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Groundwater resources in Euro-Mediterranean countries provide a large part of the population’s water supply and are affected to varying degrees by anthropogenic use and climatic impacts. In many places, significant groundwater-level declines have already been observed, indicating an imbalance between natural groundwater recharge and groundwater abstraction. The extent of changes in groundwater storage (GWS) in the period 2003–2020 is quantified for the Euro-Mediterranean region using the latest data from the Gravity Recovery and Climate Experiment (GRACE/GRACE-FO) satellite mission and recently reanalyzed ERA5-Land climate data from the European Centre for Medium-Range Weather Forecasts. The results are set in relation to the prevailing climate, the regional hydrogeological setting, and annual groundwater recharge and abstractions on country level. Analysis of the mean annual trends over the study period shows significant decreases in GWS in many countries of Europe, Northern Africa and the entire Arabian Peninsula. Overall, there are significantly negative trends in about 70% of the study region. The mean of the trends across the Euro-Mediterranean region is –2.1 mm/year. The strongest negative trends in GWS per country are observed in Iraq and Syria (–8.8 and –6.0 mm/year, respectively), but also countries in central and eastern Europe are affected by depleting aquifers. The results are a clear indicator of the already medium-term groundwater stress in the Euro-Mediterranean region, which is expected to increase in the future, and demonstrate the need for adapted strategies for sustainable groundwater management on a transregional scale in the context of climate change and population growth.

Urban karst aquifers are threatened by anthropogenic activities, especially in semiarid developing countries. Recent water-sampling campaigns assessed the extent of groundwater-quality impairment in the Wadi Shueib in Jordan by a comprehensive hydrogeological and hydrochemical characterization of groundwater, wastewater, and imported water with its endmembers Lake Tiberias, Yarmouk River and Mukheiba wellfield. The results of a ternary mixing model with the mass ratios of Cl−/Br− and Ca2+/Mg2+ indicate that Lake Tiberias and Yarmouk River provide most of the imported water at the time of sampling in 2017 and Mukheiba wellfield provided minor amounts. The similarity in seasonal variations of Br− concentrations in the springs and the imported water revealed that imported water has generally a greater impact on groundwater than wastewater. However, the Br− concentrations also showed that the spatial wastewater impact is more visible towards urbanized areas due to increased infiltration of pollutants. The analysis of historical data series demonstrated the long-term urban impact on groundwater by an electrical conductivity increase over several decades, particularly since the mid-1990s, which is related to a doubling of Jordan’s population since that time and the associated urban growth. In this context, increased chloride and sulfate concentrations are noticeable due to the increased impact of more highly mineralized imported water and wastewater and decreasing recharge of low-mineralized rainwater. This study showed the hydrochemical differences in the Wadi Shueib groundwater system and serves as an example for the spatial and long-term response of karst aquifers to anthropogenic, seasonally variable input of imported water and wastewater.

Improving water availability and distribution is key to combatting water stress in semi-arid regions. This study presents an approach that integrates hydrogeological surveys and economic analysis at a pilot site in the eastern Lower Jordan Valley to assess the potential for managed aquifer recharge (MAR). Based on exploratory drillings, hydraulic tests, water-quality analyses and previous studies, an appropriate storage space in the alluvial fans along the eastern margin of the valley was found. Average measured infiltration rates of 9.8 × 10−5 m/s in a gravel pit and average hydraulic conductivity values of 6.8 × 10−5 m/s in the aquifer indicate that up to 1 Mm3/year can be infiltrated and stored underground. It is assumed that water is trapped towards the center of the valley where the alluvial sediments inter-finger with fine-grained layers. An increase in electrical conductivity values in the same direction probably indicates irrigation return flow and the presence of evaporitic layers and suggests a water recovery before reaching these saline zones. Depending on whether the water provided by MAR is used for irrigation or domestic water purpose, an average incremental profit of 0.81 and 2.44 US$/m3 can be achieved, respectively. These calculations result in a net present value of the MAR plant of 10.6 or 31.9 million US$, respectively, over a lifetime of 30 years. The presented study serves as a basis for further investigations and planning of a MAR plant, and can be transferred and upscaled to other sites in the Jordan Valley.

Karst groundwater-dependent ecosystems (KGDEs) in the Mediterranean region are important in terms of ecosystem services and biodiversity but are increasingly under anthropogenic pressures and climate-change constraints. For this study, the ecohydrological characteristics, threats, and protection status of 112 selected KGDEs around the Mediterranean Sea, including caves, springs, rivers and wetlands, were evaluated, based on local expert knowledge and scientific literature. Results demonstrate that KGDEs contribute considerably to regional biodiversity. The diversity of karst landscapes, combined with the groundwater emergence at springs, leads to exceptional habitat diversity, particularly in arid climates, where KGDEs serve as a refuge for species that could not thrive in the surrounding environment. The most common threats identified among the selected sites are direct human disturbances, such as mass tourism or overfishing, water-quality deterioration and water shortage from aquifer overdraft and/or climate change. Although most of the selected sites are under protection, conservation measures are frequently insufficient. Such shortcomings are often caused by poor data availability, little knowledge on conservation needs of invertebrate species, and conflicts of interest with the local population. For this purpose, it is necessary to raise environmental awareness and promote interdisciplinary research, in order to monitor water quality and quantity in addition to the status of the biocenoses.

Carbonate rocks in the Mediterranean region form karst landscapes with a variety of morphological and hydrological features, and are of particular interest from a water management perspective as they represent major karst aquifers. The Mediterranean Karst Aquifer Map and Database (MEDKAM) provides a 1:5,000,000 scale map showing the distribution of carbonate and evaporite rocks that can host karst groundwater resources, with additional information on other hydrogeological settings, selected terrestrial and submarine karst springs, caves and karst groundwater-dependent ecosystems. A statistical evaluation shows that carbonate rocks cover ~39.5% of the Mediterranean region within a 250-km focus area from the coastline. North Africa has the largest continuous area of carbonate rocks, while smaller countries in the Middle East and the Dinarides have the largest proportion of carbonate rocks in relation to their total area. Carbonate rocks are also widespread in coastal areas, occurring along ~33.6% (14,000 km) of the total Mediterranean coastline, including large islands such as Crete and Mallorca, and ~25.9% (6,400 km) of the continental coastline. Two additional maps display (1) groundwater recharge, showing a climatic gradient from north to south, and (2) groundwater storage trends, indicating a mean annual karst groundwater loss from 2003 to 2020 of 436 million m3 in the 250-km area. This study quantifies the carbonate rocks in the Mediterranean region and shows their importance for groundwater resources. MEDKAM will serve as a basis for further research and improved international cooperation in karst groundwater management.

Flooding is a recurring natural phenomenon that can have both life-giving and destructive aspects. In natural environments, floods are often an important element of the seasonal hydrologic cycle that provides water and nutrients to soil, supporting unique, rich and diverse ecosystems. However, flood events can also represent a destructive force that can endanger lives and cause significant damage in urban areas. Karst areas, in particular, are unique because of their special hydraulic characteristics in terms of flood occurrence, the dependence of ecosystems on such events, and attempts to actively store and manage floods. In this article, the hydraulic response of karst aquifers to heavy precipitation events, flood generation, and engineering interventions for flood control are discussed using several examples from karst areas in the Mediterranean region. Flooding mechanisms and regulatory structures in karst poljes are considered using several typical examples from the Dinaric mountain range. In addition, different variants of groundwater abstraction for increasing storage capacity and flood control are presented using examples from France and Montenegro. Managed aquifer recharge in karst areas and adjacent aquifers is demonstrated with examples from Jordan and Algeria. Finally, failed attempts at flood storage in karst reservoirs are presented with examples from Spain and Montenegro. These examples of flood retention in karst areas show the wide range of planning and technical measures and remind us of possible risks and failures in implementation as well as some positive and negative impacts on the environment and especially on ecosystems.

Karst aquifers in semi-arid regions are particularly threatened by surface contamination, especially during winter seasons when extremely variable rainfall of high intensities prevails. An additional challenge is posed when managed recharge of storm water is applied, since karst aquifers display a high spatial variability of hydraulic properties. In these cases, adapted protection concepts are required to address the interaction of surface water and groundwater. In this study a combined protection approach for the surface catchment of the managed aquifer recharge site at the Wala reservoir in Jordan and the downstream Hidan wellfield, which are both subject to frequent bacteriological contamination, is developed. The variability of groundwater quality was evaluated by correlating contamination events to rainfall, and to recharge from the reservoir. Both trigger increased wadi flow downstream of the reservoir by surface runoff generation and groundwater seepage, respectively. A tracer test verified the major pathway of the surface flow into the underground by infiltrating from pools along Wadi Wala. An intrinsic karst vulnerability and risk map was adapted to the regional characteristics and developed to account for the catchment separation by the Wala Dam and the interaction of surface water and groundwater. Implementation of the proposed protection zones for the wellfield and the reservoir is highly recommended, since the results suggest an extreme contamination risk resulting from livestock farming, arable agriculture and human occupation along the wadi. The applied methods can be transferred to other managed aquifer recharge sites in similar karstic environments of semi-arid regions.

Storm-water harvesting and storage via managed aquifer recharge (MAR) is a promising approach to combat water scarcity in semi-arid regions, but poses a challenge for karst aquifers and regions with highly variable water availability. The infiltration of low-mineralized surface water and its impact on highly mineralized groundwater of a karst aquifer was investigated at Wala reservoir in Jordan over a period of approximately 10 years. The results show significant groundwater-level rise in a wellfield, in response to the yearly average infiltration of about 6.7 million m 3 . This corresponds to about 60 % of the yearly average abstraction of about 11.7 million m 3 , confirmed by mixing calculations with tritium. A decreasing trend in infiltration due to sedimentation is observed. Mean groundwater residence times of several thousand years, derived from carbon-14 dating, indicate a large storage capacity of the aquifer. The heterogeneous distribution of the residence times is caused by strong groundwater withdrawals and artificial recharge along with karst-specific aquifer characteristics. Temporal groundwater salinity fluctuations in the wellfield are observed after the first MAR infiltration. Enhanced groundwater flow along the wadi course was demonstrated, which is an important aspect with regards to future MAR projects in similar wadis of the region.