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:

  • 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.

  • Quantification of the distribution of coastal karst in the Mediterranean region and the bordering states.

  • Terrestrial and submarine karst springs and information on their range of discharge as well as karst caves and their spatial extent.

  • 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).

Fig. 1
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Location of the Mediterranean region, which includes all colored regions/countries and the Mediterranean focus zone (250 km), extension of the printed MEDKAM and the regions and countries of Europe, North Africa and the Middle East evaluated in this study

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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:

  1. 1.

    Karst aquifers in sedimentary and metamorphic carbonate rocks

  2. 2.

    Karst aquifers in evaporite rocks

  3. 3.

    Various hydrogeological settings in other sedimentary and volcanic formations (karst aquifers are possibly present at depth)

  4. 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.

Fig. 2
figure 2

The Mediterranean Karst Aquifer Map (MEDKAM) with references to other hydrogeological settings, as well as information on the borders between exposed and non-exposed karst, and a selection of karst springs (see table in section “Karst springs”), submarine springs (see table in section “Submarine karst springs”) and caves (see table in section “Karst caves”). An indication of the level of karstification around the Mediterranean basin is given by the Mediterranean Sea level during the Messinian salinity crisis and the almost drying up of the sea (Xanke et al. 2022). The map was created for the entirety of the countries for which this section of the figure provides only partial coverage (e.g. countries of North Africa and the Middle East)

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Table 1 Size of the hydrogeological settings and their percentages in the main geographical regions and in the corresponding whole Mediterranean region as well as in the Mediterranean focus area (250 km zone). Information on the size of the countries in this study is taken from the International Hydrogeological Map of Europe (IHME1500)
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Fig. 3
figure 3

Hydrogeological settings and their percentages in the main geographical regions (see colored areas in Fig. 1) and in the corresponding whole Mediterranean region as well as in the Mediterranean focus area (250 km zone)

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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%).

Table 2 Size of countries and percentage of different hydrogeological settings. Information on the size of countries in this study is taken from the International Hydrogeological Map of Europe (IHME1500)
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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.

Table 3 Length of the coastline of individual states, separated into continental coastline and islands, with the corresponding proportion of coastal karst. Information on the length of coastlines in this study is taken from the International Hydrogeological Map of Europe (IHME1500)
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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).

Fig. 4
figure 4

Selected examples of Mediterranean islands with important absolute or relative occurrence of coastal karst: a the Balearic Islands Ibiza, Mallorca and Menorca, b islands along the northern part of the Dinaric coast of Croatia, c the southern Italian island Sicily, d the Greek islands Crete, e Ionian islands Lefkada, Kafelonia and Zakynthos

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Table 4 Selection of the larger Mediterranean islands that are not independent states, with indication of the length of the coastline and the proportion of karst. Cyprus and Malta are included in Table 3. Information on the size of countries and the length of coastlines in this study is taken from the International Hydrogeological Map of Europe (IHME1500)
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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).

Table 5 Selected springs from the MEDKAM database with estimations for low-flow and high-flow discharge (m3/s)
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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).

Table 6 Submarine karst springs with estimations for low-flow and high-flow discharge (m3/s)
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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.

Table 7 Selected karst caves from the MEDKAM database of the Mediterranean region
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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).

Fig. 5
figure 5

Groundwater recharge, expressed as a 30-year average (1990–2019), derived from the karst-specific simulation model VarKarst-R (Hartmann et al. 2020)

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Table 8 Country-specific mean value of long-term mean groundwater recharge in carbonate and evaporite rocks (1990–2019)
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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).

Fig. 6
figure 6

Extent of trends in groundwater storage (GWS) in karst areas over the period 2003–2020 for the Euro-Mediterranean region using the latest data from the Gravity Recovery and Climate Experiment (NASA 2021) satellite mission and recently reanalyzed ERA5 land climate data (Muñoz Sabater 2019) from the European Centre for Medium-Range Weather Forecasts (modified after Xanke and Liesch 2022). The apparently coarser resolution of the GRACE data in North Africa results from the stereographic projection used, which leads to a stronger distortion in the direction of the equator

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Table 9 Country-specific trends in karst groundwater storage in carbonate and evaporite rocks (2003–2020)
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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:

  • 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.

  • 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%).

  • 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.

  • 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%.

  • 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.

  • 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%.

  • 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).

  • 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.