs and resilience funding, support the transition toward sustainable groundwater supply under changing coastal conditions. The overarching aim is to preserve freshwater access while protecting ecosystem services reliant on balanced salinity regimes.

Monitoring remains the foundation of resilient coastal groundwater management. Establishing dense networks of observation wells, salinity sensors, and surface water–groundwater interaction probes provides continuous insight into interface movement. High-quality data enable early detection of salinity incursions and permit timely adjustments to pumping and recharge operations. Data-sharing agreements among agencies, universities, and communities accelerate scenario testing and knowledge transfer. In parallel, improving the accuracy of sea level rise forecasts and aquifer property characterizations reduces uncertainty, guiding decision-makers toward prudent, proactive measures rather than reactive responses.

Ultimately, safeguarding coastal aquifers requires an integrated approach that links climate projections to groundwater stewardship. By combining field data, predictive modeling, and adaptive management, regions can build flexible groundwater budgets that absorb variability in recharge and sea level. Emphasizing prevention—through sustainable extraction, recharge enhancement, and coastal restoration—reduces the likelihood of abrupt water quality declines. The ongoing challenge is to maintain reliable freshwater supplies while preserving the ecological balance at the land-sea interface, ensuring communities thrive despite rising seas and evolving coastal hydrology.