Abstract
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.
Kurrzfassung
Carbonatgesteine bilden im Mittelmeerraum Karstlandschaften mit vielfältigen geomorphologischen und hydrologischen Merkmalen und sind aus wasserwirtschaftlicher Sicht von besonderem Interesse, da sie wichtige Karstaquifere darstellen. Die Mediterrane Karstaquifer-Karte und Datenbank (MEDKAM) bietet eine Karte im Maßstab 1:5,000,000, welche die Verteilung von Carbonat- und Evaporitgesteinen zeigt, die Karstgrundwasserressourcen beherbergen können, mit Zusatzinformationen zu anderen hydrogeologischen Einheiten, ausgewählten terrestrischen und submarinen Karstquellen, Höhlen und karstgrundwasserabhängigen Ökosystemen. Eine statistische Auswertung zeigt, dass Carbonatgesteine etwa 39.5% des Mittelmeerraums innerhalb eines 250 km breiten Fokusbereichs ab der Küstenlinie bedecken. Nordafrika hat die größte zusammenhängende Fläche aus Carbonatgestein, während kleinere Länder im Nahen Osten und in den Dinariden im Verhältnis zu ihrer Gesamtfläche den größten Anteil an Carbonatgesteinen aufweisen. Carbonatgesteine sind auch in Küstengebieten weit verbreitet und kommen entlang etwa 33.6% (14,000 km) der gesamten Mittelmeerküste vor, einschließlich großer Inseln wie Kreta und Mallorca, und etwa 25.9% (6,400 km) der kontinentalen Küste. Zwei zusätzliche Karten präsentieren (1) die Grundwasserneubildung, die einen klimatischen Gradienten von Nord nach Süd zeigt, und (2) Trends der Grundwasserspeicherung, die einen durchschnittlichen jährlichen Verlust von 2003 bis 2020 in Höhe von 436 Millionen m³ innerhalb des 250 km breiten Fokusbereichs zeigen. Diese Studie quantifiziert die Carbonatgesteine im Mittelmeerraum und zeigt ihre Bedeutung für die Grundwasserressourcen. MEDKAM wird als Grundlage für weitere Forschung und eine verbesserte internationale Zusammenarbeit im Karstgrundwassermanagement dienen.
Résumé
Les roches carbonatées de la région méditerranéenne forment des paysages karstiques aux caractéristiques morphologiques et hydrologiques variées. Elles présentent un intérêt particulier du point de vue de la gestion de l’eau, car elles contiennent des aquifères karstiques d’intérêt majeur. La carte et la base de données des aquifères karstiques méditerranéens (MEDKAM) fournissent une carte au 1:5,000,000 montrant la distribution des roches carbonatées et évaporitiques, pouvant receler des ressources en eaux souterraines karstiques, avec des informations supplémentaires sur d’autres contextes hydrogéologiques, des sources karstiques terrestres et sous-marines sélectionnées, des grottes et des écosystèmes dépendant des eaux souterraines karstiques. Une évaluation statistique montre que les roches carbonatées couvrent environ 39.5% de la région méditerranéenne contenue dans une zone de 250 km à partir du littoral. L’Afrique du Nord possède la plus grande superficie continue de roches carbonatées, tandis que les pays plus petits du Moyen-Orient et les Dinarides ont la plus grande proportion de roches carbonatées par rapport à leur superficie totale. Les roches carbonatées sont également très répandues dans les zones côtières, où elles occupent environ 33.6% (14,000 km) du littoral méditerranéen total, y compris de grandes îles comme la Crète et Majorque, et environ 25.9% (6,400 km) du littoral continental. Deux cartes supplémentaires montrent (1) la recharge des eaux souterraines, avec un gradient climatique du nord au sud, et (2) les tendances du stockage des eaux souterraines, indiquant une perte annuelle moyenne des eaux souterraines karstiques de 2003 à 2020 de 436 millions de m³ dans la zone des 250 km de la côte méditerranéenne. Cette étude quantifie l’étendue des roches carbonatées en région méditerranéenne et montre leur importance pour les ressources en eaux souterraines. MEDKAM servira de base à la poursuite des recherches et à l’amélioration de la coopération internationale en matière de gestion des eaux souterraines karstiques.
Resumen
Las rocas carbonatadas que afloran en la región mediterránea desarrollan paisajes kársticos con características morfológicas e hidrológicas muy variadas, y son de especial interés desde el punto de vista de la gestión del agua, ya que constituyen importantes acuíferos kársticos. La cartografía y la base de datos de acuíferos kársticos mediterráneos (MEDKAM) proporciona un mapa a escala 1:5,000,000 que representa la distribución de rocas carbonatadas y evaporíticas potencialmente acuíferas, con información adicional sobre otros elementos hidrogeológicos como manantiales kársticos terrestres y submarinos, cavidades y ecosistemas kársticos dependientes de las aguas subterráneas. El tratamiento estadístico de la base de datos recabada muestra que las rocas carbonatadas cubren aproximadamente el 39.5% de la región mediterránea en la franja de territorio comprendida entre la línea de costa y los 250 km tierra adentro. El norte de África presenta la mayor superficie de afloramientos continuos de rocas carbonatadas, mientras que los países de Oriente Medio y los Dinárides tienen la mayor proporción de este tipo de rocas en relación con su superficie total. Del mismo modo, estas litologías se extienden ampliamente a lo largo de las zonas costeras, y cubren aproximadamente el 33.6% (14,000 km) del litoral mediterráneo, incluidas grandes islas como Creta y Mallorca, así como el litoral continental, con alrededor del 25.9% (6,400 km) de la superficie. En la nueva cartografía temática, dos mapas adicionales muestran (1) la recarga de acuíferos kársticos, con un gradiente climático de norte a sur, y (2) la evolución del volumen de aguas subterráneas almacenado en materiales carbonatados, que indican una pérdida media anual de 436 millones de m³ de agua subterránea entre 2003 y 2020 en la franja de territorio considerada (250 km). Este estudio ha permitido cuantificar la extensión de afloramientos de rocas carbonatadas en la región mediterránea y muestra su importancia de cara a la evaluación de recursos hídricos. La cartografía MEDKAM sienta las bases para futuras investigaciones y permitirá mejorar la cooperación internacional en materia de gestión de recursos hídricos subterráneos de origen kárstico.
摘要
地中海地区的碳酸盐岩形成了多样化的岩溶景观,具有多种水文特征。从水资源管理的角度来看,这些岩溶含水层尤为重要。地中海岩溶含水层地图和数据库(MEDKAM)提供了比例为1:5,000,000的地图,展示了碳酸盐岩和蒸发岩储存岩溶地下水资源的分布情况,并附有其他水文地质背景信息,包括选定的陆地和海底岩溶泉、洞穴以及依赖岩溶地下水的生态系统。统计评估显示,在距离海岸线250公里的范围内,碳酸盐岩覆盖了地中海地区约39.5%的面积。北非拥有最大的连续碳酸盐岩区域,而中东和迪纳拉山脉的一些小国在其总面积中碳酸盐岩的比例最高。碳酸盐岩在沿海地区也广泛分布,约占地中海总海岸线的33.6%(14,000公里),包括克里特岛和马略卡岛等大岛,以及大陆海岸线的约25.9%(6,400公里)。此外,两张附加地图分别展示了(1)地下水补给情况,呈现出从北到南的气候梯度,以及(2)地下水储存趋势,指出在250公里范围内2002年至2020年间每年平均损失436百万立方米的岩溶地下水。该研究量化了地中海地区碳酸盐岩的分布,并显示了其对地下水资源的重要性。MEDKAM将为进一步研究和改进岩溶地下水管理的国际合作奠定基础。
Riassunto
Le rocce carbonatiche nella regione mediterranea formano paesaggi carsici con una varietà di caratteristiche morfologiche e idrologiche, e sono di particolare interesse dal punto di vista della gestione della risorsa idrica, in quanto corrispondenti ai più ampi acquiferi carsici. La Carta degli Acquiferi Carsici del Mediterraneo (MEDKAM) è rappresentata su una scala 1:5,000,000 che mostra la distribuzione delle rocce carbonatiche ed evaporitiche in grado di ospitare risorse idriche sotterranee. Sono inoltre presenti informazioni su altri aspetti idrogeologici, come anche alcune sorgenti carsiche subaeree e sottomarine, oltre alle grotte e agli ecosistemi dipendenti dalle acque sotterranee. Statisticamente, le rocce carbonatiche coprono circa 39.5% della regione mediterranea, considerando una fascia ampia 250 km dalla linea di costa. Nel Nord Africa sono presenti le aree più ampie di affioramento continuo di rocce carbonatiche, mentre nelle più piccole nazioni del Medio Oriente e sulle montagne dinariche si riscontrano la più ampie proporzioni di rocce carbonatiche rispetto all’area totale del singolo Paese. Le rocce carbonatiche sono anche molto diffuse nelle aree costiere, costituendo circa il 33.6% (14,000 km) dell’intera costa mediterranea, comprese isole maggiori come Creta e Mallorca, ma anche circa il 25.9% (6,400 km) della linea di costa continentale. Due carte accessorie mostrano rispettivamente (1) la ricarica degli acquiferi, che presenta un gradiente climatico da nord a sud e (2) l’andamento dell’immagazzinamento delle acque di falda, da cui si deduce una perdita media annuale di acque sotterranee carsiche dal 2003 al 2020 di 436 milioni di metri cubi in un’area di 250 km. Questo studio presenta una quantificazione delle rocce carbonatiche in area mediterranea, mostrando la loro importanza per le risorse idriche sotterranee. MEDKAM costituisce quindi una base per futuri approfondimenti di ricerca e per il miglioramento della cooperazione internazionale nella gestione delle acque sotterranee carsiche.
Apstrakt
Karbonatne stene u regionu Mediterana formiraju karstne terene različitih morfoloških i hidroloških karakteristika, i od posebnog su interesa za upravljanje vodama jer predstavljaju glavne karstne akvifere (izdani). Karta akvifera u razmeri 1:5,000,000, sa bazom podataka Mediteranskog karsta (MEDKAM), prikazuje rasprostranjenje karbonatnih i evaporitskih stena koje mogu da sadrže resurse karstnih podzemnih voda, sa dodatnim informacijama o drugim hidrogeološkim elementima, odabranim kopnenim i podmorskim karstnim izvorima (vruljama), pećinama i ekosistemima zavisnim od podzemnih voda. Statistička procena pokazuje da karbonatne stene pokrivaju oko 39.5% Mediteranskog regiona unutar fokusnog područja od obala do udaljenja od 250 km u unutrašnjost kopna. Severna Afrika ima najveću kontinuiranu površinu karbonatnih stena, dok manje zemlje na Srednjem istoku i Dinaridima imaju najveći udeo karbonatnih stena u odnosu na njihovu ukupnu površinu. Karbonatne stene su takođe rasprostranjene u obalnim područjima, i javljaju se duž oko 33.6% (14,000 km) ukupne Mediteranske obale, uključujući velika ostrva kao što su Krit i Majorka, i zahvataju još oko 25.9% (6,400 km) kontinentalnog dela uz obale. Dve dodatne karte prikazuju (1) prihranjivanje podzemnih voda, pokazujući klimatski gradijent orijentisan od severa ka jugu, i (2) trendove akumuliranja podzemnih voda, koji ukazuju na srednji godišnji gubitak karstnih podzemnih voda od 2003. do 2020. od 436 miliona m³ na fokusnom području od obale do 250 km u unutrašnjost kopna. Ova studija kvantifikuje karbonatne stene u regionu Mediterana i pokazuje njihov značaj za resurse podzemnih voda. MEDKAM će poslužiti kao osnova za dalja istraživanja i unapređenje međunarodne saradnje u upravljanju karstnim podzemnim vodama.
Povzetek
Karbonatne kamnine v Sredozemlju oblikujejo kraške pokrajine z različnimi morfološkimi in hidrološkimi značilnostmi, ki so še posebej zanimive z vidika upravljanja voda, saj predstavljajo poglavitne kraške vodonosnike. Zemljevid in podatkovna baza kraških vodonosnikov v Sredozemlju (MEDKAM) ponuja zemljevid v merilu 1:5,000,000, ki prikazuje razprostranjenost karbonatnih in evaporitnih kamnin, v katerih se lahko nahaja kraška podzemna voda. MEDKAM vsebuje dodatne informacije o drugih hidrogeoloških okoljih, izbranih kopenskih in podmorskih kraških izvirih, jamah in ekosistemih, odvisnih od kraške pozemne vode. Glede na statistične ocene karbonatne kamnine pokrivajo približno 39.5% Sredozemlja znotraj 250 km območja od obale. Severna Afrika ima največjo zvezno površino karbonatnih kamnin, medtem ko imajo manjše države na Bližnjem vzhodu in v Dinaridih največji delež karbonatnih kamnin glede na njihovo skupno površino. Karbonatne kamnine so razširjene tudi na obalnih območjih, saj se pojavljajo vzdolž približno 33.6% (14,000 km) celotne sredozemske obale, vključno z velikimi otoki, kot sta Kreta in Mallorca, ter okoli 25.9% (6,400 km) celinske obale. Dva dodatna zemljevida prikazujeta (1) napajanje podzemne vode, ki prikazuje podnebni gradient od severa proti jugu, in (2) trende zaloge podzemne vode, ki kažejo povprečno letno izgubo kraške podzemne vode od leta 2003 do 2020 v višini 436 milijonov m³ na območju 250 km. Ta študija kvantificira karbonatne kamnine v sredozemski regiji in prikazuje njihov pomen kot vir podzemne vode. MEDKAM bo služil kot osnova za nadaljnje raziskave in izboljšanje mednarodnega sodelovanja na področju upravljanja kraške podzemne vode.
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Introduction
Carbonate rocks form karst aquifers that serve as important freshwater resources for humans and ecosystems, but pose an ongoing challenge for research and management due to their unique hydraulic properties (Goldscheider and Drew 2007; Stevanović 2019). Recent evaluations by Chen et al. (2017) indicate that exposed karstifiable carbonate rocks cover 21.6% of the European land surface; however, the total area of carbonate rock occurrence is much larger, as it also includes nonexposed carbonate rocks confined by younger formations. The exposed karst surfaces correspond to the main recharge zones of karst aquifers, which are often hydraulically connected over large areas and across national boundaries (Hartmann et al. 2014). Karst aquifers are often the source of large springs (Bonacci et al. 2023; Fan et al. 2023; Stevanović and Milanović 2023) and have unique underground cave systems, both of which host diverse ecosystems (Siegel et al. 2023). Karst landscapes serve as recreational areas, and many karst features are classified as World Heritage Sites (Goldscheider 2019; Gunn 2021). Furthermore, carbonate rocks serve as important industrial raw material, host oil and gas reservoirs, contain commercial ore bodies, and provide information about the Earth’s evolution through biological, chemical, mineralogical, and isotopic signatures (Mackenzie 1978), and are part of the global carbon cycle (Hartmann et al. 2009).
Carbonate rocks are a group of sedimentary rocks composed of carbonate minerals. Typical examples are limestone, composed of calcite (CaCO3), and dolomite (or dolostone), composed of dolomite [CaMg(CO3)2] (note that the term “dolomite” is used both for the mineral and the rock). The metamorphism of limestone and dolomite leads to the formation of marble, which has a nearly identical mineralogical and geochemical composition as the parent rock but with larger crystals and less intergranular porosity (Goldscheider and Drew 2007). The peculiarity of carbonate rock is related to its strong solubility, which occurs mainly along fractures and bedding planes due to the flow of (ground)water with CO2 from the atmosphere and soil. This karstification occurs mainly in the shallow phreatic zone and is a self-reinforcing process leading to the formation of a network of channels (Ford and Williams 2007; Bakalowicz 2005) that are embedded in and interact with a rock matrix that is less karstified. Karst aquifers are characterised by a high degree of hydraulic anisotropy (higher conductivity in the direction of the channels than perpendicular to them), heterogeneity (higher porosity and permeability in the channels than in the matrix) and temporal variability in water availability (variable water level, storage volume and spring discharge) and water quality (Ravbar 2013; Stevanović 2015). Managing karst aquifers is, therefore, challenging.
The distribution of karstifiable rocks is well documented in many countries and regions; however, on a continental scale, there are only a few approaches that show the distribution of carbonate rocks. The global distribution of karst formations was first shown by Ford and Williams (1989), revised by Williams and Ford (2006) and further used in Ford and Williams (2007), albeit highly generalized. More recently, the World Karst Aquifer Mapping Project (WOKAM) has prepared a much more detailed map and database that is available in digital and printed form at scales of 1:25,000,000 and 1:40,000,000 (Chen et al. 2017; Goldscheider et al. 2020); however, the inconsistent database at the supra-regional level is the difficulty of such a large-scale map development. In recent decades, large-scale continuous maps of geologic, lithologic, and water resource information have also been developed, although the degree of accuracy generally decreases with the size of the area under consideration. The Global Lithologic Map (GLiM), for example, provides lithological information translated from existing regional geologic maps and literature; however, the merging of different datasets often involves uncertainties and regional differences (Hartmann and Moosdorf 2012). In order to better harmonize such differences, the International Hydrogeological Map of Europe (IHME1500) has been developed since the 1960s under the auspices of the International Association of Hydrogeologists (IAH) and with the support of the Commission on the Geological Map of the World (CGMW). In 2002, the Worldwide Hydrogeological Mapping and Assessment Programme (WHYMAP) was launched by the United Nations Educational, Scientific and Cultural Organization (UNESCO) and the German Federal Institute for Geosciences and Natural Resources (BGR) to map invisible subsurface water resources (Richts et al. 2011). In this context, various maps have been developed that provide global information on groundwater resources, groundwater vulnerability to flood and drought events (Richts and Vrba 2016), and there is also the already mentioned WOKAM (Chen et al. 2017).
The objective of this study is to provide a compendium of the developed Mediterranean Karst Aquifer Map and database (MEDKAM; Xanke et al. 2022), based on the International Hydrogeological Map of Europe (IHME1500). The mapping approach follows WOKAM. MEDKAM was developed at a digital scale of 1:1,500,000 and printed at a scale of 1:5,000,000, providing more detailed, more accurate and new information on the following:
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Quantification of the spatial distribution of different karstifiable rocks in the Mediterranean countries and other basic information on other aquifer types depending on their formation and aquifer properties.
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Quantification of the distribution of coastal karst in the Mediterranean region and the bordering states.
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Terrestrial and submarine karst springs and information on their range of discharge as well as karst caves and their spatial extent.
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Distribution of the long-term mean karst groundwater recharge as well as the trends in karst groundwater storage.
MEDKAM is a contribution to the World-wide Hydrogeological Mapping Programme (WHYMAP) of the UNESCO Intergovernmental Hydrological Programme (IHP) and is available for download as a digital version at the product center of German Federal Institute for Geosciences and Natural Resources (BGR; Xanke et al. 2022).
Geographical setting
The evolution of the Mediterranean karst landscape is closely linked to geotectonic processes since the late Mesozoic (Duggen et al. 2003; Cavazza and Wezel 2003), with today’s landscape strongly impacted by the collision of Eurasia and Africa since the late Cretaceous that created several mountain belts stretching from North Africa and the Iberian Peninsula across Europe, the Balkan region and the Caucasus to the Middle East. These characteristic landscapes are further shaped by local climatic conditions—for example, higher precipitation rates at higher elevations combined with the tectonically highly deformed carbonates. This leads to stronger karstification (development of, e.g. sinkholes, karren, dolines) than in the lower elevations with a predominantly dry climate and only slightly tectonically stressed terrain (hilly terrain with deep dry valleys). A partial drying of the Mediterranean Sea during the late Miocene (Messinian salinity crisis), 6–5 million years ago (Jolivet et al. 2006; Roveri et al. 2014), and subsequent water level changes in the following millions of years until today, have led to deep karstification at various levels, especially in the coastal area. As a result, some of the karst systems emerge at the current sea level, while some are also found inland above, or below, which then emerge as submarine springs near the coast (Bakalowicz 2018).
A clear delineation of the Mediterranean region is difficult because all common classifications—such as climatic (Koeppen 1936; Peel et al. 2007) or Mediterranean vegetation cover use (Olson et al. 2001) or geographically by surface catchments or bordering states—include some shortage in coverage. In each of these classifications, certain areas, often near the coast, are not included (e.g. large parts of Northern Africa when using the climatic classification approach), while areas far from the Mediterranean Sea are included (e.g. the entire basin of the Nile when using the hydrologic basin approach); also, in these cases, only surface characteristics are decisive for the delineation. In the case of the hydrogeological consideration of the Mediterranean area and in particular of the karstifiable rocks, a focus area of ~250 km around the Mediterranean Sea is suggested by Siegel et al. (2023), which on one hand includes the most important karst landscapes along the coast, considers narrow countries like Italy as a whole and includes the Mediterranean area of the aforementioned criteria. However, the decisive factor for this type of delineation is that it includes the unique deep karstification caused by the Messinian salinity crisis, which has partly reached far into the inland around the Mediterranean Sea (Audra et al. 2004; Fleury et al. 2007; Bakalowicz 2014).
The focus of the map development of Xanke et al. (2022) and the extension of the karst feature database for karst springs and caves was on the entire Mediterranean region, including countries within the extent of the printed MEDKAM (Fig. 1). The creation of the database on karst groundwater-dependent ecosystems (Siegel et al. 2023) was on the 250 km focus area around the coastline (Fig. 1).
The statistical evaluation of karst distribution in this manuscript is done for (1) the whole Mediterranean region (which includes all colored regions/countries as shown in Fig. 1), (2) the main geographical areas of Europe, Middle East and North Africa, (3) each individual country that lies to a significant extent within the 250 km focus area, as well as Portugal, which is often considered to be part of the Mediterranean region , and (4) the Mediterranean focus area (250 km zone). Countries that were not included in the statistical analysis due to their small size (<250 km2) but are located in the study area are Monaco (MC), San Marino (SM), and the Vatican City State (VA).
Materials and methods
Map of karstifiable rocks and other aquifer types
Basic mapping approach and legend
The basis for the MEDKAM is the International Hydrogeological Map of Europe (IHME1500), which consists of 30 hydrogeological map sheets (five of which are unpublished) covering almost the entire European continent and parts of the Middle East (BGR and UNESCO 2019; Günther and Duscher 2019; Duscher et al. 2015) and has recently been extended by four additional map sheets to the North African region (unpublished). The development of the IHME map series began in the 1960s under the auspices of the International Association of Hydrogeologists (IAH) and with the support of the Commission on the Geological Map of the World (CGMW). The scientific editing, cartography, printing and publication of the map sheets and explanatory notes were directed and financially supported by the German Federal Institute for Geosciences and Natural Resources (BGR) and the United Nations Educational, Scientific and Cultural Organization (UNESCO). The map has been available since 2013 in printed form and as a vector dataset in Shapefile format for geographic information system (GIS) visualization and data processing (Duscher et al. 2015). The hydrogeological content is based on contributions from the respective mapped countries, coordinated on a transnational basis. The attributes of the IHME contain different levels of lithological description that allow the assignment of the formations to the MEDKAM units. To fill in the missing parts in North Africa and the Arabian Peninsula, complete the Mediterranean mapping region and enable statistical calculation of the entire countries, the digital version of the British Geological Survey’s quantitative maps of African groundwater resources (MacDonald et al. 2012) and the United States Geological Survey’s bedrock geology of the Arabian Peninsula (Pollastro 1998) were used. They have been carefully analyzed with regard to the MEDKAM lithology and have undergone the same review process as the IHME; however, for further use of MEDKAM, only the IHME-based shapefile is available for download (see section “Map availability”).
The proposed map provides information on the distribution of carbonate rocks and evaporites in the Mediterranean and in other hydrogeological settings; however, the term “karstifiable rocks” mainly refers to carbonate rocks and evaporites, which consist of easily soluble minerals and are major karst aquifers. In the following, carbonate rocks are, therefore, referred to as karst aquifers. Carbonate rocks generally consist of more than 75% carbonate minerals (Ford and Williams 2007), although this is usually not indicated in geological and lithological maps and must be derived from general nomenclature. In the case of IHME, this nomenclature and categorization is given and is, therefore, regarded as a reliable basis—for example, limestone, dolomite or chalk are considered to be relatively pure carbonate rocks, as defined above. However, a mixture with or an alternation between sedimentary and carbonate rocks is often found in the description and requires classification by regional experts or derivation from literature and more detailed regional maps. In this assessment, it is assumed that the exposed carbonate rocks are karstified to some extent, without specifying the extent. Therefore, a lithological classification was developed following the World Karst Aquifer Mapping Project (Chen et al. 2017) and the geological units were grouped into four hydrogeologically meaningful units:
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1.
Karst aquifers in sedimentary and metamorphic carbonate rocks
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2.
Karst aquifers in evaporite rocks
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3.
Various hydrogeological settings in other sedimentary and volcanic formations (karst aquifers are possibly present at depth)
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4.
Local, poor and shallow aquifers in other metamorphic rocks and igneous rocks (no karst aquifers present at depth)
The improvement over WOKAM is that it is no longer necessary to distinguish between areas with continuous and discontinuous carbonate (or evaporite) rocks because the more detailed scale of IHME (1:1,500,000) enables a more precise delineation of the carbonate (and evaporite) rock areas. The unit “various hydrogeological settings in other sedimentary and volcanic rocks” includes a wide variety of geological formations with different hydraulic properties, but also indicates the possible presence of confined karst aquifers at greater depth. “Local, poor and shallow aquifers in other metamorphic and igneous rocks” are grouped into one unit. As crystalline rocks generally form the geologic basement, karst aquifers are usually not present at depth in these regions. This improvement allows a more detailed statistical analysis of the distribution of karst aquifers in the Mediterranean and an indication of areas where karst aquifers may be present at depth, covered by other formations.
Similar to WOKAM, the map unit “Karst aquifers in sedimentary and metamorphic carbonate rocks” includes sedimentary limestones, dolomites and chalks, which are mostly biochemical sediments, as well as limestones and dolomites that have undergone some degree of metamorphism, e.g. into marble or limestone shale. Since it is very difficult to distinguish between sedimentary diagenesis and metamorphism, these rock types are combined into a single unit as they are usually referenced by their content of carbonate minerals (>75%; Ford and Williams 2007), although this information is not usually detailed on maps or in publications (nor in the IHME), it is a common approach to rock mapping and characterization. The “Karst aquifers in evaporite rocks” mapping unit, by definition, includes chemical sedimentary rocks that consist mainly of highly soluble minerals such as gypsum, anhydrite or halite. These types of rocks typically form in basins where arid climates cause the water to become supersaturated, resulting in the precipitation of these minerals. The unit of “various hydrogeological settings in other sedimentary and volcanic formations” is probably the largest group and includes unconsolidated sediments of various compositions, including alluvial fans, sandy and gravelly sequences as well as silty and clayey low-conductivity layers. Also included here is the group of consolidated sedimentary rocks such as shale or sandstone. All of these sedimentary and volcanic rocks can be deposited over karst formations. “Local, poor, and shallow aquifers in other metamorphic and igneous rocks” are the most clearly defined. These formations generally do not include carbonate rocks and are usually dominated by low hydraulic conductivity and, hence, low aquifer productivity.
The approach to identifying the nonexposed carbonate rocks is the same as in Chen et al. (2017). Data on covered rock formation are typically poor and must, therefore, be inferred from the extent of individual geosystems (USGS 2021; Pawlewicz et al. 2002; Persits et al. 1997; Pollastro et al. 1999) or other data sources. If karst aquifers are exposed within such a delineated geosystem and are adjacent to sedimentary rocks, it is reasonable to assume that the karst is dipping beneath them. However, the focus here was on large continuous karst areas and not on small-scale sequences of subdivided rock formations as occurs, for example, in mountain ranges due to the strong thrusting and folding. Information on nonexposed carbonate rocks is important because they may contain significant karst groundwater bodies, which are often present in confined conditions.
Reclassification, evaluation and iterative improvement of the map
For the reclassification of the lithological units from the IHME to the MEDKAM classification, first, an assignment to the rock groups in the attribute table was made. If the assignment was not conclusive, the more detailed lithological descriptions in the IHME were used as a reference. In a second step, official regional karst and lithological maps were used for comparison with the IHME and, if necessary, further adjustments were made by reclassification. However, the reference maps sometimes have a different scale and, therefore, differ in the topology of the individual geological polygons, and in this case, do not always match the polygons of the IHME. In some regions, a compromise had to be made between accurate selection of individual polygons and consistent classification of karst areas over a larger area, often resulting in generalization from continuous small karst outcrops. As noted previously, the greatest difficulty is in assigning lithological descriptions of sequences of carbonate rocks and sedimentary formations to either unit—for example, marlstone and sandstone intercalations have been assigned to sedimentary formations in many areas. However, when additional carbonate intercalations are described and regional maps also indicate carbonate rocks, an assignment to the karst aquifer unit has been made.
In some regions, the various rock descriptions of the IHME are not very precise. In particular, the alternation of limestone, sand, marl or claystone is difficult to classify as pure karst because of the lack of information on the proportions of each type of rock. For this reason, an attempt has been made to make a classification based on existing geological and lithological maps and descriptions, as well as on regional experts. Nevertheless, it cannot be excluded that, especially in the case of the mixed rock sequences, the assignment could have been made to another hydrogeological unit—for example, shale is by definition a sedimentary rock, but often undergoes some degree of metamorphism (e.g. slate) and could also be classified as a metamorphic rock. In this study, however, slates are classified as sedimentary rocks. The combination “slates, quarzites, sandstones, shales” is assigned to sedimentary rocks, while the combination “slates, phyllites, mica schists and graywackes of various metamorphic degrees, partly gneissic” is assigned to metamorphic rocks. Since these descriptions are not always precisely distinguishable, by adapting them to regional lithological maps, it is possible that areas with the same IHME classification are assigned to a different MEDKAM unit.
Database on karst springs, caves and groundwater-dependent ecosystems
The database collects and standardizes existing relevant information related to karst aquifers. In particular, the data on karst springs and caves established in the WOKAM project (Chen et al. 2017) are extended and insufficient data from countries and are being improved and completed. Another new development is the inclusion of selected karst groundwater-dependent ecosystems (KGDE) compiled by Siegel et al. (2023). Due to the large amount of data and the larger scale of MEDKAM, it is possible to show different types of karst features such as the distinction between continental and submarine springs and different types of KGDEs. The supplementary data are based on literature research and data provided by regional experts. In this study, only a selection of the karst features collected for MEDKAM and stored in the database is presented. The criteria used to select the karst features shown in this paper are location within the Mediterranean area (250-km zone), regional importance such as spring flow, or length of cave. Consideration has also been given to consistent documentation of large regions; therefore, a few smaller springs or caves are also shown. It should be noted that the number of karst features, springs, caves, KGDEs stored in the MEDKAM database is only a selection of the actual number of existing features. In many countries, sufficient data sets are not publicly available, or if they are, they are not of sufficient accuracy and quality.
Karst springs are documented with their low-flow and high-flow discharge values, which represent the range of reported discharge data, but are not necessarily the minimum and maximum values. Where only one value was available, it was assigned to the appropriate category (low or high); in some cases, only the mean discharge was available. Special attention is given to coastal and submarine springs. Karst caves are documented with their depth and length where this information was available. A new and important aspect of MEDKAM is the karst groundwater-dependent ecosystems, which provide an indication of the state of water quality. Although these ecosystems are only briefly mentioned, they play an important role locally, often providing habitat for rare species and plants. A multidisciplinary characterization of selected KDGEs is documented by Siegel et al. (2023).
Assessment of karst groundwater recharge
An existing karst-specific simulation model (VarKarst-R; Hartmann et al. 2020) was used to determine the 30-year average (1990–2019) of groundwater recharge in the MEDKAM region, at a spatial resolution of 0.25° and using global climate data products as input (precipitation, temperature: GLDAS, Rodell et al. 2004; potential evapotranspiration: GLEAM, Martens et al. 2017). For the MEDKAM region, the recharge map from Hartman et al. (2020) was resolved at a higher spatial resolution and interpolated to include previously unmapped areas. This was done for simplification and presentation reasons and is, therefore, subject to some uncertainty in ~18% of the area, especially in the Middle East and North Africa. From the distribution of mean annual karst groundwater recharge, a country-specific average was calculated.
Assessment of trends in karst groundwater storage
The study of trends in karst groundwater storage was based on the work of Xanke and Liesch (2022). They investigated the extent of changes in groundwater storage (GWS) over the period 2003–2020 for the Euro-Mediterranean region using the latest data from the Gravity Recovery and Climate Experiment (GRACE/GRACE-FO; Save et al. 2016; Save 2020) satellite mission and recently reanalyzed ERA5 land climate data from the European Centre for Medium-Range Weather Forecasts (Hersbach et al. 2020). For this study, these trends in groundwater storage were masked by the MEDKAM carbonate outcrops. It should be emphasized that the GRACE data are based on an accuracy of 0.5°. The values should, therefore, always be considered as a product of their environment and only apply to the entire vertical groundwater column. Overlapping aquifers and catchments are, therefore, included in the signal and, therefore, do not apply exclusively to the karst, but are projected onto it. From the distribution of mean annual trends in karst groundwater storage, a country-specific average was calculated.
Results
Distribution of carbonate and evaporite rocks
The analysis of the spatial distribution of the different hydrogeological units (Fig. 2) shows that 26.1% of the whole Mediterranean region is covered by carbonate rocks corresponding to more than 3.2 million km2, and only 0.9% consists of exposed evaporite rocks (Table 1; Fig. 3). Much more prevalent are the various hydrogeological settings in other sedimentary and volcanic formations, covering 64.2% of the total Mediterranean region. Local poor and shallow aquifers in other metamorphic and igneous rocks are much less prevalent and account for 8.8%, much of which is found in European countries.
Within the Mediterranean focus area, 39.5% is covered by carbonate rocks and 1.7% by evaporite rocks. This shows that groundwater resources in karst areas are quite significant in the Mediterranean region. The various hydrogeological settings in other sedimentary and volcanic formations cover 49.3%, while local poor and shallow aquifers in other metamorphic and igneous rocks only account for 9.5% (Table 1; Fig. 3).
Looking at the main geographical areas (as defined in Fig. 1, including the entire territory of each country, even if not fully covered by Fig. 2), the proportion of carbonate rocks is the highest in the countries of the Middle East, at 30.5% (Table 1; Fig. 3), although the proportions vary greatly between the individual countries, ranging from 22.0% in Turkey to 72.7% in Jordan. The proportion of karst in European countries reaches 25.8%, with a wide range of proportions when looking at individual countries. Among the four largest European countries, the proportion ranges from 16.7% in Spain, 22.0% in Italy and 26.9% in Greece to 36.9% in France (Table 2). Much higher proportions are found in smaller countries such as Bosnia and Herzegovina (42.0%) or Montenegro (66.1%).
The North African countries, with 25.3%, have the lowest proportion of carbonate rocks of the three major regions, but they also have the largest and, in some cases, the most extensive continuous carbonate areas. Libya tops the list with 31.0% karst, corresponding to an area of 504.9 thousand km2. In Egypt, the proportion is 38.9%, with 402.5 thousand km2 of karst, and in Algeria, 301.5 thousand km2 of karst make up ~13.0% of the total area, although these are not large continuous carbonate areas as in Libya and Egypt, but numerous small areas.
Evaporite rocks are much rarer, occurring in only 2.5% of Middle Eastern countries, 1.1% of European countries and 0.5% of North African countries. Turkey and Spain have by far the largest areas of evaporite rocks, with 20.7 and 20.4 thousand km2 respectively, while Tunisia has the highest proportion at 5.5%.
In general, the smaller Mediterranean countries sometimes have relatively high proportions of karst in relation to their total area. Malta stands out as an island state with 100% karst. The Palestinian Territories (West Bank and Gaza Strip) also have a high proportion of karst (78.8%), as do Lebanon (68.0%) and Montenegro (66.1%). The proportion of outcropping karst areas below 10% can be found only in Portugal with 3.5%; however, this country is mostly outside the 250-km area of focus. In many countries, the exposed karst does not always represent the actual regional karst groundwater resources, as karst often extends underground (Fig. 2).
The results show that the Mediterranean region, with 2.26 million km2 of karst or 39.5% of its area, is one of the most karstified regions compared to other large countries and regions of the world. Goldscheider et al. (2020) show in the WOKAM map that China has a slightly larger karst area of 2.54 million km2, corresponding to 26.5% of its total land area, while Europe has ‘only’ 21.8%, North Africa 19.6% and Asia 18.6%. However, the proportions of karst in MEDKAM and WOKAM differ because of the different observation and classification scale (WOKAM distinguishes between continuous and discontinuous karst) and partly also because of the different lithological basis.
The presence of karst aquifers and their percentage of the total area can be an indication of the presence of karst groundwater resources and the dependency of humans and the environment on these water resources. However, climatic conditions, i.e. groundwater recharge (see section “Karst groundwater recharge”), as well as the thickness, functioning and storage capacity of karst aquifers, determine the actual availability of these karst groundwater resources.
Mediterranean coastal karst
Data on the length of coastlines vary considerably depending on the scale and source of the data. Therefore, information on country size and coastline length from other data sources may differ from the data used in this study (IHME1500), which is based on the NATO VMAP data (NIMA 2001). The total length of the continental coastline and mapped islands is ~41,600 km, of which ~24,700 km is the continental coastline and ~16,900 km is the cumulative coastline of the numerous Mediterranean islands. To assess the occurrence of both carbonate and evaporite rocks on the coast, the term coastal karst is used here to encompass both hydrogeological units.
The presence of these rocks on the coast usually implies the presence of submarine karst springs or caves, which makes these coastal sections very vulnerable to saltwater intrusion (Fleury et al. 2023). The coastal karst is, therefore, particularly worthy of protection and requires correspondingly adapted management, especially in the case of islands. The statistical analysis shows that 33.6% of the total Mediterranean coastline, including the islands, is karst, corresponding to a length of ~14,000 km. Of this, the continental coastline alone accounts for ~6,400 km of karst, or 25.9% of the total coastline (Table 3). The country with the longest coastline, including the numerous islands, is Greece with a total length of ~11,700 km, of which 42.8% is karst. If only the continental coastline of ~4,100 km is considered, 30.2% is karst. Italy follows with a total coastline of ~6,970 km, of which 19.6% is karst, and Turkey with ~5,100 km total coastline and 23.9% of karst (Table 3). Countries with a very high proportion of coastal karst are Croatia with 87.7%, Montenegro with 67.9% and Lebanon with 52.4%. Some smaller countries or countries with a short coastline, such as Malta and Bosnia and Herzegovina have a karst coverage of 100% in the studied mapping scale and resolution.
Many of the larger Mediterranean islands have a high proportion of coastal karst, such as the Balearic Islands: 43.5% of the coast of Ibiza consists of karst, Menorca has 60.7% and Mallorca has 78.0% coastal karst. The islands of the Dinarides, in particular, have a high proportion of coastal karst, with some islands consisting almost entirely of carbonate rock, such as Cres, Hvar and Dugi Otok (Fig. 4b) or Korčula or Brač (Table 4). The many Greek islands are also strongly characterised by karst, and some have a high proportion of coastal karst such as Kefalonia (87.8%), Lefkada (81.9%) or Zakynthos (60.7%; Fig. 4c). The largest Greek island, Crete, also has a high proportion of its coastline covered by karst, at 65.1% (Fig. 4d).
Karst springs
Some of the world’s largest springs in the Mediterranean region occur in the Dinarides, such as the Buna spring (Q12 in Table 5) and Vrela Trebišnjice spring (Q49 in Table 5) in Bosnia and Herzegovina, with maximum discharge of 380 and 219 m3/s, respectively. Springs in the eastern Mediterranean also show discharge maxima of several tens of cubic meters per second, such as the Afqa spring (Q1) in Lebanon with 65.5 m3/s, which is partly used for local domestic water supply (Schuler and Margane 2013), or the Kırkgöz spring (Q31) in Turkey with 62.8 m3/s. Many of the springs play an important role in regional water supply, but are also life-giving for unique ecosystems—for example, Montpellier is supplied by the Lez spring (Q37; Bakalowicz 2011), which hosts an important ecosystem and feeds the Lez River with a maximum discharge of 11.9 m3/s (Siegel et al. 2023). An example of a large karst spring on an island is the Almyros of Heraklion (Q5) in Crete with a maximum discharge of 30.0 m3/s. However, the countless small karst springs around the Mediterranean, which are only partially documented in this study, also serve the local population for domestic, agricultural, livestock and energy purposes in large parts of the Mediterranean region. Examples are the Maqar spring (Q39), one of many in the Al Jabal Al Akhdar region of northern Libya (Hamad and El Hasia 2022), or the Ain El Gudeirat spring (Q3) in the north-eastern Sinai region of Egypt, which also provides water for irrigation and local water supply (LaMoreaux and Tanner 2001).
Submarine karst springs
A selection of 39 submarine springs in the Mediterranean are presented (Table 6), most of which do not have information on their discharge due to the nature of their submarine location. For this reason, information is only available for a few springs such as for the Chekka spring (S15) in Lebanon, where a maximum discharge of 60 m3/s was measured, or for the Source de Port-Miou (S29) in France, where a maximum discharge of 50 m3/s was recorded. Additionally, the Moraig and Toix spring (S23) in Spain has a maximum discharge of 9.0 m3/s, while the Uji Ftohte (S38) in Albania measured 4.3 m3/s. A well-known and studied spring is Source de la Vise (S36) in France, which still has a maximum discharge of 0.45m3/s. These examples show that submarine springs can indeed release significant amounts of freshwater and, therefore, represent a potentially usable water resource despite the technical challenges. A special feature is the Argostoli system on the island of Kafelonia (Fig. 4e), where a submarine swallow hole infiltrates the seawater on the west coast and discharges ~2 weeks later ~15 km away on the east coast, partly through coastal springs but also through a submarine spring (proven by tracer test).
Karst caves
Table 7 gives an overview of 30 selected caves from the MEDKAM database that represent the diversity and size of the known caves in the Mediterranean focus area. The longest is the Schönberg cave system (C26) in the Alps, which is 140 km long and has a remarkable depth of 1,061 m. It is mentioned here because it is still within the 250-km zone, but is not strictly speaking part of the classic Mediterranean. Other large cave systems include the Hammam Trozza (C13) in Tunisia and the Coume Ouarnède cave system (C4) in France, which are both over 100 km long. Other deep caves include the Čehi 2 cave in Slovenia (C3) with 1,502 m depth, the Clot d’Aspres cave system (C4) in France with a depth of 1,066 m and the Željezna Jama cave (C30) in Montenegro, which is 1,027 m deep. An example of a cave on an island is the Cuevas del Drach (C7, Fig. 4a) on Mallorca, which is located on the south-eastern side and is ~1.2 km long.
Karst groundwater recharge
Karst groundwater recharge shows a clear trend from north to south (Fig. 5). Countries in the Alpine region and parts of the Dinarides have the highest average karst groundwater recharge of more than 800 mm/year in Switzerland, Austria and Slovenia. In Croatia, Bosnia and Herzegovina and Albania, the recharge is still between 500 and 800 mm/year. Lower average recharge rates of 300 to 500 mm/year are found in Italy, Serbia, France, Portugal and Northern Macedonia, while recharge rates of less than 100 mm/year are found in the MENA region and the Middle East, with Libya and Egypt having the lowest average recharge rates of 13.5 and 1.6 mm/year respectively, apparently due to the extensive desert regions in their central and southern parts (Table 8).
The average annual karst recharge volume in the Mediterranean region is 740,339 million m3, while within the Mediterranean focus area (250 km zone), it is 244,569 million m3. At the level of individual countries (the whole area of countries is considered), France has the highest mean annual karst recharge with 83,435 million m3, followed by Turkey with 39,081 million m3 and Italy with 32,418 million m3. Due to the simplified method of calculating mean annual recharge, this is only a solid estimate that should be validated and improved for some regions.
Data on recharge rates or total renewable groundwater from other sources, such as the WaterGAP model (Müller Schmied et al. 2021) or the Food and Agriculture Organisation’s AQUASTAT database (FAO 2023), show different values (even when downscaled to karst areas). This is due to the different calculation methodology and database, illustrating the challenge of obtaining accurate and reliable data on groundwater recharge.
Trends in karst groundwater storage from GRACE
It can be seen from Fig. 6 that karst aquifers in all parts of the Mediterranean are affected by a decrease in groundwater storage, although in some areas there is also a slight increase. In the whole Mediterranean karst region, a mean trend of –1.1 mm/year is detected corresponding to an annual lost volume of 2,544.3 million m3. Within the Mediterranean focus area, the mean trend is –0.3 mm/year, summing up to a total annual loss of 436.2 million m3. Of the 28 countries studied, 23 show an average negative trend in karst groundwater storage change, while only 5 show a positive trend. The strongest negative trends are observed in Malta with –19.3 mm/year, followed by Cyprus with –9.3 mm/year and Austria with –6.0 mm/year. Slightly negative trends are observed in large parts of France, particularly in the Paris region and the southern part of the country; northern Italy, the entire Middle East and parts of the Algerian highlands are particularly affected by strong negative trends. The Dinarides, some areas of eastern Turkey and the coastal fringes of the major karst platforms in North Africa are also affected; however, the latter region also has extensive areas of slightly increasing groundwater storage, as do small areas in southern Spain, southern Italy, Greece and throughout Turkey (Fig. 6). Portugal with 3.0 mm/year, Egypt with 2.1 mm/year, Libya with 1.4 mm/year, Greece with 0.3 mm/year and Lebanon with 0.03 mm/year show on average positive trends. This could either be due to a small increase in natural karst groundwater recharge, or return flows from agricultural irrigation could also be responsible for increasing groundwater storage, which would be plausible in countries with generally very low karst groundwater recharge rates. However, this would not explain small positive trends in nonarable desert regions such as Libya and Egypt. As a result of the different proportions of karst areas in the countries, the annual water loss rates are also very different—for example, Turkey has an average annual loss of 693.8 million m3, followed by France with 645.0 million m3 and Algeria with 558.8 million m3. The positive GRACE trends in Egypt and Libya, with their extensive karst aquifers, add up to a mean annual increase of 856 million and 728.8 million m3 respectively, while in countries with smaller karst areas, the increase is only a few million m3 (Table 9). The results show the high pressure on the karst groundwater resources in the Mediterranean region and, thus, also on the people and the environment that depend on them. Sustainable use is, therefore, particularly important in arid countries, especially in the context of climate change, which is predicted to be more pronounced in the Mediterranean (Zittis et al. 2019).
Conclusion and outlook
This study provides an improved detailed assessment of the spatial distribution of carbonate and evaporite rocks as karst aquifers and other hydrogeological formations in the Mediterranean region and the Mediterranean focus area defined by a 250-km zone. It includes karst formations both on the continent and on the main islands and provides the first statistical evaluation of coastal karst. It also presents a selected dataset of a unique combination of terrestrial and submarine karst springs and their range of discharge, of karst caves and their spatial extent. In addition, the results of existing studies on karst groundwater recharge and on the trends in karst groundwater storage were applied to MEDKAM, which provides more accurate and quantified information on potential karst groundwater resources.
The main findings of this study are:
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39.5% of the Mediterranean focus area (250-km zone) is covered by carbonate rocks, corresponding to more than 1.2 million km2, while only 1.7% consists of exposed evaporite rocks.
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Looking at the total area of each geographical region, Middle Eastern countries have the highest percentage of carbonate rocks (30.5%), followed by European countries (25.8%) and North African countries (25.3%). For evaporite rocks, the same order is found for the Middle East (2.5%), followed by the European countries (1.1%) and the North African countries (0.5%).
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The largest and most extensive continuous carbonate rocks can be found in North African countries with 301,000 km2 in Algeria, 402,000 km2 in Egypt and 505,000 km2 in Libya.
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The highest proportions of carbonate rocks in relation to the total area of the countries (apart from countries <20.000 km2 such as Montenegro or Lebanon) can be found in Jordan with 72.7%, Slovenia with 47.3% and Israel with 44.9%.
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Coastal karst (carbonate and evaporite rocks) is found along ~14,000 km of coastline around the Mediterranean Sea, including islands, which is 33.6% of the total coastline and ~25.9% of the continental coastline.
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At the country level, the longest discontinuous coastal karst (carbonate and evaporite rocks) is in Greece with ~5,016 km, followed by Croatia with ~3,491 km and Italy with ~1,367 km, while Malta and Bosnia and Herzegovina have a 100% proportion of karst on their coast, followed by Croatia with 87.7% and Montenegro with 67.9%.
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The karst groundwater recharge is higher in the northern part of the Mediterranean region than in the southern part and amounts to a total of ~244,600 million m3/year in the Mediterranean focus area (250-km zone). In terms of volume, France is the country with the largest average annual karst groundwater recharge (83,435 million m3).
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Negative trends in karst groundwater storage can be observed in many Mediterranean regions, with only a few areas showing slightly increasing trends such as parts of the Atlas Mountains in Morocco, Algeria, and Tunisia. Also, extensive areas in Libya and Egypt show slightly positive trends. For the Mediterranean focus area (250-km zone), an average annual loss of karst groundwater resources of 436.2 million m3 is calculated.
Carbonate and evaporite rocks and, thus, karst groundwater resources, are widespread in the Mediterranean region and play an important role in supplying freshwater to people and the environment. In the vast majority of Mediterranean countries, karst aquifers cover more than a quarter of the land area, and in some countries well over 50%, which is remarkable compared to other large regions of the world, as statistical evaluations by Goldscheider et al. (2020) show. This wide distribution also means that karst is found along the coast, especially on some of the many islands, most of which are karstified, emphasizing the vulnerability of these areas to saltwater intrusion and making balanced groundwater use particularly important there (Fleury et al. 2023).
Karst aquifers in the Mediterranean region are under increasing pressure, as indicated by the negative trends in groundwater storage, which has implications for spring discharge and thus for rivers and groundwater-dependent ecosystems, as well as for regional drinking water supplies. Recent climate projections for the Mediterranean are not optimistic, expecting rising temperatures and reduced water availability, although with regional differences (Pal et al. 2004; Mariotti et al. 2015; Lionello and Scarascia 2018; Zittis et al. 2019). According to Xanke and Liesch (2022), the causes of negative trends are diverse and can be attributed regionally to high, unsustainable groundwater abstraction as well as declining natural groundwater recharge. Positive trends, on the other hand, might regionally be associated with return flows from irrigation and not necessarily with increased groundwater recharge.
MEDKAM serves as a basis for various research and management approaches at different scales to keep the karst groundwater resources in the Mediterranean region usable for future generations. The spatial and statistical distribution of karst aquifers provides valuable information for scientific and regulatory purposes and can be used in the context of similarly scaled applications and data sets related to climatic, hydrological or geographical issues such as studies by Zhang et al. (2023), who conducted a global analysis of land use changes in karst areas and assessed their impacts on water resources. Additionally, it can be used to upscale or transfer local and regional management options to a supra-regional scale, which may involve, for example, karst-specific protection measures such as vulnerability mapping approaches (Zwahlen 2003), flood prevention and flood regulation approaches (Stevanović 2010; Jourde et al. 2014), or active karst groundwater management through managed aquifer recharge and storage (Xanke 2017). Further development of MEDKAM could include a more detailed scale, or a further subdivision into different types of carbonate rocks and their genesis or the degree of karstification, both in the subsurface and in the epikarst.
Map availability
The PDF version of the printed MEDKAM and more information on it, as well as a link to download the shapefile, which is free for noncommercial use, can be found here: https://doi.org/10.25928/MEDKAM.1.
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Acknowledgements
This study is a contribution to the project “Karst Aquifer Resources availability and quality in the Mediterranean Area” (KARMA)
Funding
Open Access funding enabled and organized by Projekt DEAL. Financial support of KIT researchers through the German Federal Ministry of Education and Research (BMBF) and the European Commission through the Partnership for Research and Innovation in the Mediterranean Area (PRIMA) program under Horizon 2020 (grant agreement no. 01DH19022A) is greatly appreciated, as well as funding of our KARMA project partners by their respective national funding agencies (MITECO for Spain, ANR for France, MIUR for Italy, CNRS-L for Lebanon, ANPR for Tunisia). Open Access funding enabled and organized by Project DEAL. The authors thank Salah Hamad for contributing data on karst springs for Libya.
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Xanke, J., Goldscheider, N., Bakalowicz, M. et al. Carbonate rocks and karst water resources in the Mediterranean region. Hydrogeol J 32, 1397–1418 (2024). https://doi.org/10.1007/s10040-024-02810-1
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DOI: https://doi.org/10.1007/s10040-024-02810-1