Evaluation of coastal aquifer vulnerability of six countries in the Mediterranean Sea area : MedProgramme UNESCO

Coastal aquifers in the Mediterranean region are increasingly vulnerable to both natural and human induced pressures. Rising sea levels, climate change, excessive groundwater extraction, and land-use changes are accelerating the salinization of groundwater, threatening water quality and availability for drinking, agriculture, and ecosystems. This report presents the evaluation of the vulnerability of coastal aquifers in six Mediterranean countries (Albania, Lebanon, Libya, Montenegro, Morocco, and Tunisia) and assesses the risks to groundwater-dependent activities. This work is an activity within Component 2 of the GEF MedProgramme Child Project 2.1 (GEF ID 9687): Mediterranean Coastal Zones, Water Security, Climate Resilience and Habitat Protection. UNESCO-IHP serves as the executing agency, and the listed countries are project beneficiaries. Coastal aquifer vulnerability is the susceptibility of groundwater to depletion or contamination, including salinization, due to natural hazards or human pressures like over-pumping or sea level rise. In this study we adopted a risk-informed approach in which we combine the concept of coastal aquifer vulnerability as defined by Lobo Ferreira (2007, “The sensitivity of groundwater quality to an imposed groundwater pumpage or sea level rise or both in the coastal belt, which is determined by the intrinsic characteristics of the aquifer”) with an additional pressure related to a decrease of groundwater recharge, and in relation to the exposure of groundwater dependent activities and their sensitivity to salinization. For this study, regional 3D numerical models (using iMOD-WQ) are applied to simulate variable density groundwater flow and salt transport and in this way assess the vulnerability of coastal aquifers to salinization in the six countries. The models incorporate a paleo-reconstruction to reproduce the historical evolution of the salinity distribution in the groundwater and obtain an approximation of the current situation. The other model input files are global datasets or estimations based on publicly available information. The model results show the Total Dissolved Solids (TDS) concentration in the groundwater across the model domains, and the dynamics of the salinization and freshening processes. By running a future scenario including (a) a sea level rise of 1m, (b) a groundwater recharge reduction of 30% and (c) a doubling of the groundwater extractions by 2120, trends of salinization and freshening are identified, and the most vulnerable areas to salinization in the six countries become apparent. To understand the risk at which groundwater dependent activities and assets are exposed, salinity tolerance thresholds were defined based on the sensitivity of key uses such as drinking water supply and irrigation, as well as the sensitivity of ecosystems. These thresholds were used to extract and analyze modelling results. Results were analyzed for the current situation (2020), and for the future (2120). To understand the changes in time and space, we compared results in 2120 with results in 2020. For drinking water supply, a threshold of 600 mg TDS/L was applied to evaluate both the depth to the saline interface and the volume of freshwater available within the top 100 meters below the surface. In agricultural areas, the mean salinity concentration within the top 50 meters was assessed, reflecting the typical depth from which irrigation water is extracted. For ecosystems, the analysis focused on changes in mean concentration (between 2120 and 2020), exceeding 1000 mg TDS/L either toward salinization or freshening, as such shifts may significantly impact ecological balance and species survival. The modelling results indicate that all countries have areas with aquifers vulnerable to salinization. In these zones, currently brackish and saline water can be found close to the surface, and risk to salinization will increase in the future. In most countries, coastal drinking water wells in urban areas, and coastal agricultural lands may be at risk of suffering from salinization, while ecosystems appear to be mostly in areas where no significant changes of salinity are expected and therefore are generally not at risk. According to the modelling results, Albania shows vulnerable aquifers in the northern and central coastal zones, which present a shallow (<5m) interface of 600 mg TDS/L and small freshwater volumes. Urban areas and agricultural land near the coast face increased risk for drinking and irrigation water under the future scenario. Ecosystems in Albania are generally located in areas where no significant changes in TDS concentration in the shallow layers is expected. In Lebanon, modelling results show a shallow interface within 5 to 10km of the coast. Cities like Beirut may face reduced fresh groundwater availability according to the scenario for 2120, while agriculture inland is generally not affected, but areas in the northern border are more vulnerable. Also, the nature areas along the coast show potential salinity changes. Also, in Libya coastal aquifers are vulnerable and show very reduced volumes of freshwater in the first meters according to the modelling results. Urban centers like Tripoli and Sirte are at risk due to increasing salinity levels and the shallowing of the interface of 600 mg TDS/L during the analyzed period. In general, nature areas are less affected by changes in salinity, but agricultural areas near Tripoli are likely to face rising groundwater salinities, and therefore increased risk. Montenegro shows only a few areas vulnerable to salinization, like the coast of the Bay of Kotor, the surroundings of lake Skadar, and the city of Podgorica. However, some of these areas (like lake Skadar) might not be well conceptualized in the model given that only global datasets were used, and therefore these results are uncertain. Agricultural and nature areas are mostly not at risk, with some coastal exceptions. Morocco also only shows some specific areas vulnerable to salinization according to the modelling results. These areas mostly correspond to the river mouths of for example, the Martil, Oued Laou, and Molouya rivers. Urban centers like Tetouan and Nador, located close to these river deltas, face salinization risks for their groundwater systems, same as agricultural lands nearby. Agriculture and nature areas located inland are generally not exposed to salinization. In Tunisia, modelling results show a more complex picture with different depths of the 600 mg TDS/L interface and encroached groundwater salinity pockets inland. In general, urban centers and coastal agriculture face increasing salinity risks, similarly to the few nature areas near the coast. The models used for the analysis were set-up using global datasets and at a temporal and spatial resolution that allowed for national-scale simulations. The use of the best-available global datasets and paleo-reconstructing processes (for groundwater salinity estimates) over the past thousands of years are among the most effective strategies to best describe the present-day groundwater salinity patterns. With the objective to assess the accuracy of the modelling results, available reports describing salinization risks in the countries were used for qualitative validation. To conduct a quantitative validation of the results, specific local hydrogeological data on salinity in groundwater across the countries and over time need to be implemented in the models. As local hydrogeological data was not available during the course of our modelling activities, the present results should only be used as a general guide to identify trends and define more targeted vulnerability studies. These findings underscore the efficiency and effectiveness of large-scale and nationwide modelling studies to identify potential areas and activities at risk of salinization, and simultaneously show the need for targeted vulnerability studies and improved local data collection to guide future water management strategies.