Fresh-saline Water Interface Research Articles

Managed aquifer recharge in island aquifer under thermal influences on the fresh-saline water interface

Storing and utilizing freshwater in coastal aquifers with managed aquifer recharge (MAR) presents significant challenges, particularly in small island settings where seawater intrusion is a major source of groundwater salinization. Existing studies usually evaluate density-dependent flow under isothermal condition, which neglects the potential impact of temperature on fluid density. To fill the knowledge gap, we performed both laboratory experiments and numerical modeling to investigate the impact of warmer water injection on freshwater storage and extracted water quality for island aquifer. It is found that warmer water (70 °C) injection compromise the “barrier effect” achieved by normal water injection, resulting in a reduced ability to prevent coastal seawater intrusion by, creating an upward hydraulic gradient beneath the injection well. In addition, warm water injection result in a 5 % decrease in freshwater storage capacity for island aquifer. However, by employing a combination of receding tide cycles and intermittent injection-extraction operations, the duration of lowered saline concentration within the extracted water can be extended by approximately 10 % with an 11 % reduction in the overall extracted saline concentration. This study provides valuable insights into the influence of warmer water injection on coastal aquifer quality. The findings from both the laboratory experiments and numerical simulations enhance the understanding of thermal density-dependent dynamics of coastal aquifers, offering valuable guidance for effective freshwater storage and usage in island settings.

An Automatic-Vertical Profile Monitoring System for Fresh-Saline Water Zones in Coastal Aquifer.

Coastal aquifers are complex systems governed by fresh-saline water interactions and ocean tidal effects. The vertical electrical conductivity (EC) and temperature (T) are general indicators for detecting the fresh-saline water interface (FSI) and sea water intrusion in groundwater wells located in coastal aquifers. In this method brief, we developed a cost-effective Arduino-based automatic-vertical profile monitoring system (A-VPMS) to continuously record vertical EC and T in groundwater wells, with the aim of testing its effectiveness in spatiotemporal monitoring of the FSI in a coastal aquifer located in eastern Korea. By analyzing the high-density EC and T data obtained by the A-VPMS, we evaluated the characteristics of the FSI, such as depth and spatial distribution. Our established EC and T data collection method using the A-VPMS proved to be efficient and reliable, providing an excellent tool for fine-scale temporal and spatial understanding of sea water intrusion. The results of this study demonstrate the potential of the A-VPMS for continuous monitoring of the FSI in coastal aquifers, which is crucial for sustainable management of groundwater resources.

Injection of desalination brine into the saline part of the coastal aquifer; environmental and hydrological implications

Seawater desalination, specifically reverse osmosis (RO), has become an important water resource, especially in arid and semi-arid regions. The desalination process generates a brine solution that is usually discharged to the nearshore environment, negatively impacting the marine ecosystem. A different method of disposing of the brine solution is needed to restore and maintain the marine environment. One such method is injecting the desalination brine into the saline part of the coastal aquifer. This study aims to investigate the hydrological and environmental impacts of such injection using groundwater flow and solute transport numerical models, showing the fresh-saline water interface (FSI) response at different injection rates, depths, and distances from shore. Moreover, this study investigates the recovery evolution of the aquifer after injection stops. We also analyze the hydrological response when pumping saline groundwater (SGW) (for desalination) or freshwater (for water supply) simultaneously with brine injection. Results show that brine injection creates a high salinity plume that pushes the FSI landward and salinizes the aquifer. After 20 years of injecting 5 million m3 y−1, 17 million m3 of freshwater are lost due to salt contamination. It is also shown that by injecting further offshore and in shallower depths, the impact on the FSI and the aquifer is reduced. Furthermore, pumping SGW simultaneously with brine injection negates the brine plume effect on the FSI and results in a more stable interface. It is noted that aquifer recovery is a long process and even after 100 years of recovery, the aquifer is not fully rehabilitated. This paper shows for the first time the hydrological implications of brine injection into the saline part of the coastal aquifer and demonstrates its potential utility for desalination plants in protecting the environment.

Application of Cl/Br ratio to demarcate the fresh-saline water interface in coastal aquifers of northern Tamilnadu, Southern India

Geochemical parameters, chloride and bromide were used as tracers to demarcate the extent of saline water intrusion in the northern part of Tamilnadu coastal aquifer which served as a hub for major pumping well fields for supply of drinking water to Chennai city. 158 groundwater samples were collected during pre & post monsoon period to analyse the hydrochemical variations across this temporal stretch and two major water types were interpreted from the analytical data. During pre-monsoon period, Na–Cl is dominant followed by Na–HCO3 type and during post monsoon, the groundwater in the central and eastern part shift towards Na–HCO3 type from Na–Cl type. Bromide is absent in the freshwater samples, while the mixed water samples have varying bromide concentrations. In the study area, Br concentration in groundwater ranged from 0 to 22.1 mg/l while the seawater sample had Br concentration of 63 mg/l. The Cl/Br ratios of groundwater of the study area ranged from 0.01 to 188 while that of the seawater figured at 288. The groundwater of the eastern and north-eastern portion of the study area has Cl/Br ratio between 75 and 188 and this value has a similar signature to that of seawater. The continental part of the study area has Cl/Br ratio less than 50 which corresponds to that of the intermittent zone between the seawater and rainwater. Spatial distribution of Cl/Br ratio and their variation across distance from the coast indicate the remnants of seawater in the aquifer material upto 19–20 km inland and the ratio tends to decrease beyond 20 km distance. The study explores the applicability of the Cl/Br ratios for defining the extent and movement of he freshwater-saline water interface in a coastal region, which can be used by the water managers, administrators and planners for effective management and protection of available groundwater resources.

Haline Convection within a Fresh-Saline Water Interface in a Stratified Coastal Aquifer Induced by Tide

Sea-tide effects on the fresh-saline water interface (FSI) in a stratified coastal aquifer are examined through laboratory experiments. The physical model, a two-dimensional rectangular flow tank, is filled with layered aquifers and aquitards. The aquifers serve as the main entrances/exits of water to/from the system through significant horizontal flows, creating unstable conditions of heavier saline water above lighter freshwater for short periods of time. Several processes create mixing; this instability results in haline convection, creating downward fingering, stable rising of horizontal saltwater front, and unstable upward fingerings of flushing freshwater. The time lag between the sea tide fluctuations and the emergence of adequate fresh- and saltwater is higher in a stratified system compared to a homogeneous system. Furthermore, longer tide cycles lead to the enlargement of the FSI’s toe horizontal movement range. The combination of tidal forcing with a layering aquifer structure leads to a wider FSI by creating a significant salt- and freshwater mixing inside each layer, vertical flows between the layers, and saltwater bodies at isolated areas. Haline convection within the FSI might be the reason for the wider fresh-saline interfaces that are found in field studies.

Using radium isotopes to constrain the age of saline groundwater, implications to seawater intrusion in aquifers

In this paper, we present a method for groundwater dating, using radium isotope ratios. We use this method to constrain ages of saline groundwater in two aquifers of different depth and age (shallow Pleistocene and deep Cretaceous) along the Israeli coast, with implications to seawater intrusion. In the Pleistocene aquifer, long to short-lived isotope ratios (e.g. 226Ra/223Ra and 226Ra/228Ra) in groundwater from both shallow and deep sub-units, up to >700 m from the sea, were far from equilibrium with radioactive parent ratios (238U/235U and 230Th/232Th, respectively). This suggests young ages, on the order of decades, which further implies high flow rates of ≥10 m yr−1, probably related to recent over-pumping and seawater intrusion. On the other hand, most Cretaceous aquifer groundwater showed close to or higher than equilibrium 226Ra/223Ra, which implies ages of >1000 years, with the exact minimum age dependent on radium adsorption in saline water. The higher than equilibrium ratios are attributed to a borehole stagnation effect, which results in the preferred decay of the short-lived 223Ra. Three samples showed significantly lower than equilibrium 226Ra/223Ra (226Ra/223Ra = 4.5–7.6), which suggests ages of several hundreds to slightly over 1000 years. These results, together with recent relatively young (Holocene) 81Kr and 39Ar apparent ages of Yechieli et al. (2019), challenge the accepted paradigm of a Neogene age for the Cretaceous saline water and suggests that this water recently had hydraulic connection with the sea. The use made in this study of radium isotopes to constrain groundwater ages, in particular its relevance to the time window of 100’s to 1000’s years, is an important addition to the family of groundwater dating tools, which should be further elaborated on. It is especially important to dating of water from the fresh-saline water interface, since it is less sensitive than other methods to the mixing with fresh meteoric water.

Three-dimensional configuration and dynamics of the fresh–saline water interface near two saline lakes with different levels (Middle East)

A typical fresh–saline water interface in a coastal aquifer is characterized by saline-water circulation below the interface and freshwater flow above. Both flows are perpendicular to the shoreline. The flow pattern near two separated saline lakes is more complicated. For example, in the Middle East, the Dead Sea northern basin and the evaporation ponds of the Dead Sea Works are adjacent to each other but separated. The northern basin level is dropping by 1.1 m/year and the evaporation ponds’ levels are increasing by 0.2 m/year. The fresh–saline water interface in such situation is numerically simulated. Streamlines parallel or semiparallel to the shoreline are significant. Moreover, the fresh–saline water interface intrudes landward adjacent to the higher saline lake and is pushed lakeward adjacent to the lower saline lake. The simulation results support field observations showing that the interface migrates vertically at a faster rate relative to the changes in the water table and the lake levels.

Geophysical Assessment of Seawater Intrusion into Coastal Aquifers of Bela Plain, Pakistan

Seawater intrusion is a major challenge in many coastal areas all around the world, mainly caused by over-exploitation of freshwater resources, climate change, and sea-level rise. Consequently, seawater intrusion reaches several kilometers inland, thus making the freshwater resources polluted and unsuitable for human use. Conventionally, the fresh-saline water interface is delineated by the number of laboratory tests obtained from boreholes. However, such tests suffer from efficiency in terms of data coverage, time, and cost. Hence, this work introduces Dar-Zarrouk (D-Z) parameters, namely transverse resistance (Tr), longitudinal conductance (Sc), and longitudinal resistivity (ρL) computed from non-invasive vertical electrical sounding (VES). Two-dimensional (2D) imaging of D-Z parameters provides a clear distinction of fresh-saline aquifers. Such techniques remove ambiguities in the resistivity interpretation caused by overlapping of fresh and saline aquifers during the process of suppression and equivalence. This study was carried out by 45 VES along five profiles in the coastal area of Bela Plain, Pakistan. D-Z parameters delineate fresh, brackish, and saline aquifers with a wide range of values such as freshwater with Tr > 2000 Ωm2, Sc < 3 mho, and ρL > 20 Ωm; saline water with Tr < 1000 Ωm2, Sc > 25 mho, and ρL < 5 Ωm; and brackish water with Tr between 1000–2000 Ωm2, Sc from 3 to 25 mho, and ρL between 5–20 Ωm. The D-Z results were validated by the physicochemical analysis using 13 water samples and local hydrogeological setting. The obtained results propose that D-Z parameters can be used as a powerful tool to demarcate the fresh-saline aquifer interface with more confidence than other traditional techniques. This geophysical approach can reduce the expensive number of borehole tests, and hence contributes to the future planning and development of freshwater resources in the coastal areas.

The effects of long-term saline groundwater pumping for desalination on the fresh–saline water interface: Field observations and numerical modeling

This study tests for the first time the long-term effects of pumping saline groundwater (SGW) as feed for a desalination plant on a coastal aquifer. Field measurements combined with 3D modeling of the hydrological conditions were conducted to examine the effects of SGW pumping on the aquifer system. The plant is next to the city of Almeria (South East Spain) and has been operating since 2006. It uses multiple beach wells along the shore to draw SGW from beneath the fresh–saline water interface (FSI) of the Andarax coastal aquifer. The long-term impact of the intensive pumping on the aquifer was assessed by electrical conductivity profiles in three observation wells during 12 years of pumping. The FSI deepened with continuous pumping, reaching a decrease of ~50 m in the observation well closest to the pumping wells. A calibrated three-dimensional numerical model of the Andarax aquifer replicates the freshening of the aquifer due to the continuous pumping, resulting in a salinity decrease of ~16% in the vicinity of the wells. The salinity decrease stabilizes at 17%, and the model predicts no further significant decrease in salinity for additional 20 years. Submarine groundwater discharge is lowered due to the SGW pumping and ~19,000,000 m3 of freshwater has not lost to the sea during the 12 years of pumping with a rate of ~1,100,000 m3 yr−1 after 6 years of pumping. After pumping cessation, hydrostatic equilibrium would take about 20 years to recover. This work presents the complex dynamics of the FSI due to the SGW pumping for desalination in the first real long-term scenario. It shows by combining field work and numerical modeling, a significant freshening of the aquifer by pumping SGW, emphasizing an additional advantage and the effectiveness of this use as a negative hydraulic barrier against seawater intrusion.

High Resolution Monitoring of Seawater Intrusion in a Multi-Aquifer System-Implementation of a New Downhole Geophysical Tool

Monitoring of seawater intrusion is extremely important for the management of coastal aquifers, and therefore requires reliable and high-frequency monitoring tools. This paper describes the use of a new near field and downhole geophysical tool that monitors seawater intrusion in boreholes with high vertical resolution. This sensor is further used to study the impact of pumping on water electrical conductivity profiles (ECP) at the fresh-saline water interface. The new device was installed in a confined calcareous sandstone aquifer along the northern Israeli coast. The site includes two monitoring wells and one pumping well located at distances of 50, 75 and 125 m from shoreline, respectively. The new geophysical tool, called the subsurface monitoring device (SMD), was examined and compared to water an electric conductivity profiler (ECP) and a conductivity temperature depth (CTD) driver’s data. All methods show similar salinity trends, and changes in pumping regime were clearly identified with both the SMD and CTD. The advantage of using the SMD tool is the high temporal and spatial resolution measurement, which is transferred via internet and can be analyzed and interpreted in real time. Another advantage of the SMD is that it measures the electrical resistivity of the aquifer mostly outside the well, while both water ECP and the CTD measure in-well electrical conductivity; therefore, are subjected to the artefact of vertical flow in the well. Accordingly, while the CTD shows an immediate and sharp response when pumping is stopped, the SMD provides a gradual electric conductivity (EC) change, demonstrating that stability is reached just after a few days, which illustrates, more precisely, the hydrological response of the aquifer.

Toxicity assessment of heavy metals and organochlorine pesticides in freshwater and marine environments, Rosetta area, Egypt using multiple approaches

Egypt faces a rapidly increasing deterioration of its surface water owing to the discharges of contaminated effluents. The present study focuses on assessing the levels of toxic heavy metals and organochlorine pesticides (OCPs) in water, sediments and Phragmites australis of freshwater and marine environments at Rosetta area. Fourteen sites were sampled including 4 sites within the marine environment and 10 sites distributed along the Rosetta River Nile. Potential sources of pollution were spatially assessed using remote sensing and geographic information system techniques. To quantify the environmental impact of metals on the aquatic system, metal index (MI) in freshwater, heavy metal pollution index, and ecological risk index (Er) in sediments as well as bioaccumulation factor (BAF) in P. australis were calculated. Remote sensing results showed that drainage canals (954.8 km), cultivations (1252.4 km2) and urbanized zones (88.6 km2) are the major sources of contamination in the studied area. Results of MI indicated that water of the Rosetta area was moderately affected by the metal pollution. Er values for metals in the investigated sites showed low potential ecological risk (< 40). High values of Er were observed in coastal marine water and the nearby estuary. OCPs in water and sediment samples were below the detection limits. Mean Ni and Co concentrations of metals in P. australis were higher than the critical limits of these ions. Results of BAF and regression analysis recommend using P. australis in remediation of metals from the aquatic system in such settings worldwide. The present study shows that using integrated remote sensing and chemical analyses could provide a regional and cost-effective assessment tool of environmental toxicity in fresh-saline water interface.

The effect of pumping saline groundwater for desalination on the fresh–saline water interface dynamics

Over the past few decades, seawater desalination has become a necessity for freshwater supply in many countries worldwide, particularly in arid and semi-arid regions. One potentially high-quality feed water for desalination is saline groundwater (SGW) from coastal aquifers, which has lower fouling propensity than seawater. This study examines the effect of pumping SGW from a phreatic coastal aquifer on fresh groundwater, particularly on the dynamics of the fresh–saline water interface (FSI). Initially, we constructed a 3D finite-element model of a phreatic coastal aquifer by using the FEFLOW software, which solves the coupled variable density groundwater flow and solute transport equations. Then, we compared and validated the results of the model to those of a field-scale pumping test. The model indicates that pumping SGW from a coastal aquifer freshens the aquifer and rehabilitates parts that were salinized due to seawater intrusion – an effect that increases with increasing pumping rate. In addition, when simultaneously pumping fresh groundwater further inland and SGW from below the FSI, the freshening effect is less pronounced and the salinity of the aquifer is more stable. In line with the results of the model, the field experiment revealed that salinity in the observation well decreases over the course of pumping. Taken together, our findings demonstrate that, in addition to providing a high-quality source feed water for desalination, pumping SGW does not salinize the aquifer and even rehabilitates it by negating the effect of seawater intrusion. These findings are important for planning shoreline desalination facilities and for managing arid coastal regions with lack of water supply and over exploited aquifers.

Delineation of Saline-Water Intrusion Using Surface Geoelectrical Method in Jahanian Area, Pakistan

Groundwater is the main supply of fresh water in many parts of the world. The intrusion of saline water into the fresh water is a serious threat to groundwater resources. Delineation of fresh-saline aquifer zones is essential to exploit the potable fresh water. The conventional method to differentiate fresh-saline water interface is to collect and test groundwater samples from boreholes using a number of laboratory tests. However, such techniques are expensive and time consuming. A non-invasive geoelectrical method, in combination with borehole data and physicochemical analysis, is proposed to assess the fresh-saline aquifers. This investigation was conducted in Jahanian area of Pakistan with forty-five vertical electrical soundings (VES) using Schlumberger array, nine bore wells and fifty physicochemical samples. The fresh-saline aquifers are delineated by aquifer resistivity and Dar-Zarrouk parameters namely transverse unit resistance and longitudinal unit conductance. The aquifer potential of fresh-saline water zones is estimated by the aquifer parameters namely transmissivity and hydraulic conductivity. Integration of subsurface resistivity with hydrogeological information reveals the subsurface formation of five layered succession, that is, topsoil having dry strata with resistivity greater than 30 Ωm, clay containing saline water with resistivity less than 15 Ωm, clay-sand with brackish water having resistivity between 15 and 25 Ωm, sand containing fresh water with resistivity ranging from 25 to 45 Ωm and gravel-sand having fresh water with resistivity greater than 45 Ωm. The geoelectrical columns and geological cross-sections constructed by the aquifer resistivity provide effectiveness of the interpretations for the evaluation of fresh-saline aquifers. The results of physicochemical analysis using WHO guideline validate the fresh-saline aquifer zones delineated by the geophysical method. This investigation contributes towards predicting the fresh-saline water interface using inexpensive geoelectrical method.

The Interrelations between a Multi-Layered Coastal Aquifer, a Surface Reservoir (Fish Ponds), and the Sea

This research examines the interrelations in a complex hydrogeological system, consisting of a multi-layered coastal aquifer, the sea, and a surface reservoir (fish ponds) and the importance of the specific connection between the aquifer and the sea. The paper combines offshore geophysical surveys (CHIRP) and on land TDEM (Time Domain Electro Magnetic), together with hydrological measurements and numerical simulation. The Quaternary aquifer at the southern Carmel plain is sub-divided into three units, a sandy phreatic unit, and two calcareous sandstone (‘Kurkar’) confined units. The salinity in the different units is affected by their connection with the sea. We show that differences in the seaward extent of its clayey roof, as illustrated in the CHIRP survey, result in a varying extent of seawater intrusion due to pumping from the confined units. FEFLOW simulations indicate that the FSI (Fresh Saline water Interface) reached the coastline just a few years after pumping has begun, where the roof terminates ~100 m from shore, while no seawater intrusion occurred in an area where the roof is continuous farther offshore. This was found to be consistent with borehole observations and TDEM data from our study sites. The water level in the coastal aquifer was generally stable with surprisingly no indication for significant seawater intrusion although the aquifer is extensively pumped very close to shore. This is explained by contribution from the underlying Late Cretaceous aquifer, which increased with the pumping rate, as is also indicated by the numerical simulations.

Geophysical Investigation of Fresh-Saline Water Interface: A Case Study from South Punjab, Pakistan.

The importance of the study of fresh-saline water incursion cannot be over-emphasized. Borehole techniques have been widely used, but they are quite expensive, intrusive, and time consuming. The electrical resistivity method has proved very successful in groundwater assessment. This advanced technique uses the calculation of Dar-Zarrouk (D-Z) parameters, namely longitudinal unit conductance, transverse unit resistance, and longitudinal resistivity has been employed by using 50 vertical electrical sounding points to assess the groundwater and delineate the fresh-saline water interface over 1045 km2 area of Khanewal in Southern Punjab of Pakistan. The x-y plots and maps of D-Z parameters were produced to establish a decipherable vision for the occurrence and distribution of different water-bearing formations of fresh-saline water aquifers through a complicated situation of intermixing of different resistivity ranges for fresh-saline water bodies. This technique is useful to reduce the ambiguity produced by the process of equivalence and suppression which cause intermixing in differentiating fresh, brackish, and saline aquifers during interpretation. The fresh-saline water interface is correlated very well with the previous studies of water quality analysis carried out in Khanewal area. The results suggest that the D-Z parameters are useful for demarcating different aquifer zones. The behavior and pattern of D-Z parameters with respect to occurrence and distribution of different water-bearing formations were effectively identified and delineated in the study area.

Tide-induced fluctuations of salinity and groundwater level in unconfined aquifers – Field measurements and numerical model

The responses of the fresh-saline water interface (FSI) and the groundwater level (GWL) to the Mediterranean Sea tide were monitored in the coastal aquifer of Israel, modeled numerically and analyzed using cross-correlation analysis. Different time-lags between sea level fluctuations and hydraulic head and salinity fluctuations were detected for the FSI and the GWL. At the FSI, the time-lag of hydraulic head behind the sea level is much shorter than the lag of the salinity at the same point. Surprisingly, similar time-lags behind the sea level were measured for both the hydraulic head at the GWL and the salinity at the FSI, both at the same distance from the shoreline. Results from a numerical model, simulating the flow and transport processes at the field scale, agree with field measurements. In both, the GWL and the salinity in the FSI fluctuate almost simultaneously, while the hydraulic head in the FSI reacts faster to sea level fluctuations. The actual movement of the fresh water body, which is controlled by the unsaturated flow in the capillary fringe (‘capillary effect'), lags behind the pressure head fluctuations in the deeper parts of the aquifer, which is controlled by saturated parameters of the aquifer. The overall results agree with the conceptual mechanism suggested by Levanon et al. (2016), in which the effect of sea tide on the coastal groundwater system comprises two main processes: (1) tidal fluctuations at the sea floor boundary which cause pressure wave propagation into the aquifer, and (2) attenuation at the GWL due to the capillary effect which control also the change in the salinity and the actual movement of the FSI.

Sea water intrusion into coastal aquifers -concepts, methods and adoptable control practices

Seawater intrusion is one of the most wide-spread and important processes that degrades groundwater quality by raising salinity to levels exceeding acceptable drinking water and irrigation standards, and endangers future exploitation of coastal aquifers. In India, the coastal region occupies some of the most potential aquifer systems of the country. Extensive research is being carried out in many parts of the world with the objectives of understanding the mechanism of seawater intrusion and improving the methods to control it in order to protect groundwater resources in coastal aquifers. The developed numerical models are based on different concepts. The investigations on possible impacts of climate change and seawater level rise on seawater intrusion in coastal aquifers revealed the severity of the problem and the significance of the landward movement of the dispersion zone under the condition of seawater level rise due to climate change. Sea level rise caused by global warming has become a root cause, pressure from the increase in the quantity of saltwater that many will try to enter the fresh water aquifer. Future sea-level rise due to climate change is expected to occur at a rate greatly exceeding that of the recent past. For effective management of coastal aquifer system, it is necessary to thorough understand the aquifer geometry, distribution of the fresh water and saline water in the system, optimum pumping rates, movement of the fresh water saline water interface, tidal influence into the aquifer. Safe yield of the aquifer has to be evaluated and accordingly the extraction has to be restricted, remedial measures have to be done wherever sea water intrusion has taken place. This paper describes about sea water intrusion processes, extent, estimation and various control measures to combat the problem and for sustainable use of fresh water from different case studies undertaken at India, Australia, Netherlands, Brazil and United Kingdom.

Trace elements (Li, B, Mn and Ba) as sensitive indicators for salinization and freshening events in coastal aquifers

The current global intrusion of seawater into coastal aquifers causes salinization of groundwater and thus significant degradation of its quality. This study quantified the effect of seawater intrusion and freshening events in coastal aquifers on trace elements (Li, B, Mn and Ba) across the fresh-saline water interface (FSI) and their possible use as indicators for these events. This was done by combining field data and column experiments simulating these events. The experiments enabled quantification of the processes affecting the trace element composition and examination of whether salinization and freshening events are geochemically reversible, which has been seldom investigated.The dominant process affecting trace element composition during salinization and freshening is ion exchange. The results of the experiments show that the concentrations of major cations and Li+ were reversible during salinization and freshening, whereas B, Mn2+ and Ba2+ were not. During salinization, Li+ and B were depleted due to sorption by 10 and 100μmol·L−1, respectively, to about half of their expected conservative concentrations. The relative depletion of Li+ increased with distance from the shore, representing the propagation of salinization. Ba2+ and Mn2+ were desorbed from the sediment during salinization and enriched by tenfold in the aqueous phase compared to their concentration in seawater (~0.1 μeq·L−1). During freshening both were depleted by almost tenfold compared to their concentration in fresh groundwater (~0.7 μeq·L−1). The depletion of Mn2+ is a sensitive marker for freshening because Mn2+ has a strong affinity to the solid phase. Moreover, this study shows that both Mn2+ and Ba2+ can be used as sensitive hydrogeochemical tools to distinguish between salinization and freshening events in the FSI zone in coastal aquifers.

Fluctuations of fresh-saline water interface and of water table induced by sea tides in unconfined aquifers

This study examines effects of tides on fluctuations of the fresh-saline water interface and the groundwater level in unconfined coastal aquifers using a two-dimensional numerical model. The time-lags of the simulated hydraulic heads and salinities fluctuations compared to sea level fluctuations are analyzed using cross-correlation analysis. The results show that both the fresh-saline water interface and the groundwater level are affected harmonically by sea tide fluctuations. However, significantly different time-lags are obtained between the hydraulic head in the deeper and upper parts of the aquifer, and between head and salinity in the fresh-saline water interface. The hydraulic head in the deeper part of the aquifer responses much faster to sea level fluctuations than in the upper part. Surprisingly, a similar difference is detected between the time-lag of the hydraulic head in the fresh-saline water interface and the time-lag of the salinity at the same location. Furthermore, the time-lag of the salinity in the fresh-saline water interface is similar to the time-lag of the water table. We suggest a comprehensive mechanism for tidal influence on the coastal groundwater system, in which two main processes act simultaneously. First, sea tide causes a pressure head wave which propagates into the saturated zone of the aquifer, governed by the diffusivity of the aquifer (Ks/Ss). Second, this pressure head wave is attenuated at the water table due to the unsaturated flow within the capillary fringe which occurs during groundwater level oscillations. Because the tidal forcing acts on the sea-floor boundary and the attenuation of the groundwater level due to capillary effect acts on the groundwater table, two dimensional distributions of time-lag and hydraulic head amplitude are created. The capillary effect in the unsaturated zone plays a key role not only in the water table fluctuations as shown previously, but also on the salinity fluctuations in the fresh-saline water interface. Unsaturated flow dynamics controls the actual movement of the entire fresh water body, which results in simultaneous fluctuations of the groundwater level and the fresh-saline water interface.

Shape of the shallow aquifer at the fresh water–sea water interface on a high-energy sandy beach

Inland aquifers are connected to the ocean through permeable coastal sediments. The mixing zone between sea water and fresh-groundwater forms a so-called subterranean estuary. The dynamics and the shape of this interface influence fluxes of fresh water to the coastal zone. On tidal sandy beaches, tide and waves actions lead to the formation of an intertidal saline circulation cell that complements the structure of the subterranean estuary between sea water and fresh water. The upper beach corresponds to the recharge zone, where large volumes sea water penetrate the sediment at high tide, whereas the lower beach corresponds to the discharge zone of older pore water that remains in the sand for several tidal cycles. The objective of our study was to characterize the shape and the evolution of this saline cell in a high-energy macro-tidal beach, the Truc Vert beach (SW France). In order to assess the distribution of saline, brackish and fresh water in this high-energy beach aquifer, we conducted electrical resistivity tomography (ERT) measurements from the lower beach face to the sand dune in winter 2012–2013. We also measured water table elevation and salinity in three piezometers situated at the base of the dune and in pore waters at low tide on tidal cross-shore transects. Results show that the intertidal saline plume can be identified and localized with the ERT method. The fresh-saline water interface is almost vertical in the upper beach. The saline circulation cell occupies the upper part of the tidal beach over a thickness of 5 m deep. Below this saline cell, a brackish water flows to the ocean: about 30% of fresh water was found in pore water in the lower part of the beach. Shifting of the vertical front between saline and fresh waters of about 2–5 m occurred during the spring–neap tidal cycle. This migration is small relative to the beach size, suggesting that the transition zone is relatively stable. The variations in salinity of the circulation cell suggest that the position of the saline plume is controlled by the spring tide level. Our results differ slightly from studies based on modelling and piezometric measurement made in less exposed micro-tidal sandy beaches. Our data would allow to better constrain numerical models that would enable to quantify water residence time and solute fluxes in subterranean estuaries of high-energy beaches.