Geometry and dynamics of the freshwater—seawater interface in a coastal aquifer in southeastern Spain

<p>Greater groundwater abstraction combined with possible reductions in recharge rates are likely to be detrimental to the long-term viability of groundwater resources (Mehdizadeh, 2019). An additional issue specifically affecting coastal aquifers is saltwater intrusion (SI). The key processes governing SI have been long understood but monitoring the ingress of saline water into coastal aquifers and especially its risk to abstraction sources is still a complex and costly exercise (Graham, 2018). Here we build on evidence that self potential (SP) could be a useful tool for remotely tracking the movement of saline-freshwater interfaces associated with SI.  The work reported describes SP response, along with water level, temperature and electrical conductivity measurements from an array of piezometers under ambient and pumped conditions on a beach aquifer located on Benone Strand, on the northern tip of Northern Ireland, UK. These data are supplemented by time-dependent electrical resistance tomography (ERT) obtained from the BGS PRIME system.</p><p>Self potential voltages arise from subsurface pressure and concentration gradients (Jackson et al., 2012). These gradients can cause ion separation, which gives rise to an electrical potential and a flow of electrons in order to maintain electrical neutrality. The potentials (typically in the millivolt range) can be detected and logged in the field using installed electrodes. There are two main types of SP; electro-kinetic potentials (V<sub>EK</sub>), due to differential flow velocities, and exclusion-diffusion potentials (V<sub>ED</sub>), due to ion concentration gradients with different mobilities. SP has been shown to have a response to pumping tests in (Jackson et al., 2012), though this was limited in scope. In a longer-term study, tidal signatures in SP were recorded in a Chalk borehole less than 2 km inland from the English Channel (MacAllister, 2016). Separating out these two sources of SP can be challenging.</p><p>Comparing SP and ERT responses coupled with groundwater level changes show tidal responses with are related to depth below surface and distance from the sea. In addition, results pumped well water levels appears to indicate that the drop in SP is not correlated with the expanding cone of depression from pumping, as the high pressure gradients that occur at the start of pumping has not induced an electrokinetic response. This is in contrast with the results obtained from (Jackson et al., 2012) at an inland site on the Cretaceous Chalk. This, therefore, points to the change in SP being induced by local movements of the saline-freshwater interface in the vicinity of the pumping wells, where a more progressive response is induced by changes in groundwater flow.</p>

Seawater intrusion occurs commonly in coastal aquifers around the world, threatening the availability and usability of fresh groundwater resources for vegetation and human uses. The rapid growth of the world population and urbanization requires sound strategies for protection and management of the freshwater resources, especially coastal groundwater. This goal can only be achieved through proper understanding of the processes that underlie seawater intrusion in coastal aquifers. Major insights have been gained over the past several decades, in particular, roles of the density-driven flow in driving and maintaining the invasive flow of saltwater. However, most studies have linked the seawater intrusion process merely to the level of salt content through the free convection induced by the salinity contrast between groundwater and seawater but ignored their temperature difference. In reality, the thermal contrast between coastal groundwater and marine seawater may range up to 15°C in absolute value with either warmer or colder seawater. Such thermal contrast can alter seawater circulation through the coastal aquifer, which in turn affects the biogeochemical reactions of land-sourced pollutants in the aquifer prior to discharge to the marine ecosystem.This research aimed to investigate the combined effects of salinity and temperature contrasts on the interactions between freshwater and seawater in unconfined coastal aquifers. Using laboratory experiments and numerical simulations, this research explored the coastal groundwater dynamics under various boundary settings with regards to thermal variations, tidal forcing and seasonal changes of seawater temperature. Findings from this research provided insights into the importance of temperature variations on various key processes in coastal aquifers under the condition of either static sea level or tidal oscillation.The effects of temperature contrast were first investigated for the seaward boundary of static level using physical experiments and numerical models in combination with tracer tracking. With the static sea level, the thermal contrast induced long-term impacts on the aquifer and altered background flow patterns and transport activities. The position of the saltwater wedge toe was modified significantly by the presence of temperature gradient either landward or seaward. Colder seawater enhanced the advancement of saltwater while warmer seawater hindered it. More importantly, the seawater circulation pattern changed dramatically in the latter case. A second circulation cell was discovered for the first time near the seaward boundary. The regular landward circulation cell was pushed to the vicinity of the interface where considerably larger velocity was observed. In-depth sensitivity analysis revealed the important role of spatial correlation between temperature-induced and salinity-induced density gradients, especially at the base of the aquifer, in driving the formation of the new cell.Both laboratory experiments and numerical simulation were carried out to investigate the thermal effects under the condition of tidal oscillation. The responses of the saltwater wedge were found to be similar to those under the static sea level, in particular, the retreat and advance of the wedge with warmer and colder seawater, respectively. The mixing zone widened as a result of the tidal fluctuation. Meanwhile, the upper saline plume and the freshwater discharge zone expanded in the warmer seawater case and contracted with colder seawater. The increased seawater temperature also intensified water exchange across aquifer-ocean interface, seawater circulation and the submarine groundwater discharge. Furthermore, tidally induced seawater circulation intensified with increased contribution to the submarine groundwater discharge compared with density-driven seawater circulation. All these characteristics were persistent over a range of tidal amplitudes. These results shed light on the importance of the thermal effects and have important implications for the assessment of the biogeochemical processes in coastal aquifers.The seasonal variations of the temperature contrast were then examined based on numerical simulations. The results showed clearly seasonality of the aquifer – ocean exchange and seawater circulation induced by the seasonal variation of seawater temperature in both cases with the static sea level and tidal conditions. Compared with the cases of the isothermal condition, all fluxes increased during colder months and decreased during warmer months. The periodic oscillation of the thermally induced density gradient resulted in a continuously changing mode of saltwater flow in the saltwater wedge. The flow path and transit time of circulating seawater shortened considerably in comparison with that in the isothermal case. This finding is particularly important for the evaluation of transport of land-sourced contaminants to the marine environment.The insights into the thermal effects on coastal unconfined aquifers gained from laboratory experiments and numerical simulations were applied to calculate a thermal impact factor and a thermal sensitivity index for aquifers along global coastlines based on local conditions of freshwater temperature and temperature contrast. The results suggested that the temperature effect is significant and would either amplify or reduce the impact of sea level rise on the vulnerability of coastal aquifers over a large proportion of the global coastlines.

Seawater intrusion into coastal aquifers is a major problem in almost all parts of the world. The increasing demand for fresh water in coastal regions is being met by the coastal aquifers. The development and management of fresh groundwater resources in coastal aquifers are seriously constrained by the presence of seawater intrusion. For proper management of coastal aquifers, it is necessary to assess the extent of saltwater intrusion in the aquifers. In the present study a numerical model based on solute transport, which can simulate seawater intrusion, is studied and is applied to an actual field situation. The extent and pattern of seawater intrusion in the region is simulated under the present situation. Simulations are also carried out to find the effect of pumping rate on the intrusion pattern in the vertical direction. The model is applied to an actual field problem. The model is calibrated in steady state to estimate model parameters such as conductivity and specific yield. Simulation runs are performed for the model to calculate groundwater heads in the region for a period of 180 days. When these heads are compared with the observed heads it yields a correlation coefficient of 0.75. The model is then calibrated in the transient state and when the transient heads are compared with observed heads a correlation coefficient of 0.98 is obtained. The model is then applied to simulate seawater intrusion in the horizontal direction. It is seen that the advancement of the seawater intrusion front is at maximum for a depth of 28–40m from the ground surface. The simulation is also carried out to see the effect of the pumping rate on the advancement of the seawater intrusion front in the vertical direction in the region along the three pumping wells namely Karichal, Pollinkudi and Adimalathura. The rate of advancement in the intrusion front in the Pollinkodi pump well is found to be greater than the other pumping wells.

Ocean tides cause water table overheight near the seaward boundary of coastal aquifers which can have a large impact on groundwater flows and saltwater intrusion. Despite this, regional-scale coastal groundwater flow models often neglect the effects of ocean tides or alternatively assume that the influences of ocean tides and sea level are constant over time. These simplifications are often required because simulation of the phase-resolved tidal signal at a coastal boundary is computationally demanding. Our objective was to derive a phase-averaged tidal boundary condition for application along gently sloping coastlines that enables simulation of real, complex tidal signals combined with sea-level changes due to meteorological effects. The boundary condition extends an existing analytical solution of tidal overheight by including an empirical correction function for conditions where the assumptions of the existing solution are violated. The correction function was developed by conducting a parametric study on an idealized two-dimensional cross-sectional coastal aquifer model and considering the effects of parameters including horizontal hydraulic conductivity, vertical anisotropy, specific yield, aquifer thickness, and beach slope. The performance of the new phase-averaged tidal boundary condition was assessed using a three-dimensional groundwater flow model of the island of Spiekeroog, Northwest Germany, which comprises complex coastal morphology with non-planar beach slopes. Simulations applying the new boundary condition were compared to simulations that adopted a phase-resolved tidal boundary condition and to observed groundwater levels. Accounting for time-varying changes in the tidal signal and sea level was found to be critical to simulate observed data and to adequately reproduce the transient tidal overheight simulated by the phase-resolved model. The performance of the new phase-averaged boundary condition in the regional-scale model varied depending on how the coastal morphology was represented in the boundary condition with local morphological features increasingly important for lower intertidal vertical infiltration capacity (low isotropic hydraulic conductivity or high vertical anisotropy) or specific yield. In conclusion, the new boundary condition presented overcomes current limitations in simulating transient tidal overheight in regional-scale groundwater models.

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

Unconfined coastal aquifers with a sloping aquifer bed are ubiquitous. However, most analytical solutions previously developed to estimate pumping‐induced seawater intrusion considered a horizontal aquifer bed. In this study, we first developed a steady‐state analytical solution to quantify the maximum safe pumping rate in sloping unconfined coastal aquifers with a fixed‐flux inland boundary condition, using the single potential approach. The analytical solution corrected by an empirical factor is validated against the numerical simulation, which reproduces the corresponding numerical results well. It is found that coastal aquifers with a higher aquifer bed toward inland (i.e., a positive sloping angle) yield a higher maximum safe pumping rate than that of coastal aquifers with a horizontal aquifer base, since the elevated freshwater head prevents seawater intrusion and enhances groundwater pumping. Specifically, for the aquifer bases with the sloping angle of 0.01 and −0.01, the maximum safe pumping rate is increased by 42.6% and decreased by 48.4%, respectively, in comparison to the case of a horizontal aquifer base, suggesting that neglecting the aquifer base slop can result in a significant error in estimating the maximum safe pumping rate. Moreover, the sensitivity analysis reveals that the maximum safe pumping rate increases with a lower hydraulic conductivity and a larger dispersivity. Our results provide valuable insights into the effect of the aquifer base slope on the maximum safe pumping in coastal aquifers, and analytical solutions developed can serve as a powerful tool for quick assessment of pumping‐induced seawater intrusion in sloping unconfined coastal aquifers.

Late and postglacial sea-level changes in The Kattegat: The consequences to different coastal aquifers (on the islands Zealand and Anholt)

Freshwater stored in a coastal aquifer is extensively extracted through pumping wells due to high water demand in coastal area (touristic, industrial, and public use). To enhance freshwater security and to avoid contamination of water reserves, seawater intrusion becomes a topic of great interest for hydrogeologists. During the last few decades, hydrogeologists have provided a deeper understanding of the prediction, processes, investigative tools, and management of such systems. The majority of the studies quantifies these hydrogeological systems using traditional density-dependent flow and transport models and does not consider the effect of the chemical reactions. Interdependence of density-dependent flow and chemical reactions and their effects on the porosity and permeability is the most important key toward a reliable modeling of these complex systems. Seawater intrusion can increase by the solid matrix dissolution processes in the saltwater–freshwater mixing zone in the case of a coastal carbonate aquifer. The dissolution of such rocks can easily induce a development of porosity and permeability as a result of the mixing processes. The increase of permeability would enhance further seawater flux to the freshwater side. In this work, a relatively complete modeling scheme is presented to quantify and predict this risk. The modeling of such a problem requires a set of highly nonlinearly coupled equations. In this regard, GEODENS code used in this work can solve these equations by a finite element procedure; it can handle density-dependent flow, transport, and geochemical reactions in porous media. Its main purpose is to represent the physicochemical processes in the subsurface system. The code is used to simulate the effect of calcite dissolution during seawater intrusion in a coastal carbonate homogeneous aquifer.

Artificial recharge (AR) is gaining importance as a management tool in water stressed regions. The need to prove recovery performance requires new monitoring tools for AR systems. A novel combination of environmental isotope tracers (B, Li, O, H stable isotopes) was tested for the monitoring of AR of tertiary treated, desalinated domestic wastewater into a coastal dune aquifer in Flanders, Belgium. No significant isotope fractionation was observed for the treatment process, which includes low pH RO desalination. The wastewater, after infiltration through ponds and before recovery through pumping wells is characterized by low molar Cl/B ratios (3.3 to 5.2), compared to 130 to 1020 in the wider study area, delta(11)B values close to 0% per hundred, rather homogeneous delta(7)Li values (10.3 +/- 1.7% per hundred), and a 18O and 2H enrichment with respect to ambient groundwater due to evaporation in the infiltration ponds. This confers to the AR component a unique isotopic and geochemical fingerprint. Immediately downstream of the pumping wells and in the deeper part of the aquifer no evidence of AR wastewater could be found, indicating a high recovery efficiency. In the wider area and in the deeper part of the aquifer, isotopes evidence mixing of coastal rain and a fresh paleo-groundwater component with residual seawater as well as interaction with the aquifer material. Combining several isotope tracers provides independent constraints on groundwater flow and mixing proportions as a complement to hydrodynamic modeling and geochemical studies.

Mapping of the vulnerability to marine intrusion “in coastal Cherchell aquifer, Central Algeria” using the GALDIT method

Despite its purported importance, previous studies of the influence of sea-level rise on coastal aquifers have focused on specific sites, and a generalized systematic analysis of the general case of the sea water intrusion response to sea-level rise has not been reported. In this study, a simple conceptual framework is used to provide a first-order assessment of sea water intrusion changes in coastal unconfined aquifers in response to sea-level rise. Two conceptual models are tested: (1) flux-controlled systems, in which ground water discharge to the sea is persistent despite changes in sea level, and (2) head-controlled systems, whereby ground water abstractions or surface features maintain the head condition in the aquifer despite sea-level changes. The conceptualization assumes steady-state conditions, a sharp interface sea water-fresh water transition zone, homogeneous and isotropic aquifer properties, and constant recharge. In the case of constant flux conditions, the upper limit for sea water intrusion due to sea-level rise (up to 1.5 m is tested) is no greater than 50 m for typical values of recharge, hydraulic conductivity, and aquifer depth. This is in striking contrast to the constant head cases, in which the magnitude of salt water toe migration is on the order of hundreds of meters to several kilometers for the same sea-level rise. This study has highlighted the importance of inland boundary conditions on the sea-level rise impact. It identifies combinations of hydrogeologic parameters that control whether large or small salt water toe migration will occur for any given change in a hydrogeologic variable.

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.

&amp;lt;p&amp;gt;Most, if not all, models of real aquifers go through a calibration process to adjust their hydraulic and solute transport parameters in order to bring the simulations outputs closer to the field observations. In coastal aquifers, the datasets are commonly composed of head time series, solute concentrations from water samples, and water and formation electrical conductivity, these last being of particular importance in coastal settings due to their relevance for seawater detection. Argentona is a well-instrumented field site of a coastal alluvial aquifer located 40 km NE of Barcelona, where a 2-year Cross-Hole Electrical Resistivity Tomography (CHERT) experiment was performed. CHERT provided high resolution electrical resistivity data in depth and allowed the visualization of dynamic aquifer processes. In the present work, we test the calibration of the Argentona SWI model using both the hydrological and the geophysical datasets. To do so, a density-dependent groundwater model was combined with CHERT forward modeling within a parameter calibration framework. In the process we pay attention to the CHERT capacity to recover aquifer salinities, to the coupling of the hydrological and geophysical simulations through petrophysics, to the use of the field specific relations and to the inverse problem parametrization, among other things. Pre-calibration analysis showed the sensitivity of the formation electrical resistivities to the porosities and to the petrophysical parameters, so the inverse problem solves for hydraulic transmissivities, porosities and petrophysical parameters. From the comparison of the preliminary results from the hydrological and the hydrogeophysical calibration, we observe that they point towards a better calibration of model porosities when the electrical resistivity is included in the inverse problem. The results will be compared to other parameter estimation methods, such as laboratory tests, the tidal method and heat tests, also performed at the Argentona site. We will conclude on the added value of the geophysical dataset in the calibration process, the possible improvements and drawbacks of the method.&amp;lt;/p&amp;gt;

Coastal aquifers are subject to seawater intrusion. Therefore, managing freshwater aquifers in coastal areas remain challenging. At present, determining safe yields from the coastal aquifers to prevent seawater intrusion is primarily based on the use of numerical simulation-optimization models or by the use of analytical models based on the Ghyben-Herzberg principle. This study examines the cause and effects of seawater intrusion into a coastal aquifer, Lincoln Basin in southern Eyre Peninsula, South Australia and shows that application of simple techniques would have prevented seawater intrusion. Three freshwater lenses, Lincoln A, B, and C of the Lincoln Basin, located about 13 km southwest of Port Lincoln township, have been developed as a town water supply source in 1960. The capacity of the basin has been assessed by three long-term pumping tests. Based on pump tests results, three areas were developed to supply 2×106 m3 per year distributed across three lenses as lens A : four wells to supply 0.84×106 m3, lens B: four wells to supply 0.5×106 m3 and lens C: four wells to supply 0.66 ×106 m3. Neither recharge to the freshwater lenses nor a water balance had been assessed, and a precautionary approach to groundwater extraction was not followed. The apparent driver for managing the basin was demand for the township. In this study, we assessed the recharge using two methods; water-table fluctuation (WTF) and the conventional chloride mass balance (CMB) method. Total recharge to the freshwater lenses is estimated at 1.6×106 m3 per year which is less than the average annual groundwater extraction from the basin during the 1961-1977 periods (average 2.14×106 m3). As a result mining of the groundwater storage has occurred in the basin leading to saline intrusion, upconing and lateral flow of brackish water into wellfield areas. The total volume extracted from the basin was 35×106 m3, which exceeded the average recharge over the 15 year period, 24×106 m3. Using analytical methods, the seawater/freshwater interface movement from its original position was estimated to be 35 m in lens A, 337 m in lens B and 188 m in lens C. For each pumping well at maximum discharge rate, the transient interface location directly underneath the well was calculated. This results in interface rises under pumping wells in lens A of 3.8 m, lens B of 0.5 m, and in lens C about 0.7 m. According to the risk-based groundwater allocation method, maximum extraction would have been as a proportion of 25% of the annual recharge. Thus, maximum annual abstraction limits for lens A, B and C would have been 210×103 m3, 72×103 m3 and 130×103 m3, totaling 412×103 m3.

Aquifer storage and recovery have gained attention as a solution that utilizes submarine groundwater discharge (SGD) as a surrogate water resource to alleviate water scarcity and fill the demand gap. Characterizing SGD is crucial for using coastal groundwater and improving understanding of the interaction between continental water and seawater. This study employs fiber-optical distributed temperature sensing (FODTS) and the heat tracer to quantify the groundwater flux in a coastal aquifer in northern Taiwan. The fluxes in different sections along the borehole were estimated from the temperature response caused by the active heating tests and campier groundwater flux under different tidal conditions, providing information on potential water resources for water resource planning and management. According to the active heating tests, the material of the sections with high-temperature response mainly consists of a gravel–sand mixture. Based on the estimations of groundwater fluxes along the well, the sections with low sensitivity of temperature response have low hydraulic conductivity and low groundwater flux. The estimated thermal parameters at the site are consistent with those obtained from the borehole samples in the laboratory tests. The groundwater fluxes in different sections are calculated based on the temperature response observed from the FODTS. The groundwater fluxes along the well vary between 0.02 and 1.77 m/day. There are considerable differences between the estimated fluxes during the tidal cycle in a heterogeneous coastal aquifer, indicating the high uncertainty of estimated SGD along coastlines.