Fresh Groundwater Lenses Research Articles

Time Scale of the Formation of Fresh Groundwater Lens on an Atoll Island under the Influence of Land Reclamation

Time Scale of the Formation of Fresh Groundwater Lens on an Atoll Island under the Influence of Land Reclamation

Island hydrogeology in the tropics: Constraining a 3D variable-density groundwater flow and solute transport model with geophysics

Saltwater intrusion is the greatest risk to coastal community water supplies where they are dependent on fresh groundwater as the main source of supply. For small, fractured bedrock island aquifers, the fresh groundwater lens dynamics and transition zone geometry are complex. This study investigated the impacts of projected increases in groundwater pumping on a fresh groundwater lens to evaluate changes to the lens geometry and localized up-coning from the deeper, more saline aquifers beneath a small bedrock island in the tropics. A combination of traditional hydrogeological datasets and an airborne electromagnetic survey were used to develop a three-dimensional density-dependent groundwater flow and solute transport model using the SEAWAT code. This investigation represents one of very few studies that have taken such an approach. The model was calibrated using observed groundwater hydraulic heads and chloride concentration data, and calculated chloride values based on bulk conductivity measurements determined from inverted geophysical data. A staged calibration approach was adopted, firstly assessing the time-average lens extent and geometry, and secondly considering the seasonal groundwater level response. In the calibration, the geophysical data helped constrain the lens geometry in the absence of hydraulic head and chloride data. The calibrated model was used to test scenarios where groundwater pumping rates were increased above the current demand of 452 m3 d−1, showing that the lens is likely to be stable, i.e., its available storage is not expected to contract excessively, for extraction rates of up to 3,000 m3 d−1. The combined use of geophysical data and a numerical modeling approach was advantageous in investigating the lens characteristics. It also demonstrated how these techniques can be used together to evaluate coastal water resources and to manage water supply risks for coastal communities. The study demonstrated that the freshwater lens can likely support the freshwater demands of the remote community, and is a preferred option compared with high-cost and more complicated options such as seawater desalination and managed aquifer recharge.

Using electrical resistivity tomography method to investigate fresh groundwater lenses on Dong Island, China

Using electrical resistivity tomography method to investigate fresh groundwater lenses on Dong Island, China

Imaging freshwater and saline aquifers beneath Bradford County, Pennsylvania, USA, using Audio-Magnetotelluric (AMT) data

We investigate the utility of audio-magnetotelluric (AMT) data for detecting high electrical conductivity zones indicative of fresh and saline groundwater lenses at depths of < ∼1000 m at seven survey sites in the southwestern corner of Bradford County, PA. Dimensionality analysis indicates the AMT data can be interpreted using two-dimensional (2-D) models. 2-D electrical resistivity models obtained from inverting the AMT data reveal conductive zones at depths of <30 m at all sites, between depths of 50–150 m at six sites, and between depths of 400–600 m at one site. The models show slightly lower resistivities (∼15–30 Ω m) in the deeper zones compared to the shallower zones (>30 Ω m). Consistent with well log data from two boreholes in the study area and salinity estimates obtained from the imaged resistivities, we interpret the <30 m deep conductive zone as the freshwater aquifer, and the two deeper conductive zones as salty/briny groundwater lenses. Our salinity estimates for the deeper zones vary from 1 to 380 (unitless Practical Salinity Scale), which agrees well with previous estimates of Appalachian brine salinity of 10–343. We attribute the high-salinity groundwater lenses to Appalachian Basin brine migrating upwards from deeper in the basin via bedrock fracture and fault systems. Our findings indicate AMT surveying can be useful for imaging fresh and saline aquifers at shallow depths (< ∼1000 m), at least within the northeastern section of the Appalachian Basin in Pennsylvania.

Multi-dimensional management framework on fresh groundwater lens of Kish Island in the Persian Gulf, Iran

Groundwater in arid and semi-arid coastal aquifers is vulnerable to seawater intrusion and quality deterioration despite being one of the most reliable sources of water supply due to the increasing number of development plans and competition between water consumers. A multi-dimensional groundwater management framework is developed to trade-off between groundwater abstraction, allocation equity, groundwater quality, and energy considerations in the reverse osmosis (RO) filtration process in the fresh groundwater lens of Kish Island, Iran. An arid island confined in the Persian Gulf is modeled using 3D simulation and three well-occupied multi-objective evolutionary optimization algorithms. Four objectives include: maximizing the groundwater abstraction, minimizing the Gini coefficient (allocation inequity), minimizing the total energy required to pass saline water through the RO membrane to reach the standard total dissolved solids (TDS), and minimizing the average TDS concentration of water abstraction positions from 11 management zones have been considered over a 50-year management horizon. Solutions obtained in the simulation-based constrained multi-objective optimization framework allow managers to choose from 587 Pareto optimal solutions. They provide an abstraction scheme with a range of 1.44 to 4.53 MCM/yr, a Gini coefficient of 0 to 0.98, filtration energy of 988,562 to 1,935,760 kWh/yr, and an average TDS of 19,663 to 21,351 mg/L. The Pareto optimal solutions can help decision-makers decide on the multi-dimensional problems of sustainable coastal groundwater management and show patterns among different objectives.

Evaluation of Fresh Groundwater Lens Volume and Its Possible Use in Nauru Island

A proper management of fresh groundwater lenses in small islands is required in order to avoid or at least limit uncontrolled saltwater intrusion and guarantee the availability of the resource even during drought occurrences. An accurate estimation of the freshwater volume stored in the subsoil is a key step in the water management decision process. This study focused on understanding the hydrogeological system behaviour and on assessing the sustainable use of the groundwater resource in Nauru Atoll Island (Pacific Ocean). A first phase, concerning the hydrogeological characterization of the island, highlighted the occurrence of few drought-resilient freshwater lenses along the seashore. The second part of the study focused on the characterization of a freshwater lens found in the northern coastal area and identified such area as the most suitable for the development of groundwater infrastructures for water withdrawal. The characterization activities allowed quantifying the freshwater lens thickness and volume in order to assess the capability to satisfy the population water demand. A geo-electrical tomography survey was carried out, and a 3D density-dependent numerical model was implemented in SEAWAT. The model results demonstrated that in small islands freshwater can unexpectedly accumulate underground right along the seashore and not in the centre of the island as is commonly believed. Furthermore, the model can constitute a useful tool to manage the groundwater resources and would allow the design of sustainable groundwater exploitation systems, avoiding saltwater intrusion worsening.

Impact of major nearshore land reclamation project on offshore groundwater system

Offshore groundwater (OG) systems are widely distributed in the coastal areas, where nearshore land reclamation projects are constructed for seaports, airports, and urban other infrastructures. However, there is almost no study on the interaction between these projects and the OG system. This paper presents the first such an effort using the Hong Kong International Airport near Lantau Island, as a case study by developing a three-dimensional density-dependent flow and transport model to investigate the interaction between the reclaimed airport and the OG system. Results show that the offshore relatively fresh groundwater may flow from the nearby Lantau Island through the submarine confined aquifer. The higher pore water pressure in the basal aquifer beneath the airport is generated by the regional groundwater system of the nearby Lantau Island. Besides, the reclaimed airport also serves as extra aquifer space for the storage of fresh groundwater resources. The expansion of the 3rd runway will further increase the water table and move the center of the water mound toward the 3rd runway area. The groundwater system in the airport will reach a new steady state 50 years after the expansion, and the volume of the fresh groundwater lens is calculated to be ~5.2 × 107 m3 with a maximum water table of 1.4 m above mean sea level. The findings of this study have implications on the large-scale nearshore land reclamation which may be affected by the OG system in other coastal areas.

Freshwater lens assessment of karst island water resources: towards an interdisciplinary protocol

Fresh groundwater lenses on karstic oceanic islands form a vital resource sustaining local populations. However, this resource is susceptible to saltwater intrusion through human drivers (over-abstraction) and natural processes (variable precipitation and storm surges). There is a paucity of means to assess the risks that freshwater lenses are exposed to. This is partly driven by a poor understanding of the root causes of saltwater intrusion, which leads to potentially inappropriate freshwater management strategies. Thus, effective management of these freshwater lenses requires a baseline understanding of the processes that drive saltwater intrusion and the degradation of freshwater lenses, and the temporal and spatial variability of these processes. Dynamics of such freshwater lenses involve an interplay between physical, chemical, and socio-economic processes; therefore, finding a solution necessitates an interdisciplinary approach and a range of data collection strategies. This approach was formalized in a Freshwater Lens Assessment Protocol (FLAP). Results from the research developed and tested on Bantayan Island in the Philippines reveals a sufficient freshwater lens to support the current and projected population; however, local officials are operating abstraction wells from the wrong locations on the island. Such locations are utilized due to ease of access to existing infrastructure and government boundaries, but do not consider technical factors that influence saltwater intrusion. FLAP is an appropriate, cost-effective, interdisciplinary tool that uses a pragmatic approach to data collection, interpretation, and integration into an observational model. Continuous adjustments are possible through ongoing monitoring of the model, offering opportunities to evaluate the efficacy of resource management strategies.

Influence of Land Reclamation on Fresh Groundwater Lenses in Oceanic Islands: Laboratory and Numerical Validation

AbstractFresh groundwater lenses in tropic oceanic islands are important freshwater resources for local inhabitants and ecosystems. These reef‐carbonate islands typically have a dual‐aquifer structure, with poorly consolidated Holocene sediments deposited unconformably on Pleistocene limestone reef deposits. Land reclamation has been carried out on the reef flats or shallow water areas in coral islands, a very common practice in the Indian Ocean and the South China Sea. However, there is a lack of comprehensive understanding of how the groundwater system will respond to land reclamation. In this study, the impact of land reclamation on the fresh groundwater lens in an island is investigated using multiple methods: sand‐tank experiments, sharp‐interface analytical solutions, and numerical modeling. This study demonstrates that land reclamation can increase the freshwater storage of the lens and shift the water divide toward the reclamation area. Furthermore, a lower permeability fill material especially with a greater scale or thickness leads to higher freshwater storage. However, the expansion of the lens will be truncated due to the high‐permeability of the lower layer. These observations and findings from the laboratory experiments and two‐dimensional numerical simulations are further ground‐truthed by the three‐dimensional modeling of Yongxing Island in the South China Sea. The findings of this study provide a comprehensive understanding of the impact of land reclamation on island groundwater system and the theoretical supports for Small Island Developing States to use the reclamation not only for urban development but also for extra aquifer to enhance the water resource sustainability under the climate change.

Analytical Solutions for Fresh Groundwater Lenses in Small Strip Islands With Spatially Variable Recharge

AbstractGroundwater lens may provide the primary freshwater supply for the inhabitants of small islands. The lens profile and volume are sensitive to spatially variable recharge caused by topography, soil type, land cover, evapotranspiration and human activities. We derive analytical solutions based on the sharp‐interface approximation to describe transient and steady‐state lens profiles for spatially variable recharge rates in small strip islands. The derived solutions are validated by a steady‐state solution for simple spatial recharge patterns and numerical simulations. Numerical investigations show that the spatial recharge pattern has a significant impact on the lens profile. For a given constant total recharge, smaller recharge areas or a more concentrated recharge rate can result in a larger lens volume. The volume increase can be over 20% compared with the lens volume of a uniform recharge. For the steady‐state lens profile corresponding to a point recharge, the squared lens thickness peaks at the recharge location and linearly decreases to the boundary, resulting in a triangle shape for the profile of squared lens thickness. Moreover, compared with recharges in other locations, recharge in a small area at the domain center has the greatest impact. Uncertainty analysis can be conveniently implemented based on the derived analytical solution to investigate the lens behavior under spatially variable recharge with uncertainties. Results show that the lens volume tends to have a larger variance for a recharge rate with a higher variance and longer correlation length.

Experimental and modeling investigation of pumping from a fresh groundwater lens in an idealized strip island

Groundwater lens floating on the seawater in surficial coastal aquifers is an important freshwater supply for small island inhabitants. This study addresses an unanswered, simple but important question for groundwater resources management in small islands: how much freshwater can be sustainably withdrawn from a groundwater lens in small islands. We derive an approximate analytical solution to describe the freshwater-seawater sharp interface profile in an idealized strip island subject to a pumping well located at the domain center to determine the critical or maximum allowable pumping rate that prevents saltwater upconing. A simple formula of the critical pumping rate is provided as a function of hydrogeologic parameters, including the recharge rate, densities, hydraulic conductivity and well penetrating depth. It shows that the critical pumping rate increases linearly with the recharge rate and decreases linearly with the hydraulic conductivity and the squared well penetrating depth. Laboratory visualization experiments and numerical simulations were conducted to validate the derived analytical solution. Results indicate that the maximum pumping rate can be as high as 50% of the total recharge for a single well located at the top of the domain center. Our solutions and results provide useful insights for sustainable groundwater management in small islands and can potentially render significant social and economic impact for inhabitants on small islands.

Cycling of redox-sensitive trace metals in barrier island freshwater lenses

Freshwater lenses connect the terrestrial and marine realm via groundwater discharge at the edges of islands and serve as drinking water resources. We studied the redox-sensitive metals U, Mo, V, and Tl along the redox gradient of fresh groundwater lenses on Spiekeroog Island, northern Germany. Groundwater solute concentrations were linked to groundwater age and redox characteristics. We further quantified the contribution of precipitation, sea spray, and aquifer matrix to the groundwater metal concentrations and evaluated the sink and source function of the aquifer under oxic and reducing conditions.We found that biogeochemical processes altered the concentrations of the trace metals. In young, oxygen to nitrate reducing zones, the aquifer matrix represented the major metal source to the groundwater. For Tl, rain was an additional important (anthropogenic) source. Under manganese and iron oxide to sulfate reducing conditions, U and Tl were sensitive to redox dependent removal, whereas Mo and V were less affected by reductive precipitation/adsorption. In detail, 99% of dissolved Tl, 88% of U, 66% of Mo, and 44% of V were removed to the solid phase in comparison to values from less reducing zones. Large parts of the western freshwater lens on Spiekeroog were anoxic. For this reason, the delivery of aquifer derived metals to the ocean via fresh groundwater discharge appeared to be limited. Higher U, Mo, V, and Tl concentrations were observed in the presently developing young freshwater lens in the east of Spiekeroog Island. This suggests that less reducing groundwater lenses may be a source of these metals to the adjacent beach/coastal seawater. Especially for V, freshwater discharge from sandy coastal aquifers may be important, as groundwater concentrations exceeded seawater concentration under oxic as well as anoxic conditions. Regarding the suitability of the freshwater as drinking water, all measured trace metal concentrations were classified as uncritical.

Fresh groundwater lens dynamics of a small bedrock island in the tropics, Northern Australia

Fresh groundwater lens dynamics and transition zone geometries on small tropical islands have been investigated in a wide range of geological environments. However, the understanding of multi-aquifer fractured bedrock systems and how they respond to episodic wet season rainfall recharge is still limited. This study used a comprehensive suite of isotopic tracers, hydrogeological and near-surface geophysical methods to characterise the multi-layer aquifer system on Milingimbi, a small bedrock island located in the tropics of the Arafura Sea, Northern Australia. Near-surface geophysics was used to determine the subsurface structure of the aquifer, including the spatial extent and thickness of the fresh groundwater lens and the shape of the transition zone. Pore water chloride profiles, hydrochemistry, and δ18O and δ2H data supported the geophysics results. The lens was found to be 40 m thick on average, with an up to 70 m thick transition zone underneath. Water level time series data over 4 years showed that there was a strong tidal signal observed in the groundwater wells screened in the deeper aquifer and that the aquifer system showed a dynamic response to the wet season rainfall. Time series chloride and δ18O and δ2H rainfall and production bore data suggest that there was a freshening (i.e. lower salinity input) of the lens as a result of recharge during the wet season, which occurs up to 6 months after the event. Groundwater residence time indicators showed that the mean residence time in the lens was at least 25 years and the rate of recharge to the system was up to 200 mm y−1. The comprehensive data set, which is rather unique in its wide range of methods that were applied, resulted in a hydrogeological conceptual model of the multi-layer bedrock aquifer system of the island that also provides insight into the fresh groundwater lens and the transition zone geometry of similar island aquifer systems.

Small atoll fresh groundwater lenses respond to a combination of natural climatic cycles and human modified geology

Freshwater lenses underlying small ocean islands exhibit spatial variability and temporal fluctuations in volume, influencing ecologic management. For example, The Palmyra Atoll National Wildlife Refuge harbors one of the few surviving native stands of Pisonia grandis in the central Pacific Ocean, yet these trees face pressure from groundwater salinization, with little basic groundwater data to guide decision making. Adding to natural complexity, the geology of Palmyra was heavily altered by dredge and fill activities. Our study based at this atoll combines geophysical and hydrological field measurements from 2008 to 2019 with groundwater modeling to study the drivers of observed freshwater lens dynamics. Electromagnetic induction (EMI) field data were collected on the main atoll islands over repeat transects in 2008 following ‘strong’ La Niña conditions (wet) and in 2016 during ‘very strong’ El Niño conditions (dry). Shallow monitoring wells were installed adjacent to the geophysical transects in 2013 and screened within the fresh/saline groundwater transition zone. Temporal EMI and monitoring well data showed a strong contraction of the freshwater lens in response to El Niño conditions, and indicated a thicker lens toward the ocean side, an opposite spatial pattern to that observed for many other Pacific islands. On an outer islet where a stand of mature Pisonia trees exist, EMI surveys revealed only a thin (<3 m from land surface) layer of brackish groundwater during El Niño. Numerical groundwater simulations were performed for a range of permeability distributions and climate conditions at Palmyra. Results revealed that the observed atypical lens asymmetry is likely due to more efficient submarine groundwater discharge on the lagoon side as a result of lagoon dredging and filling with high-permeability material. Simulations also predict large decreases (40%) in freshwater lens volume during dry cycles and highlight threats to the Pisonia trees, yielding insight for atoll ecosystem management worldwide.

Modelling Groundwater Hydraulics to Design a Groundwater Level Monitoring Network for Sustainable Management of Fresh Groundwater Lens in Lower Indus Basin, Pakistan

The over-extraction of groundwater from thin fresh groundwater lenses is a threat to the livelihood of farmers in the Lower Indus Basin (LIB). It is essential to monitor and regulate this pumping to sustain fresh groundwater lenses. In this study, we applied a modelling approach in combination with geostatistical analysis to identify the critical locations to monitor the groundwater levels for sustaining fresh groundwater in the LIB. Our approach included four steps: (i) simulating temporal heads using a calibrated hydrogeological model; (ii) sampling monitoring locations using a hexagonal pattern of sampling; (iii) applying principal component analysis (PCA) of the temporal head observations, and selecting high scoring locations from the PCA; and (iv) minimizing the observation points to represent the water level contours. The calibrated model was able to replicate the hydro-dynamic behavior of the study area, with a root mean square of 0.95 and an absolute residual mean of 0.74 m. The hexagonal pattern of spatial sampling resulted in a 195 point network, but PCA reduced this network to 135 points and contour classification reduced it even further to 59 points. The 195, 135, and 59 point networks represented the water levels with average standard errors of 0.098, 0.318, and 0.610 m, respectively. Long-term simulations with increased pumping showed that the water levels would best be assessed by 195 monitoring points, although 135 and 59 points would represent the depleting area but would not capture the water logging area.

Regional and well-scale indicators for assessing the sustainability of small island fresh groundwater lenses under future climate conditions

Salinization of the freshwater lens and well fields under future climate and groundwater demands threaten the sustainability of groundwater abstraction in small islands. In this paper, two simple indices are proposed for a comprehensive assessment of freshwater salinization in small islands. Values of these indices are computed based on a sharp-interface finite element numerical model with the interface matched to the lower limit of freshwater. The first index relates rainfall percentiles with the computed freshwater volume and provides regional assessment of changes in the volume stored in the freshwater lens over time. The second is a sustainability index, based on computed saltwater ratios in pumping wells as the performance indicator, which represents the well salinization risk at a spatial scale. The proposed methodology is illustrated with the example of Tongatapu Island. Freshwater lens dynamics and well salinization under various scenarios of dry, median, and wet general circulation model predictions-based unsteady recharge, sea level rise, and pumping demands were evaluated for the period 2010–2099. The freshwater lens of Tongatapu is dependent on rainfall, and the 60-month rainfall percentile is highly correlated with the total freshwater volume. Public well field indicates sustainability indexes of 45% and 100% under the current pumping conditions for the driest and the wettest scenarios, and with increased pumping and sea level rise, they reduce to 30% and 90%, respectively. Thus, management of pumping rates is essential for future freshwater sustainability. This methodology can be used for first-hand temporal and spatial estimations of small island freshwater lens salinization considering both the regional and well scales.

Development of fresh groundwater lens in coastal reclaimed islands

Groundwater serves as a significant freshwater supply for coastal regions. In many coastal areas, extensive islands have been created by filling the sea. These reclaimed islands not only provide valuable space for urban development, but also can serve as valuable aquifers. Recharge of fresh water from rainfall can flush out salt water and eventually create a freshwater lens beneath the reclaimed island. However, how a fresh groundwater lens grows in reclaimed islands and the factors that control its growth are unclear. Here we use a series of three-dimensional density-dependent flow and transport models to investigate the process of fresh-salt water mixing and quantify the potential maximum volume of fresh groundwater that can occur beneath reclaimed islands. Our results show that the recharge rate controls the growth rate and the initialization of the fresh groundwater lens, whereas the permeability of the underlying fill materials and bedrock determines the shape of freshwater lens beneath the island but has a minor influence on freshwater volume. Increasing porosity enlarges the total water volume but diminishes the proportion of fresh water as the larger volume of original salt water in the aquifer requires more recharge to flush and dilute; longitudinal dispersivity promotes the fresh-salt water circulation in a certain range but limits the maximum fresh volume storage, and vertical transverse dispersivity facilitates extending mixing areas to enlarge the freshwater lens. Heterogeneity in the hydraulic conductivity field increases the variability of groundwater flow, restricts the average speed of fresh-salt water circulation, and results in a decrease in the growth rate and maximum volume of the freshwater lens. Our study has large implications for planning and management of reclaimed lands for water sustainability in the future in coastal areas.

Up-coning Caused by Excess Pumping during Drought Had Reduced Storage Volume of Fresh Groundwater Lens on Atoll Island

Up-coning Caused by Excess Pumping during Drought Had Reduced Storage Volume of Fresh Groundwater Lens on Atoll Island

Groundwater geochemistry fluctuations along a fresh-saltwater gradient on the carbonate islands of the lower Florida Keys

Climate change will have long-lasting effects on the availability of fresh water on small, carbonate islands that have isolated fresh groundwater lenses, particularly as sea level rises and rainfall regimes shift. The carbonate islands of the Florida Keys provide an ideal location to study the effect of variable rainfall on the aqueous geochemistry of the islands' groundwater. In a rainfall-driven carbonate system, the expectation is that limestone dissolution will occur within the vadose zone resulting in increased ions in the groundwater. However, geochemical processes are also affected by the salinity of groundwater and the extent of the mixing zone between fresh and salt water. We chose two islands to conduct the study of the shallow groundwater: the largest island in the lower Florida Keys, Big Pine Key (BPK), and a smaller island, Upper Sugarloaf Key (SLK). From May 2011 through April 2012, monthly groundwater samples were collected from 24 shallow (1 m deep) wells located along a fresh to saline gradient on both islands. Groundwater chemistry was compared with rainfall amounts from a weather station on BPK. Saturation indices for aragonite and calcite, generated with geochemical modeling in PHREEQC, were compared to conservative mixing between Gulf of Mexico water and freshwater. Equilibrium to supersaturated conditions with respect to carbonate minerals dominated in all of the groundwater samples. Saturation indices varied with rainfall with the most supersaturated samples observed after a large rain event and samples approaching equilibrium after the longest period without rainfall. Calcium in excess of what would be expected from conservative mixing of fresh water and seawater was observed in all groundwater samples and was elevated at near-shore locations, especially on BPK. Contrary to expectations, dissolution resulting from mixing of freshwater and seawater was not supported in the shallow groundwater. Instead, dissolution within the narrow vadose zone from rain events likely resulted in the excess calcium in the groundwater. Seasonal fluctuations in groundwater composition were primarily observed on the smaller island and were related to the fresh water balance, changing rapidly after a heavy rain event, and suggest that a size threshold has been surpassed for a stable lens. Rising seas will further decrease lens extent and vadose zone depth, reducing the potential for future limestone dissolution.

Estimating the freshwater-lens reserve in the coastal plain of the middle Río de la Plata Estuary (Argentina)

Drinking-water supply is one of the main issues that populations face in many coastlands. Shallow coastal aquifers are often characterized by the presence of lens-shaped freshwater floating on the saline groundwater plume of marine origin. These groundwater lenses are commonly associated with landforms, such as littoral ridges and dunes and in many cases they represent the main source of water supply in remote coastal areas. At the right side of the middle Río de la Plata estuary (Argentina) the aquifer system is generally saline. Elongated and thin sandy beach ridge systems emerging from the general flat morphology of the marsh-flood plain are capable of storing precipitations forming freshwater lenses, which to date are the main freshwater supply for inhabitants. The aim of this study is to identify and delimitate the presence of such valuable freshwater reserves in order to provide the first necessary guidelines for the water management plan in this area, which has never been implemented since, to the Authors' knowledge, no specific investigation had been carried out before this study. To achieve this goal, Vertical Electrical Sounding, groundwater electrical conductivity measurements, water balances and groundwater chemical analyses were performed and interpreted together. The whole dataset was processed to define the electro-stratigraphic model of the study area and to produce the map of the electrical conductivity of the shallow aquifer. In addition, a three-dimensional model of the fresh water reservoir has been implemented for a better understanding of the relationship between geomorphology and groundwater. Results point out that a total freshwater volume of 78,259,700m3 is stored into a continuous lens and the annual average recharge from precipitation amounts to 6,303,500m3. Although preliminary, this work provides the basic knowledge on the potential fresh groundwater lenses and provides important information for addressing a sustainable use of the freshwater resource.