HS8.2.10

Salinization is one of the main threats to groundwater quality around the world, particularly in arid and semi-arid regions (IPCC, 2007). Some of the major causes of high salinity include natural geological conditions, seawater intrusion, climate change affecting patterns of precipitation and evaporation, overexploitation of groundwater and poor irrigation practices (Amer and Vengosh, 2001; Russ et al., 2020). Salinity can reduce the availability of water for humans and wildlife and can negatively impact crop productivity and promote desertification. Desert regions in Somalia, Ethiopia and Kenya have natural characteristics that favour high salinity in groundwater. 80% of the population in the region depends on groundwater (UNICEF, 2020), and 69% of groundwater sources have salinity levels above the WHO health-based drinking water guideline of 1500 µS/cm.

Here, we use machine learning to spatially predict patterns of high salinity with a dataset of 6300 groundwater quality measurements and various environmental predictors. More than 60 predictor variables were tested and 100 iterations of the random forest were performed. Most of the salinity data were clustered, which can lead to sampling issues due to spatial autocorrelation (SAC). As traditional non-spatial validation methods ignore SAC in the data and therefore do not guarantee independence between training and testing data, we instead use spatial cross-validation to address this spatial phenomenon as well as variograms to identify the extent of autocorrelation among variables. Preliminary results indicate that fractured ancient marine deposits, recharge, precipitation, evaporation and proximity to the ocean are the main factors related to high salinity levels. The model performs well with a combined overall accuracy of ~80% and an Area Under the Curve (AUC) of 0.80. Predictive spatial maps of groundwater salinity will be presented along with an analysis of the drivers of salinity.

Figure 1. Topographic map of the study area and salinity concentration represented by electrical conductivity (EC).

How to cite: Araya*1, D., Podgorski1, J., and Berg1, M.: Mapping groundwater salinity in the arid region of the Horn of Africa, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-848, https://doi.org/10.5194/egusphere-egu22-848, 2022.

In many regions of the Mediterranean basin, both climate variability and human pressure threaten groundwater quality and quantity. Uncertainties concerning future precipitation, temperature patterns, and water accessibility provide a challenge in understanding how groundwater reacts to the variability of the hydrological forces. This aspect is of fundamental interest for water resources planning and management, especially in those areas where groundwater is the main water source. The proposed research looks for correlations between meteorological drought indexes and groundwater levels (GWLs) to provide qualitative information about GWLs response to precipitation and temperatures changes. The GWLs time series refer to nine monitoring wells located in the coastal karst aquifer of Salento (Puglia, Southern Italy). The aquifer is challenging because of the highly complex geological, geomorphological, and hydrogeological structure and regional size. In such complex environment and under climate changes, a high and unrestricted exploitation for irrigation, industrial, and drinking purposes may deteriorate the qualitative and quantitative status of groundwater. Such features often prevent the recognition with sophisticated methods of the relationship between hydrological and hydrogeological time series, especially under data scarcity. Searching for these relationships, three correlation coefficients were applied at different time scales, with reference to the period between July 2007 and December 2011, between the SPI (Standardized Precipitation Index) and SPEI (Standardized Precipitation and Evapotranspiration Index) and GWLs time series. Results of the three coefficients outline a positive and statistically significant correlation between time series, generally for long time scales, highlighting the slow response of GWLs to precipitation. Despite the complexity of the aquifer, it linearly reacts to precipitation and temperature variability in the long term, acting as a low-pass filter with a notable inertial behavior in response to meteorological events. The aquifer response is different compared to dry and recharge periods. In most cases, the decreasing GWL courses agree with the dry SPI and SPEI ones, while the increasing GWL courses are less congruent during wet periods. This characteristic reveals crucial in defining correct measures of protection and safeguard of groundwater resources during periods of meteorological drought.

Results suggest that the selected approaches are worthy of interest for those areas characterized by severe stress conditions due to long drought periods and under excessive groundwater exploitations, demonstrating the generality of their applicability also under data scarcity.

How to cite: Alfio, M. R., Balacco, G., and Fidelibus, M. D.: Correlation between groundwater levels and meteorological indicators in the coastal karst aquifer of Salento (Southern Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1649, https://doi.org/10.5194/egusphere-egu22-1649, 2022.

In coastal aquifers, many factors, including sea-level oscillations, cyclic flow, aquifer geologic structure, hydraulic properties of the aquifer, seawater density, variation in groundwater recharge, and human activities that involve groundwater exploitation influence the 3D spatial distribution of salt content of groundwater. Its changes over time under the natural and human forcing locally reflect on the width and elevation of the mixing (transition) zone, with different response times compared to applied stresses depending on the aquifer size and hydrogeological features at the monitoring sites.

Groundwater depth profiles of EC in coastal aquifers allow identifying the features of the transition zone. Reliable information on the EC vertical distribution comes only from wells reaching saltwater beneath freshwater, with screens along with the entire aquifer thickness and crossing zones of prevalent horizontal flow with negligible vertical components.

The study shows the temporal evolution of transition zones reconstructed from combining periodical EC depth profiles carried out over five decades in a few special deep wells. Such wells pertain to the regional net for groundwater monitoring of the Salento karst coastal aquifer (Southern Italy). The aquifer coincides with the geological basement of the Salento Peninsula, which is a carbonate formation of the Upper Cretaceous–Palaeocene. It comprises layers and banks of fractured and karstified limestone and dolomitic limestone. Gentle folds and normal and strike-slip faults dislocate the basement. Groundwater flows in phreatic conditions with max hydraulic heads around 3 m AMSL and low hydraulic gradients. It may be locally in confined conditions because of low permeability carbonate levels or when the carbonate basement top is below mean sea level. Hydraulic conductivity is highly anisotropic because of the combination of major and minor discontinuities and surface and subsurface karst features, thus conditioning the groundwater flow. Lateral seawater intrusion and saltwater up-coning cause diffuse and progressive groundwater salinization from the 1960s because of over-exploitation.

Starting from an initial well net of deep wells set in the 1970s to monitor groundwater salinization, the number of deep wells changed over time. Some of the oldest wells are no longer operational because of obstruction, while others are more recent. As a consequence, the available EC depth profiles cover, for each well, different periods from 1974 to 2021. The evolution of EC vertical distributions allows recognizing the effects of climate variations (wet periods and droughts) that influence the hydrodynamics of the aquifer and unveiling critical transitions triggered by such extremes. Data evolution allows clarifying the system’s response to long-term exploitation in a more effective and comprehensive way than the only variations in groundwater levels. Because of the regional scale of the flow system, the high natural storage, and high groundwater residence times, this response shows lags compared to disturbances (as exploitation, recharge variability, droughts). The significant storage acts as a buffer, allowing cushioning from their adverse effects. Over time, the transition zone deforms with distinct upward expansion leading, in the most severe cases, to the disappearance of the freshwater of low salinity observed in the wells in the 1970s.

How to cite: Parisi, A., Balacco, G., and Fidelibus, M. D.: Temporal evolution of transition zone by EC depth profiles (Salento karst coastal aquifer, Southern Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4544, https://doi.org/10.5194/egusphere-egu22-4544, 2022.

The characterization of fresh submarine groundwater discharge (FSGD) in coastal aquifers has been the object of study in many investigations due to the importance of water as a strategic resource for populations. However, investigating processes that occur in the part of the aquifer located under the sea entails greater difficulties. 
The objective of this study has been to characterize FSGD in the coastal alluvial aquifer of Maresme, located 40 km north of the city of Barcelona. To study the marine part of the aquifer with good spatial resolution, the geophysical method of continuous resistivity profiling (CRP) has been chosen. Marine profiles, parallel and perpendicular to the coastline, have been done using a boat in a shallow water area to obtain electrical resistivity data of the seabed covering 3 km². Data acquisition has been carried out in two field campaigns, one in the dry season and another in the wet season. 
From the results obtained, it has been possible to observe different electrical resistivity values in marine sediments along the coast. These variations have also been identified between the two campaigns, being the wet season the one with the highest electrical resistivity values. This study shows that CRP is a non-invasive method that allows the detection of resistive zones of marine sediment that have been related to preferential discharge areas.

Acknowledgments
This work was partly funded by the Spanish Government (grant no. PID2019-110212RB-C22) and the project TerraMar (grant no. ACA210/18/00007) of the Catalan Water Agency.

How to cite: Tur-Piedra, J., Folch, A., Queralt, P., Marcuello, A., and Ledo, J.: Using continuous resistivity profiling (CRP) method to identify electrical resistivity variations related to FSGD areas., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4975, https://doi.org/10.5194/egusphere-egu22-4975, 2022.

Studies on groundwater salinity stratification interested an area under a semi-arid climate located within an industrial zone in the Taranto Province (South-Eastern Italy). Fresh groundwater circulates in a Mesozoic carbonate coastal aquifer affected by salinization due to saltwater mixing. A clay formation along the coast prevents direct contact with present seawater. 
The area is home to many companies of national and international importance that require water for their processes. The solution is to use locally available saltwater, considered a nonvaluable resource, because of the lack of surface water and use restrictions of good quality fresh groundwater. 
The study covers the area of a company that uses its domain for both quarry and landfill activities. Because fresh groundwater resources are protected, the law authorizes the company to exploit only salt groundwater to humidify the pet-coke or lope piles stored in the landfills. The moistening is intended to avoid the atmospheric dispersion of polycyclic aromatic hydrocarbons.
In the study area, with ground-level elevations around 50 m AMSL, the water table is above 2.5 m AMSL. A borehole had to reach a depth of 300 m to locate saltwater. Prospecting during drilling, including temperature and EC logs, allowed the reconstruction of the saline stratification. The elevation of saltwater top and the thickness and position of the transition zone make such stratification differ from that expected from the usual simplified laws describing the balance between fresh and salt waters in coastal aquifers. The recognized density stratification likely depends on the aquifer boundary conditions (included fresh groundwater exploitation rate), tectonic assets, and karst development. Further research may provide useful information on disturbances to fresh and salt water equilibrium that may result from direct pumping of salt water.  

How to cite: Specchio, V., Parisi, A., and Piccinni, A. F.: Salt groundwater use for industrial purposes in compliance with the equilibrium between freshwater and saltwater, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5404, https://doi.org/10.5194/egusphere-egu22-5404, 2022.

The groundwater, which present 30.1% of the global freshwater, is at risk of being contaminated by saltwater intrusion. The saltwater intrusion is the induced flow of seawater into freshwater aquifers. Saltwater intrusion can occur due to natural processes as well as over-abstraction of groundwater from coastal aquifers. Numerical modelling is a useful tool in helping hydrologists to understand and predict how saltwater intrusion occurs in coastal aquifers. These numerical models are based on the governing equations of groundwater flow and contaminant transport.

 In this paper, the extension of saltwater intrusion into the coastal aquifers, of the Bouficha region, has been investigated by modelling using SEAWAT in MODFLOW. The abstraction from the Bouficha groundwater had increased more than fourfold between 1993 and 2021. Numerical groundwater modelling is a powerful tool for evaluation, development and management of groundwater resources of this basin. A numerical groundwater model for Bouficha groundwater was developed using MODFLOW software (pm8) to simulate regional groundwater changes in the Bouficha groundwater under steady and transient state. The flow model was calibrated based on 29-years historical period.

For controlling the quality of Bouficha groundwater, and as the Bouficha groundwater is a costal aquifer, a transport model related to salinity was developed using SEAWAT package, in MODFLOW, based on historical salinity data of 28-years. The transport model was successfully calibrated in the steady and transient state.

The transport model was applied to examine how far the seawater transition zone will moved based on five future scenarios (pumping and climate change). The five flow scenarios were used to predict the salinity distribution in the Bouficha groundwater, using SEAWAT package, by extended the transport model until 2050.

The scenarios results indicate the total deterioration of the Bouficha groundwater’s quality. The predicted salinity shows that the Bouficha groundwater will be in critical status.

The sources of the Bouficha groundwater quality degradation are multiple: the seawater intrusion from the sea and from the Sebkha and a chloride input from agricultural activities (as transport boundary conditions).

How to cite: Aouiti, S., Hamzaoui–Azaza, F., and Zammouri, M.: Prediction of saltwater intrusion’ dynamics in coastal aquifer using modeling techniques: a case study in Northeastern Tunisia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7658, https://doi.org/10.5194/egusphere-egu22-7658, 2022.

Groundwater resources are inevitably considered as the primary source of high-quality water for the Mediterranean region. In critical cases, groundwater is essential to complement inadequate, uncertain, expensive, or even lacking surface-water sources. Especially in coastal areas of the Mediterranean, which show increasing development and population growth, karst aquifers represent vital freshwater sources. Karst aquifers are rather complex water systems, and therefore, though to manage, predict, and protect. Groundwater modeling has proved to be a very effective tool for groundwater management. Physically-based modeling is usually applicable to porous aquifers; numerical modeling application to karst aquifers is very challenging because of their complexity, which combines discontinuity, conduit, and porous medium domains. The need to forecast groundwater quantity and quality in karst aquifer systems is high, with groundwater salinity being very critical. Artificial Intelligence (AI) algorithms have been proved to be an effective alternative in simulating groundwater quality and quantity variables. This study aims to develop and test the performance of 6 AI algorithms to forecast groundwater electrical conductivity (EC) in the highly complex, coastal karst aquifer system of Salento (Puglia, Southern Italy). The AI algorithms applied were: 1) Multilayer Perceptron (MLP), 2) Long short-term memory (LSTM), 3) Bidirectional LSTM (BiLSTM), 4) Convolutional Neural Network (CNN), 5) Recurrent Neural Networks (RNN), and 6) Support Vector Machine (SVR). Except for SVR, which is considered a machine learning (ML) algorithm, all the other approaches are deep learning (DL) neural network architectures. Models’ development was based on 3-year groundwater EC daily data from 7 sensors. Other variables used for EC modeling were groundwater level and temperature, precipitation, and air temperature. The above variables were combined in 11 input variable experiments. In addition, various realizations of training times windows were developed under five scenarios. The total number of trained EC models was 2184. The results show AI models can efficiently provide a 30-day groundwater EC forecast for a wide range of EC values varying from slightly saline (0.7-2 mS/cm) to very saline (25-45 mS/cm). BiLSTM proved to be the most effective algorithm, while the least but still effective algorithm was SVR, thus showing the superior performance of DL algorithms compared to legacy ML approaches. Experimental results showed that increasing the number of input variables did not improve the performance of models. In contrast, including 2-time windows for training (one short-term and one long-term) increased it.

How to cite: Pisinaras, V., Nikolaidiou, A., Semertzidis, T., Daras, P., Balacco, G., Fidelibus, M. D., and Tziritis, E.: Forecasting electrical conductivity of a coastal karstic aquifer with artificial intelligence methods, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8124, https://doi.org/10.5194/egusphere-egu22-8124, 2022.

Groundwater salinization is a complex and dynamic process often related to multiple causes, such as seawater intrusion, soil salinization associated with water irrigation, and geogenic factors such as evaporate dissolution. Consequently, a reliable assessment of salinization risks depends heavily on the ability of statistical methods to accurately capture the spatial variability and interrelation among salinization indicators. Geostatistical methods are often used to identify and map salinization-affected regions, investigate how salinization indicators influence groundwater mechanisms, and eventually design optimal groundwater management policies.

In the context of the MEDSAL Project (www.medsal.net), this study reviews the recent key applications of geostatistical methods to address problems relevant to groundwater salinization. The basic principles of geostatistics are briefly described, and several studies are discussed that employ geostatistical and multivariate tools for identifying salinization sources, clarifying the relationship among salinization indicators and groundwater processes, and facilitating uncertainty propagation in physically-based models of the groundwater systems affected by salinization.

The literature review identifies most used methods and offers several recommendations in terms of future directions and challenges on the role of geostatistics for improved mapping of the spatial and/or spatiotemporal distribution of geochemical data related to salinization. These recommendations include the integration of geostatistics and machine learning methods for improved understanding and modeling of groundwater salinization processes, as well as the application of modern geostatistical simulation algorithms, accounting for diverse information sources, for exploring parameter uncertainty in spatially distributed hydrogeochemical models.

How to cite: Panagiotou, C. F., Kyriakidis, P., and Tziritis, E.: Review of application of geostatistical techniques to groundwater salinization problems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9666, https://doi.org/10.5194/egusphere-egu22-9666, 2022.

In coastal aquifers seawater intrusion is a worldwide problem caused by natural processes but significantly worsened by aquifer overexploitation for irrigation and drinking water supply, land subsidence, sea level rise and climate changes, which contribute to the reduction of groundwater natural recharge.

Seawater intrusion represents a relevant environmental issue along the coastal aquifers of the Mediterranean Sea, including the coast south of the Venice Lagoon, a peculiar ecosystem characterized by a fragile equilibrium between reclamation and irrigation activities, whereby salinization is significantly reducing the annual local crop production of about the 25% on average.

Here, we present the test case of Ca’ Bianca, located near the city of Chioggia - Italy. A numerical flow and transport model has been set up with SEAWAT, aimed at reproducing the complex saltwater intrusion dynamics in the area. To pursue this goal, real field water table and concentration measurements are combined to aid in the calibration and validation of the model. Particular attention is devoted to the evaluation of the dynamics and uncertainty associated with seawater levels, an essential forcing of the model. Then, mitigation strategies, such as drains supplying freshwater in the first layer of soil, are simulated to test their effectiveness against the saltwater intrusion in a way that their application can be reproduced also in other sites affected by the same phenomenon.

Results show a good match between the simulations and the data, with errors of about 10 cm for the water table, which is acceptable if we consider the scale of the project and its topographical and stratigraphical uncertainties. Even though matching observed concentrations proved to be more difficult, the model realistically reproduces the saltwater spatio-temporal behaviour. The comparison between the scenarios with and without mitigation strategies shows that, in the latter case, significant enhancement in crop production can be achieved.

As a future development, climate change effects on the sea levels will be considered and predictive scenarios will be developed and quantitatively analysed.

How to cite: Botto, A., Camporese, M., and Salandin, P.: Saltwater intrusion modelling for the safeguard of crop production in the Mediterranean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10559, https://doi.org/10.5194/egusphere-egu22-10559, 2022.

Groundwater salinization occurs when high concentrations of water-soluble salts are present in groundwater systems and is regarded as one of the most worldwide, severe, and complex phenomena affecting coastal aquifers. Salinization might occur, for example, from: (i) from marine sources via seawater intrusion, seawater ingression, (ii) underground or terrestrial sources (e.g., natural soils and rocks through the dissolution of soluble minerals), and (iii) salt and saline fluids from anthropogenic activities.

The analysis of salinization-related data by multivariate statistical methods is often undertaken in the context of efficient groundwater management. For example, clustering algorithms are used to delineate hydrogeohemically distinct water classes, whereas dimensionality-reduction algorithms are being used to decipher underlying natural and anthropogenic influences responsible for these distinct water classes. However, most of these algorithms do not explicitly account for spatial information and/or constraints, which can often have a significant impact on the classification of groundwater quality samples.  In the context of the MEDSAL Project (www.medsal.net), such spatial effects are incorporated in the clustering procedure via the inclusion of pertinent hydrological attributes, namely, hydraulic head and conductivity data, along with pair-wise distances between sample locations.

The application of the proposed spatially explicit clustering approach is illustrated using groundwater quality samples collected from the Rhodope coastal aquifer, located at north-eastern Greece. Sampling locations were grouped into four hydrogeochemically distinct water classes using k-means clustering with and without accounting explicitly for spatial information. Principal component analysis (PCA) was used to decipher underlying natural and anthropogenic influences responsible for these distinct water classes. The first four principal components (PCs) explained more than 83% of the total variance in water quality variables, from which the major component was found to be associated with salinization processes.

How to cite: Kyriakidis, P., Panagiotou, C., and Tziritis, E.: Accounting for hydrological controls when clustering groundwater quality data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11480, https://doi.org/10.5194/egusphere-egu22-11480, 2022.

The evolution of groundwater salinization often is difficult identify, track and characterize. Additional factors such as hydro-climatic variability, pumping, pollution, and mixing with other groundwater end members in the borehole add complexity to the hydrochemical signal. Different isotope methods can provide insight into the inherent dynamics of groundwater salinization. The application of different isotope systems has been studied both theoretically using reactive transport models and in the field in different case studies. Tritium, SF₆ and ¹⁴C, in combination with ¹³C as a marker of geochemical interaction, provide a straightforward access to information on the hydrodynamics of the flow system. Combined with data on salinity and on the geochemical fingerprint of salinization, these residence time tracers provide a first insight into the expected dynamics of changes. Radium isotope ratios allow an even more detailed reconstruction as the sorption of radium depends on the salinity of ambient groundwater and affects the transport behaviour. The chromatographic effect induced by salinity dependent transport behaviour can be used to date the onset of salinization. These concepts have been validated both by applying coupled groundwater flow and reactive transport models with PhreeqC and Geochemical Workbench and practically in several test sites. The transport modeling indicated that especially the combination of conservative residence time tracers and non-conservative tracers yield information on the dynamics of groundwater salinization, especially at time scales of decades to centuries and more. The verification of these concepts in case studies in Saudi-Arabia, in the Dead Sea valley and in the coastal aquifer of Samos within the PRIMA project MEDSAL confirms the viability of isotope methods in groundwater salinization studies.  

How to cite: Külls, C. and Bassukas, D.: Groundwater salinization dynamics - an isotope approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12355, https://doi.org/10.5194/egusphere-egu22-12355, 2022.

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.

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_EK), due to differential flow velocities, and exclusion-diffusion potentials (V_ED), 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.

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.

How to cite: Butler, A., Rowan, T., Jackson, M., McDonnell, M., Aguila, J. F., Benner, E., Flynn, R., Donohue, S., and Hamill, G.: Self Potential Monitoring of Saline Intrusion Dynamics in a Coastal Sand Aquifer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12643, https://doi.org/10.5194/egusphere-egu22-12643, 2022.

Groundwater salinization has been a phenomenon of increasing concern in the circum-Mediterranean region. Future hydroclimatic and social changes in the area are expected to affect the dynamics of salinization, and to create further pressure on water resources. However, these dynamics are system-specific. To effectively mitigate predicted impacts, local water management needs to prioritize actions based on the temporal dynamics of the change. The conceptual modeling approach BC2C has been applied to the coastal aquifer on the island of Samos, Greece. The approach takes into account the flow dynamics, depending on physical parameters of the flow system, and connects biogeophysical changes of the land-use system to a response function of aquifer salinity. This straightforward modeling approach based on analytical functions yields a time-impact relationship of propagation and recession for given interventions. At the MEDSAL project pilot site of the Kampos plain in Samos, the BC2C model generates a response time between 15 years for the shallow aquifer and up to 150 years for the coupled deep aquifer system. This insight into the dynamics of groundwater salinization can be used as a guide and baseline for future groundwater management plans, and highlights the importance of a medium to long-term perspective. 

How to cite: Külls, C. and Bassukas, D.: Groundwater salinization dynamics - a conceptual modeling approach to prioritize water management plans in a changing environment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12667, https://doi.org/10.5194/egusphere-egu22-12667, 2022.

Due to greater ground-water abstraction, rising demand for water, possible reductions in recharge rates and rising sea levels, costal aquifers are under ever increasing threat of Saline Intrusion (SI) (Mehdizadeh, 2019). Though the mechanismins of SI have long been understood, the ability to monitor and warn in advance of the ingress of saline water into costal aquifers has remained costly and complex (Graham, 2018). The work reported here describes initial efforts to develop and results from, a vertically profiling Self Potential (SP) device. The device was used to monitoring the position of a well parametrized saline front in a costal aquifer, located on Benone Strand, Co. Derry,  on the northern tip of Northern Ireland, UK, as part of the SALine INtrusion in coastal Aquifers project.

Naturally arising voltages, Self Potential (SP), are formed when pressure and concentration gradients move though the subsurface. The gradients cause ion separations, which create electrical potentials and a flow of electrons in order to maintain electrical neutrality. The SP signals (usually in the millivolt range) can be detected, relatively inexpensively (in comparison to resistivity imagining) with reference electrodes and a high impedance voltage logger. The positioning of the electrodes is key as it has only been possible, until now, to measure the voltage between two points. There are two key types of SP, in hydrology, electro-kinetic potentials (V_EK), due to differential flow velocities, and exclusion-diffusion potentials (V_ED), due to ion concentration gradients with different mobilities. Understanding the source mechanims in these voltages is complex, but evolving. Previous work has shown that self-potential rises before a saline breakthrough into a borehole (Graham, 2018).

A novel vertically travelling (or trolling) SP electrode was repeatedly used in a number of satellite boreholes during a pumping test; in order to look at the changes in the vertical gradient of SP. The pumping test took place over three days, during which initially fresh water was abstracted from the main pumping well. Resistivity imagine was used as a benchmark. It was shown that the vertical SP profile changed as the salt content of the pumped water increased (i.e. the saline front moved inland). This change in SP could not be explained by pressure changes – gradients of 50mV inside a single borehole were observed. The data showed SP profiles that varied widely before, during and after the pumping test, as saline water is drawn progressively towards the pumping well, offering far more data than a single stationary electrode. Demonstrating that these signals change in advance of the saltwater arriving at the pumping well, but also that this method could be used as an inexpensive way to safeguard costal aquifers in the future.

References

Graham, M. T. (2018). Self-Potential as a Predictor of Seawater Intrusion in Coastal. Water Resources Research.

MacAllister, D. a. (2016). Tidal influence on self-potential measurements. Journal of Geophysical Research: Solid Earth.

Mehdizadeh, S. a. (2019). Abstraction, desalination and recharge method to control seawater intrusion into unconfined coastal aquifers. Global Journal of Environmental Science and Management, 5, 107-118.

How to cite: Rowan, T., Macdonnel, M., Aguila, J., Benner, E., Bulter, A., Jackson, M., Tompson, C., Flynn, R., Donoue, S., and Hamill, G.: Monitoring experimentally induced Saline Intrusion through vertical Self Potential profiling at costal aquifer in Northern Ireland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12754, https://doi.org/10.5194/egusphere-egu22-12754, 2022.

Seawater intrusion is a global phenomenon occurring in many coastal aquifers. The excessive and uncontrolled withdrawal of groundwater and/or reduction in recharge to aquifers decrease the freshwater hydraulic head and can result in the saline front advancing inland toward abstraction boreholes. Sea-level rise due to the climate change can exacerbate these effects.  Old saline groundwater related to eustatic effects resulting from climate change during the last post-glacial period can also occur in coastal aquifers. According to the Ghyben-Herzberg principle, the depth of fresh-saline groundwater interface is mainly controlled by density and, in turn, by salinity. However, aquifer geometries and intrinsic heterogeneity of the geological medium, can affect the fresh-saline groundwater interface position and the response times to the forcing that control the salinization processes. Therefore, the knowledge of the response dynamics of the aquifer conditioning the position of the interface are essential to design countermeasures to compensate the salinization processes. Results of the monitoring of electric conductivity, temperature, pH and Eh log profile at about 30 m deep boreholes in the highly anthropized coastal plain of Muravera, in south-eastern Sardinia (Italy), are presented. Since the early fifties, in the plain area the natural hydrodynamic equilibrium between groundwater, surface-water, and seawater has been deeply modified by the construction of dams across the Flumendosa river, embankments, and the development of agriculture, tourism, and aquaculture activities along the coast. Moreover, abandoned branches of the river have been salinized by a fishpond that created a direct opening to the sea. According to a geological–depositional model based on sequential stratigraphy, the geometry of the aquifers in the Muravera coastal plain has been defined integrating stratigraphic, geophysical, geochemical, and isotopic data. A complex multilayer aquifer, mostly phreatic and locally confined, has been recognized. Results of the monitoring campaigns showed that the position of the fresh-saline groundwater interface along the plain cannot be explained by the Ghyben-Herzberg model. In the north area of the Muravera Plain, where the semi-confined condition of the aquifer occurs, the position of the interface doesn’t change significantly. Moreover, the lowering of pH as conductivity increases suggests high residence time of saline groundwater in the aquifer and interaction with marshes sediments. In the central sector of the plain, in unconfined conditions, the deepening of the interface as the piezometric head increases occurs and has been related to the higher transmissivity of the aquifer and a recharge rate coming from the Flumendosa river during extreme rainfall events. The multilayered aquifer geometry and the relationship between surface waters and groundwater have been recognized as responsible for the recharge rate of the aquifer and for the relative position of the freshwater–saltwater interface.

How to cite: Da Pelo, S., Porru, M. C., Piscedda, F. A., Testa, M., Lorrai, M., Botti, P., Lobina, F., Arras, C., Buttau, C., Loi, A., Funedda, A., Biddau, R., and Cidu, R.: Monitoring of physical-chemical parameter depth profile to assess sea water intrusion phenomena in a coastal multi-layer aquifer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13533, https://doi.org/10.5194/egusphere-egu22-13533, 2022.