Karst and artificial recharge: Theoretical and practical problems

Journal of Hydrology 408 (2011) 189–202

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Karst and artificial recharge: Theoretical and practical problems A preliminary approach to artificial recharge assessment Walid Daher a,b, Séverin Pistre a, Angeline Kneppers c, Michel Bakalowicz a,b,⇑, Wajdi Najem b a b c

HydroSciences Montpellier, cc MSE, Université Montpellier 2, F-34095 Montpellier Cedex 5, France CREEN, ESIB, Université St. Joseph, Campus des Sciences et Technologies, BP 11-514 Riad El Solh, Beirut, Lebanon Schlumberger Water Services, Les collines de l’Arche, Bat. Madeleine D, 76 route de la Demi-Lune, 92057 Paris La Défense Cedex, France

a r t i c l e

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Article history: Received 7 October 2010 Received in revised form 14 June 2011 Accepted 13 July 2011 Available online 16 August 2011 This manuscript was handled by Philippe Baveye, Editor-in-Chief, with the assistance of Barbara J. Mahler, Associate Editor Keywords: Karst aquifer Managed Aquifer Recharge (MAR) Methodology Lebanon

s u m m a r y Managed Aquifer Recharge (MAR) is an emerging sustainable technique that has already generated successful results and is expected to solve many water resource problems, especially in semi-arid and arid zones. It is of great interest for karst aquifers that currently supply 20–25% of the world’s potable water, particularly in Mediterranean countries. However, the high heterogeneity in karst aquifers is too complex to be able to locate and describe them simply via field observations. Hence, as compared to projects in porous media, MAR is still marginal in karst aquifers. Accordingly, the present work presents a conceptual methodology for Aquifer Rechargeability Assessment in Karst – referred to as ARAK. The methodology was developed noting that artificial recharge in karst aquifers is considered an improbable challenge to solve since karst conduits may drain off recharge water without any significant storage, or recharge water may not be able to infiltrate. The aim of the ARAK method is to determine the ability of a given karst aquifer to be artificially recharged and managed, and the best sites for implementing artificial recharge from the surface. ARAK is based on multi-criteria indexation analysis modeled on karst vulnerability assessment methods. ARAK depends on four independent criteria, i.e. Epikarst, Rock, Infiltration and Karst. After dividing the karst domain into grids, these criteria are indexed using geological and topographic maps refined by field observations. ARAK applies a linear formula that computes the intrinsic rechargeability index based on the indexed map for every criterion, coupled with its attributed weighting rate. This index indicates the aptitude for recharging a given karst aquifer, as determined by studying its probability first on a regional scale for the whole karst aquifer, and then by characterizing the most favorable sites. Subsequently, for the selected sites, a technical and economic feasibility factor is applied, weighted by the difficulties that could occur when trying to undertake a recharge operation at a selected site from the surface. Each site is finally rated by its rechargeability index – the product of two factors, the intrinsic rechargeability and the feasibility index. ARAK was applied to the region of Damour, Lebanon, on the Mediterranean coast where uncontrolled exploitation of public and private wells led to its partial salinization by seawater. A MAR system in Damour region represents an interesting solution to cope with salinization and the insufficiency of the resource. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Managed Aquifer Recharge (MAR) is a sustainable technique that has generated successful results at social, economic and political levels and is expected to solve more water supply and management problems, especially in semi-arid and arid zones. As such,

⇑ Corresponding author at: HydroSciences Montpellier, cc MSE, Université Montpellier 2, F-34095 Montpellier Cedex 5, France. Tel.: +33 634 140 143; fax: +33 467 144 774. E-mail addresses: [email protected] (W. Daher), [email protected] (S. Pistre), [email protected] (A. Kneppers), michel.bakalowicz@ gmail.com (M. Bakalowicz), [email protected] (W. Najem). 0022-1694/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2011.07.017

MAR was identified as a key strategy for integrated water resources management, especially in water scarce regions. MAR techniques are of substantial interest for karst aquifers, particularly in Mediterranean coastal regions where this is generally the most common type of aquifer. Karst aquifers are the most exploitable existing groundwater resources in these areas (Margat, 2008), but they are highly sensitive to overexploitation and seawater intrusion, even under natural conditions (Fleury et al., 2007). Karst aquifers are known to be highly heterogeneous, formed by a complex conduit system that is generally impossible to locate. Moreover, the resources are very hard to exploit, with permeability coefficients ranging from 109 to 101 m s1, and flow velocities ranging from a few centimeters a day to hundreds of meters an

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hour. Note, however, that karst aquifers can be particularly favorable for very high exploitation rates when adequate wells are located close to springs, as is the case, for instance, at the Lez spring in France (1.5 m3 s1; Fleury et al., 2009) and the Figeh spring close to Damascus in Syria (3.5 m3 s1; Kattan, 1997). These examples show that, despite high seasonal discharge variability, karst aquifers may be exploited at flow rates close to the annual average and even higher for shorter periods of time. Active management of groundwater resources is therefore probably the most suitable approach for karst aquifers (Detay, 1997), but the main difficulty concerns the method of assessing the appropriate location for a productive well (Bakalowicz, 2005). This could explain why, as compared to projects in porous media, application of MAR for karst aquifers is still marginal, with the exception of a test project in Australia (Vanderzalm et al., 2009) and a few similar pilot projects under way in USA and elsewhere. Even in porous media, MAR projects can be jeopardized by a lack of comprehensive knowledge on the MAR approach (Dillon, 2005), in spite of the sound existing knowledge base on MAR (Bouwer, 2002; Dillon, 2002). Similarly, past MAR operations in karst aquifers were unfortunately attempted without proper aquifer characterization – only technical and practical matters were considered (Guo et al., 2008). The PhD research work of Massaad (2000) conducted in the late 1960s, and based on sound hydrogeological reasoning, is noteworthy. The main goal was to mitigate the problem of aquifer salinization resulting from seawater intrusion during the long summer low stage in the Hazmieh-Hadeth karst aquifer in Lebanon. Massaad proposed an Aquifer Storage and Recovery (ASR) operation using water from Beirut River. The first tests were conducted in the early 1970s, but the injection system was only implemented in the early 2000s. The system is currently operational 6 months a year, with a 500 m3/h injection rate into a single injection well (oral communication from the Beirut and Mount Lebanon Water Authority). However, the efficiency of this very unique experiment cannot be fully interpreted due to the lack of adequate follow-up monitoring. Only a few partial observations in wells close to the pumping station indicate that the salinity has not significantly decreased. All hydrogeologists now agree (Bakalowicz, 2005) that karst groundwater resources should be explored, exploited, managed, and protected through tailored approaches that take the specific features of karst aquifers into account, most notably the existence of preferential pathways that allow very fast surface to spring flow. Artificial recharge must be approached in an appropriate and rational manner to ensure the success of MAR in karst aquifers. Accordingly, the conceptual methodology for Aquifer Rechargeability Assessment in Karst (ARAK) was developed and is presented hereafter. It is based on the consideration that karst aquifers are generally unsuitable for MAR since they are naturally recharged and substantial groundwater may be stored in them. Application of MAR in karsts is considered to be an improbable challenge to fulfill since conduits may very quickly drain off recharge water without significant storage and/or change in saltwater intrusion patterns, or because recharge water cannot infiltrate. The ARAK method aims at determining: i) the ability of a given karst aquifer to be artificially recharged and managed; and ii) the best sites for implementing artificial recharge. It opens the door to a new cost-effective tool, especially for developing countries seeking new water resources along with sustainable management methods.

2. Overview of Managed Aquifer Recharge Many countries currently have some sort of enhanced or Managed Aquifer Recharge system, mostly for alluvial and porous

aquifers and to fulfill various objectives (Gale and Dillon, 2005). MAR is being adopted to an increasing extent to provide a large storage capacity to capture seasonally or intermittently available excess water for subsequent beneficial use, while often also enhancing water quality. Other objectives include: (i) cycling water through the sub-surface for purification, often using infiltration ponds or induced riverbank filtration, (ii) restoring partially depleted aquifers to buffer critical loss of porosity (e.g. land subsidence), or to remediate polluted aquifers through water recharge, (iii) mitigating the impacts of storms or floods, while storing water from flood peaks, (iv) acting as a hydraulic barrier, mitigating saltwater intrusions in coastal areas, and generally intrusion of impaired water. Natural recharge is, to an increasing extent, impaired by various types of land use. MAR is thus no longer considered as a mere water storage technique but rather as an integrated water resources management (IWRM) tool to be adopted as a balancing process for safe and sustainable water resource development, allocation and monitoring. This addresses current and future social, economic and environmental needs. As such, MAR could be defined as a situation whereby water that is non-native to the targeted aquifer is infiltrated or injected into an aquifer through a human implemented and controlled system. Hence, the site and recharge system are managed and monitored on a scale at which aquifer and groundwater property changes can be measured, with the aim of preserving or enhancing water quantity and quality. In many cases, MAR could be a cost-effective tool for IWRM, especially in regions where water demand exceeds the resource (Detay, 1997). However, Pyne (2005) considers that the economic factor is the most important of all the drivers. In other words, the following key factors should be considered when implementing MAR: (i) the existence of a socioeconomic need; (ii) of a suitable aquifer for storage and/or filtering; (iii) of a sufficient and adequate water resource for the projected lifespan of the MAR system (e.g. 20 years); and (iv) the technical and financial feasibility of the project. In addition, there should be a strong political willingness to sustain the region’s economy (Detay, 1997). Regardless of the economic, political or social aspects related to the technical feasibility of a MAR project, a certain number of hydrological and hydrogeological imperatives should be fulfilled. Hydrologically, a sufficient and adequate water resource is required to supply the water needed for aquifer recharge, e.g. surface water from rivers, storm or flood water, treated sewage effluent, industry effluent and desalinated water. Moreover, accurate knowledge of climate and weather variations and the impact of precipitation, temperature and hygrometry on the water source are required. The following hydrogeological conditions are necessary: – Aquifer hydrogeological characteristics that facilitate the introduction and storage of water. In an unconfined aquifer, for instance, the unsaturated zone must be thick enough to allow a significant rise in the saturated zone during the recharge phase. Moreover, the potentiometric surface should not be too close to the ground surface in order to avoid interference with soil. In a confined aquifer, the system’s physical or chemical boundary conditions need to be sufficient to allow injection of the additional volume of water by displacing the native water. – A transmissivity coefficient in the saturated zone that is sufficiently high to allow significant storage, but it cannot be too high or the added volumes of water could drain out too quickly from the aquifer. The aquifer heterogeneity needs to be characterized so as to identify high and low transmissivity flow zones impacting dispersivity, and thus impacting the recovery efficiency and/or the natural treatment provided by the aquifer.

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– The geochemistry of the recharge water, the native water and the aquifer rock should be characterized. Fluid–fluid and fluid–rock interactions can have a negative impact on the MAR system, but natural treatment processes and the removal of nutrients, disinfection by-products and other undesired xenobiotics or pathogenic microorganisms can be an additional benefit. – The above site-specific hydrological and hydrogeological conditions will determine the recoverable volume of water and the efficiency of the recharge and recovery rate of the MAR system, and hence its success. 3. Recharge conditions in karst aquifers Of all existing aquifers, karst aquifers are unique. They are hydrological systems shaped by the flow and dissolution conditions that mainly prevail in carbonate rock, limestone and dolomite outcrops. These are present in over 10% of ice-free continental areas and underlie all sedimentary basins, accounting for about 20% of all sedimentary rock, while providing groundwater to approximately 25% of the world population (Ford and Williams, 2007). Carbonate rock that emerges at some stage during geological times can be considered as potentially karstified (Bakalowicz, 2005). Consequently, carbonate possibly karstic aquifers may be one of the most important aquifer formations in the world, along with alluvia (Margat, 2008). They provide water for approximately 50% of the population around the Mediterranean Basin. Karst systems are considered difficult to exploit and protect because of their specific heterogeneity. As such, they are often poorly managed and not adequately considered as an alternative to surface water. However, the increase in water needs and forecasted impacts of climate change, as well as the degradation of water resources, highlights the importance of active management of karst groundwater resources, including MAR techniques using wells. Over the next decade, this is a prime objective in many arid and semi-arid regions, especially around the Mediterranean Basin. The question is, are karst aquifers rechargeable? Are (all) MAR techniques applicable to karst aquifers? As all aquifers are naturally recharged, hereafter we will use the terms rechargeable and rechargeability to describe the ability of an aquifer to be artificially recharged, in line with the terms commonly used for batteries. In response to these questions, it is necessary to discuss what is known from karst aquifers to be (un)favorable to a managed recharge.

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caves such as the Mammoth – Flint Ridge cave system in Kentucky, USA (Palmer, 2000), while a rise in the base level results in large karst aquifers with a huge storage capacity (El Hakim and Bakalowicz, 2007; Fleury et al., 2009). In the same manner, the Messinian salinity crisis that occurred in the Mediterranean Basin around 5.5 Ma caused a very substantial lowering of about 1500 m below the present sea level (Clauzon, 1982). It also led to major downward karst development, reaching a new base level by turning the main valleys into deep gorges, such as the Rhone canyon in France, as well as the submarine springs formed around the Mediterranean due to this particular geological event (Blavoux et al., 2004; Bakalowicz et al., 2007; Fleury et al., 2007). The physical heterogeneity of the karst system induces typical functional heterogeneity. Pipe flow conditions prevail in the conduit system, at atmospheric or higher pressures when conduits are deep and under confined conditions, while slow to very slow flow conditions prevail away from the conduits, in parts connected to the conduits by high head water loss. For instance, there is a very broad range of permeability, i.e. more than six orders of magnitude (Kiraly, 1998). Many structural or functional difficulties or particularities may be encountered while studying these highly distinctive aquifers (Bakalowicz, 2005), such as: – The impossibility of defining a Representative Elementary Volume (REV), as for other types of aquifers, particularly from hydraulic tests; – The inability of conventional hydrogeology tools, such as well observations, surface and borehole geophysics, pumping tests and potentiometric maps, to provide information on the whole karst aquifer; those tools mostly inform at a very local level (Mangin, 1994; Smart, 1999; Worthington, 1999; Ford and Williams, 2007); – The difficulty in determining karst aquifer limits and boundary conditions from geological and potentiometric maps, and the storage capacity, which is essential for any exploitation or recharge operation. – The complex structure and functioning of karst aquifers are illustrated by the large variety of hydrogeological models designed to represent karst systems for the purpose of developing mathematical models, protection scenarios or exploitation schemes. These specificities of karst aquifers explain why, to date, karst aquifers have been neglected as a potential environment for MAR systems.

3.1. Main characteristics of karst aquifer

3.2. Natural recharge in karsts: epikart and infiltration issues

The word ‘‘karst’’ refers to both a surface and a subsurface landscape, and by extension to a hydrological system shaped by the flow and dissolution conditions occurring mainly in carbonate rock (Bakalowicz, 2005). Carbonate rock is soluble in the presence of acid dissolved in water, most commonly resulting from CO2 solution. The process that combines solution of carbonate rock and flow in openings is named ‘‘karstification’’ (Ford and Williams, 2007). The resulting karst aquifers are characterized by marked heterogeneity related to the presence of open fractures and drainage systems, underground conduits organized in a hierarchical system. Different studies (Bakalowicz, 2005; Atkinson et al., 1978) showed that a minimum of a few thousand years are required for the development of a network of conduits. Consequently, any change in the conditions controlling groundwater input or output (climate change, base level change, tectonic uplift or subsidence) modifies the karst organization – multi-storey karst systems with very complex superimposed structures may develop multi-storey drainage systems. Base level lowering is illustrated by multi-storey

In the karst landscape, the epikarst (Fig. 1) corresponds to the upper part of karst formations, possibly covered with soil and/or sediment. An epikarst is a shallow, discontinuous zone with high permeability and porosity, estimated between 5% and 10% (Smart and Friederich, 1986; Williams, 1985). It is related to karst and surface processes that are far greater than those occurring in the underlying infiltration zone, and due to the enlargement of joints by acidic water from CO2 released below the overlying green cover (Fig. 1). Slow infiltration of water close to the surface can feed an almost continuous and permanent flow to the phreatic zone, comparable to percolation flow in porous aquifers. Consequently, the epikarst is actually an extension of the soil, acting as an interface between the atmosphere/biosphere and the karst aquifer, while playing an integral part in the morphogenesis and development of the karst structure by distributing water in the infiltration zone. This water seeps slowly and then dissolves rock close to the surface, or it runs fast in open discontinuities and dissolves deep rock (Bakalowicz, 2004). Because of this contrast in permeability between the epikarst or epikarstic zone (Mangin, 1994) and the

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Fig. 1. Synthetic representation of the epikarst, showing the discontinuous perched saturated zone, drained by a slow two-phase flow through fine joints (percolation flow), or by a fast concentrated infiltration flow through open fractures and conduits (from Mangin, 1994, modified after Bakalowicz, 2005).

underneath infiltration zone (magnitude of 103 m s1 or even more), it is a saturated zone in the vicinity of the surface. The hydrodynamic parameters drastically decrease between the epikarst and the underlying infiltration zone. This involves the formation of a seasonal, even locally permanent, saturated zone with a dominant horizontal flow that feeds the infiltration zone in a slow long-term manner. Hydrochemical and isotope studies from various regions have demonstrated that the infiltration delay can range from several days to a few months (Bakalowicz et al., 1974; Williams, 1983; Klimchouk and Jablokova, 1989; Perrin et al., 2003). As such, the epikarst and its temporary perched aquifer can be considered a ‘buffer’ zone, delaying infiltration by storing water in a discontinuous saturated zone. The epikarst is interrupted by surface karst features, including shafts, closed depressions, sinkholes and swallow holes which are the openings of conduits connecting the surface to the deep karst and the conduit system of the phreatic zone (Bakalowicz, 2004; Mangin, 1994). These interruptions in the epikarst enable direct inflow to the phreatic zone and generally serve as overflows of the epikarst, allowing a flush flow effect between the surface and the conduit system (Pinault et al., 2001). The epikarst distributes recharge water into different infiltration processes: – fast infiltration, in free surface flow conditions, circulating in the largest open and connected discontinuities, quite similar to surface runoff patterns; – slow infiltration, circulating in fine cracks and rock porosities, generally a two-phase flow pattern identical to the infiltration process in porous media; – infiltration delayed by storage in the epikarst and therefore subject to evapotranspiration processes, including geochemical and isotopic changes and volume reduction. 3.3. Artificial recharge issues in karst aquifers In the natural infiltration process, part of the recharge water does not flow through the epikarst. This water directly reaches the infiltration zone either as a concentrated flow through swallow holes in rivers, sinkholes fed by the epikarst overflow or as a diffuse flow through the upper infiltration zone with its soil or sedimentary cover when there is no epikarst. Artificial recharge operations

must prevent water flows through concentrated fast infiltration flow paths because of potential contamination risks and of rapid transfer to springs through conduits. Direct injection into the phreatic zone should be avoided because of the near-impossible ability to determine the most appropriate location. If reached by the injection wells, the conduit system could rapidly evacuate the injected water towards the springs. At sites without conduits, the permeability could be too low to allow injection of significant volumes. It would be impossible to identify an appropriate recharge site for injection into the phreatic zone. Due to these karst hydrology features, MAR operations should be designed to avoid direct injection into the phreatic zone while making use of slow and delayed epikarst infiltration. Injected water thus resides in the epikarst until it naturally infiltrates, via different pathways, and reaches the saturated phreatic zone. For artificial recharge, an epikarst must consequently be considered as a recharge regulator, as it is in the natural recharge context. The epikarst zone must be used maximally in artificial recharge operations, due to its dual role of: (i) spreading the injected water, preferably through slow and diffuse infiltration, and (ii) favoring the natural treatment process, which could decrease the project’s operational cost. A comprehensive study of the karst system must be conducted prior to any MAR operations. It is crucial that selected MAR sites be considered within the overall framework of the considered karst aquifer, i.e. the MAR site must be managed with respect to the karst drainage structure in a manner that optimizes use of the relevant storage function. If this prerequisite condition is not fulfilled it will be hard to monitor and assess the efficiency of the operation of the specified MAR system. Consequently, when a MAR system is designed and before it is implemented, the karst aquifer must be characterized at the appropriate scales in order to obtain accurate information with respect to the following criteria: (1) Related to hydrogeology: – Ability to infiltrate water from the ground surface or to inject it in wells. – Ability of the aquifer to retain injected water, or at least not to discharge it too fast through its conduit system. (2) Related to technical and economic issues:

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– Capacity to protect and/or enhance the quality of the stored water (regulations including sanitary well protection zone), quality of the water available for injection, surveillance of potential contamination through natural recharge at the river basin scale or through existing wells, self-purification capacities, etc. – The required infrastructures (water transport to injection site, energy, treatment, injection system, land purchase, etc.). These factors or criteria are based on problems encountered in past or current projects (see for instance the special issue of the Hydrogeology Journal, 10 (1) February 2002). For carbonate–rock aquifers, it is essential to gain further insight into the specific criteria for recharge project implementation. Carbonate formations have high spatial variability of transmissivity, and high related vulnerability due to the presence of preferential fast flow conduits with low dispersion. Finally, MAR projects in karsts should be focused on studying the infiltration conditions of the recharge site. It is strongly recommended that water be injected far from karst features such as vertical shafts or closed depressions, which are generally connected to the conduit system, so as to ensure that the recharge water spreads towards the phreatic zone through diffuse and slow flow conditions. Injected water should be subject to geo-purification during infiltration towards the saturated zone. However, some karst aquifers have both a developed conduit system and a large storage capacity due to their complex evolution (El Hakim and Bakalowicz, 2007). We think that artificial recharge could be done in a different way in these karst aquifers, i.e. by combining pumping at rates higher than the natural low flow discharge, as done at large Mediterranean karst springs (Fleury et al., 2009; Kattan, 1997), and injection into wells reaching the depleted storage parts of the phreatic zone. This is the proactive management strategy lauded by Detay (1997). Proactive management of groundwater resources is not considered in this paper with respect to karst aquifers with a large storage capacity.

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of conduits through various existing exploration methods, karst aquifer functioning cannot be efficiently modeled other than by lumped models (see for example Bakalowicz, 2005; Fleury et al., 2007, 2009). Distributed models, even though they satisfactorily simulate the functioning at the aquifer scale, do not output realistic results on local patterns of hydraulic characteristics. It is thus irrelevant to identify and characterize the rechargeability of a given karst aquifer using a general mathematical model with values of the main hydraulic parameters at the aquifer system scale as input variables, and the rechargeability parameters at regional and local scales as output variables. On the basis of present karst aquifer knowledge and study methods, it was determined that the most efficient approach to adopt is based on the use of physical data combined with field observations. The ARAK approach is thus comparable to the methods used for karst aquifer vulnerability mapping, whereby different scenarios are proposed to define protection zones and limits to be considered when springs or groundwater resources are to be exploited for potable water in karst terrain (Doerfliger et al., 1999; Petelet-Giraud et al., 2000; COST-Action 620, 2005; Kavouri et al., 2011). The intrinsic vulnerability of a karst aquifer depends on the ability of water to flow easily through the infiltration zone and into the saturated zone. Hence, the methodology is based on a multicriteria analysis, such as a mapping method using indexes with weighting criteria (Point Count Systems Models), which is relevant when using field data and independent criteria. These methods are currently the most accepted (Aller et al., 1987; Civita and De Malo, 1998; Gogu and Dassargues, 2000). RISKE is one of the various proposed methods considered, it is based on the following criteria: (i) rock type, (ii) infiltration, (iii) soil, (iv) karstification, and (v) epikarst (Petelet-Giraud et al., 2000), in its advanced version PaPRIKa (Kavouri et al., 2011), which attempts to quantify the ability of water to flow through the infiltration zone and reach the phreatic zone. 4.1. Principles of the approach

4. The ARAK methodology The Analysis of Rechargeability of an Aquifer in Karst (ARAK) methodology is proposed to answer the following questions: – Is a given karst aquifer rechargeable from its surface through a MAR operation, or not? – Can this recharge be achieved for a given MAR implementation site? The rechargeability of a target karst aquifer must be assessed at aquifer and local levels. Aquifer rechargeability concerns only manmade or artificial recharge aspects. It is the ability of a given aquifer to be recharged by means of a specific system designed to enhance its natural recharge. While natural recharge applies to whole aquifer formation outcrops, enhanced recharge is necessarily implemented at a designated site. Rechargeability of a karst aquifer must therefore be considered at two different scales, i.e. the aquifer system and the MAR implementation site. The aquifer may not be considered as rechargeable if the karst drainage structure is extremely developed. In such cases, it would not be possible to guarantee that injected water would be stored for a sufficient period of time or would act as a hydraulic barrier against saltwater intrusion. If the aquifer features are suitable for artificial recharge, then the method can be applied to analyze rechargeability at the system scale so as to identify potential sites favorable for enhanced recharge. Because of the complex structure and functioning of karst aquifers and the impossibility of detecting the location and dimensions

The proposed approach consists of developing a simple but easily applicable methodology that can define the feasibility of MAR for specific karst aquifers. It answers the above questions in a sequential manner, and at three levels: (i) analysis of the rechargeability of a karst aquifer at a regional level; (ii) determination and definition of potential sites for recharge at a local level; and (iii) evaluation, comparison and ranking of sites by taking hydrogeological, technical and socio-economic factors into consideration. The ARAK approach consists of the following five steps: Mapping each criterion, indexed from 0 to 4, from the least to the most favorable for recharge; (1) Discretization of criterion maps; (2) Computation of the intrinsic rechargeability index Ir at every mesh of the grid. The mesh size is generally determined on the basis of the Digital Terrain Model (DTM) resolution (generally 50 m  50 m). The total index is the sum of the criterion indexes bearing the rate of the criterion and its weighting factor; (3) Validation of the final map and computation of potential recharge sites; (4) Application of a feasibility factor (Fe) concerning the technical, economic and social feasibility of a recharge operation for every selected site. Sorting potential sites with respect to the intrinsic factor Irt and the socioeconomic factor Fe. Direct mapping of the ‘‘vulnerability’’ criteria into the ‘‘rechargeability’’ criteria is not feasible since the ARAK methodology criteria have to be considered when assessing the karst

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rechargeability. For instance, a very vulnerable zone, i.e. a zone where water infiltrating from the surface rapidly reaches the phreatic zone and possibly the spring, and a zone that is totally invulnerable because of its impermeability, are both unfavorable for recharge operations.

4.2. Criteria selected for the ARAK method The ARAK method assesses the capacity of a karst aquifer to retain, store and transfer the injected water volume within an adequate duration. The resulting map provides a basis for the determination of the ‘‘best rechargeable’’ sites, i.e. the most appropriate sites for artificial recharge from the karst surface. In this approach, the criteria should allow characterization of the water transfer operation that occurs from the surface to the phreatic zone, without determining the local details of this operation in the phreatic zone. The criteria must thus be selected to meet the following constraints: – Relevance: every criterion must physically describe an elementary part or a specific character of the aquifer having an impact on its rechargeability. – Independence: all selected criteria must be independent of each other, i.e. no characteristic or functioning process in the aquifer should be given or interpreted from two sources. However, this condition is hard to fulfill because some characteristics of karst aquifers may have implicit links. – Accessibility: every criterion must be easily identified and quantified for the whole study area. – Additivity: in order to pool all criteria so as to compute the intrinsic rechargeability, they must concern the same variable, i.e. the rechargeability, related to the different components controlling the recharge. Infiltration through carbonate rock depends on several factors, including the ground slope, the type of rock, the abundance of joints and their enlargement by karst processes at the surface and underground. The ARAK methodology considers four criteria controlling the recharge processes: (i) Epikarst E, including the epikarst and its soil cover, i.e. all surface karst landforms; (ii) aquifer Rock type R; (iii) Infiltration potential I; and (iv) the degree of Karstification K of the entire system, which includes all underground karst landforms, and defines the transfer conditions through the infiltration and phreatic zones of the whole aquifer, without considering the local status related to the conduit system. While there may be some good local indicators of efficient drainage in the infiltration zone, from the surface it is generally impossible to locate conduits in the infiltration and phreatic zones and to assess their pattern and degree of development. Criterion K is therefore only a rechargeability characteristic at the aquifer scale – it cannot highlight favorable sites. Criterion I, the Infiltration potential, depends only on the ground slope, i.e. maximum where the slope is minimum and vice versa. Criterion E, the epikarst, must be clearly differentiated from criterion K. It characterizes the development of a high permeability zone in the shallow part of the infiltration zone in carbonate rock, controlling infiltration flow towards the phreatic zone. Rock criterion R indicates the initial characteristics of the rock, whatever the extent of karstification. These four criteria are assessed from topographical and geological maps at appropriate scales, and from field measurements in order to account for the specific structural and functional features of the considered karst aquifer.

Fig. 2. The four criteria defining the intrinsic rechargeability of a karst aquifer from the surface.

4.3. Definition of the criteria The four selected criteria must be defined as precisely as possible in order to avoid misinterpretation and/or redundancy. Two of them are related to karst development (Epikarst and Karst), while the two others (Rock and Infiltration) are related to hydrogeology and hydrology conditions. They are defined as follows (Fig. 2, Table 1):

4.3.1. Epikarst E criterion The Epikarst criterion E characterizes the epikarst zone, including the epikarst and its soil or sedimentary cover. It is defined by its thickness, generally 0–20 m, its rate of development, including the density of open joints, and its lateral continuity in the area where it can be observed. As mentioned in the section describing epikarst, the presence of a well-developed epikarst, able to retain infiltrated water and to spread it in a temporarily and spatially diffuse, slow and uniform manner to the underneath infiltration zone, is considered the most favorable configuration for a MAR operation in a karst aquifer. Consequently, the characterization of the epikarst hydrogeological functioning is essential for the proper indexing of this criterion. Observations in quarries and road embankments, as well as temporary perched springs at the epikarst base and shallow wells can provide such information. However, care should be taken to avoid confusing such temporary springs with overflow springs directly connected to the conduit system draining the phreatic zone. Distinctions can be made through spring hydrograph analysis. Detecting the presence and extension of an epikarst and assessing its development is complicated by the soil or sediment cover. In some cases, the epikarst may be absent because it is locally eroded, or unobservable when covered with a thick continuous sediment layer. Epikarst identification is therefore based on field observations associated with the use of aerial photographs allowing mapping of surface karst features. Criterion Epikarst E is thus considered to include the ‘‘surface karst features’’, ‘‘soil cover’’ and ‘‘epikarst’’ functions. Note that a thick soil or sediment cover enhances the formation and development of covered underlying epikarst (Song, 1986). An epikarst is indexed by attributing the value (0) when the epikarst is totally absent or in the presence of swallow holes, sinkholes, dolines, vertical shafts and closed depressions that serve as fast infiltration paths, thus forming a ‘‘hole’’ in the epikarst formation, and the value (4) when it is well developed, i.e. a more than 5 m thick epikarst, spatially continuous, with the presence of temporary springs and/or shallow wells, indicating a good storage capacity

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W. Daher et al. / Journal of Hydrology 408 (2011) 189–202 Table 1 Considered criteria and definition of their rating in the ARAK methodology. Criteria

Class

Description

Index value

E Epikarst

E0

Total absence of epikarst, presence of karst depressions, shafts, swallow holes, etc. Epikarst: thickness < 1 m, discontinuous and fairly developed; presence of temporary springs with flow rates of l/h; karst depressions (poljes, dolines with flat bottom, dry valleys) Epikarst: 1 m < thickness < 5 m, developed and laterally discontinuous; presence of temporary springs with flow rates of l/mn; pavement and karren field Epikarst: thickness >5 m, developed and laterally continuous; presence of temporary springs with flow rates of l/s; thin, discontinuous soil cover Epikarst: thickness >5 m, developed and laterally continuous; presence of temporary springs; presence of continuous soil and/or sediment cover

0

E1

E2

E3 E4

1

2

3 4

R Rock

R0 R1 R2 R3 R4

Marly formations (35–65% of clay) Marl containing limestone (25–35% of clay) Marly limestone (10–25% of clay) Thick bedded, highly fractured limestone and dolomite Thin bedded, moderately fractured limestone and dolomite, with eventual thin marly interbeds

0 1 2 3 4

I Infiltration

I0 I1 I2 I3 I4

Slope > 50% 30% < Slope < 50% 15% < Slope < 30% 5% < Slope < 15% 0% < Slope < 5%

0 1 2 3 4

K Karstification

K0

Very developed and functional binary karst; high velocities in tracing tests (>100 m/h) Low functional binary/high functional unary karsts; high velocities in tracing tests (50 < V < 100 m/h) Low functional unary karsts; velocities in tracing tests (V < 50 m/h); variable chemical composition and flow rates Very low functional karst; absence of variability in chemical and physical compositions and flow rates; absence of witness of quick water circulation Karst system compared to fractured systems or non functional system

0

K1 K2 K3 K4

(Bakalowicz, 2004). The intermediate values (1, 2, 3) are attributed to epikarsts relative to their thickness and spatial continuity.

4.3.2. Rock R criterion The Rock criterion, R, is defined by the characteristics of rock outcrops, by considering their lithologic nature (limestone or dolomite, marl, marly limestone, non-carbonate rock), thickness and structure. In addition, this criterion accounts for fracturing at the outcrop local or regional scale due to the prevailing tectonic features, while also considering surrounding zones, up to 100 m away based on field observations, where breaching zones could be noted. Criterion R is defined from geological maps, field observations, and data from boreholes or any underground work. Criterion R (lithology and fracturing) is mapped from 1:50,000 or larger scale geological maps. It must be supplemented by field investigations, particularly to monitor the lithofacies, type of fracturing and thickness of layers. The lithological description of a formation from a map is very general and does not account for the presence of vertical or lateral variations in facies that might affect the R indexation. As the geological map identifies all outcropping formations as carbonate rock or not, it is very important to initially select the carbonate formations and then sort them from the least to most favorable to recharge, based on available data and field investigations. The classification of these formations is based on their ability to infiltrate water and serve as an underground reservoir where water will be stored, independently of karst and epikarst development. This is done by allocating value (0) to the marl and all noncarbonate rocks unfavorable for a surface recharge operation and increasing to value (4) for thin bedded, well fractured limestone and fractured dolomite, weathered in dolomitic sand allowing easier horizontal flow and storage. Thick, massive, well fractured limestone and dolomite where water may flow easily along extended joints

1 2 3 4

are rated (3), because they are favorable to the development of long vertical fractures that facilitate deep water infiltration, but not as much as thin bedded, fractured limestone. Marly limestone and chalk are rated (2), while calcareous marl is rated (1). 4.3.3. Infiltration I criterion Infiltration criterion I considers only the ability of the ground surface in infiltrating water. It does not consider the infiltration conditions, such as slow and fast, concentrated or diffuse, which are related to karst and epikarst development and rock properties. The infiltration criterion depends only on the terrain slope. Accordingly, infiltration is most probable where the ground is horizontal and least probable where it is very steep, which favors surface runoff. Criterion I is a key factor favoring infiltration in carbonate rock, thus allowing the development of karst features. Obviously it is not crucial for implementing a MAR system. The classes of criterion I are determined from a DTM. The different slope classes range from the value (0) for ground surface with slopes > 50%, of negligible interest with respect to surface recharge operations, (1) for slopes of 35–50%, (2) for slopes of 20–35%, (3) for slopes of 5–20%, and (4) for ground surfaces with slopes < 5%, thus providing the best conditions for recharge water infiltration. The value of the slope in each mesh depends on the scale of the DTM. However, the class range is large enough to smooth these differences and avoid any discrepancy when changing the mesh dimensions. 4.3.4. Karstification K criterion The Karst criterion K characterizes the whole karst system. It defines the development of the conduit system, i.e. the degree of karstification of the system. It is based mainly on available data on all of its aspects to obtain maximum information about the

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studied karst, such as the hydrodynamics from spring hydrograph analysis, data from hydraulic and tracing tests, geophysical surveys, geochemical data, including those obtained using natural tracers and isotopes, as well as speleological and field observations. All such data enhances mapping of the karst aquifer functionality, with the lowest value (0) allotted to the most functioning karst aquifer capable of very rapid flushing of the recharge water, similarly to a pipe network, or conduit flow type, with an allogenic recharge from surface streams through swallow holes, whereas the value (4) is given to a less functioning or developed karst system, or a diffuse flow type. Intermediate values are attributed to aquifers with a highly developed conduit system with only authigenic recharge (1), or with a less to poorly developed conduit system (2, 3). Although the karst criterion is not discriminant for selecting potential recharge sites, it is essential for showing whether the aquifer is rechargeable or not overall. 4.4. Computation of the rechargeability index 4.4.1. Rechargeability of a karst aquifer The ARAK methodology includes several steps that are presented hereafter. This methodology proposes and yields only potential surface recharge sites for the considered karst aquifer. During the first stage, each of these four criteria is mapped and indexed from 0 to 4 based on their ability to favor recharge, with the lowest index 0 allotted to sites that are the least favorable for recharge operations and the highest index 4 given to the most favorable sites (Table 1). During the second stage, every resulting map is computerized and divided into a discretized predefined grid that allows calculation per grid of the value of the intrinsic rechargeability index Irt according to Eq. (1):

Irt ¼ aEi þ bRj þ cIk þ dKl

ð1Þ

where a, b, c and d are coefficients weighting each criterion. Consequently, the sequence generally proposed to determine potential recharge sites follows the 5-step approach described above. The mapping step is carried out using dedicated software integrating GIS technology. Field work provides supplementary information from direct observations. Acquisition points are referenced and transferred onto the map relevant to each criterion, thus constituting a database for site observations. They enable backups or updating of the proposed index. As long as such observations are compatible with the set indexing, the final criteria map will be rigid and consistent and can thus serve as the final intrinsic rechargeability map. 4.4.2. Indexes and weighting rates The weighting rates reflect the assumed importance of each criterion they represent in any MAR operation. The weighting rates of the four criteria were empirically defined while considering at all times the hydrogeological impact that each criterion may have on natural infiltration conditions from the surface and on the surface recharge operation. a, b, c and d represent the weights of the E, R, I, K criteria, respectively. Formula (1) is subject to the following constraints: – a P b P c P d P 0.1 – the sum of the weighting indexes a + b + c + d = 1 Several scenarios were tested on the Damour study site in order to find the best combination of weighting indexes that would define the ability of the aquifer to be recharged while discriminating potential MAR sites. We consider that an epikarst should play an essential role in a MAR recharge operation from the surface in a car-

bonate aquifer because it allows slow and delayed infiltration, shallow temporary storage, and dispatches water in the underlying infiltration zone. We retained the following combination of values: a = 0.6; b = 0.2; c = 0.1; d = 0.1; which seem to be the best for characterizing the rechargeability at the aquifer scale and identifying favorable sites. Note, however, that this combination of weighting rates is a first try of the method, and tested only at one study site. Rock criterion R was attributed a medium weight (b = 0.2) that accounts for the rock formation and structure. Infiltration criterion I was given a low weight (c = 0.1) due to its marginal importance in MAR operation in karsts. Finally, karst criterion K was given a low weight (d = 0.1) because of the relatively low importance of karst development in determining and controlling its potential rechargeability, when the epikarst serves as the main structure for dispatching artificial recharge water. The intrinsic rechargeability Irt is then calculated in every mesh of the grid according to Eq. (1). Irt reflects the ability and suitability of a given karst system for a MAR operation based on the above four hydrogeological criteria that characterize the karst (E, K) and the hydrogeology (R, I), and thus control the recharge conditions and operation (Fig. 2). Accordingly, the Irt index is the response of the karst aquifer at every mesh of the grid to a recharge operation. Irt values range from (0) to (4), whereby (0) indicates the absence of any aspect for rechargeability, whereas (4) marks the most interesting rechargeability aspects. The Irt map is validated by comparison with individual criterion maps, including site observations in order to reveal any discrepancy that could be corrected in the final map. The intrinsic rechargeability of the karst system is then assessed by checking the Irt values. Most Irt values under 2 reflect a negative hydrogeological response of the karst aquifer with regard to a recharge operation, whereas the opposite highlights the possibility of artificially recharging it. The rechargeability of a karst aquifer is thus defined at the system level by setting a simple threshold value (Irt 6 2) under which no further exploration for rechargeability feasibility is required. In contrast, if Irt > 2, then the karst aquifer has recharge potential. The next step is the identification of these potential recharge sites locally. 4.4.3. Intrinsic rechargeability of potential sites After determining whether a karst is suitable for MAR, the Irt map is thoroughly checked, with meshes having the highest values representing the best potential recharge sites. The sites are sorted by values ranging from highest to the lowest. Five to ten sites may be selected for further detailed study. The same method for characterizing the local hydrogeological rechargeability is then used based on the same criteria. The mapping scale should be adapted to the local settings, i.e. 1:5000 to 1:10,000 scale. The main goal is to better characterize criteria E and K. The Epikarst criterion is determined for an adequate and meticulous characterization by detailed field investigations, such as observations in trenches, shallow boring, geophysical, hydraulic and tracing tests. The Karst criterion rating should be rechecked for consistency, particularly by studying the hydrodynamic behavior of the aquifer and data from boreholes and pumping and tracing tests in the phreatic zone, in order to check the degree of development and functioning of its karst structure. The different sites are compared in order to select the site(s) most appropriate, from a hydrogeological standpoint, for a recharge operation from the surface. This local approach of the method has not yet been tested. 4.4.4. Feasibility factor Fe and total rechargeability index Ir Once a site has been selected for recharge based on the hydrogeological criteria, additional factors need to be taken into consideration, such as the technical, financial, social and economic feasibility of the project. A feasibility factor Fe is thus introduced

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which considers the constraints, such as the availability and quality of the water to inject, the recharge system to deploy, the cost of land purchase, the distance to the area to be supplied, and the monitoring and protection zones to set up to evaluate the capital and operational costs for the lifespan of the project, while taking all potential risks into account. An adequate study of the above points facilitates calculation of a factor Fe ranging between 0 < Fe < 1, where not at all feasible is allotted the value 0 to fully feasible (1), so the final rechargeability index Ir will be in the 0–4 range. The total rechargeability index Ir is calculated for every potential preselected MAR site. It is the product of the intrinsic rechargeability index Irt of a site with the relevant factor Fe, with the two indexes Fe and Irt having the same weight. The total indexes of rechargeability Ir are sorted for ranking the most favorable sites. This table of Ir values is subdivided into five rechargeability classes indicating the suitability of every site for recharge operations, i.e. ranging from (0) non-rechargeable to (4) for the most favorable sites for recharge.

5. Application to the Damour karst aquifer 5.1. The issues The complete contamination of Beirut’s main aquifer by seawater intrusion in the late 1960s led authorities to choose the Damour upper aquifer as a main substitute. Currently, this aquifer is one of the three water supply sources for Beirut and its suburbs. A study for a dam project on the Damour River was launched despite the hazards that could be encountered if such a dam were built in karstic terrain. Consequently, the Damour aquifer is of crucial importance since it is also affected by seawater intrusion, which might contaminate it if no control strategy is adopted. This would involve initially providing information to authorities so as to facilitate their management of the aquifer and prevent irreversible future damage. The Damour aquifer is exploited through public and private wells. Its situation is representative of the exploitation of groundwater in Lebanon (Aulong et al., 2008). The fourteen public wells have pumped large quantities of water since 1991 and are operated and controlled by the Beirut Water Authority (EBML). Currently the wells pump a total of 9.5 hm3/year for 6–7 operational months per year. In addition, private wells for industrial and domestic use were drilled in the area, with most of them being unaccounted for and unlicensed. The regional municipal authorities estimate that there were 500 private unlicensed wells in 2009. MAR was previously undertaken at the Damour site in 1975, where five gravity recharge wells were drilled on the river bank. It seems that they were never successfully operated and the site was abandoned.

5.2. The study area The area under investigation lies along the coastal zone of Lebanon in the eastern part of the Mediterranean Basin, about 10 km south of Beirut. It is located between 35°260 –35°300 E and 33°420 33°460 N. It has an average N–S length of 6 km and an E–W width of 5.5 km, covering an area of about 30 km2. It is bounded to the north by Naame valley, to the south by Damour River, to the east by the village of Baouarta and to the west by the Mediterranean Sea. There are four main villages within these boundaries: Mechref, Damour, Haret el Naame and Naame. The main source of groundwater in this area comes from the Upper Cenomanian–Turonian carbonate aquifer, located between two impermeable formations.

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The study area has a Mediterranean climate, with a moderately cold and rainy winter extending from October to April, a hot and moist summer extending from June to September, and two transitional periods from September to November and from April to May. The average annual temperature of the Damour area, as measured by the Beirut International Airport (BIA) weather station (about 5 km north of the study area) is about 19.9 °C, with a monthly maximum of 26.6 °C in August and a minimum of 13.3 °C in January. The amount of precipitation on the western slopes of Mount Lebanon ranges from 950 to 1300 mm/year. Maximum rainfall occurs in January, while the minimum occurs in July and August. Precipitation records from 2001 indicated an annual level of 825 mm. The annual average humidity of the study area is about 70% according to the BIA database. By applying Thornthwaite’s and Turc’s formulas, the actual evapotranspiration (ET) value is estimated at 605 mm/year. The rock outcrops in the study area range from Upper Jurassic to Quaternary (Fig. 3). The greater part of the study area is occupied by the Cenomanian C4 and Turonian C5 carbonate formation, where fracturing and karstification are common. The oldest rocks are the Upper Jurassic limestone J7, outcropping in the middle course of the Damour River valley. The youngest Quaternary deposits are found mainly in the western coastal plain, in addition to some alluvial deposits along the Damour River valley. The other formations are outcropping chronologically from the east and southeast to the west, because of a regional westward dip. Several regional and local faults are widespread in the study area, mainly oriented E–W. The geology of Lebanon is known thanks to the works of Dubertret (1951), who drew up most of the 1:50,000 maps during the 1940s and 1950s. The regional geology was revised by Walley (1998). The Damour area was mapped at 1:20,000 scale by Guerre in 1973 in an original unpublished draft. While the 700 m thick Cenomanian–Turonian carbonate formation in Lebanon was considered as a unique limestone reservoir overlying a 500 m thick impermeable series of lower Cretaceous age formations, Guerre showed that the Cenomanian (C4)–Turonian (C5) series is composed of three parts with different hydrogeological behaviors: the lower limestone C4a and the upper limestone C4c–C5 form two independent aquifers separated by aquiclude marl to marly limestone (C4b), with all three having a thickness of about 230 m. These Cenomanian–Turonian series are covered with a 600 m thick series of marls and marly chalk (C6), deposited during the Senonian to Paleocene. In the Damour area, most of these marls were eroded after its post-Eocene emersion and they outcrop only as outliers along the Quaternary coastal plain. The Quaternary marine and continental deposits (Q) outcrop along the coast of the study area between Damour and Naame villages. Recent alluvia are deposited in the lower Damour River valley. They cover about 5 km2 of the study area and are unconformably overlying the Cretaceous formations. Their thickness varies from a few to several meters, and they form the cultivated coastal plain. The regional uplift, which created the Mount Lebanon range and the regional westward dip in the study area, is due to the recent activity of the northern part of the Dead Sea Transform DST Fault System (Walley, 1998). The dip reaches its maximum (55°) at the eastern part of the area. It decreases westward and ranges from 8° to 15° on the C4c–C5 plateau and then increases to 45° in the C6 formation. These changes are either local, due to faulting, or regional, corresponding to successive down-bendings parallel to the coast. As a result of this tectonic activity, the study area is crossed by some major faults, in addition to many minor and local faults. Most of them are normal faults with relatively thick sub-parallel shattered zones, with an approximate E–W to NNE–SSW trend.

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Fig. 3. Damour geological map, according to an unpublished map done by A. Guerre in 1973, showing the faults and location of public and some private wells, and the proposed location of the injection gallery pilot test.

Many of them cross the study area from east to west and disappear into the sea (Fig. 3). The study area is characterized by two main topographical features. The first is a coastal plain at the shoreline that slopes very gently from east to west and is highly cultivated. The second feature is a tilted plateau in the eastern part of the area, reaching its highest point in the study area at 572 m above mean sea level. It also slopes downward from east to west, where it intersects the coastal plain. Several valleys cut through the study area. Some are deep and well developed, while others are not as well defined. Many of these valleys are located on fault zones. They are all dry most of the time, except for the Damour River valley, which has meanders, especially at its lower part. Despite the well-developed karst features in Lebanon, especially in the Cenomanian–Turonian carbonate formation, karst features are unknown in the study area, except for a few very small caves located along the sides of the dry valleys. However, epikarst development is clearly visible on the plateau. The regional morphology of the plateau is fluvial although the local valleys are dry, as is the case in karsts. This is interpreted as the consequence of the recent erosion of the Senonian marl formation C6, which only recently uncapped the C4c–C5 limestone and therefore did not allow karst development, as can be observed in the Mechref area where the C6 marl capping the C4c–C5 limestone is not totally eroded, keeping the limestone from weathering and hampering natural recharge, thus preventing karst development. Field evidence shows that the limestone plateau between Damour River and Naame was never recharged by concentrated, allogenic surface flows. The only known karst and paleokarst features are located on the southern side, on the left bank of the Damour River valley. In conclusion, the whole limestone plateau from Mechref to Naame can be considered as poorly karstified.

5.3. Hydrogeological setting Given the stratigraphic log of the study area and the associated hydraulic characteristics, the geologic units may be classified into hydrogeological units as follows: – The lower Cenomanian limestone Unit (C4a) is the lower aquifer. It is deep and confined in the study area and it is overlain by the aquiclude C4b. – The Middle Cenomanian Unit (C4b), more than 200 m thick, is the aquiclude separating the lower from the upper aquifer. – The upper Cenomanian–Turonian limestone Unit (C4c–C5), up to 250 m thick, forms the upper, unconfined aquifer. It represents the major aquifer in the Damour area from which all groundwater is extracted. – The Senonian marly formation (C6) is also an aquiclude locally covering the C4c–C5 unit between Mechref and Naame. No information is available on its continuity to the west below the Quaternary sediments, as a possible confining unit of the upper aquifer. – The Quaternary deposits form a discontinuous aquifer where existing water lenses are pumped and used for irrigation on the coastal plain. There is no evidence of connections/relationships between the Quaternary aquifer and the upper aquifer. Due to the westward dipping of strata, the major aquifer (C4c– C5) outcrops at the surface in the largest part of the study area. The aquifer is thus unconfined where it outcrops and it is confined in some areas in the western part, where it is overlain by the Senonian formation. The major aquifer is geologically very homogeneous regionally and in its thickness, consisting of thin bedded pure reef limestone. The only heterogeneity is related to faulting,

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which created narrow crushed strips, which are considered as impermeable boundaries according to patterns elsewhere in Lebanon.

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lower aquifer via some of the faults. This assumption has not yet been checked. 5.4. Mapping of rechargeability

5.3.1. Discharge of the upper aquifer There are no existing springs discharging groundwater from the upper aquifer. It is thus assumed that it discharges either into the Quaternary coastal aquifer, if the Senonian formation is discontinuous, or at submarine springs. The coastal aquifer has never been studied and has been heavily exploited for irrigation, to the point that it is currently saline. Submarine springs are a well-known phenomenon in coastal karstified aquifers, especially along the eastern Mediterranean coast. They are strongly linked to karst features developed during the Messinian salinity crisis that occurred in the Mediterranean Basin 5.5 My ago (Fleury et al., 2007). A thermal infrared survey along the Lebanese coast covering the offshore segment of the study area was carried out by the Food and Agricultural Organization of the United Nations (FAO) in 1973. It located two thermal anomalies, located about 100 m from the shore at an approximate depth of 30 m below sea level. They were interpreted as being the discharge of fresh groundwater at two submarine springs. However, contrary to several other sites along the Lebanese coast where karst submarine springs were identified and studied (Fleury et al., 2007), these anomalies were not observed afterwards. A localized submarine discharge from the upper limestone aquifer may occur during high recharge periods. 5.3.2. Hydraulic characteristics Previous unpublished studies (Khadra, 2003) estimate that the water table is very close to sea level. Unfortunately no precise piezometric data are available. The hydraulic head in the eastern part of the aquifer does not exceed 5.5 m asl, with westward regional groundwater flow in line with the surface topography, where the high water head values are in the east and southeast and they decrease in the west and northwest. The piezometric surface in the eastern part of the aquifer does not exceed 4 m asl, with a maximum of around 5.5 m asl in the southeast. Pumping activity lowered the water level in the Naame area, not far from the shore, from around 4 m to 1.5 m asl. Moreover, the aquifer is highly heterogeneous, with transmissivity values ranging from 105 m2/d to 13,220 m2/d. According to Khadra (2003), the aquifer is being affected by seawater intrusion, with a saltwater encroachment rate estimated to be about 200 m/year landward and with an annual deficit in the water budget of around 5–6 hm3/year. Consequently, the aquifer is expected to be totally contaminated within a very short period at the current pumping rate if no management policy is applied to the aquifer. However, this assumption is not based on reliable piezometric and pumping rate data. The aquifer water budget was recalculated during this study. According to the climatic data issued by the Beirut International Airport weather station, which is the closest and most representative station, the mean annual rainfall is about 825 mm and the mean annual actual evapotranspiration is around 605 mm. Since there is no surface runoff, we consider that 220 mm represents the yearly recharge to the aquifer, i.e. 3.3 hm3/year for a recharge area of about 15 km2. According to the Water Authority, total withdrawals via pumping activity are about 10.5 hm3. This means that there should be a theoretical deficit of almost 7.2 hm3/year, i.e. more than twice the recharge. However, salinization of the aquifer is still very limited, and chemical data from the exploited wells show a chloride content that generally below the limit of potability. This means that the aquifer is certainly recharged by infiltration from the seasonal Naame River north of the study area and/ or the Damour River south of it, or by upward seepage from the

As previously discussed, rechargeability mapping depends on the definition of the four criteria classes. In the Damour sector and further to our site observations in the Cenomanian–Turonian carbonate outcrops, the following class values were allotted: - Criterion E: E = 0 for the bottom of dry valleys running along the faults. In fact, the limestone layers were eroded and no epikarst could develop. E = 1 for the sides of these valleys where there was less erosion. E = 2 for perched areas located between Senonian outcrops at the coastal strip and the upper tilted plateau at the level of Mechref village. E = 3 for epikarst formations with overlying soil cover. Since there was no evidence of continuous epikarst and seasonal shallow springs (E = 4), class 3 was attributed to it. - Criterion R: R = 4 value was allotted for C4c–C5 outcropping rocks because the limestone was composed of relatively thin highly fractured beds. The fault zones with a value of R = 3 were not considered because their widths never exceeded 50 m, which could not be mapped at 1:20,000 scale. All other geologic formations were rated 0 or 1, except for the lower aquifer, which was also rated 4 because it had the same rock characteristics. - Criterion K: K = 2 value was allotted for the C4c–C5 karst aquifer, while bearing in mind that little reliable information is available on karst development in the study area. In addition, this karst aquifer does not present any concentrated discharge at a spring at which hydrodynamical or chemical studies could be undertaken. As surface and underground karst features are completely unknown throughout the area, the K value is certainly underestimated, and consequently so is the ability of the aquifer to be artificially recharged. - Criterion I: a DTM was provided to identify the different infiltration classes according to the slope. Accordingly, the maps of each of the four criteria were acquired and the relevant intrinsic rechargeability index Irt map was computed based on the chosen weighting rates (Fig. 4). 5.5. Interpretation and discussion The intrinsic rechargeability index map shows (Fig. 4) that despite the underestimate of K criterion most of the considered aquifer is in class 2.4–3.19 (very favorable), with some sites in class 3.2–4 (extremely favorable). The aquifer presents several sites that would be suitable for an artificial recharge operation. They are located on the plateau at elevations of 50–250 m asl. The technical and economic feasibility factor can be roughly estimated, because a precise, detailed chart for assessing Fe could not be drawn up for just this test site. There are some constraints since the suitable sites are located within urbanized zones. In the Damour aquifer, the main role of a MAR operation should be to shift the saltwater/fresh-water interface toward the sea and to recharge the aquifer in order to enhance exploitation yields. Ideally, this could be achieved by inputting a regular and continuous source of fresh water parallel to the sea upstream of the public pumping wells located in the Mechref, Damour and Naame areas. From a technical viewpoint, any recharge operation should be done using either injection wells or infiltration ponds in the epikarst. In this respect, infiltration ponds require large areas of land for

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Fig. 4. Map of the intrinsic (hydrogeological) rechargeability index Irt of Damour aquifer according to maps of the four criteria Epikarst, Infiltration, Karst development and Reservoir Rock considered for determining rechargeability from the ground surface.

adequate implementation. However, the recharge well solution has many limitations, including the difficulty of implementing injection wells in a medium with such highly heterogeneous transmissivity, as reported by Khadra (2003), and the need to implement a 1500 m long line of wells parallel to the coast in order to cope with the low ability to disperse recharge water uniformly from wells in the epikarst and the infiltration zone. The local regulations and the high land prices, especially in the the Mechref sector, must be considered as unfavorable conditions that lower the feasibility factor Fe. Moreover, the water authority would have to pump river water at below 50 m asl in order to inject it into wells at more than 250 m asl. This would be a very costly, energy-consuming operation, while Lebanon has long been undergoing an energy crisis. All of these social and economic conditions mean that trying to implement a MAR operation in the Damour C4c–C5 karst aquifer by using the above techniques would yield a Fe factor significantly lower than 1, probably less than 0.5, i.e. a final rechargeability index lower or equal to 2 for all considered sites with a favorable rechargeability index. A MAR operation from infiltration ponds or trenches or from shallow injection wells would thus be unfeasible. Therefore, instead of the aforementioned techniques and based on the site conditions, an injection sub-horizontal well or gallery in the infiltration zone (Fig. 3) is proposed under the favorable surface sites identified around the Mechref village. This gallery option has the following major advantages: (1) The river water level is at the level of the gallery entrance, i.e. the recharge water would flow in by gravity. (2) The gallery would provide a uniform distribution of water, in the same way as an epikarst that spreads water uniformly in the infiltration zone, just over the saturated zone. (3) The gallery would use the open fractures and fissures in the karst aquifer to enhance recharge. Vertical shafts and voids encountered during the drilling operation would be avoided during the recharge operation. (4) Existing faults could be used or avoided depending on their status of compression or distension, being clogged or active as required by the recharge operation. However, no recharge would be done in the shattered zone especially for major faults.

Two gallery sites were selected, the first one at 30 m asl, close to the main road, the second site at 45 m asl, also on the right bank of the Damour River, 1 km upstream from the first site. The gallery will be oriented in the south–north direction, approximately parallel to the sea shore and perpendicular to the dip and faults. The second site is more suitable than the first one because of its higher elevation and its position upstream from all of the exploited wells. However, the second gallery site will be longer and thus more expensive than the first one. The gallery will be drilled at 1 m diameter or more, in two parts: the first part, about 900 m long (site 1), crossing the faulted zone up to the fault F11b (fig. 3), will be cemented. The second part, between fault F11b and fault F11a, the injection part (1000 m long), will remain uncased. The general slope of the injection part will be less than 1%. Several observation wells reaching the groundwater and preferably the bottom of the aquifer should be drilled on both sides of the gallery, which will be located from the ground surface by geophysics. For the drilling, there will be some limited geotechnical risks in the crushed faulted zones. These risks are well known and should be controlled from the 4 km long by 4 m diameter gallery drilled in the same limestone in Kesrwan region north of Beirut. 6. Conclusion Managed Aquifer Recharge is a very promising technique and although it may be difficult to quantify the risks of a MAR system not meeting the project expectations, progress is being made based on past successes and failures. As the performance of a MAR system depends considerably on the hydrogeologic conditions, sound characterization before construction followed by testing through the deployment of a pilot MAR system to validate the parameters of the study is essential before deployment of a MAR system at full operational scale. The high heterogeneity and vulnerability of karst aquifers, due to the complexity of their structures and functioning systems, has certainly hampered the development of MAR operations until now. The lack of a rational approach to assess the rechargeability of a given karst aquifer and identify the best locations for conducting MAR operations has also certainly played an important role. However, water resources offered by karst aquifers commonly found in carbonate rock, combined with the high increase in water

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demand in both the short-term (including seasonal) and long-term calls for the development of simple but efficient methods to determine the rechargeability of a karst aquifer and to calculate potential recharge sites in a manner that integrates the MAR technology into the overall integrated water resources management of the water supply, as recommended by the United Nations. The ARAK methodology was developed to reconcile both the theoretical and practical problems encountered while analyzing karst aquifers to identify sites for MAR operations from the surface in aquifers with low storage capacity. Although it appears to be a relatively simple methodology, it is based on generally available information such as geological maps, land occupation maps, Digital Terrain Models (DTMs), and existing geophysical and hydrogeological data. Through a simple sequence of steps, it provides a useful tool to help authorities and water managers determine the potential of MAR when relying on karst aquifers. The first case study carried out using the ARAK approach was in the region of Damour, located in southern Lebanon. This coastal aquifer, which is one of the main water supply sources for the capital Beirut, has been undergoing long-term saltwater intrusion, with a marked shortage and deterioration in water quality, thus usually rendering unsuitable as a potable water supply. Although feasibility studies for a dam on Damour River are currently being undertaken by local authorities, a MAR pilot project would validate the feasibility of MAR as an alternative or complementary solution to the dam project. Water authorities consider that an aquifer recharge project could offer solutions to both the shortage in water supply and the saltwater intrusion problem, while also being an alternative or complement to the proposed dam project. Completion of Damour project is essential for testing the basic assumptions and validating the ARAK methodology. Tests must also now be carried out on different types of karst systems, with authigenic and allogenic recharge, with high or low storage capacity, with well or poorly developed conduit systems. Acknowledgements This research was supported by Schlumberger Water Services, which funded the PhD work of Walid Daher, and by the French research agencies IRD and CNRS, which supported the scientific cooperation with St. Joseph University of Beirut. References Aller, J.R., Bennet, T., Feher, J.H., Petty, R.J., Hackett, G., 1987. DRASTIC: a standardized system for evaluating groundwater pollution potential using hydrogeological settings. US Environmental Protection Agency, EPA/600/2-87036, 455 p. Atkinson, T.C., Harmon, R.S., Smart, P.L., Waltham, A.C., 1978. Palaeoclimatic and geomorphic implications of 230Th/234U dates on speleothems from Britain. Nature 272, 24–28. Aulong, S., Bouzit, M., Dörfliger, N., 2008. Cost effectiveness analysis of water management measures in two river basins of Jordan and Lebanon. Water Resources Management. doi:10.1007/s11269-008-9297. Bakalowicz, M., 2004. The epikarst, the skin of karst. In: Jones, W.K., Culver, D.C., Herman, J.S. (Eds.), The Epikarst Conference. Karst Water Institute Special Publication n 9. The Karst Water Institute, Shepherdstown, WVA, pp. 16–22. Bakalowicz, M., 2005. Karst groundwater: a challenge for new resources. Hydrogeology Journal 13 (1), 148–160. Bakalowicz, M., Blavoux, B., Mangin, A., 1974. Apports du traçage isotopique naturel à la connaissance du fonctionnement d’un système karstique – teneurs en oxygène 18 de trois systèmes des Pyrénées, France. Journal of Hydrology 23, 141–158. Bakalowicz, M., El-Hajj, A., El Hakim, M., Al Charideh, A.R., Al-Fares, W., Kattaa, B., Fleury, P., Brunet, P., Dörfliger, N., Seidel, J.L., Najem, W., 2007. Hydrogeological settings of karst submarine springs and aquifers of the Levantine coast (Syria, Lebanon). Towards their sustainable exploitation. In: Pulido Bosch, A., Lopez Geta, J.A., Ramos Gonzalez, G. (Eds.), TIAC’07. Coastal aquifers: challenges and solutions. Hidrogeologia y aguas subterraneas, vol. 23. IGME, Almeria, Spain, pp. 721–732.

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