Assessing Groundwater Recharge Dynamics in Coastal Aquifers under Climate Change Scenarios using Isotopic Tracers and Numerical Modeling

Project Abstract

Coastal aquifers are increasingly experiencing altered recharge regimes and salinization pressures driven by climate variability and sea-level rise, necessitating a comprehensive evaluation of groundwater recharge dynamics under shifting hydrological and geochemical conditions. This study integrates isotopic tracers (d18O, d2H, 3H, 14C, and 13C) with high-resolution numerical modeling to quantify recharge fluxes, identify dominant recharge pathways, and distinguish between natural and anthropogenic influences in a representative coastal setting. Field campaigns were conducted over two hydrological years to capture seasonal and interannual variability, including pore water sampling from multiple aquifer depths, surface water–groundwater interactions at seepage zones, and town-scale well hydraulics to calibrate models. Isotopic end-members and groundwater age indicators were used to discriminate meteoric recharge from seawater intrusion and to resolve lag times between rainfall events and aquifer responses. A coupled hydrogeochemical framework was developed to simulate recharge processes in a variably saturated zone, incorporating soil–water–rock interactions, preferential flow through fractures, and dynamic recharge height under sea-level fluctuations. The numerical model employed (a calibrated MODFLOW-6/SEAWAT-3D-Flow- and solute-transport suite) was parameterized with site-specific hydraulic conductivity fields, porosity, dispersivity, recharge intensity, and boundary conditions informed by climate projections downscaled to the watershed scale. Scenario analyses encompassed Representative Concentration Pathways (RCP 4.5 and RCP 8.5) and local sea-level rise trajectories to assess impacts on recharge distribution, freshwater lens sustenance, and salinity intrusion risk. The integrated framework enables partitioning of recharge contributions from direct rainfall infiltration, diffuse percolation through unsaturated zones, and indirect recharge via riverine and irrigation return flows, while accounting for isotopic age dating to trace groundwater persistence and renewal rates. Sensitivity analyses identified critical controls on recharge dynamics, including evapotranspiration, soil moisture storage, land-use changes, and coastal pore-water mixing. The results reveal spatially heterogeneous recharge patterns, with higher recharge zones linked to permeable lithologies and higher rainfall intensity events contributing disproportionately to the aquifer system. Under future climate scenarios, recharge amplitude shows a tendency toward increased episodic fluxes but with elevated attenuation due to prolonged drought periods and heightened evaporation, potentially accelerating seawater intrusion in low-lying sectors. The isotopic signatures provided robust constraints on model calibration, reducing equifinality and improving confidence in forecasts of aquifer resilience. Policy-relevant outputs include threshold rainfall regimes required to sustain freshwater lenses, optimal well-field placements to mitigate salinity risk, and adaptive groundwater management strategies aligned with climate projections. The study demonstrates the efficacy of combining isotopic tracers with advanced numerical modeling to resolve complex recharge processes in coastal aquifers, offering a transferable methodology for similar settings facing climate-driven hydro-geochemical challenges.

Project Overview

What This Project Is About

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The project looks at how groundwater in coastal areas gets replenished, especially as the climate changes. It uses simple tools to trace how water moves underground and combines real-world data with computer models to understand recharge rates and pathways.

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The Problem It Addresses

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Coastal aquifers can be overused or contaminated, and climate change can alter rainfall, sea levels, and evaporation. This makes it hard to predict how much groundwater is recharged and how vulnerable the supply is. The project aims to fill gaps in understanding recharge under changing conditions.

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Objectives of the Project

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1. Explain how coastal groundwater is replenished in simple terms.
2. Identify factors that control recharge under different climate scenarios.
3. Learn how isotopic tracers can reveal water sources and movement.
4. Use a basic numerical model to simulate recharge and flow.
5. Evaluate potential risks to groundwater quality and availability.

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What You Will Do Step by Step

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1. Gather geographic and climate data for a chosen coastal area.

2. Collect or review isotopic data to distinguish water sources.

3. Learn simple data analysis to interpret tracers (what they reveal about origin and travel time).

4. Build or adapt a straightforward groundwater model to simulate recharge scenarios.

5. Run scenarios for different climate futures and assess outcomes.

6. Compare model results with observations where available.

7. Discuss uncertainties and limitations of the approach.

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Expected Outcome

Students will produce a clear, practical understanding of how coastal groundwater recharge responds to climate change, plus a simple model and data interpretation guide that can be extended in future work. The project aims to inform local water managers about recharge trends and potential risks to groundwater supply.