Effects of different boundary conditions on the simulation of groundwater flow in a multi-layered coastal aquifer system (Taranto Gulf,southern Italy)

The evaluation of the accuracy or reasonableness of numerical models of groundwater flow is a complex task, due to the uncertainties in hydrodynamic properties and boundary conditions and the scarcity of good-quality field data. To assess model reliability, different calibration techniques are joined to evaluate the effects of different kinds of boundary conditions on the groundwater flow in a coastal multi-layered aquifer in southern Italy. In particular, both direct and indirect approaches for inverse modeling were joined through the calibration of one of the most uncertain parameters, namely the hydraulic conductivity of the karst deep hydrostratigraphic unit. The methodology proposed here, and applied to a real case study, confirmed that the selection of boundary conditions is among the most critical and difficult aspects of the characterization of a groundwater system for conceptual analysis or numerical simulation. The practical tests conducted in this study show that incorrect specification of boundary conditions prevents an acceptable match between the model response to the hydraulic stresses and the behavior of the natural system. Such effects have a negative impact on the applicability of numerical modeling to simulate groundwater dynamics in complex hydrogeological situations. This is particularly important for management of the aquifer system investigated in this work, which represents the only available freshwater resource of the study area, and is threatened by overexploitation and saltwater intrusion.     

Salt water intrusion in the coastal aquifer of the southern Po Plain, Italy

Saltwater has invaded the coastal aquifer along the southern Adriatic coast of the Po Plain in Italy. The topography, morphology and land use of the region is complex: rivers, canals, wetlands, lagoons, urban, industrial and agricultural areas and tourist establishments all coexist in a small area. Water table and iso-salinity maps show that in four study areas (Ancona-Bellocchio, Marina Romea, San Vitale Forest, Cervia) out of five, the water tables are below sea level and saltwater has replaced freshwater in the aquifer. The fifth area (Classe Forest) has a relatively pristine freshwater aquifer thanks to an average water-table height of 2 m above sea level, a lower hydraulic conductivity (< 7.7 m/day) and a continuous dune system along the coast. Only in this area is the topography high enough to maintain freshwater heads that can counteract saltwater intrusion according to the Ghyben-Herzberg principle. Furthermore, the climate, with an average yearly precipitation of 606 mm and an average temperature of 14.4°C, allows for little recharge of the aquifer. Ongoing subsidence, encroachment of sea water along rivers and canals, as well as drainage from agricultural land also enhance the salinization process.     

Source and mobility of minor and trace elements in a volcanic aquifer system: Mt. Vulture (southern Italy)

In this paper we provide a geochemical investigation on 34 groundwater samples in the Mt. Vulture volcanic aquifer representing one of the most important groundwater resources of the southern Italy pumped for drinking and irrigation supply. The present study includes the first data on the abundance and mobility of minor and trace elements and the thermodynamic considerations on water–rock interaction processes in order to evaluate the conditions of alkali basalt weathering by waters enriched in magma-derived CO₂. The results highlight the occurrence of two hydrofacies: bicarbonate alkaline-earth and alkaline waters deriving from low-temperature leaching of volcanic rocks of Mt. Vulture, and bicarbonate-sulfate-alkaline waters (high-salinity waters) related to prolonged water circulation in alkali and feldspathoids-rich pyroclastic layers interbedded with clay deposits. The Al-normalized relative mobility (RM) of metals in Vulture's aquifer varies over a wide range (10^− 1 < RM < 10⁴), confirming that the basalt weathering is not a congruent and isochemical process. Chemical equilibrium studies show that the bicarbonate alkaline-earth and alkaline waters, having a short interaction with silicate minerals, plot very close to the kaolinite–smectite stability boundary, whereas the high-salinity waters fall in the stability field of smectite and muscovite because of prolonged interaction with alkali and feldspathoids-rich pyroclastic layers. Overall, for the bicarbonate alkaline-earth and alkaline waters, the release of toxic metals in solutions is related to the spatial variation of host-rock geochemistry, the high-salinity waters, collected near urban areas, show values higher than legal limits for Ni and As, likely as a consequence of anthropogenic contribution.     

Olocenic alluvial aquifer of the River Cornia coastal plain (southern Tuscany, Italy): database design for groundwater management

 Hydrogeological research is in progress, utilizing GIS methods, with the principal aim of modelling the Olocenic alluvial aquifer of the River Cornia coastal plain (southern Tuscany, Italy), which has been exploited for drinking water, irrigation, and industrial uses. A consequence of exploitation has been the appearance of wide seawater intrusion. The alluvial aquifer has recently been subjected to new well fields for the supply of drinking water, with an increase of total average discharge of about 4×10⁶ m³/year. This paper presents results obtained from updating and integrating basic knowledge and structuring the database. The hydrogeological study allowed the recognition of the extension of areas that are characterized by a hydraulic head under the sea level, the progressive salinization of the aquifer, and the increase of water deficit in the aquifer which is produced by a progressive extraction of water superior to the natural recharge. In addition, benefits and disadvantages resulting from the location of new well fields in a hydrogeologically favourable zone, and the boundary conditions for much of the area studied have been defined. The GIS was used as support for making and updating the tabular and spatial database with the aim of integrating the local and regional hydrogeological knowledge. This study will permit the realization of a numerical simulation of the groundwater flow of the aquifer aimed at correcting the management of water resources, by means of the GIS-modelling integration. Received: 23 June 1998 · Accepted: 16 November 1998     

Study on the groundwater sustainable problem by numerical simulation in a multi-layered coastal aquifer system of Zhanjiang,China

Assessing sustainability of coastal groundwater is significant for groundwater management as coastal groundwater is vulnerable to over-exploitation and contamination. To address the issues of serious groundwater level drawdown and potential seawater intrusion risk of a multi-layered coastal aquifer system in Zhanjiang, China, this paper presents a numerical modelling study to research groundwater sustainability of this aquifer system. The transient modelling results show that the groundwater budget was negative (\(-3826\times 10^{4}\) to \(-4502\times 10^{4 }\,\hbox {m}^{3}/\hbox {a}\)) during the years 2008–2011, revealing that this aquifer system was over-exploited. Meanwhile, the groundwater sustainability was assessed by evaluating the negative hydraulic pressure area (NHPA) of the unconfined aquifer and the groundwater level dynamic and flow velocity of the offshore boundaries of the confined aquifers. The results demonstrate that the Nansan Island is most influenced by NHPA and that the local groundwater should not be exploited. The results also suggest that, with the current groundwater exploitation scheme, the sustainable yield should be \(1.784\times 10^{8}\, \hbox {m}^{3}/\hbox {a}\) (i.e., decreased by 20% from the current exploitation amount). To satisfy public water demands, the 20% decrease of the exploitation amount can be offset by the groundwater sourced from the Taiping groundwater resource field. These results provide valuable guidance for groundwater management of Zhanjiang.     

Arsenic enrichment in unconfined sections of the southern Gulf Coast aquifer system, Texas

Groundwater arsenic concentrations exceeding the federal drinking water standard are common in the southern Gulf Coast aquifer system in Texas, including in aerobic, unconfined groundwater which provides much of the municipal and domestic water supplies for the region. The objective of this study was to determine geochemical factors affecting the occurrence and distribution of groundwater As in unconfined portions of the southern Gulf Coast aquifer system through a comparative transect study of groundwater across three major hydrostratigraphic units (the Catahoula Formation, Jasper aquifer and Evangeline aquifer) and analysis of regional water quality data. Results show that As concentrations decrease with increasing distance from the Catahoula Formation, which is consistent with Miocene volcanic ash as the main source of As to groundwater in the region. Arsenic concentrations correlate with V, SiO₂ and K, all of which were released during weathering of volcanic sediments and their degradation products. In all three units, carbonate weathering and active recharge in the unconfined zones result in circum-neutral pH and oxidizing groundwater, which are typically amenable to As immobilization by adsorption of arsenate onto mineral oxides and clays. However, As concentrations exceed 10 μg/L in approximately 30% of wells. Silica that was co-released with As may compete for sorption sites and reduce the capacity for arsenate adsorption.     

Morphometric analysis of sinkholes in a karst coastal area of southern Apulia (Italy)

Salento, the southern portion of Apulia region (SE Italy), is a narrow and elongated peninsula in carbonate rocks, with prevailing low coastlines, locally interrupted by high rock cliffs. The long stretches of low coasts are marked by typical karst landforms consisting of collapse sinkholes. As observed in many other karst coastal settings worldwide, development of sinkholes may be particularly severe along the coasts, where both natural and anthropogenic processes contribute to accelerate the dissolution of carbonate rocks and subsidence processes, even influencing the coastline evolution. Following a previous study, where the main features of sinkholes at Torre Castiglione (Taranto province) were investigated and described, and a preliminary susceptibility map produced, at the light of updated data and elaborations in the present paper we perform a detailed morphometric analysis on the sample of identified sinkholes. The main morphometric parameters generally used for sinkhole characterization have been considered in this study: shape of the sinkhole, azimuth and length of maximum and minimum axes, depth, elongation ratio, and distance from the shorelines. Each of them is described, both as individual parameter and in conjunction with the others, in the attempt to identify the main factors controlling development of sinkholes in the area, and their evolution as well. With regard to this latter aspect, beside simple morphometry of the sample of sinkholes at Torre Castiglione, we also focused our attention on the likely relationships existing between distribution and shape of the sinkholes and the tectonic discontinuities. To investigate the matter, a three-stage analysis has been carried out in this study by means of: field measurements of the fractures bounding the sinkholes, field measurement of the long axes azimuth of the elongated sinkholes, comparison of the previously described sets with the strikes of the main regional geological structures. The obtained results show, in addition to the coincidence of the main regional discontinuity systems with the major axis of elongated sinkholes, a clear control exerted by development and evolution of the sinkholes on the formation of coastal inlets and bays. Eventually, the approach here presented may be applied in other karst coastal sinkhole-prone areas, to gain new knowledge on the genesis and evolution of coastal sinkholes, and to properly evaluate the hazard they pose to the anthropogenic environment.     

A conceptual and numerical model for groundwater management: a case study on a coastal aquifer in southern Tuscany, Italy

Ongoing hydrogeological research aims to develop a correct management model for the Plio-Pleistocene multi-aquifer system of the Albegna River coastal plain (southern Tuscany, Italy); overexploitation of this aquifer for irrigation and tourism has caused seawater intrusion. The conceptual model is based on field and laboratory data collected during the 1995–2003 period. Meteoric infiltration and flows from the adjoining carbonate aquifer recharge the aquifer. Natural outflow occurs through a diffuse flow into the sea and river; artificial outflow occurs through intensive extraction of groundwater from wells. Water exchanges in the aquifer occur naturally (leakage, closing of aquitard) and artificially (multiscreened wells). The aquifer was represented by a three-dimensional finite element model using the FEFLOW numerical code. The model was calibrated for steady-state and transient conditions by matching computed and measured piezometric levels (February 1995–February 1996). The model helped establish that seawater intrusion is essentially due to withdrawals near the coast during the irrigation season and that it occurs above all in the Osa-Albegna sector, as well as along the river that at times feeds the aquifer. The effects of hypothetical aquifer exploitation were assessed in terms of water budget and hydraulic head evolution.     

Stability assessment and characterization of colloids in coastal groundwater aquifer system at Kalpakkam

Natural colloids have a potential role in facilitating the transport of radionuclides in groundwater. To assess the role of mobile colloidal phases in radionuclide transport, characterization of colloids for size, size distribution, zeta potential, surface charge and elemental composition is required. Groundwater samples were collected from 12 borewells in the study area and were characterized with respect to physicochemical parameters. Water quality parameters such as pH, temperature, specific conductance, TDS and dissolved O₂ were measured in situ. Based on salinity, two groundwater types were identified in the study area: (1) freshwater type and (2) brackish type. Laboratory and field-scale turbidity measurements in addition to quantitative analysis of major ions were carried out. It was observed that the colloid concentrations are reduced at higher salinity. Zetasizer, particle size analyzer, scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction techniques were used to characterize the colloidal particles in groundwater. Colloids were present in all the groundwater samples, the concentration varying between 0.05 and 6 mg/L. Colloid concentration was greater in wells with low ionic strength and the number of colloidal particles varied between 3 × 10⁹ and 4 × 10¹¹ particles/L. The average colloid size varied between 200 and 350 nm for various samples. The zeta potential of the colloidal particles varied between −25.5 and –34.0 mV. SEM analysis of colloidal particles revealed the presence of clays particularly kaolinite and that the mineral composition of colloids reflects the mineralogy of the aquifer. For proper risk assessment and remediation studies, the role of colloids in radionuclide transport assumes greater importance. This study highlights the need for, and relevance of, detailed colloid characterization to assess its role in the migration of radionuclides from near surface disposal facilities.     

Hydrogeological and multi-isotopic approach to define nitrate pollution and denitrification processes in a coastal aquifer (Sardinia,Italy)

Agricultural coastal areas are frequently affected by the superimposition of various processes, with a combination of anthropogenic and natural sources, which degrade groundwater quality. In the coastal multi-aquifer system of Arborea (Italy)—a reclaimed morass area identified as a nitrate vulnerable zone, according to Nitrate Directive 91/676/EEC—intensive agricultural and livestock activities contribute to substantial nitrate contamination. For this reason, the area can be considered a bench test for tuning an appropriate methodology aiming to trace the nitrate contamination in different conditions. An approach combining environmental isotopes, water quality and hydrogeological indicators was therefore used to understand the origins and attenuation mechanisms of nitrate pollution and to define the relationship between contaminant and groundwater flow dynamics through the multi-aquifer characterized by sandy (SHU), alluvial (AHU), and volcanic hydrogeological (VHU) units. Various groundwater chemical pathways were consistent with both different nitrogen sources and groundwater dynamics. Isotope composition suggests a mixed source for nitrate (organic and synthetic fertilizer), especially for the AHU and SHU groundwater. Moreover, marked heterotrophic denitrification and sulfate reduction processes were detected; although, for the contamination related to synthetic fertilizer, the attenuation was inefficient at removing NO₃^? to less than the human consumption threshold of 50 mg/L. Various factors contributed to control the distribution of the redox processes, such as the availability of carbon sources (organic fertilizer and the presence of lagoon-deposited aquitards), well depth, and groundwater flow paths. The characterization of these processes supports water-resource management plans, future actions, and regulations, particularly in nitrate vulnerable zones.     

Geophysical characterization of the complex dynamics of groundwater and seawater exchange in a highly stressed aquifer system linked to a coastal lagoon (SE Spain)

Anthropogenic pressure influences the two-way interactions between shallow aquifers and coastal lagoons. Aquifer overexploitation may lead to seawater intrusion, and aquifer recharge from rainfall plus irrigation may, in turn, increase the groundwater discharge into the lagoon. We analyse the evolution, since the 1950s up to the present, of the interactions between the Campo de Cartagena Quaternary aquifer and the Mar Menor coastal lagoon (SE Spain). This is a very heterogeneous and anisotropic detrital aquifer, where aquifer–lagoon interface has a very irregular geometry. Using electrical resistivity tomography, we clearly identified the freshwater–saltwater transition zone and detected areas affected by seawater intrusion. Severity of the intrusion was spatially variable and significantly related to the density of irrigation wells in 1950s–1960s, suggesting the role of groundwater overexploitation. We distinguish two different mechanisms by which water from the sea invades the land: (a) horizontal advance of the interface due to a wide exploitation area and (b) vertical rise (upconing) caused by local intensive pumping. In general, shallow parts of the geophysical profiles show higher electrical resistivity associated with freshwater mainly coming from irrigation return flows, with water resources mostly from deep confined aquifers and imported from Tagus river, 400 km north. This indicates a likely reversal of the former seawater intrusion process.     

Rapid late Pleistocene/Holocene uplift and coastal evolution of the southern Arabian (Persian) Gulf

The coastline along the southern Arabian Gulf between Al Jubail, Kingdom of Saudi Arabia, and Dubai, UAE, appears to have risen at least 125 m in the last 18,000 years. Dating and topographic surveying of paleo-dunes (43–53 ka), paleo-marine terraces (17–30 ka), and paleo-marine shorelines (3.3–5.5 ka) document a rapid, > 1 mm/a subsidence, followed by a 6 mm/a uplift that is decreasing with time. The mechanism causing this movement remains elusive but may be related to the translation of the coastal area through the backbasin to forebulge hinge line movement of the Arabian plate or, alternatively, by movement of the underlying Infracambrian-age Hormuz salt in response to sea-level changes associated with continental glaciation. Independent of the mechanism, rapid and episodic uplift may impact the design of engineering projects such as nuclear power plants, airports, and artificial islands as well as the interpretation of sedimentation and archeology of the area.     

Geochemistry of regional aquifer systems hosted by carbonate-evaporite formations in Umbria and southern Tuscany (central Italy)

In central Italy Mesozoic carbonates represent the principal reservoir of freshwater of the region. The hydrogeological setting is linked to the geological evolution of the Apennine chain and is generally characterised by a lower aquifer and one or more shallower aquifers separated by thin aquicludes. In these systems, groundwater composition is the result of a complex array of regional and local geochemical processes. The main geochemical processes are the dissolution of calcite, the influx of deeply derived CO₂ related to a regional process of mantle degassing, dedolomitization and mixing with deep saline fluids. The occurrence of saline fluids, characterised by a Na–Cl(HCO₃) composition, is related to the presence of a deep regional aquifer at the base of Mesozoic carbonates. The extremely high pCO₂ values computed for the saline waters suggest that the deep aquifer is also a structural trap for the mantle derived CO₂ during its ascent towards the surface. In central Italy, geological and geophysical data highlight the presence of two different crustal sectors: the eastern sector, where the geometry of the Apennine thrust belt is still preserved, and the western sector, where the compressive structures are dislocated by important extensional deformations. In the western sector, the normal faults disrupting the compressive structures allow the mixing of the deep Na–Cl(HCO₃) fluids with the shallow groundwater causing a salinity increase and the natural deterioration of groundwater quality.     

Origin of water salinity in a lake and coastal aquifer system

The studied area is in the Algerian Northeast. A number of interacting factors at the site are able to modify salinity. The main influences are: (1) The lithology, (2) Fedzara Lake, (3) marine waters, and (4) a prolonged drought. (1) The lithology is very clearly delineated by the outcrop of metamorphic formations (gneiss, marbles, micaschists, and quartzites) and sedimentary formations (limestones, clay, sandstones, and sands). All these formations can influence the chemical composition of waters. (2) Fedzara Lake evacuates its salted waters via the Wadi Meboudja, which connects with the Wadi Seybouse and the phreatic surface. These lake waters might induce changes in salinity. (3) The marine waters represent the third source of salinity change. Over-pumping of water from wells and drillings can cause an imbalance to the fresh–salted water interface. (4) The effects of a local prolonged drought of more than 10 years may increase water salinity. Over-pumping to augment water supply generates a fall in water levels and the drier atmosphere increases evapotranspiration. Both may contribute to increased salinity. Results of water analysis were used to observe the evolution of these various factors.     

Electrical resistivity imaging for the characterization of the Montaguto landslide (southern Italy)

Long runout landslides involve a massive amount of energy and can be extremely hazardous owing to their long movement distance, high mobility and strong destructive power. Numerical methods have been widely used to predict the landslide runout but a fundamental problem remained is how to determine the reliable numerical parameters. This study proposes a framework to predict the runout of potential landslides through multi-source data collaboration and numerical analysis of historical landslide events. Specifically, for the historical landslide cases, the landslide-induced seismic signal, geophysical surveys, and possible in-situ drone/phone videos (multi-source data collaboration) can validate the numerical results in terms of landslide dynamics and deposit features and help calibrate the numerical (rheological) parameters. Subsequently, the calibrated numerical parameters can be used to numerically predict the runout of potential landslides in the region with a similar geological setting to the recorded events. Application of the runout prediction approach to the 2020 Jiashanying landslide in Guizhou, China gives reasonable results in comparison to the field observations. The numerical parameters are determined from the multi-source data collaboration analysis of a historical case in the region (2019 Shuicheng landslide). The proposed framework for landslide runout prediction can be of great utility for landslide risk assessment and disaster reduction in mountainous regions worldwide.     

The impact of hydrological conditions on salinisation and nitrate concentration in the coastal Velez River aquifer (southern Spain)

This study reports the impact of hydrological conditions on salinisation and nitrate concentrations of a coastal aquifer located at the Mediterranean Sea, southern Spain. Eighty-two samples of ground- and surface water taken during two extreme hydrological events between 1994 and 1996 at 25 different wells were evaluated with regard to hydrochemistry, focusing on nitrate concentrations and salinisation, which constitute the main hazard of this aquifer. Furthermore, hydrochemical data were analysed by principal component analysis (PCA). Additionally, in 2007 13 ground- and surface water samples taken at 12 different locations were analysed for stable isotopes of D/¹⁸O, and one sample was analysed for ¹⁵N. Since 1993 until present saltwater intrusion was observed only during dry hydrological conditions in 1994; it showed an irregular salinisation pattern probably related to locally elevated hydraulic conductivities. Nitrate concentrations increase significantly during wet hydrologic conditions owing to uptake of nitrate by rising groundwater. Stable isotopes of groundwater reveal an Atlantic origin of the precipitation that recharges the aquifer and a minor amount of groundwater recharge by the water coming from the La Viñuela reservoir, which is used for irrigation over the aquifer. ¹⁵N isotopes point to a considerable input of nitrates derived from organic fertilisers.     

Chemical and physical parameters as trace markers of anthropogenic-induced salinity in the Agua Amarga coastal aquifer (southern Spain)

Agua Amarga coastal aquifer in southern Spain has been the subject of chemical and physical measurements since May 2008 in order to monitor the potential effects of water withdrawal for the Alicante desalination plants on the salt marsh linked to the aquifer. Electrical conductivity contour maps and depth profiles, piezometric-head contour maps, hydrochemical analyses, isotopic characterizations and temperature depth profiles show not only the saltwater intrusion caused by water abstraction, but also the presence of a pronounced convective density-driven flow below the salt marsh; this flow was a consequence of saltwork activity in the early 1900s which generated saline groundwater contamination. The influence of a seawater recharge programme, carried out over the salt marsh in 2009–2010, on the diminishing groundwater salinity and the recovery of groundwater levels is also studied. Based on collected field data, the project provides a deeper understanding of how these successive anthropogenic interventions have modified flow and mixing processes in Agua Amarga aquifer.     

A thirty-year artificial recharge experiment in a coastal aquifer in an arid zone: The Teboulba aquifer system (Tunisian Sahel)

With the increased demand for groundwater resulting from fast demographic growth, accelerated urbanization, economic and agricultural activity diversification, and the increase of per capita consumption, ground water resources, in particular in coastal regions, remain relatively low, compared to demand. The groundwater quality and piezometric variations result mainly from intensive exploitation, agricultural activities and the intrusion of seawater. This phenomenon is observed mostly in semi-arid areas, such as the oriental Sahel of Tunisia, where an apparent reduction in rainfall in recent years can be seen. Groundwater becomes overexploited especially as its natural recharge by rainwater does not succeed in maintaining the hydrologic balance. The imbalance between water demand and resources induces the degradation of the water quality. In such a case, the artificial recharge of water-table aquifers by water from dams is a credible alternative to improve the hydrodynamic and physicochemical conditions of the groundwater. Like most coastal aquifers, the Teboulba water-table aquifer is threatened by overexploitation for at least three decades. This threat appears by a considerable piezometric level drop and by water salinisation, due to seawater intrusion. Given this alarming situation, since 1971, artificial recharge through wells with surface water from a dam was tested in order to restore the water levels and to improve water quality. The piezometric and chemical surveys of the Teboulba aquifer permitted one to describe the temporal and spatial piezometric and geochemical conditions of the aquifer and to show the effect of the artificial recharge. Indeed, the artificial recharge undertaken since 1971 made the geochemical and piezometric conditions of the Teboulba aquifer improve. This example is a rare, well-documented case-study of the benefits of artificial recharge in a coastal aquifer, over the long term.     

Hydrogeochemistry of the Saloum (Senegal) superficial coastal aquifer

Seawater has entered and concentrated in the Saloum hydrologic system up to 100 km upstream, contaminating both the surface water and the superficial 'Continental terminal' (CT) groundwater resources, and large proportions of cultivated lands. In the areas affected by saltwater contamination, chloride concentrations as high as 3,195 mg/l have been measured in the groundwater samples collected from wells located in the vicinity of the Saloum River, making the water inadequate for drinking water purposes. This paper presents the results of a study designed to characterise the current extent of the saline groundwater and the mechanism of saline surface water body/fresh groundwater mixing in relation to the groundwater flow trends. It also describes the groundwater chemical and isotopic composition and geochemical processes controlling the chemical patterns. Four major water types occur in the study area, namely Na-rich saline groundwater, Ca-Na-rich saline groundwater, Na-dominant fresh groundwater and Ca-dominant fresh groundwater. A hydrogeochemical zonation of the aquifer, based on the presence of different water families and on the groundwater flow, led to the identification of the main processes controlling the groundwater chemistry. Cation exchange reactions on the kaolinite clay mineral, calcite dissolution in the eastern zone where calcite minerals have been identified, reverse cation exchange reactions in the saline-contaminated band along the Saloum River and, to a lesser extent, a gypsum dissolution are the predominant processes. Results of i¹⁸O and iD analysis in 15 groundwater samples compared with the local meteoric line indicate that the groundwater has been affected by evaporation, and the groundwater is isotopically lighter as the depth of water table increases. In this study, i¹⁸O data were used in conjunction with chloride data to identify the source of high chloride. Results show a departure of the contaminated water samples from the seawater mixing line, which indicates that other processes rather than mixing between modern seawater and native groundwater alone increase the chloride concentrations.