Freshwater-seawater Interface Research Articles

Effects of global and climate change on the freshwater-seawater interface movement in a Mediterranean karst aquifer of Mallorca Island

Karst aquifers are globally prized freshwater sources, posing a significant preservation challenge. These aquifers typically exhibit dual or even triple porosities, encompassing matrix, fractures-fissures and conduits, rendering them highly responsive to variations in chemical characteristics and hydraulic head. In coastal regions, these aquifers often possess extensive subsurface conduit networks intricately linked to the rock matrix, facilitating groundwater discharge into the sea. Therefore, they display acute sensitivity to seawater intrusion, swiftly reacting to changes in precipitation and pumping regimes. This makes them exceptionally vulnerable to short-term meteorological fluctuations and long-term climate change. Their high heterogeneity leads to uneven penetration of the freshwater-seawater interface, causing rapid seawater intrusion inland over significant distances. The Mediterranean region, characterized by water deficit and water stress, faces strong impacts from climate change, featuring a warming atmospheric trend exceeding the global average, along with diminished rainfall exacerbating water scarcity. Increasing water demands for agriculture, urban development, and the growing tourism industry, because of global change, are worsening water stress. Our primary research objectives were analyzing the environmental consequences of global and climate change on seawater intrusion in Mediterranean coastal karst aquifers, with a focus on the role of the double-flow model, thus contributing to the understanding of the processes involved. To achieve this, we selected a study region on Mallorca Island in the western Mediterranean, where a karst aquifer system discharges into the sea. We employed various study methods, notably hydrochemical techniques and multi-isotopic analysis, encompassing the examination of 2H and 18O isotopes in water, 87Sr/86Sr ratio, Sr and B concentrations, and δ11B in water. A key finding is the rebound effect, wherein aquifers recontaminate due to solute molecular back-diffusion following cessation of extractions and the retreat of marine intrusion, providing insight into the impact of climate and global change on Mediterranean karst aquifers.

An Analytical solution for groundwater lens pumping in a three-dimensional rectangular island

Groundwater is an important freshwater supply for residents on islands without surface reservoirs, where groundwater floats on top of seawater, forming freshwater lenses. Previous analytical solutions of groundwater lens are developed based on two-dimensional models. In this study, we first derived an analytical solution for steady-state groundwater lens profiles, i.e., the freshwater-seawater interface, under pumping conditions for a three-dimensional rectangular island. Using this solution, critical or maximum pumping rates without extracting saltwater can be conveniently estimated for wells or drains at different locations in a three-dimensional rectangular island. Numerical models are simulated to validate the analytical solution. Results show that the critical pumping rate of a single well initially increases with the aspect ratio, i.e., the ratio between island length and width, and reaches a constant, which is limited by the short island side. In addition, the critical pumping rate of a single well is the largest in the center of the domain and decreases as the well approaches the domain boundary, as expected. When a rectangular island has an aspect ratio equal to one (i.e., a square island), the critical pumping rate is close to a circular island. For islands with a high aspect ratio, the critical pumping rate increases with the pumping drain length, which may be simplified to a two-dimensional strip island model. The derived analytical solution can be easily extended to include multiple pumping wells and drains, and can potentially be used to optimize groundwater pumping in oceanic islands.

Probing Individual Particles Generated at the Freshwater-Seawater Interface through Combined Raman, Photothermal Infrared, and X-ray Spectroscopic Characterization.

Sea spray aerosol (SSA) is one of the largest global sources of atmospheric aerosol, but little is known about SSA generated in coastal regions with salinity gradients near estuaries and river outflows. SSA particles are chemically complex with substantial particle-to-particle variability due to changes in water temperature, salinity, and biological activity. In previous studies, the ability to resolve the aerosol composition to the level of individual particles has proven necessary for the accurate parameterization of the direct and indirect aerosol effects; therefore, measurements of individual SSA particles are needed for the characterization of this large source of atmospheric aerosol. An integrated analytical measurement approach is required to probe the chemical composition of individual SSA particles. By combining complementary vibrational microspectroscopic (Raman and optical photothermal infrared, O-PTIR) measurements with elemental information from computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM-EDX), we gained unique insights into the individual particle chemical composition and morphology. Herein, we analyzed particles from four experiments on laboratory-based SSA production using coastal seawater collected in January 2018 from the Gulf of Maine. Individual salt particles were enriched in organics compared to that in natural seawater, both with and without added microalgal filtrate, with greater enrichment observed for smaller particle sizes, as evidenced by higher carbon/sodium ratios. Functional group analysis was carried out using the Raman and infrared spectra collected from individual SSA particles. Additionally, the Raman spectra were compared with a library of Raman spectra consisting of marine-derived organic compounds. Saccharides, followed by fatty acids, were the dominant components of the organic coatings surrounding the salt cores of these particles. This combined Raman, infrared, and X-ray spectroscopic approach will enable further understanding of the factors determining the individual particle composition, which is important for understanding the impacts of SSA produced within estuaries and river outflows, as well as areas of snow and ice melt.

Trace Metal Partitioning in the Salinity Gradient of the Highly Stratified Estuary: A Case Study in the Krka River Estuary (Croatia)

A size partitioning of several trace metals (Zn, Cd, Pb, Cu, Ni, Co, Mn, Fe and Al) between five size fractions (<3 kDa, 3 kDa–0.1 µm, 0.1 µm–1.2 µm, 1.2 µm–5 µm and >5 µm) was studied in the vertical salinity gradient of the highly stratified Krka River estuary. The results indicated a dominant river source for Zn, Co, Mn, Fe and Al and a diluting effect on Cd, Pb and Ni. The truly dissolved fraction (<3 kDa) dominated the Zn, Cd, Cu, Ni and Co pool, and a large part of Pb, Mn, Fe and Al was present in >5 µm particles. Pb, Mn, Fe and Al were closely related, showing a precipitation and colloidal aggregation in the surface layers and dissolution in the seawater layer. The highest percentage (30–37%) of colloids (3 kDa–0.1 µm) in the dissolved pool was found for Pb, Cu, Fe and Al. Differences in size distribution between low and high river flow periods revealed that Zn, Pb, Co, Mn, Fe and Al are introduced by the river mostly in the 3 kDa–5 µm size range. Therefore, a low percentage of colloidally bound metals compared to other coastal areas can be explained by a limited riverine input of terrigenous material, characteristic for this estuary. Correlation with PARAFAC components revealed associations of Cu with protein-like substances and Co with humic-like substances. The accumulation of Cu at the freshwater-seawater interface coupled with an increase of its colloidal fraction was observed, apparently governed by biologically produced organic ligands.

A framework for parameter estimation using sharp-interface seawater intrusion models

Sharp-interface seawater intrusion models present shorter run times than variable density codes, which makes them practical for regional, decision-support groundwater modeling. Although parameter estimation and uncertainty analyses are essential steps for model-based decision making, their implementation in seawater intrusion models has remained limited. Few guidelines are available and it is unclear which observations should be used, what processing they require and what weighting strategy should be used. A framework has been developed for parameter estimation using a regional sharp-interface decision-support model applied to a real-world example in the Magdalen Islands (Quebec, Canada). This framework included the assimilation of head observations collected from shallow wells, deep open wells and pumping wells, as well as freshwater-seawater interface observations derived from deep open wells, TDEM (time-domain electromagnetic) and ERT (electrical resistivity tomography) geophysical surveys. A model was developed using MODFLOW-SWI2 in which fast model run times allowed the estimation of numerous parameters, including a heterogeneous hydraulic conductivity field with pilot points. Following parameter estimation with PEST, the uncertainty of several model forecasts, i.e. the volume of freshwater and the interface elevation near municipal pumping wells, was examined with the first-order second moment (FOSM) approach and a data worth analysis was carried out. While the observations presented a low signal-to-noise ratio, parameter estimation was effective to reduce the uncertainty of model forecasts. Interface observations, and particularly geophysical observations, were most useful to reduce predictive uncertainties. Head and interface observations from deep open wells were biased and could not be suitably reproduced by the model. The framework developed here is relatively straightforward and could be implemented more systematically. The study also provides recommendations to guide future data collection strategies in coastal aquifers.

A review on cesium desorption at the freshwater-seawater interface

A review on cesium desorption at the freshwater-seawater interface

Landscape controls on riverine export of dissolved organic carbon from Great Britain

The dissolved organic carbon (DOC) export from land to ocean via rivers is a significant term in the global C cycle, and has been modified in many areas by human activity. DOC exports from large global rivers are fairly well quantified, but those from smaller river systems, including those draining oceanic regions, are generally under-represented in global syntheses. Given that these regions typically have high runoff and high peat cover, they may exert a disproportionate influence on the global land–ocean DOC export. Here we describe a comprehensive new assessment of the annual riverine DOC export to estuaries across the island of Great Britain (GB), which spans the latitude range 50–60° N with strong spatial gradients of topography, soils, rainfall, land use and population density. DOC yields (export per unit area) were positively related to and best predicted by rainfall, peat extent and forest cover, but relatively insensitive to population density or agricultural development. Based on an empirical relationship with land use and rainfall we estimate that the DOC export from the GB land area to the freshwater-seawater interface was 1.15 Tg C year−1 in 2017. The average yield for GB rivers is 5.04 g C m−2 year−1, higher than most of the world’s major rivers, including those of the humid tropics and Arctic, supporting the conclusion that under-representation of smaller river systems draining peat-rich areas could lead to under-estimation of the global land–ocean DOC export. The main anthropogenic factor influencing the spatial distribution of GB DOC exports appears to be upland conifer plantation forestry, which is estimated to have raised the overall DOC export by 0.168 Tg C year−1. This is equivalent to 15% of the estimated current rate of net CO2 uptake by British forests. With the UK and many other countries seeking to expand plantation forest cover for climate change mitigation, this ‘leak in the ecosystem’ should be incorporated in future assessments of the CO2 sequestration potential of forest planting strategies.

Impact of projected climate change on seawater intrusion on a regional coastal aquifer

Sea level rise (SLR) is one of the prime consequences of global warming as pointed out by the Intergovernmental Panel on Climate Change (IPCC). SLR adversely affects coastal regions; triggers coastal erosion, inundation, and affects the freshwater–seawater interface as well. This paper presents the results of a study in which a coastal aquifer under changing climate was simulated using a three-dimensional groundwater model. The study area covers a part of the coastal aquifer in Ernakulam district in the State of Kerala, India. Support Vector Machine (SVM) was used for projection of future sea levels under the representative concentration pathways (RCPs) 4.5 and 8.5, based on the projections of Phase 5 of the Coupled Model Intercomparison Project (CMIP5). Both thermosteric and halosteric components were taken into account in the projection of sea level. It was observed that sea level changes are significantly influenced by the halosteric effect. Results indicate that SLR in the year 2050 with respect to the levels in 2014 will be about 8.64 cm and 12.96 cm under RCPs 4.5 and 8.5, respectively. The repercussions of this rise in sea level on seawater intrusion into the coastal aquifer were evaluated by performing simulations with SEAWAT. Results of the study indicate that the effect of this SLR on seawater intrusion is negligible.

Redox condition of saline groundwater from coastal aquifers influences reverse osmosis desalination process

Reverse osmosis (RO) seawater desalination is a widely applied technological process to supply potable water worldwide. Recently, saline groundwater (SGW) pumped from beach wells in coastal aquifers that penetrate beneath the freshwater-seawater interface is considered as a better alternative water source to RO seawater desalination as it is naturally filtered within the sediments which reduces membrane fouling and pre-treatment costs. The SGW of many coastal aquifers is anoxic – and thus, in a low redox stage – has elevated concentrations of dissolved manganese, iron and sulfides. We studied the influence of the SGW redox stage and chemistry on the performance - permeate flux and fouling properties - of RO desalination process. SGWs from three different coastal aquifers were sampled and characterized chemically, and RO desalination experiments were performed under inert and oxidized conditions. Our results show that all three aquifers have anoxic saline groundwater and two of them have intensive anaerobic oxidation of organic matter. Two aquifers were found to be in the denitrification stage or slightly lower and the third one in the sulfate reduction stage. Our results indicate that the natural redox stage of SGWs from coastal aquifers affects the performance of RO desalination. All SGW types showed better RO performance over seawater desalination. Furthermore, air oxidation of the SGW was accompanied with pH elevation, which increased the membrane fouling. Hence, keeping the feed water unexposed to atmospheric conditions for maintaining the natural reducing stage of the SGW is crucial for low fouling potential. The observed benefits of using naturally reduced SGW in RO desalination have significant implications for reduction in overall process costs.

Investigation of the elevation of saltwater wedge due to subsurface dams

AbstractSubsurface dams are rather effective and used for the prevention of saltwater intrusion in coastal regions around the world. We carried out the laboratory experiments to investigate the elevation of saltwater wedge after the construction of subsurface dams. The elevation of saltwater wedge refers to the upward movement of the downstream saltwater wedge because the subsurface dams obstruct the regional groundwater flow and reduce the freshwater discharge. Consequently, the saltwater wedge cannot further extend in the longitudinal direction but rises in the vertical profile resulting in significant downstream aquifer salinization. In order to quantitatively address this issue, field‐scale numerical simulations were conducted to explore the influence of various dam heights, distances, and hydraulic gradients on the elevation of saltwater wedge. Our investigation shows that the upward movement of the saltwater wedge and its areal extension in the vertical domain of the downstream aquifer become more severe with a higher dam and performed a great dependence on the freshwater discharge. Furthermore, the increase of the hydraulic gradient and the dam distance from the sea boundary leads to a more pronounced wedge elevation. This phenomenon comes from the variation of the freshwater discharge due to the modification of dam height, location, and hydraulic gradient. Large freshwater discharge can generate greater repulsive force to restrain the elevation of saltwater wedge. These conclusions provide theoretical references for the behaviour of the freshwater–seawater interface after the construction of subsurface dams and help optimize the design strategy to better utilize the coastal groundwater resources.

Subsurface Investigation of Freshwater-Seawater Interface on Gowa-Takalar Coastal Aquifer, Indonesia

Gowa-Takalar Coastal Aquifer is an unconfined aquifer located in the southward of Makassar City. The Gowa-Takalar area has a heavy groundwater exploitation particularly on dry season. In this area, the groundwater is utilized to support water need for agriculture activities, marine tourism facilities, and new settlement development. The highly economic activities in this area, it causes an intensive groundwater exploitation. Since this area is located around coastal area, this area has highly susceptibility to suffer from seawater intrusion. In order to maintain the continuity of groundwater supply, a special study is needed on hydrodynamics of fresh-sea water interface as the basis for the formulation of a sustainable management model. This study as an early step of groundwater management research emphasizing on basic of aquifer and groundwater data. The research method consisting of reconnaissance study for geological & hydrogeological mapping, groundwater chemistry laboratory analysis, and geo-electrical investigation. Geological condition of this area is composed of alluvial and coastal deposits, volcanic deposits, and marine sedimentary rocks interbedded with volcanics. These materials form three hydrogeological characteristics: extensive and moderately productive aquifer, locally productive aquifers, and poorly productive aquifers of local importance. The chemical composition of groundwater from dug wells show the chloride concentration vary from 3.55 ppm to 81.65 ppm, with chloride-bicarbonate ratio is 0.008 – 0.035 indicating aquifer without sea water invasion. However, based on vertical electrical sounding, the resistivity layers vary from 0.02 to 62.50 ohm-m indicating the present of seawater intrusion into coastal aquifer.

Integrated seawater intrusion study of coastal region of Thiruvallur district, Tamil Nadu, South India

The groundwater is the main resource of water for irrigation activity in river lacking area. The freshwater–seawater interface in the study region that has existed 3 km away from the coast in the year 1969 has been found to be migrated to distance of 13 km from the coast during the year 2007 noticed by the Central Ground Water Board (Central ground water board’s district groundwater brochure, Thiruvallur district, Tamil Nadu, 2007). Integrated geochemical and geophysical techniques were carried out in the study area to decode subsurface geologic pattern and delineate the seawater–freshwater zones. Total numbers of 50 samples were collected from the entire study area and analyzed for major ions. The considerable samples are brackish scenery of groundwater water at low depth. Chadha and Piper’s plots categorize the coastal groundwater into Na–HCO3, Ca–Na–HCO3, Ca–HCO3, and Na–Cl water facies, with Ca–HCO3 as the dominant. Cl/CO3 + HCO3 ratio, Cl/HCO3, and ionic strength, Mg/Ca and Cl/HCO3 ratios show that most of the samples in the study area are affected by seawater intrusion, which is also confirmed by the geophysical method. The results of vertical electrical sounding carried out in the study area reveal the low transverse resistance and high longitudinal conductance. It suggests the brackish nature of the groundwater in the eastern part of the study area may be due to the seawater intrusion. The final map using GIS platforms productively delineates the location that is really undergoing seawater and freshwater zone is migrated toward the inland. The article suggested further studies to arrest the migration of sea/freshwater interface into the land and avoid overexploitation of groundwater to further development.

Dispersion effects on the freshwater–seawater interface in subsea aquifers

Recent recognition of the widespread occurrence of freshwater beneath the ocean has renewed interest in approaches to understand and predict its extent. The most straightforward methodologies are based on the sharp-interface approximation, which neglects dispersive mechanisms. The understanding of dispersion effects on freshwater extents in coastal aquifers is based almost entirely on onshore aquifer situations. This study explores dispersion in offshore coastal aquifers, in terms of the steady-state freshwater extent, seawater circulation and freshwater discharge, through numerical experimentation. Results show that increasing dispersion causes a seaward shift in the interface toe location, as expected, whereas the interface tip shows a non-monotonic relationship with dispersion that depends on the contrast between aquifer and aquitard hydraulic conductivities. Higher dispersion leads to enhanced seawater recirculation rates and freshwater discharge, as opposed to non-monotonic relationships obtained previously for onshore aquifers. The mixing zone at the toe widens as dispersion increases, similar to onshore cases, whereas the mixing zone at the tip has a surprisingly non-monotonic relationship with dispersion. The dispersion relationships revealed in this study can be explained by counteractions between dispersion, density and advective forces, and refraction across the aquifer-aquitard interface, which in combination produce offshore aquifer behaviour that differs, in some ways, to the manner in which onshore aquifers respond to dispersive processes. Consequently, previous empirical corrections to sharp-interface methods (to account for dispersive effects) applied to onshore coastal aquifers are ineffective in their application to offshore settings.

Assessment of sedimentary heterocyst glycolipids as tracers of freshwater input to the Changjiang Estuary and East China Sea

Heterocyst glycolipids (HGs) are biomarkers of important freshwater and marine diazotrophs known as heterocystous cyanobacteria. Recent research has found that HGs with hexose moieties (C6) are derived from free-living, mostly freshwater species while those with pentose moieties (C5) are derived from diatom-symbiont, mostly marine species. Here, we analyzed 39 surface sediments from a study area ranging from the Changjiang Estuary to the offshore East China Sea for HGs to investigate distribution patterns at this freshwater-seawater interface. We detected both C5 and C6 HGs in these samples; however, they had different spatial distributions: the C6 HG abundance was highest at the freshwater end of the estuary and decease offshore, whereas the abundance of C5 HGs displayed almost inverse pattern. We hypothesized that such distributions were likely caused by the mixing of freshwater and seawater, leading to a mixture of HGs derived from inland waters and/or soils and the marine environment. The proportion of C6 HGs to total HGs, defined as the FHG index, decreased with distance offshore, possibly reflecting the delivery of freshwater into the ocean. The FHG index changed curvilinearly with salinity, likely resulting from concentration-based weighting of end-member contributions. The FHG index correlated linearly (r2 = 0.76) with δ13Corg, suggesting that FHG could be used to indicate relative contributions of freshwater-delivered terrestrial organic matter in this estuarine and coastal region. However, the FHG index did not linearly correlate with the branched and isoprenoid tetraether (BIT) index, another proxy used for terrestrial input, likely because the BIT index is predominantly controlled by variations in marine crenarchaeol production.

The Numerical Simulation of the Freshwater-Seawater Mixing in the Single Phase Coastal Aquifer

The interaction between freshwater and seawater usually occurs in the coastal aquifer. To simply explain this condition, describing the interface between them is conducted. This approach is a multiphase condition. Unfortunately, this condition is on immiscible condition, whereas the miscible condition should be obtained. The miscible condition of freshwater and seawater along coastal aquifer occurs on the single phase. This condition neglects the concept of the interface between them. The purpose of this research is to simulate the mixing of freshwater and seawater in the coastal aquifer by using the numerical method. Groundwater flow equation is governed involving the effect of contaminant concentration. On the other hand, the conservative groundwater contaminant transport equation is used. The discretization process uses the Finite-Difference Method. Coupling two equations will obtain the spreading of contaminant concentration along the model domain. The result shows that the freshwater-seawater interface does not appear and the change of concentration can be reviewed in every section and every point of the computational domain.

First application of the marine differential electric dipole for groundwater investigations: A case study from Bat Yam, Israel

The marine differential electric dipole (DED) is applied for the first time to study a subseafloor groundwater body in the coastal region of Bat Yam, Israel. Previous marine long-offset transient electromagnetic applications detected this freshwater body underneath the Mediterranean seafloor. We have applied the novel DED method for the first time in the marine environment to further investigate this natural phenomenon. The main objectives are to locate the freshwater-seawater interface at the western aquifer edge and to identify the mechanism controlling this freshwater occurrence beneath the seafloor. The acquired step-on signals allow one to detect the freshwater body in the vicinity of the Israeli coastline at a depth of approximately 70 m beneath the seafloor. However, aquifer thickness is only poorly determined and may vary between 40 and 100 m. A lateral resistivity contrast is observable between adjacent 1D inversion models and also apparent in data profile curves that constrain the seaward extent of the detected resistive body to a distance of less than 4 km from the coastline. A subsequent 2.5D forward-modeling study aims to find a subseafloor resistivity distribution that adequately explains all measured DED data simultaneously. The results further constrain the lateral extent of the resistive aquifer to approximately 3.6–3.7 km from the Israeli coast. Furthermore, the data indicate that the aquifer system may be susceptible to seawater intrusion, as a superior data fit is achieved if a brackish water zone of approximately [Formula: see text] with a lateral extent of less than 300 m is located at the head of the freshwater body.

Dynamic response of an Arctic epishelf lake to seasonal and long-term forcing: implications for ice shelf thickness

Abstract. Changes in the depth of the freshwater–seawater interface in epishelf lakes have been used to infer long-term changes in the minimum thickness of ice shelves; however, little is known about the dynamics of epishelf lakes and what other factors may influence their depth. Continuous observations collected between 2011 and 2014 in the Milne Fiord epishelf lake, in the Canadian Arctic, showed that the depth of the halocline varied seasonally by up to 3.3 m, which was comparable to interannual variability. The seasonal depth variation was controlled by the magnitude of surface meltwater inflow and the hydraulics of the inferred outflow pathway, a narrow basal channel in the Milne Ice Shelf. When seasonal variation and an episodic mixing of the halocline were accounted for, long-term records of depth indicated there was no significant change in thickness of ice along the basal channel from 1983 to 2004, followed by a period of steady thinning at 0.50 m a−1 between 2004 and 2011. Rapid thinning at 1.15 m a−1 then occurred from 2011 to 2014, corresponding to a period of warming regional air temperatures. Continued warming is expected to lead to the breakup of the ice shelf and the imminent loss of the last known epishelf lake in the Arctic.

Response Behaviors of Heavy Metals at Tidal Currents Interface and Salinity Interface in the Estuary Area

In China, trace metals (such as dissolved Cu, Zn, Pb, Cd, and Cr) are the routine mandatory monitoring indicators in existing Freshwater Quality Standard (FWQS, 2002) and Seawater Quality Standard (SWQS, 1997). Using Daliao River estuary as an example, we tried to research the general response patterns of these metals in estuarine system. Firstly, we determined the location of FSI (freshwater-seawater interface) and TCI (tidal currents interface), which were based on biological, chemical and geographic similarities by descriptive and quantitative methods, respectively. Secondly, two hypotheses were proposed to explain the difference based on distribution coefficient Kd along salinity and SS (suspended solids) gradient, these results revealed that the indicators for As, Cd and Cr may be prone to be affected by salinity, and the ones in the second pattern may be more sensitive to SS. Finally, we classified the response behaviors into two patterns based on the changing trends along the distance and magnitudes affected by the FSI and TCI: 1the obvious boundary break point from FSI for As, Cd and Cr; 2 the boundary zones from TCI for Zn, Cu, Pb and Ni. The partitioning behaviors showed similar changing trends at particle and sediment phases, especially for particle phase, depending on the concentration of SS along the salinity. The difference between general behaviors for the two patterns mainly existed at water phase. In general, this paper highlighted the need to consider the background difference of trace metals and identified the sensitive indicators controlled by different environment factors (such as salinity and SS) based on the native estuarine characteristics. Our work will assist the implementation of updating the current water quality standards in China.

A coupled stochastic inverse/sharp interface seawater intrusion approach for coastal aquifers under groundwater parameter uncertainty

This paper presents an alternative approach to deal with seawater intrusion problems, that overcomes some of the limitations of previous works, by coupling the well-known SWI2 package for MODFLOW with a stochastic inverse model named GC method. On the one hand, the SWI2 allows a vertically integrated variable-density groundwater flow and seawater intrusion in coastal multi-aquifer systems, and a reduction in number of required model cells and the elimination of the need to solve the advective-dispersive transport equation, which leads to substantial model run-time savings. On the other hand, the GC method allows dealing with groundwater parameter uncertainty by constraining stochastic simulations to flow and mass transport data (i.e., hydraulic conductivity, freshwater heads, saltwater concentrations and travel times) and also to secondary information obtained from expert judgment or geophysical surveys, thus reducing uncertainty and increasing reliability in meeting the environmental standards. The methodology has been successfully applied to a transient movement of the freshwater-seawater interface in response to changing freshwater inflow in a two-aquifer coastal aquifer system, where an uncertainty assessment has been carried out by means of Monte Carlo simulation techniques. The approach also allows partially overcoming the neglected diffusion and dispersion processes after the conditioning process since the uncertainty is reduced and results are closer to available data.

Saline Groundwater from Coastal Aquifers As a Source for Desalination.

Reverse osmosis (RO) seawater desalination is currently a widespread means of closing the gap between supply and demand for potable water in arid regions. Currently, one of the main setbacks of RO operation is fouling, which hinders membrane performance and induces pressure loss, thereby reducing system efficiency. An alternative water source is saline groundwater with salinity close to seawater, pumped from beach wells in coastal aquifers which penetrate beneath the freshwater-seawater interface. In this research, we studied the potential use of saline groundwater of the coastal aquifer as feedwater for desalination in comparison to seawater using fieldwork and laboratory approaches. The chemistry, microbiology and physical properties of saline groundwater were characterized and compared with seawater. Additionally, reverse osmosis desalination experiments in a cross-flow system were performed, evaluating the permeate flux, salt rejection and fouling propensities of the different water types. Our results indicated that saline groundwater was significantly favored over seawater as a feed source in terms of chemical composition, microorganism content, silt density, and fouling potential, and exhibited better desalination performance with less flux decline. Saline groundwater may be a better water source for desalination by RO due to lower fouling potential, and reduced pretreatment costs.