Freshwater-saltwater Interface Research Articles

Climate change impact on groundwater resources in sandbar aquifers in southern Baltic coast

Shallow coastal aquifers are vulnerable hydrosystems controlled by many factors, related to climate, seawater-freshwater interactions and human activity. Given on-going climate change, sea level rise and increasing human impact, it is especially true for groundwater resources situated in sandbars. We developed numerical models of unsaturated zone water flow for two sandbars in northern Poland: the Vistula Spit and the Hel Spit using HYDRUS-1D. The simulations were performed for three types of land use: pine forest, grass cover and bare soil, for 2024–2100 based on weather data and sea level rise forecasts for two emissions scenarios (RCP 4.5 and RCP 8.5). The results present prognosis of groundwater recharge, water table level and water content changeability in near-term (2023–2040), mid-term (2041–2060), and long-term period (2081–2100). Expected sea level rise and decreasing hydraulic gradient of the sandbar aquifers will probably cause in-land movement of the freshwater–saltwater interface, leading to significant decrease or complete salinization of groundwater resources. The study shows that holistic monitoring including groundwater level and salinization, sea level rise, and metheorological data (precipitation amount and variability, temperature) is crucial for sustainable management of vulnerable aquifers located in sandbars.

Groundwater, salt and contaminant transport in a coastal aquifer system with a low‐permeability layer in the intertidal zone

AbstractLow‐permeability layer (LPL), formed by natural deposit or artificial reclamation and commonly found below the intertidal zone of coastal groundwater system, can retard the ingress of seawater and contaminants, and shorten the travel time of the land‐sourced contaminant to the marine environment compared with a homogenous sandy coastal aquifer. However, there is limited understanding on how an intertidal LPL, a condition occurred in a coastal aquifer at Moreton Bay, Australia, influences the groundwater and contaminant transport across the shallow beach aquifer system. We characterized the aquifer hydrological parameters, monitored the in situ groundwater heads, and constructed a 2‐D numerical model to analyses the cross‐shore hydrological processes in this stratified system. The calibrated model suggests that in the lower aquifer, the inland‐source fresh groundwater flowed horizontally towards the sea, upwelled along the freshwater–saltwater interface, and exited the aquifer at the shore below the LPL. Whereas in the upper aquifer, the tidally driven seawater circulation formed a barrier that prevented fresh groundwater from horizontal transport and discharge to the beach above the LPL, thereby directing its leakage to the lower aquifer. A contaminant represented by a conservative tracer was ‘released’ the upper aquifer in the model and results showed that the spreading extent of the contaminant plume, the maximum rate of contaminant discharge to the ocean, and its plume length decreased compared with a simulation case in a homogenous sandy aquifer. Sensitivity analysis was also conducted to investigate the characteristics of the LPL, including its continuity and hydraulic conductivity, which were found to vary along the beach at Moreton Bay. The result shows that with a lower hydraulic conductivity and continuous layer of LPL reduced the groundwater exchange and contaminant transport between upper and lower aquifer. The findings from the combined field and modelling investigations on the impact of an intertidal LPL on coastal aquifer systems highlight its significant implications to alter the groundwater and mass transport across the land–ocean interface.

Permeability–porosity model considering oxidative precipitation of Fe(II) in granular porous media

Despite decades of research into the interactions between groundwater and seawater, the evolution of permeability produced by iron precipitation has received little attention, resulting in uncertain predictions of permeability in porous media. This study establishes a novel permeability–porosity model for estimating permeability considering the oxidative precipitation of Fe(II) in granular porous media. The accuracy of this model is validated by a series of sand column tests investigating iron precipitation driven by subsurface freshwater–saltwater mixing. The experiments showed a localized pattern of iron precipitation concentrated near the freshwater–saltwater interface in those columns, where a low-permeability zone with intensive precipitation was generated, subsequently leading to a decrease in sand permeability of 45%. Extracted samples analyzed by scanning electron microscopy showed quartz sand particles coated by iron precipitates. With those successful experiments, the newly developed model is validated and fully capable of estimating the sand permeability underlying this geochemical process, providing predictions to within an order of magnitude. The present experimental results and model predictions bring valuable insights into how the pore matrix of a granular porous media evolves during iron precipitation in the subsurface groundwater–seawater mixing zone.

Relationship between hydro-environmental variables and Coilia nasus catch in a highly turbid macrotidal estuary in Japan

The effect of estuarine hydro-environmental parameters and tidal phases on the fish catch in the macrotidal Chikugo River estuary was investigated. This study focuses on the catch data of Etsu (Coilia nasus) fish, obtained during their spawning season from 2009 to 2020, employing traditional fishing techniques at a site situated 14.6–16 km upstream from the river mouth. It analyses various hydro-environmental parameters and identifies optimal ranges of environmental parameters. The optimal range of salinity (0.04–0.2) and suspended sediment concentration (SSC, 16–118 mg/L) indicated a higher fish catch at the saltwater-freshwater interface and upstream of the estuarine turbidity maximum (ETM) zone. By developing a generalized additive model (GAM) to analyze the relationship between CPUE and environmental parameters, the study assesses the relative importance of each variable in explaining CPUE variance. GAM exhibited a reasonable predictive capability (R2 = 0.509) and indicated the relative importance of each variable (SSC-24.69%, salinity-21.81%, temperature-24.41%, discharge-9.16%, and MTR-19.91%) in explaining the CPUE variance. Additionally, it aims to verify the maturity status and spawning migration patterns of C. nasus fish through standard-length distribution analysis during the spawning season. The standard-length distribution and comparison with previous literature confirmed that fishes caught by the fisherman belong to the category of matured fishes aged 2–3 years. The results indicated that the high CPUE of matured C. nasus at the upstream of ETM and freshwater-saltwater interface was related to the spawning migration and not to their feeding behaviour at the ETM. This research contributes to fisheries management strategies and conservation efforts in the Chikugo River estuary by providing valuable guidance for habitat restoration, informing fishing regulations, and understanding the behaviour of C. nasus.

"Load-Unload" Effect of Manganese Oxides on Phosphorus in Surface Water of the Pearl River Estuary

Phosphorus (P) conveyed by surface runoff plays an essential role in regulating nutrient balance and primary production in estuarine waters. In this study, basic physiochemical properties, total phosphorus (TP, including speciation), particulate iron (PFe), particulate manganese (PMn), and particulate aluminum (PAl) of the surface water in the Pearl River Estuary (PRE) in different seasons were determined to investigate the spatiotemporal distribution characteristics of P and to identify the crucial factor controlling P migration and transformation in the freshwater-saltwater interaction zone. TP concentrations (28.88-233.68 μg·L-1) decreased with increasing salinity gradient owing to deposition and dilution. The proportions of P speciation followed a decreasing order as dissolved inorganic phosphorus (DIP, 37.3%) > particulate inorganic phosphorus (PIP, 22.7%) > dissolved organic phosphorus (DOP, 21.0%) > particulate organic phosphorus (POP, 19.0%). PIP was positively related to PFe, PMn, and PAl (P < 0.05), confirming their concurrent migration behaviors. In addition, the increase in salinity promoted the desorption of phosphate on the suspended particulate matters, which mainly took place near the freshwater-saltwater interface. A significant positive correlation (P < 0.001) between the solid-liquid phase partitioning coefficient (Kd) of phosphate and salinity indicated that PIP was present mainly in more stable forms in the brackish water. Most importantly, a better relationship between Kd and PMn (P < 0.01) supported our scientific hypothesis of the "load-unload" effect of Mn oxides on P:particulate-carrying phosphates transported from the freshwater zone tend to be desorbed and released into the brackish water.

Analytical estimation of sea-level rise impacts on the freshwater lenses of elliptical islands with sloping shorelines

The vulnerability of island freshwater lenses to rising sea levels has been assessed previously using sharp-interface analytical solutions that consider islands as circular or two-dimensional strips, or the lenses are treated as entirely underlain by seawater. However, many islands do not fit these basic conditions. The current study develops an analytical solution for the freshwater-saltwater interface in elliptical islands where the interface intercepts the aquifer base, and explores (for the first time for island lenses) the applicability of a dispersion correction to the analytical solution. An analysis of the lens response to sea-level rise (SLR) and the accompanying land-surface inundation (LSI) shows that for lenses entirely underlain by seawater, the primary impact of SLR is likely the associated LSI. Counterintuitively, lenses in contact with the aquifer basement may experience an increase in the freshwater volume with SLR, although this requires that LSI is limited (e.g., the shoreline is steep) and the water table is sufficiently deep (below the land surface) to allow it to rise commensurately with SLR. The analysis also shows that as the aspect ratio of the elliptical island increases (i.e., larger values approach strip islands and circular islands have a value of 1), the lens volume decreases (for a given island area). Thus, circular islands have the maximum freshwater storage per island area. Specifically, for typical island aquifer conditions, freshwater storage decreased over 50% as a/b increased from 1 to 10. The results obtained from this study and the new analytical solution are expected to assist in the rapid assessment of the freshwater resources of elliptical-like islands.

Growth retardation and suppression of ubiquitin-dependent catabolic processes in the brackish water clam Corbicula japonica in response to salinity changes and bioaccumulation of toxic heavy metals

The brackish water clam (Corbicula japonica) is constantly exposed to stressful salinity gradients and high levels of heavy metals in the freshwater-saltwater interface of estuary environments, which are introduced from upstream regions and land. To identify the key molecular pathways involved in the response to salinity changes and heavy metal bioaccumulation, we obtained the transcriptomes of C. japonica inhabiting different salinities and heavy metal distributions in Gwangyang Bay (Korea) using RNA sequencing. Among a total of 404,486 assembled unigenes, 5534 differentially expressed genes were identified in C. japonica inhabiting different conditions, 1549 of which were significantly upregulated and 1355 were significantly downregulated. Correlation analyses revealed distinct gene expression patterns between the low and high conditions of salinity and heavy metal bioaccumulation. Functional annotation revealed significant downregulation of genes involved in “ubiquitin-dependent protein catabolic process,” “tricarboxylic acid cycle,” and “intracellular protein transport” in C. japonica from the high condition compared to the low condition. Transcription and translation pathways were significantly enriched in the high condition. Additionally, upon comparison of the low and high conditions by qRT-PCR and proteasome enzyme activity analyses, our findings demonstrated that environmental stress could suppress the ubiquitin-proteasome complex (UPC). Additionally, transcriptomic changes under high salinity stress conditions may be related to an increase in cellular protection by defense enzymes, which leads to more energy being required and a disruption of energy homeostasis. Ultimately, this could cause growth retardation in the clam C. japonica. In summary, this study provides the first evidence of UPC suppression induced by a combination of high salinity and heavy metal bioaccumulation stress in C. japonica, which could compromise the survival and growth of estuarine bivalves.

Dynamic influence of land reclamation on the nitrate contamination and saltwater redistribution

Previous research concerning the effect of land reclamation on seawater intrusion mostly focused on the modification of the saltwater wedge and the dynamics of freshwater-saltwater interface after land reclamation, utilizing both analytical and numerical models. So far, the impact of land reclamation on the recharging and accumulation of land-based pollutants such as nitrate has been disregarded. In this work, we are the first to examine the impact of land reclamation on the discharge of nitrate together with the movement of saltwater. The influence of reclamation area and filled soil permeability on nitrate pollution and saltwater redistribution is revealed using a series of field-scale simulations based on numerical models including density flow combined with reactive transport. It was discovered that land reclamation might, on the one hand, result in a substantial redistribution based on the initial saltwater-freshwater interface and, on the other hand, significantly modify the nitrate discharge. This in total would drastically alter the distribution of nitrate in the subsurface. The reclamation area and the permeability of the reclamation material are the two elements that determine the amount of variance. For the cases with hydraulic conductivities increasing from 5 to 50 m/d, the salt mass reduction rate showed a trend of first increased (84.78 %–95.58 %) and then slowly decreased (95.58 %–74.01 %). Meanwhile, the nitrate reduction rate decreased from 80.08 % to 12.93 %, when hydraulic conductivities increased from 5 to 50 m/d. It was also found that coastal nitrate accumulation was always intensified with the enlargement of the reclamation area. Finally, we are able to assist engineers in optimizing their land reclamation strategies by taking into account both the degree of saltwater intrusion and nitrate enrichment.

Groundwater Dynamics near the Saltwater–Freshwater Interface in an Island of Seto Inland Sea

Groundwater dynamics near the saltwater–freshwater interface were investigated in an island of the Seto Inland Sea, using multiple tracers (δD, δ18O, Cl−, SF6, and 14C) at two coastal groundwater monitoring wells at depths of 10–40 m. The groundwater recharge area and age were also estimated using these tracers. Additionally, bedrock groundwater at a depth of 40 m at the 2.7 m altitude was brackish and considered to be near the saltwater–freshwater interface, and a mixture of seawater (2–3.5%) and fresh groundwater (97–98%) was estimated by the Cl− concentration. Based on the δ18O of fresh groundwater estimated from the seawater mixing ratio, the recharge area was estimated to range from near to above the summit; however, this region is unlikely to be the actual recharge area, as the groundwater may be old freshwater that was recharged during a previously colder period. Groundwater dating using SF6 and 14C suggests that the fresh groundwater originated during the last glacial period (assumed 20,000 years ago) and that the 40 m deep bedrock groundwater is a mixture of old water (0–28%), 30 m deep groundwater (76–100%), and stagnant seawater (1–3%).

Parametrical Study of Freshwater–Saltwater Interface Dynamic

In this paper, a saltwater intrusion model, in view to study the dynamics of the interface between the saltwater and the freshwater in a coastal aquifer, is established. This dynamic is caused by an injection of saltwater and a freshwater pumping through a well located in a given position. From the flow model in each phase, we defined an appropriate hypothesis to obtain our global model based only on the height of the interface. The numerical simulation of our model led us to study the effect of the parameters and obtain some empirical laws of the pollution time versus the distance well-injection area and pollution time versus the pumping flow.

Rainfall-Runoff Time Lags from Saltwater Interface Interactions in Atlantic Coastal Plain Basins

The dynamic behavior of the freshwater-saltwater interface (FSI) in coastal aquifers can introduce unexpected lags between recharge and stream discharge, especially when recharge is forced by long-term cyclical precipitation patterns. This work seeks to assess these FSI impacts at the watershed scale. Recharge-discharge time lags were evaluated in 68 watersheds overlying the Floridan Aquifer System in the coastal region of the southeastern United States (Florida, Georgia, and South Carolina). Utilizing the strength of the Atlantic multidecadal oscillation (AMO) signal in this region, 10–20 year averaged recharge and discharge time series were used for the selected watersheds. Lags of 10–25 years between recharge and discharge were found in 16% of the basins considered, possibly induced by a dynamic FSI which responded slowly to the AMO-scale recharge signal. Freshwater storage coefficients (S) were estimated from time series of change-in-storage and groundwater level, with 11 basins showing S>1.5 indicating water storage well above that expected for unconfined aquifers. These 11 basins with both multidecadal recharge-discharge time lags and high S values showed a positive linear relationship between time lag and FSI depth with slope 0.016 yr/m (R-squared = 0.30). These large time lags may be directly impacting the management of these basins as they obscure water and solute mass balances in the southeastern US.

Impact of beach face slope variation on saltwater intrusion dynamics in unconfined aquifer under tidal boundary condition

Density-dependent saltwater flow in coastal aquifers varies with the seaside tidal and beach slope conditions. This paper presents a set of laboratory-scale saltwater intrusion experiments under different beach slopes (15°, 20°, 25°, and 30°) for unconfined aquifer conditions. Experimental porewater pressure measurements were utilized for quantitative analysis. A new G channel-based image analysis technique is used for experimental image analysis and freshwater–saltwater interface identification. Experimental results were numerically validated using the two-dimensional Finite Element subsurface FLOW (FEFLOW) model. The time-varying analysis revealed that saltwater intrusion occurs rapidly on flatter beach slopes. In the case of a steeper beach slope, saltwater intruded less. Steeper slopes take more time to reach a quasi-steady condition. Tidal oscillations alter hydraulic gradient changes across the beach slope. This gradient change generated clockwise circulating saltwater flow within porous media in the inter-tidal zone. This circulating flow resulted in the formation of the upper saline plume (USP). The USP expanded with time and moved in a downward direction. Finally, a deformed elliptic-shaped USP was observed under quasi-steady-state conditions. Submarine groundwater discharge (SGD) pathways move the saltwater region through the intermediate zone of a saltwater wedge and USP on varying beach slopes. It is evident that SGD particles move along the saltwater–freshwater interface (SWI) zone and rise upward (up to the intersection point). The tracer experiments were started after attaining the quasi-steady state condition and continued till the tracer reached the intersection point of saltwater level and sloping beach face. The experimental data sets can be used as benchmark test cases.

Future inundation of coastal on-site wastewater treatment systems in a region with pronounced sea-level rise

An often-overlooked impact of rising sea levels is the concurrent rise in coastal groundwater tables, which can result in subsurface inundation of below-ground infrastructure. This is an important consideration for rural coastal communities, many of which rely on on-site wastewater treatment systems (OWTS). Subsurface inundation and flooding of OWTS leads to treatment performance issues, potentially resulting in enhanced contaminant transport to coastal groundwater resources and the ocean via submarine groundwater discharge. We use SEAWAT to develop a variable-density groundwater flow model for the groundwater system underlying a densely populated rural community on the north shore of Nova Scotia, Canada. We then impose various climate change scenarios (sea-level rise, changes in recharge) to evaluate the risk of OWTS inundation from rising groundwater. We find that under current conditions as many as 9 % of OWTS in this small but densely populated watershed are either inundated or completely flooded. This number that could grow to 27 % of OWTS for the climate change scenario with the highest recharge and sea-level rise. As a secondary objective, we track the location of the modeled saltwater-freshwater interface, and investigate the potential salinization of groundwater resources used for drinking water supply. The modeled interface moved landward by ≤ 20 m and proved to be less of a concern than OWTS inundation, except for shoreline dwellings. We also use the modelling results to develop adaptation recommendations for OWTS installation regulations. This research contributes to an increasing number of groundwater modelling studies focusing on the impacts of sea-level rise on coastal subsurface infrastructure and provides important insight for rural coastal communities.

Salinity and Temperature Variations near the Freshwater-Saltwater Interface in Coastal Aquifers Induced by Ocean Tides and Changes in Recharge

The temperature distribution of shallow sectors of coastal aquifers are highly influenced by the atmospheric temperature and recharge. However, geothermal heat or vertical fluxes due to the presence of the saline wedge have more influence at deeper locations. In this study, using numerical models that account for variable density, periodic oscillations of temperature have been detected, and their origin has been attributed to the influence exerted by recharge and tides. The combined analysis of field data and numerical models showed that the alternation of dry and wet periods modifies heat distribution in deep zones (>100 m) of the aquifer. Oscillations with diurnal and semidiurnal frequencies have been detected for groundwater temperature, but they show differences in terms of amplitudes and delay with electrical conductivity (EC). The main driver of the temperature oscillations is the forward and backward displacement of the freshwater–saltwater interface, and the associated thermal plume generated by the upward flow from the aquifer basement. These oscillations are amplified at the interfaces between layers with different hydraulic conductivity, where thermal contours are affected by refraction.

Coastal groundwater model calibration using filtered and amplified hydraulic information retained in the freshwater–saltwater interface

Coastal groundwater model calibration using filtered and amplified hydraulic information retained in the freshwater–saltwater interface

Saline lakes as barriers against pollution: a multidisciplinary overview

Saline lakes are subject to numerous environmental impacts closely related to human activities, including mining, desiccation and pollution. These lakes are mostly located in arid and semi-arid regions located in endorheic basins, which combined with the low precipitation and high evaporation rates typical for these regions, may lead to the accumulation of pollutants. Saline lakes are polluted mainly through inputs of agricultural wastewater along with organic and inorganic wastes from domestic and industrial sources. The excessive anthropogenic loading of nutrients, especially nitrogen (N), can result in eutrophication of these ecosystems. Here, the importance of pollution in saline lakes was assessed, placing special emphasis on N pollution and its attenuation in aquatic interfaces. Two main interfaces are present in saline lakes: (1) the sediment-water interface; (2) the freshwater-saltwater interface. While the former is the most important one for microbial turnover, the latter plays also a key role in pollution attenuation based on solute transport by density-driven flows. The study of how saline lakes can attenuate N pollutants (e.g. nitrate) is a step forward in preserving the good ecological status of these ecosystems. This work highlights the need of integrating a wide range of techniques from different disciplines such as limnology, hydrogeology, geochemistry or microbial ecology for such studies. The effective application of conservation policies by managers will depend on the degree of knowledge of these systems.

Saltwater Intrusion into Coastal Aquifers and Associated Risk Management: Critical Review and Research Directives

Basack, S.; Loganathan, M.K.; Goswami, G., and Khabbaz, H., 2022. Saltwater intrusion into coastal aquifers and associated risk management: Critical review and research directives. Journal of Coastal Research, 38(3), 654–672. Coconut Creek (Florida), ISSN 0749-0208. Coastal regions mainly rely on sources of local fresh groundwater for domestic, irrigational, and industrial usages, which are vulnerable to high-risk of getting intruded by saltwater. Excessive pumping of fresh groundwater initiates advances of saltwater-freshwater interface inward due to hydraulic equilibrium and continuity. This introduces saline water intrusion into coastal aquifers. This is also caused by natural hazards like sea-level rise and storm-surge. The saltwater intrusion in coastal aquifers contaminates the freshwater storage, thereby emerging as a major environmental issue. To incorporate adequate coastal groundwater control and management techniques that are effective and conveniently implementable, understanding the phenomenon of saline water intrusion and the risk assessment is of utmost importance. Several scientific contributions including theoretical (analytical and numerical) solutions, experimental (laboratory and field) results, design recommendations, and risk analysis are available, indicating remarkable advances in the research area. The authors have attempted to summarize the significant contributions over the last few decades in each of these study aspects through extensive literature survey and critical analysis of the existing knowledge. It is observed that risk prevention and control methodologies such as qanat-well structure, shallow and deep wells might not be effective in many coastal areas as the complex intrusion process is yet to be understood clearly. Moreover, the high intensity coastal hazards that often occur due to climate change continue to make aquifers more vulnerable, adversely affecting the coastal groundwater management. The paper presents a critical overview of existing studies on saline water intrusion into coastal aquifers and associated risks and management techniques. Furthermore, adequate research directives with recommendations for future development are also provided.

Analytical and numerical model of saltwater intrusion in Jeneponto Regency, South Sulawesi Province Indonesia

Abstract This paper assesses the extent of saltwater intrusion (SWI) in coastal aquifers in two sub-districts of Jeneponto Regency, Indonesia, for the first time using an analytical method in the form of a sharp-interface approach and numerical dispersive simulation of a finite difference solution developed by SEAWAT, based on field observations and secondary field data. In general, the SWI extents obtained from the analytical solutions for four aquifers in the region are around 30% greater than numerical solutions. The results show that the maximum SWI extent occurs in the Binamu2 aquifer in Jeneponto Regency, where the analytical solution and numerical simulation produced SWI extents of 850.4 m and 510 m, respectively. The higher results in the sharp interface model compared to the numerical model are most likely due to the pushing seaward effect caused by the presence of circulating flow inside the saltwater wedge below the freshwater-saltwater interface, which is considered in the numerical solution but not in the analytical solution, as explained in the previous SWI study. The results also revealed that the SWI conditions in Jeneponto Regency are still within reasonable limits, but groundwater exploitation control is critical in the region to maintain this situation.

Seawater Intrusion in Extremely Heterogeneous Laboratory-Scale Aquifer: Steady-State Results

This work used experimental and numerical methods to investigate seawater intrusion (SWI) in a complex heterogeneous laboratory-scale aquifer. We started the analysis with a homogeneous isotropic aquifer as a reference case, then moved to heterogeneous layered aquifers. The study also investigated block-wise synthetic aquifers with different configurations. The seawater wedge toe length generally decreased under heterogeneous conditions, while the freshwater–saltwater dispersion/mixing zone generally increased when compared to the homogenous case. The saltwater–freshwater interface shows a distinct gradient change across boundaries at differing hydraulic conductivities. This was attributed to streamline refraction, which caused a reduction to the angle of intrusion when transitioning from high to low permeability zones and vice versa. The refraction also affected the mixing zone, where additional spreading was also observed when transitioning from high to low permeability zones and vice versa. When low permeability zones predominated the shoreline at the saline water boundary, this produced a shorter saline wedge in the horizontal direction, but it was more expanded vertically. This study provides insight into the general processes of SWI in heterogeneous aquifers and could be used as a basis for defining conceptual models of real-world systems. It highlights the capabilities of the image analysis to capture small perturbations.

A chemical and isotopic approach to investigate groundwater dynamics in a coastal aquifer

The hydrogeological processes in two different zones of a sandy coastal aquifer of the province of Buenos Aires, Argentina, have been assessed on the basis of hydrochemical tools and environmental isotopes (2H and 18O). In general, groundwater in the sand dune barrier is either a Ca-HCO3 or a Na-HCO3 type, whereas in the coastal plain it is a NaCl type. In general terms, the isotopic composition of groundwater matches the average isotopic composition of the rainfall, which would indicate that there is no significant fractionation during the recharge process. However, the interpretation of data allowed the detection of different processes— rapid infiltration, selective infiltration, evaporation, mineral dissolution and mixing—in certain locations. The last process mentioned includes both natural lateral and vertical interface mixing, as well as the one generated by anthropogenic action (saline intrusion). There is a relationship between the variability of the isotopic contents and the thickness of the unsaturated zone. In the northern zone, its lesser thickness causes greater variability in isotopic contents, whereas in the southern zone a greater thickness is suggested by more homogeneous, clustered values. In both zones, selective infiltration was observed; they were represented by both highly depleted and highly enriched isotopic contents, as well as evaporation processes occurring prior to infiltration. Besides, in both zones it was possible to recognise the effects of the displacement of the freshwater–saltwater interface towards the continent as a consequence of intensive groundwater exploitation. The mixing percentage of salt water is about 51% for both conservative elements oxygen-18 and chloride. The importance of the association of hydrochemical and isotopic methodologies stands out as a tool in the sustainable management of groundwater resources in coastal sand-dune regions.