The karst environment

Dissolution of soluble rocks is the main process that governs the landscape in karst environments, characterized by distinctive landforms and a peculiar hydrologic behavior dominated by subsurface drainage (White 1988; Ford and Williams 2007). Karst landforms develop especially in carbonate rocks, but also in other soluble rocks such as gypsum (Gutiérrez and Cooper 2013), halite (Frumkin 2013), and even quartzite. Evaporite karst is widely distributed and accounts for a significant proportion of the engineering problems. Dissolution and subsidence processes in these karst systems commonly occur at much higher rates than in carbonate karst areas, due to the high solubility and low mechanical strength of gypsum and halite. In fact, some of the longest cave systems in the world are developed in gypsum units with limited thickness (Klimchouk 2000).

Karst occurs in around 20 % of the emerged Earth’s surface and provides approximately 20–25 % of the world’s drinking water. Karst systems can be extremely complex, heterogeneous and unpredictable due to the wide range of geological and hydrological controlling factors (Andriani and Parise 2015). Engineers may find excellent solid karst rock for founding a structure at a site, and highly difficult and hidden conditions in an adjacent location within the same geological setting. Moreover, karst environments are among the most vulnerable settings in the world from the environmental perspective, largely due to direct connection between surface waters and high permeability aquifers (Parise and Gunn 2007; Parise et al. 2008, 2009). Planning engineering works in karst environments should take into account such peculiarities, and incorporate concepts and measures aimed at reducing as much as possible the negative impacts on ecosystems and landscapes, with particular regard to pollution events.

Engineering problems in karst

The design and development of engineering projects in karst environments require specific approaches aimed at minimizing the detrimental effects of hazardous processes and environmental problems. Karst hydrological systems are highly vulnerable to pollution, and to land-use changes as well (White 2002). Moreover, water table declines related to groundwater over-exploitation or dewatering for tunneling and mining may lead to multiple negative effects, such as enhanced sinkhole hazard (Fig. 1), underground disappearance of drainages, deactivation of springs, drying-out of wells, or depletion of non-renewable resources (e.g., Song et al. 2012; Valenzuela et al. 2015). In addition, subsurface dissolution of soluble rocks creates voids and undermines the overlying material, eventually leading to ground subsidence processes through various mechanisms (Waltham et al. 2005; Gutiérrez et al. 2014).

Fig. 1
figure 1

Cover collapse sinkhole induced by groundwater withdrawal in an abandoned pivot irrigation crop, Al Jouf region, northern Saudi Arabia (photograph taken by F. Gutiérrez)

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Karst aquifers are extremely vulnerable to pollution, largely due to the direct connection between the surface and the subsurface drainage and the rapidity of the water flow in conduit networks. In contrast with other aquifers dominated by granular or fracture permeability, depuration capability is very low and the dispersal of the pollutants can be very fast and frequently difficult to foresee. Engineering works involving the construction of heavy structures, water impoundment, changes in the surface drainage, surface and underground excavations, among others, may trigger or create favorable conditions for sinkhole activity, which in turn facilitate the infiltration of pollutants. Excavations may create topographic depressions that may be affected by runoff concentration and enhanced infiltration, often modifying the groundwater flow paths and rates (White 2002). They may also reduce the thickness and strength of cavity roofs, triggering sinkholes (Fidelibus et al. 2011). Tunnels and mine galleries, frequently excavated after dramatic water table declines by pumping (dewatering), may cause dramatic changes in the local hydrology (Milanovic 1981, 2002), leading to the formation of sinkholes, intercepting karst conduits and causing dangerous inrushes of water under pressure (Bonetto et al. 2008; Lucha et al. 2008; Palma et al. 2012). Water inrushes and instability problems are especially common when the excavation works intercept pervious and mechanically weak breccia pipes rooted in deep-seated cavities developed in limestone or evaporites.

Karst terrains, due to rapid internal drainage through conduits and cave passages within high diffusivity aquifers are highly prone to groundwater flooding (DeWaele et al. 2011). The groundwater level in karst aquifers may experience dramatic rise in a short-time lapse in response to recharge events like a rainstorm. Engineering projects and urban development may lead to a substantial increase in the flood hazard by adversely altering the surface and underground hydrology. For instance, they may affect the runoff coefficient and the inflow of water at sinkholes and other absorption features, altering the characteristics of flood hydrographs in flash springs. Since floods related to groundwater discharge in karst settings may occur very rapidly and with low frequencies (high return period), the potential risks posed to infrastructures, engineering works, and other facilities linked to the built up environment may be difficult to anticipate and manage (López-Chicano et al. 2002; Parise 2003; Bonacci et al. 2006; Bailly-Comte et al. 2008; Jourde et al. 2014).

Sinkholes

Sinkhole formations are commonly the main source of engineering problems in karst environments (Gutiérrez 2010; Parise 2010). Ground settlement associated with the development of subsidence sinkholes, regardless of the mechanisms involved, may cause severe damage in any human structure. Moreover, collapse sinkholes, typically occurring in a catastrophic way, may lead to human life losses by directly swallowing structures and people, or by causing fatal accidents in transportation infrastructure. Other potentially hazardous problems associated with sinkholes include (1) flooding of depressions due to runoff concentration, water table rise, and/or backflooding in the karst conduit network; (2) water losses in reservoirs through pre-existing or induced sinkholes (Milanović 2000); (3) instability in artificial underground openings related to sinkholes and subsidence structures; (4) flooding of tunnels and mines; and (5) differential compaction of soils underlain by highly irregular rockhead.

The strategies and solutions that may be applied to mitigate sinkhole problems are highly variable and largely depend on the kind of engineering structure, the karst setting, and the typology and size of the sinkholes. A sound geological model, taking into consideration the peculiarities of karst and its interactions with the human environment, is essential for the design of cost-effective and successful risk reduction programs. Several key aspects should be taken into consideration regarding the sinkhole hazard (Gutiérrez et al. 2014):

  1. (1)

    Subsidence sinkholes result from subsurface dissolution (hydrochemical component), and downward displacement by internal erosion and/or gravity-driven deformation of the undermined material (mechanical component). A common misleading concept is to attribute sinkhole occurrence in carbonate karst areas to contemporaneous dissolution. However, the effects of limestone dissolution, characterized by a slow kinetics, are in most cases negligible at a human time scale. Those sinkholes are very probably related to old pre-existing cavities that have become unstable due to anthropogenic changes in the karst environment. Conversely, dissolution of the more soluble evaporite rocks (e.g., gypsum, halite) may contribute significantly to sinkhole development in the short term, by creating cavities and reducing the rock mass strength. Under adequate conditions, some subsidence processes like collapse and suffosion may operate at high rates, regardless of the solubility of the karst rocks.

  2. (2)

    The probability of occurrence of sinkholes and the subsidence rates tend to be higher in evaporite karst terrains. Evaporite formations frequently include salt units that may play an instrumental role in active subsidence phenomena However, these extremely soluble rocks are usually unappreciated or disregarded because they rarely crop out (e.g., Gutiérrez and Cooper 2013; Gutiérrez et al. 2015).

  3. (3)

    Site investigations should include the characterization of the karstification style and its degree of development (e.g., Waltham and Fookes 2003), since they determine the type of sinkholes that may develop. The solutional enlargement of discontinuity planes typically produces fissure- and conduit-like opening of limited size. These features are particularly abundant in the epikarst (Williams 2008), where the rockhead underlying the soil mantle may display a highly irregular geometry with pinnacles and cutters. Downward migration of cover deposits by internal erosion through those voids may result in the formation of cover collapse and cover suffosion sinkholes. Despite their limited diameter, these are the sinkhole types that account for the vast majority of the damage (Waltham et al. 2005). Large caves may occur at a wide range of depths within the bedrock. The upward propagation of these cavity roofs by stoping may produce breccia pipes and large catastrophic bedrock collapse sinkholes. This is frequently considered as the main threat in karst areas and the focus of many investigations. However, the spatial frequency of these karst features is typically very low, as well as the probability of occurrence of the resulting sinkhole types (Beck 2005). The large arched roofs of cavities in resistant limestones may have high safety factors. Interstratal dissolution, whereby specific soluble beds within the bedrock are dissolved, is particularly common in stratigraphic sequences including evaporite layers, especially salt (Fig. 2). Differential dissolution over relatively large areas may lead to the development of large sagging sinkholes, in which subsidence rates may reach very high values. These subsidence depressions, in contrast with the common conception, do not require the presence of cavities for their development. Eventually, overpressure conditions in phreatic conduits during recharge events may also contribute in deteriorating the mechanical properties of the rock mass, thus favoring the collapse of cavities and the development of sinkholes. This effect may act particularly in evaporite karst (Iovine et al. 2010), while in most of the cases dealing with carbonate rocks the impact of this mechanism on sinkhole development is difficult to prove.

    Fig. 2
    figure 2

    Sagging and collapse paleo-sinkhole exposed in a cutting along the Madrid-Barcelona high-speed railway near Zaragoza city, NE Spain. This subsidence structure is most probably related to interstratal dissolution of glauberite (photograph taken by F. Gutiérrez)

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  4. (4)

    Sinkhole mapping and characterization should be a priority in site investigations, since a significant proportion of the subsidence damage is frequently related to pre-existing buried sinkholes. Relevant attributes to be determined for each sinkhole include state of activity, precise limits, and subsidence mechanism. Regarding the limits, some aspects should be considered: (a) areas affected by subsidence are frequently larger than those mapped on the basis of geomorphic features; (b) sinkholes may expand through time; (c) a reasonable set-back distance (however, not easy to be defined) should be established around the sinkholes.

  5. (5)

    The development and evaluation of sinkhole susceptibility and hazard models is generally not a practical approach for most site investigations. The production of those prognostic models require a large amount of data, tend to yield optimistic hazard estimates and their validity may rapidly decrease, since continuously changing human factors may overwhelm the contribution of the more static natural factors. Nonetheless, the analysis of the spatial distribution patterns of the sinkholes and their relationships with various natural and anthropogenic factors may provide valuable clues for identifying the most hazardous zones (e.g., sinkholes clusters and alignments, spatial association with wells) (Galve et al. 2009).

  6. (6)

    A significant proportion of the damaging sinkholes is induced by anthropogenic changes in the karst system (Newton 1987; Waltham et al. 2005) (Figs. 1, 3). The potential impact of an engineering project on sinkhole risk should be evaluated in the early stages of its design. Gutiérrez et al. (2014) present a checklist of potential human activities that may accelerate or trigger the formation of sinkholes.

    Fig. 3
    figure 3

    Collapse sinkhole developed in La Loteta Reservoir (Ebro Valley, NE Spain) along a major leakage path associated with the left abutment of the dam. The reservoir is located in a 6 km long karst depression related to interstratal dissolution of evaporites (Gutiérrez et al. 2015) (photograph taken by F. Gutiérrez)

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  7. (7)

    A critical design parameter is the maximum sinkhole diameter at the time of formation or that of sinkholes with a specific recurrence. This information may be obtained from magnitude and frequency scaling relationships, although their production requires relatively large sinkhole inventories with chronological data (Taheri et al. 2015). Nevertheless, useful data in this parameter can be inferred from the historical and geological record.

Two main concepts may be applied alternatively or jointly for sinkhole risk mitigation: (1) Prevention, through the avoidance of the subsidence features and the areas most susceptible to sinkhole development. (2) Application of corrective measures aimed at reducing the activity of the processes involved in sinkhole development, including sinkhole remediation, and/or protecting the structures by means of subsidence-proof engineering designs (Milanović 2000; Zhou and Beck 2011; Gutiérrez et al. 2014). Most engineering measures applied to prevent subsidence in sinkhole areas are based on the following concepts (1) induce the collapse of shallow soil cavities (dynamic compaction); (2) improve the geotechnical properties of the cover (compaction grouting); (3) seal the openings associate with the rockhead (cap grouting and dental grouting); (4) filling cavities by grouting or direct access, in the case of large bedrock caverns; (5) rigid foundations that distribute the load of the structure over large areas, in order to span potential sinkholes; (6) piles that transfer the structural load to a depth below the cavernous bedrock. A major challenge is the development of early warning systems able to anticipate the development of collapse sinkholes (Jones and Blom 2014).

Pollution of karst groundwater

Karst aquifers are the most vulnerable to pollution due to their high hydraulic conductivity and limited self-depuration capacity. In the case of unconfined aquifers, groundwater is directly exposed to the rapid incorporation of chemical and organic substances that may infiltrate and percolate to the water table. Confined karst aquifers may be also easily polluted by fast lateral migration of pollutants. Several methods have been developed to assess the vulnerability of karst aquifers such as DRASTIC (Aller et al. 1987), EPIK (Döerfliger and Zwahlen 1997), PI (Goldscheider 2005), COP (Vías et al. 2006), the Slovene approach (Ravbar and Goldscheider 2007), and PaPRIKa (Kavouri et al. 2011). They can be very useful for anticipating and assessing potential impacts of engineering works on water resources. The results of the assessments may be used as a basis to incorporate modifications in the engineering projects in order to minimize their environmental impacts.

Delineation of sanitary protection zones is another delicate task, since the catchment boundaries may be very difficult to ascertain and can vary through time (Gunn 2007). Some authors suggest the establishment of more than three protection zones considering the specificity of karst. Industrial pollutants, mining waters, pesticides and nitrates used in agriculture, inappropriate landfills, uncontrolled waste disposal, as well as waste waters represent major and common threats to the karstic groundwater.

Management of karst settings

Management of karst environments is a very delicate matter due to the unique direct interaction between surface and subsurface environments, and the frequent ground instability problems related to underground karstification. These circumstances are frequently disregarded in land-use planning and development, resulting in severe problems with high economic impacts. Karst environments, due to their complex peculiarities, require specific investigation methods in order to properly manage and safeguard the sensitive geo-ecosystems and natural resources associated with them. To illustrate the importance of proper management in karst regions, we focus in this section on the Dead Sea, one of the areas in the world where in recent times strong changes to the environment have been caused by human activities.

For decades, enlightening lessons have been learned from the karst hazards related to the over-exploitation of water resources in the Dead Sea watershed, Middle East. The Dead Sea is an attractive place for tourism and the potash industries because its salinity is ten times higher than in the average ocean water. In Jordan, the growth in the investments for the economic valorization of the coastal zones and, concomitantly, the rapid deterioration of the environment, offer a unique opportunity to study the Human–Environment relationships within the Anthropocene.

The 80 by 16 km terminal lake is shared by Israel, Palestine, and Jordan, three nations among the poorest in the world with respect to soft water resources. Around 19 million people use freshwater withdrawn from the watershed of the Jordan River, which is the main drainage that feeds the Dead Sea. The decline of the lake level was first noticed in the 1960s. Since then, it has been dropping at an increasing pace, from 60 cm/year in the 1970s to as much as 1.5 m/year in the 2010s. In 2015, the water level is 33 m lower than that in the 1960s’, with the consequent lakeward migration of the shoreline. The water table in the surrounding areas and groundwater circulation have also been affected (Salameh and El-Naser 2000a, b; Sivan et al. 2005; Yechieli 2006; Yechieli et al. 2006; Kafri and Yechieli 2010).

The Dead Sea is the lowest emerged place on Earth (428 m b.s.l., as recorded in 2015). It is a terminal lake lying in a pull-apart basin associated with the Jordan-Dead Sea Transform fault system. For centuries, the potential evaporation rate was around 2500 mm/year. The continuous base level drop is responsible for the development of a highly dynamic human-induced salt karst system due to the replacement of the Dead Sea brine by brackish water aggressive with respect to halite (Yechieli et al. 2006; Closson and Abou Karaki 2009). In the 1990s, sinkholes and subsidence features have appeared at increasing rates all along the coast. Geophysical studies have shown that sinkholes mostly occur in areas where a particular halite layer, deposited some 10,000 years ago, is present several tens of meters below the ground surface (Abelson et al. 2003; Yechieli et al. 2006). Most sinkhole sites occur associated with discontinuities (i.e., Quaternary faults and fractures) and the feather-edge of the halite layer (Closson and AbouKaraki 2013; Ezersky and Frumkin 2013). Concomitantly, during the last two decades, five-star hotels and industrial infrastructures have been developed by the Arab Potash Company along the Jordan coast.

Chronology of damage and collapses in dike 18 of the Arab Potash Company

The chronological analysis of subsidence problems in dike 18 illustrates the importance of proper management in a hazardous environment characterized by rapid changes. Moreover, it raises questions about what would be the best strategy to build sustainable infrastructures in the Anthropocene.

In 1982, after the completion of the Arab Potash Company solar evaporation system at the southern sector of the Dead Sea, the very first sinkholes were observed in different places all around the Lisan Peninsula (Fig. 4). In 1992, the company modified the design of an initially projected 12 km long dike (dike 18 of saltpan SP-0A), in order to bypass a 1.6 km long lineament defined by more than 70 sinkholes occured after a period of intense rains.

Fig. 4
figure 4

Location map of salt evaporation pond SP-0A threats by geohazards related to the Dead Sea level lowering

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From January 1996 to December 1997, during the SP-0A construction phase, several incidents occurred (Tabbal and Mansour 2009): vertical settlement of around 2–3 m in very soft clays; artesian conditions associated with sand and salt layers; and development of sinkholes. The 12 km long dike was built to create a 95 Mm3 pond at a cost of $32M over a ground composed of soft to very soft silt-clay and massive salt rock (Lisan marls).

In 1997, a leak was discovered. Technical studies were conducted in collaboration with local and international experts, including Sir Alexander Gibb & Partners, which designed the dam. The reports indicated that the problem could be due to the development of underseepage processes that caused internal erosion, as a consequence of an artesian water basin with high water level. But others believed that it was due to a sinkhole that formed back in 1996.

In early 1998, during impounding operation, a landfill jetty 300 m long, 30–60 m wide, was rapidly built from dike 18 toward the basin center due to seepage flow through a large channel below the dike (Fig. 4). Since that time, this zone remained very active (Closson et al. 2007). Sinkholes are frequently observed and filled as soon as they appear. In May 2009, a ~20 m wide, partially filled, sinkhole was observed at the entrance of the jetty. This collapse and many others provide evidence of active piping in that zone from at least 1996, before the first filling of the reservoir. Moreover, this area and its surroundings are located within an active subsidence zone detected with differential radar interferometric techniques, applied to ERS-1 images acquired in 1992–1993 (Closson et al. 2007). It is also associated with the projection of a kilometer-sized canyon related to the uplift of the Lisan diapir.

In 2001, dike 18 was seriously damaged and forced security engineers to empty salt pan SP-0A. The production unit went back to service 5 years later, in September 2006, after the remediation of its 12 km dike at a cost of $16 M, half the original price of the basin. However, field surveys carried out in the time span 2004–2009 have shown that, although repair works increased the safety coefficient, they failed to stop the problems. Many cracks and backfilled sinkholes were continuously located.

At the end of July 2008, a 400 m long dike segment was enlarged to improve safety conditions. In fact, its width has tripled since 1997 (Fig. 4). In early 2011, the company launched a bid to fill underground cavities between stations 6 + 100 and 6 + 250 (i.e., a 150 m dike segment) with cement and thus protect a fragile part of dike 18. The works included drilling of around 40 boreholes to an average depth of 40 m through the dike body and foundation soil. Subsurface cavities and boreholes were filled with a grout mixture.

At the end of December 2012, between stations 1 + 000 and 2 + 000, a single circular structure ~300 m in diameter was identified within SP-0A from a Worldview-2 image acquired the April 2nd, 2011 (Fig. 4). This circular feature was unknown by the security engineers working for the company (Royal Haskoning). The “circle” was not alone but associated with other circular features identified as sinkholes and dating back to the mid-2000s. This “finding” raised lots of questions regarding the origin of the underlying cavity, the capabilities of sinkholes’ prediction models developed in Jordan and in Israel, as well as the strategies, approaches, and methods used by engineers and geophysicists to deal with such features. The analysis of satellite images revealed that this unique depression appeared after September 2006. Evidence of underground water circulation in that zone dates back to the mid-1980s. Processing of satellite radar data has revealed that the overall diameter of the subsidence area could be around 600 m across, threatening the stability of a section more than 1 km long of dike 18, which holds 90 Mm3 of Dead Sea brine.

In April 2013, a new tender announcement was launched to raise the height of dike 18 between station 5 + 300 and 11 + 750 (i.e., 6450 m long dike segment) and perform risk control works. In February 2015, the SP-0A production unit was definitely amputated from the first three kilometers of dike 18 in order to avoid complete destruction (Fig. 4). Four causes have been identified that justify this costly decision: (1) underground water circulates below the pond, from the Lisan Peninsula to the Dead Sea and the Araba Wadi; (2) Dead Sea brine seeping from SP-0A; (3) underground water circulation below the Lynch Strait owing to the supply of Wadi Araba (via the flood channel over the Jordan-Israel border), and to brine seeping from a juxtaposed saltpan; (4) natural settlement of the recently emerged coastal zones.

Lessons learned

With the benefit of hindsight, it is possible to judge that the expansion scheme around the Lisan was a questionable concept. Since its inception in 1998, SP-0A was out of service for 25 % of its lifetime. Repairs with a total cost of US$ 16 M represent an extra expenditure of half the original price of the project. In early 2015, 15 % of the salt pan was amputated.

The idea of the expansion scheme dates back to the 1980s. At that time, the Dead Sea sinkhole problem was spatially very limited and only known by few people. In the early 1990s, during the design phase, dozens of sinkholes occurred across the trace of the future dike 18. In 1993, consultants were contacted and provided advice. This warning did not stop the project. Arab Potash is a profitable company and engineers had found technical solutions to build up a lucrative dike system in the southern Dead Sea basin. A comprehensive environmental impact study, also dealing with the likely behavior of the coastal environment of the Dead Sea with a lower lake level, was not performed.

In case the lake level and water table changes were taken into account during the feasibility studies, the lifespan of major infrastructures such as dikes, bridges, and hotels would be drastically affected. For decades, scientists have developed concepts and robust methods to predict the consequences of lowering a base level and the associated groundwater table. Lowering the groundwater table and displacing the brackish water-saline water interface may induce rapid karstification where the optimal geological conditions are met. The wrong approach of neglecting the dynamic nature of environmental parameters is dangerous.

There are now many signs indicating a destabilization of the coast. Landslides are becoming more frequent and severe. Costly damages recorded in the last two decades underline the need for engineers to think differently about the safety and sustainability of the infrastructures they design. Techniques taught in the universities, and successfully applied in many places worldwide, are insufficient in the Dead Sea dynamic environment without adaptation and preventive planning. Among other things, cost-effective monitoring and early warning systems (for instance, the integrated use of surface and satellite monitoring, specifically designed for the peculiarity of the site under study) should be set up to avoid accidents as much as it is technically possible.

In recent years, several efforts have been carried out aimed at evaluating the impact of human activities on the karst environment. For instance, the Karst Disturbance Index (KDI), proposed by van Beynen and Townsend (2005), and later modified by North et al. (2009), measures the disturbance induced by man on karst, taking into account five categories (geomorphology, atmosphere, hydrology, biota, and cultural factors), each one described by several attributes. The approach followed in the evaluation of the KDI makes the results readily understandable to policy-makers and the wide public. A further approach was the Karst Sustainability Index (KSI; van Beynen et al. 2012) that takes into account 25 indicators related to the environment, economy, and society. It was created as a standardized metric of sustainable development practices in karst settings, aimed at comparing sustainability practices temporally and spatially to highlight those specific areas where remedial policies or actions are needed. Implementation of indices as KDI and KSI, which are specifically developed for karst, may represent an important step in a correct definition of the problems related to this fragile environment, aimed at proper land-use planning and management.

Managing karst water sources—an engineering approach

Although karst aquifers are one of the main sources for water supply worldwide, they are also one of the most problematic due to their unstable flow. The great variations of the water reserves and the minimal flow during recession periods pose significant problems of many waterworks worldwide that rely on karst waters. In areas with arid or semi-arid climate with low precipitation and huge evaporation, aquifer over-exploitation and depletion of non-renewable groundwater resources are very common. Many countries are using far more water than they have or can recover by natural replenishment. Saudi Arabia and Yemen, where there is massive abstraction of non-renewable fossil groundwater, are some of the relatively recent and well-known examples (Stevanovic 2013).

Due to high extraction rates, some huge and well-known springs become dry. The Ras el Ain spring in Syria, near the Turkish border, no longer flows although it used to discharge between 34.5 and 107.8 m3/s (Burdon and Safadi 1963). Over-exploitation of groundwater resources caused serious lowering of the water table. This was the result of very ambitious plans to exploit water resources for summer crops, and decisions by individual farmers to install numerous drilled wells in the entire catchment area.

During periods of minimal flow followed by increased demand, which may last for months in drylands, the risk of conflicts among water users significantly increases (Stevanović 2015). The Edwards aquifer in Texas, the Zagros aquifers in Iraq and Iran, or the Eocene age aquifers in Somali regions, namely the Togdheer, Sool, and Sanaag in Horn of Africa (Fig. 5), are also highly vulnerable to forced over-pumping during drought seasons. Some coastal karst aquifers suffer from deep inland salt water intrusions as a result of disturbance of the freshwater–saline water interface (e.g., Mediterranean islands and peninsulas, Yucatan Peninsula).

Fig. 5
figure 5

Extracting groundwater from shallow wells dug out in diluvium is the single solution for water tapping after springs along the foothill of karst Karkar Fm. (behind) dried out (Buuhoodle, Puntland province, Somalia; after Stevanović et al. 2012)

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Appropriate engineering solutions for similar cases could be identified, but probably not in all the situations, and not everywhere. If an aquifer is well karstified and with adequate storage in its deeper parts, it is possible to regulate and manage the minimal flow by various engineering interventions, thus satisfying the water demands of direct consumers, as well as the dependent ecosystems, by ensuring ecological flow downstream. These interventions are not very different from managing flow in the open water reservoirs (Fig. 6). Counting on water replenishment during the following wet season, over-pumping and groundwater extraction during a limited time period are possible in regions with moderate climate and rainfalls, well distributed throughout the year (Stevanović et al. 2007, 2010). For instance, climatic and hydrogeological preconditions in southern Europe and the Mediterranean basin are relatively favorable for such kinds of interventions.

Fig. 6
figure 6

Typical annual hydrograph of a karst spring with potential exploitation capacity (Q expl) bigger than natural minimal discharge and still much lower than the available dynamic groundwater reserves accounted as annual average discharge (Q av) (after Stevanović 2015)

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During the twentieth century and especially in its second half, many successful projects were implemented in different karst regions around the world. Most of them were in Europe, the Middle East, and China. They included different engineering works and intakes such over-pumping from deep siphons, construction of well batteries, galleries, plunging ponors, water channeling, building weirs, or underground dams.

It is therefore possible to distinguish the following two main groups of engineering regulation for controlling karstic groundwater (Stevanović 2015):

  1. 1.

    Regulation of the discharge zone

  2. 2.

    Regulations in wider catchment area

Paloc and Mijatović (1984) additionally distinguish two kinds of artificial regulation of the discharge zone: (1) by lowering the water table, and (2) by rising the water table (e.g., additional storage resulting from underground dam).

One of the most successful and largest recent projects in regulation of karst aquifers is the Montenegro regional water supply system, which supplys with water all municipalities and tourist resorts along the coast. It is designed on a maximum capacity of 1.5 m3/s; the total pipeline length is 140 km. This new system, functioning since 2010, has solved the acute problem of water shortage related to the small discharge of previously tapped coastal springs in summer months, when the tourist season dictates the maximal water demands.

The tapping of Bolje Sestre sublacustrine spring in Skadar Lake basin (Fig. 7) was firstly proposed by Radulović (2000) because of the shallow depths of submerged discharge points and its assessed sufficient and stable flow throughout the year. After conducting complex surveys, which included hydrological, hydrochemical, hydrogeological investigations, tracing tests, exploratory drilling, and identification of the position of the main groundwater transient channel, several options of tapping freshwater flow were evaluated (Stevanović 2010). Finally, following the request of reducing as much as possible the mixture of underground and lake waters, a concrete elliptical structure—coffer dam has been designed and constructed. Considering the importance and influence of lake water fluctuations on the intake, a special removable spillway section (rubber gate) has been installed on the dam crest (Fig. 7). The idea is to allow the non-captured spring water to overflow easily into the lake and to manipulate the water level inside the coffer dam when the lake level rises (annual amplitude can be 5–6 m). The two automatic and remotely operated compressors are used to pump the air into a rubber tube and activate the gate to rise when necessary.

Fig. 7
figure 7

Intake Bolje sestre and rubber gate between the two compressor stations. The source is supplying water to the entire coastal area of Montenegro (photograph taken by Z. Stevanović)

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All these engineering interventions, however, require available water resources and their proper assessment. Otherwise, applied measures may result in only a temporary or short-term effect and negative environmental impacts. Moreover, not all springs and aquifers can be physically regulated, and systematic and comprehensive research are always the preconditions for successful works. In particular, knowledge on aquifer characteristics (the discharge regime and the position of the hydraulic head in the aquifer), the thickness of the saturated zone, and the aquifer storage capacity are of key importance (Stevanović et al. 2010). A number of failures confirmed that karst is a risky environment for different aspects of karst water utilization and control (LaMoreaux et al. 1989; Milanović 1981, 2002). The sentence “expect the unexpected” is commonly used by many hydrogeologists and engineers dealing with karst and its properties.

Conclusions: Guidelines for builders wishing to develop industrial or tourism activities in karst areas

This final section provides some essential guidelines for industrial and touristic development in karst areas; the indications here presented derive mainly from analysis of the existing literature, to which the reader is invited to refer for further details.

Suitable foundations must be chosen and piling must be done carefully. To prevent infrastructures from falling into sinkholes, a type of foundation known as “linked foundations” can be used. Preventing damage to buildings, as well as utilities such as water and electricity, must be considered. Flexible pipes can be used to prevent cracking.

Roads, bridges, dikes must be planned carefully when subsidence features are present as they are a serious hazard to motorists.

Water extraction must be planned carefully, to avoid dissolution of soluble materials. Dissolution, however, can be mitigated with the use of careful planning and management. On a national scale, it is important to help the public to recognize that areas prone to dissolution are potentially dangerous. On a local scale, it is important to understand the recharge patterns within the karst.

Karst-water systems can rapidly transmit pollutants. They are sensitive to industrial and agricultural pollution, and require careful exploitation and protection. The complexity of hydrogeological systems mandates thorough studies to determine whether a specific site is, or can be rendered, suitable for a facility. Important components of hydrogeology studies are field mapping of structural-stratigraphic units; interpretation of multi-year air photos and differential interferograms (DInSAR/PS/SBAS techniques); test drilling and geophysical analyses; seasonal variation in water-levels; spatial variation of hydraulic characteristics of aquifer and aquiclude; velocity and direction of movement of ground water within aquifers; determination of control for recharge, discharge, and local base level; and evaluation of the effects of man’s activities.

In conclusion, we stress once again the peculiarity of karst, and the need to deal with engineering works, land-use planning, and land management in karst territories taking into account the main features of this environment, that make it unique and extremely fragile and vulnerable. Safeguard and conservation of the karst landscape and the precious natural resources therein contained (first and foremost, groundwater in karst aquifers) should become a priority, especially in those countries where karst is the predominant landscape, and in trans-boundary aquifers.