Palabras clave Acuífero costero, intrusión de agua de mar, sobreexplotación, pozo de observación, control, gestión.

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1 OVEREXPLOITATION CONSEQUENCES AND MANAGEMENT CRITERIA IN COASTAL KARSTIC AQUIFERS Lugi Tulipano Full Professor of Applied Hydrogeology. Department of Hydraulics, Transportation and Roads University of Rome «La Sapienza» ABSTRACT The correct management of coastal aquifers needs an accurate knowledge of factors regulating the equilibrium between fresh groundwater and saltwater present in the aquifer due to seawater intru - sion. Strong saline contamination phenomena can affect groundwater under over-exploitation, due to either mixing with saltwater coming from the deepest part of the aquifers (upconing) or dire c t l y f rom the coast (lateral intrusion). Lateral intrusion of seawater is particularly evident in karst are - as where conduits, which in undisturbed conditions feed coastal springs, can turn to easy way for seawater intrusion when fresh water potential reduces with respect to sea level due to overe x p l o i - tation. A powerful tool for groundwater management in coastal aquifers is the monitoring thro u g h o b s e rvation-wells of the fre s h w a t e r-saltwater equilibrium: monitoring allows the acquisition of basic data useful for calculating the parameters able to quantify the exploitation effects. Key Words Coastal aquifer; seawater intrusion; overexploitation; observation-well; monitoring; management. RESUMEN Una correcta gestión de los acuíferos costeros necesita de un conocimiento preciso de los factore s que regulan el equilibrio entre el agua dulce y el agua salada procedente de la intrusión. Los fenó - menos de fuerte contaminación salina pueden afectar al agua subterránea sometida a sobre e x p l o - tación, debido bien a la mezcla con agua salada procedente de las partes más profundas de los a c u í f e ros (ascensos o domos salinos) o directamente de la costa (intrusión lateral). La intru s i ó n lateral del agua de mar es particularmente evidente en los acuíferos costeros, donde los conductos que en condiciones no influenciadas alimentan a manantiales costeros, pueden transformarse fácil - mente como consecuencia de la intrusión de agua de mar cuando el potencial del agua dulce dis - minuye respecto al nivel del mar debido a la sobreexplotación. Una poderosa herramienta para la gestión del agua subterránea en acuíferos costeros es el control del equilibrio entre el agua dulce y el agua salada a través de pozos de observación, control que permitirá la adquisición de datos básicos útiles para calcular los parámetros que puedan cuantificar los efectos de la explotación. Palabras clave Acuífero costero, intrusión de agua de mar, sobreexplotación, pozo de observación, control, gestión. 113

2 CAPTACIÓN, GESTIÓN, CONTROL Y SEGUIMIENTO INTRODUCTION Karstic aquifers normally represent in the world high quality water resources; in coastal regions often karst groundwater represents the unique available resource. Consequently, in most cases the exploitation of these resources is very heavy. Groundwater salinisation induced by overexploitation represents a serious problem particularly affecting Mediterranean coastal karstic aquifers. Mediterranean European countries have, in fact, peculiar climatic (low precipitation rate and high evapo-transpiration), demographic (increasing immigration and internal migration) and economical (agricultural practices and tourism) characteristics which cause an increasing exploitation of groundwater resources and high production of pollutants; these factors determine in the same time water-scarcity conditions, high waterdemand and, also, high risk of pollution. Coastal karstic aquifers exhibit more serious problems for their exploitation than the porous ones. The main difference is linked to the peculiar characteristic of their functioning. Groundwater flows, in fact, mainly through karstic channels. The mouths of these karstic channels act as coastal springs; depressing the hydraulic head, karstic channels become easy ways for penetration of seawater inland. This normally results in a salinisation process, which advances very fast and interests, in a short time and irregularly, zones very far from the coast. Due to this peculiar hydrogeological environment, it is difficult to forecast in karstic coastal aquifers the progress of seawater intrusion and its consequences on groundwater quality under over-exploitation conditions. The definition of rules for a correct management of karstic groundwater resources requires deep knowledge of the aquifer behaviour, which can be obtained only through the acquisition of field data whose elaboration can allow the calculation of specific parameters able to describe the hydrogeological status of the aquifer. GEOLOGICALAND HYDROGEOLOGICAL CHARACTERISTICS OF A COASTAL KARSTIC AQUIFER The definition "Coastal Karstic Aquifer" implies that: the aquifer is constituted by carbonate rocks, generally very permeable due to fracturing and karst phenomena; the aquifer is bordered by the sea, which level represents the base level of groundwater flow; groundwater floats on intruding seawater. Carbonate rocks are generally sedimentary limestones, dolomitic limestones and dolomites, but also can be metamorphic marbles. The total permeability is ensured by a primary porosity (due to syngenetic porous and stratigraphic discontinuities) and by a secondary porosity, due to fractures and karst processes. In the peculiar case of coastal carbonate aquifers, karst phenomena, besides those due to the well-known interaction phenomena between carbonate rocks and fresh water are the result of the action of brackish and saltwater as well. Due to the changes in geological time of the position of sea level and, consequently, the change of elevation of transition zone, karst processes (dissolution, precipitation and dolomitisation) took place at each sea level stand, creating mainly horizontal karst levels separated by no karstified rocks, which can be permeable only for fissuring. This way, a coastal carbonate aquifer is generally constituted by a series of preferential sub-horizontal pathways where most part of the groundwater flow takes place (Tadolini and Tulipano, 1977a). Normally the hydraulic head of groundwater decreases towards the coast, where it reaches more or less the zero value. Discharge of groundwater occurs through coastal subaerial or submarine springs. Depending on the relative prevalence of fracturing or karst features along the coast, springs can be diffuse or concentrated in type, these last representing the mouth of karstic channels (Tulipano, 2002). 114

3 OVEREXPLOITATION CONSEQUENCES AND MANAGEMENT CRITERIA IN COASTAL KARSTIC AQUIFERS Fresh water floats on saltwater of marine origin due to the difference in density. Mainly due to dispersion, a transition zone exists between fresh and saltwater: within the transition zone water salinity rapidly increases from that of fresh waters towards that of saltwater. In natural conditions the thickness of the transition zone depends on the amplitude of seawater level fluctuations (periodical, due to tides, and a-periodical, due to atmospheric pressure variations) and on the yearly amount of infiltration. In the zones very close to the coast the thickness of the transition zone is of the same order of magnitude of the sea level fluctuations, while inland can reach tens of meters (Tadolini and Tulipano, 1970). EQUILIBRIUM BETWEEN FRESH AND SALT WATER In a coastal aquifer under natural conditions, the basic Ghyben - Herzberg law can still describe the equilibrium state. This law can be considered valid if the weighted average density of both fresh water and brackish water of the transition zone is taken into account in the calculation of the depth of the top of underground salt water (Cotecchia et al., 1986). Obviously, the law describes limit conditions: if the equality of the pressure of salt and fresh waters is respected, in theory no flow of groundwater toward the sea is possible. Thus, in normal undisturbed conditions, the pressure of fresh water has to be higher than the pressure of salt water: therefore, groundwater flow toward the sea is possible. If it would be possible to measure the elevation of the top of underground salt water, a theoric piezometric surface could be drawn by considering in each measurement point the ratio between the elevation of the top of underground salt water (bottom of the transition zone) and the δ f /(δ s δ f ) value, being δ f and δ s respectively the density of fresh and salt waters. This theoric piezometric surface should be always lower than the real piezometric surface in natural conditions. CONCEPT OF OVEREXPLOITATION It is well known that overexploitation means that the abstraction of groundwater from the aquifer is greater than a certain limit; if exploitation overcomes this limit, some consequences are observable. While in continental aquifers the consequences are the lowering of the piezometric surface and the progressive reduction of spring s discharge, in coastal aquifers a regime of over exploitation leads to an increase of salinity of the fresh water body. In continental aquifers it is enough easy to establish the limit of exploitation as the abstraction of a volume per year equal to the total amount of the annual recharge. Clearly this does not consider the environmental damages and other effects on water quality. Form the hydrogeological point of view, the final consequence is that the water table will reach the base level: consequently, no flow, no springs (Custodio, 2002). Whit reference to a coastal aquifer, the concept of over exploitation is, up today, only linked to the extent of the effects that this regime involves, namely, the progressive salinisation of groundwater caused by mixing with salt water drawn by lateral intrusion or upconing phenomena. The evaluation of the limit of withdrawals, over which a coastal aquifer has to be considered overexploited, is very difficult. In any case, in the practice, this limit is linked to the method of the exploitation, to the total amount and temporal distribution of the recharge, and to the local hydrodynamic characteristics of the aquifer. The fundamental difference between a continental aquifer and a coastal one concerns just the effect of the recharge. In a continental aquifer recharge determines the uplift of the piezometric surface; the extent of the uplift, in theory, not considering the continuous discharge at the springs, is equal to the height of infiltrating rain divided by the mean porosity of the rock matrix. In a coastal aquifer, at the same time as the recharge proceeds, the entire fresh water column overlain the salt water expands and meanwhile sinks: the 115

4 CAPTACIÓN, GESTIÓN, CONTROL Y SEGUIMIENTO depth reached by the bottom of the fresh water column differs from the original position by a thickness in theory equal to the fresh water height infiltrated into the aquifer, multiplied by the value of the fresh water salt water densities ratio. In other words the effect of recharge is negligible as far as the variation of piezometric surface is concerned, while it is considerable with respect to the variation of the elevation and thickness of the transition zone (figure 1). Figure 1 Expansion and displacement of fresh water body due to recharge. The definition of the limit for the exploitation of a coastal aquifer and in particular of a karst coastal aquifer seems to be an open problem. If the total volume of the annual recharge was yearly pumped, what could be the final consequence? In theory the aquifer would reach static equilibrium conditions (real piezometric surface coinciding with the Ghyben Herzberg one), but, in practice, a complete substitution of fresh water by salt water into the aquifer has to be expected. Therefore, only a percentage of recharging volume can be exploited: how much is it? Only the accurate knowledge of the functioning of an aquifer and its reaction to the exploitation can give an answer to this q u e s t i o n. MEASURING THE EFFECT OF OVEREXPLOITATION The qualitative signal offered by an overexploited aquifer is the remarkable increase of saline content of the water pumped from the capture works; the TDS mapping allows evidencing the zones where the drainage of seawater (due to lateral intrusion or upconing phenomena) is in progress. In order to plan recovery actions, it might be necessary to quantify the disruption of freshwater saltwater equilibrium induced by overexploitation. Of course, the best tool for this purpose is the direct survey in a network of observation wells penetrating into the underground salt-water body. The thermo-resistivity logging in these wells represents the most helpful method to directly examine and follow in time the behaviour of the fresh, brackish (transition zone) and salt-water bodies. Resistivity (or conductivity) and temperature profiles provide numerous important data as thickness of fresh groundwater, position and thickness of the transition zone, depth of salt-water top, pattern of salinity stratification. All these data should be carefully verified, because they are crucial for the calculation of some parameters suitable for the description of the evolution of the equilibrium between fresh and salt water. Salinity profiles are obtainable transforming the resistivity and thermal profiles through a calibration. It is well known that the water resistivity depends on water chemical composition. Thus, a correct calibration cannot be made with reference to artificial saline solutions of a unique salt. Conversely, it is correct to perform the calibration on a series of water samples whose chemical composition is similar to that peculiar to the waters of the explored aquifer. The analysis of the salinity distribution along the water column of an observation well allows the calculation of some parameters suitable for describing the status of the equilibrium between fresh and salt waters in the aquifer (Tulipano and Fidelibus, 2002). The most significant of them will be examined in the following paragraphs, with reference to the terms and definitions in figures 2 and

5 OVEREXPLOITATION CONSEQUENCES AND MANAGEMENT CRITERIA IN COASTAL KARSTIC AQUIFERS Figure 2 - Scheme of the the parameters obtainable from the salinity profiles derived form thermo-caonductivity logs carried out in observation-wells reaching salt water beneath groundwater in a coastal aquifer. Thickness of the transition zone (z t ) The thickness of the transition zone can be established recognising the elevation of its top (h t t ) and bottom (h b t ). The first is indicated by a sudden change of the salinity gradient at the passage from fresh water to transition zone, the second by the starting of salinity gradient equal to zero. Starting from a thickness determined by natural factors (water table fluctuations caused by periodic and non-periodic sea level oscillations, atmospheric pressure variations and recharge), the transition zone furtherly expands under over-exploitation. In fact, the decrease of the groundwater hydraulic head causes the transport of salts upwards. This determines the increase of the weight of the Z ft column, which in turn involves the downward displacement of the top of salt water. The final consequence is, in most cases, the expansion upward and downward of the transition zone. Theoric sharp interface (hsi) The whole water column overlying the top of salt water, of thickness Z ft and average weighted density δ ft, can be split up in two columns: a fresh water column (having a density δ f corresponding to the lowest salinity, S f, met at the top of the column of the well, normally ranging in 117

6 CAPTACIÓN, GESTIÓN, CONTROL Y SEGUIMIENTO coastal karst aquifers from 0.3 and 0.5 g/l) and a salt water column (of thickness Z x, having the density δ s of the underground salt water found in the well, normally g/l). To obtain the elevation (h si ) of the theoric sharp interface, the thickness Z x has to be added to the measured elevation of the top of the salt water (h bt ). The temporal variation of hsi reflects the upward transport of salts. The real Ghyben - Herzberg K coefficient The general Ghyben - Herzberg equation states that: h = [δ f / (δ s - δ f ] * t Where: h = elevation of the sharp interface t = fresh water hydraulic head and δ f / (δ s - δ f ) is the K coefficient. In presence of a transition zone, a real value of K (K r ) can be calculated using δ f t (the average density of the whole water column overlying the top of underground salt water) instead of δ f, and δ s (the real density of the salt water met by the well). K r values allow quantifying, in another way, the disequilibrium between fresh and salt water. The δ f t value increases thanks to the increase of the thickness of the transition zone and the consequent salinisation of the fresh groundwater: so, overexploitation is indicated by the increase of the K r v a l u e. The disequilibrium index t The Ghyben - Herzberg equation, using K r and the h bt values, gives a theoric hydraulic head t t. This parameter represents the hydraulic head that groundwater should have to maintain the salt water at the measured depth (h bt ). The difference t between theoric value t t and the real value t m, measured in the observation well can be positive or negative and can be considered a disequilibrium index. Equivalent salt water potential (t s ) The whole water column of thickness Z b a n d density δ b, which overlies the reference point of elevation hrd, can be transformed in an equivalent column of salt water, Z e q, of density δ s. The elevation of t s is obtained adding the Z e q thickness to the h r d e l e- vation. The equivalent salt water potential represents Figure 3 - Schematic vertical section of a coastal aquifer with indication of the parameters used for the calculation of the disequilibrium index. 118

7 OVEREXPLOITATION CONSEQUENCES AND MANAGEMENT CRITERIA IN COASTAL KARSTIC AQUIFERS the real groundwater hydraulic head (positive or negative) with respect to the adopted sea level. T h i s parameter reflects the equilibrium condition as well and, obviously, is strictly related to the t value. Thickness of fresh water column (z f ) Another parameter is the thickness of the fresh water column overlying the top of the transition zone (z f ). The temporal variation of z f d e p e n d s on the upward or downward displacement of the top of the transition zone and can be used as indicator of overexploitation of the aquifer. Fresh water component height (H si = h si + t m ) The water column above the theoric sharp interface represents the height of the fresh water component of the whole Z f t column; the variation of the theoric sharp interface in time allows the calculation of the variations of the real fresh water reserve. MONITORING Monitoring is the first technical step for a correct management of a coastal aquifer. The final t a rget of monitoring is to recognise the beginning of salinisation due to overexploitation and to obtain the needed data for planning the recovery actions. In the same time the first results of monitoring will help in the reconstruction of the conceptual model of the aquifer. It is evident that a preliminary action of the monitoring consists in the evaluation of the real amount of exploitation (areal distribution of groundwater intakes, characteristics and operation of intake works, etc.) in relation with the annual amount of the recharge. Arough picture of the state of equilibrium between fresh and salt water can be obtained from the areal distribution of salinity values measured in the pumped waters. New geophysical methods are also useful, in karst environment, to obtain preliminary information on the depth of underground salt waters. Groundwater temperature logs in drilled wells, through which vertical and horizontal sections can be drawn, are good tools in order to recognise, at basin scale, the distribution of fresh, brackish and salt waters (figure 4) (Tulipano and Fidelibus, 1988). Remote sensing techniques are essential to locate and classify the point of groundwater discharge into the sea ( Tulipano, 2002). The main target of monitoring is the acquisition of the previously mentioned parameters, which allow quantifying the state of equilibrium between fresh and salt waters. The identification of zones at high salinisation risk (high groundwa- Figure 4 Horizontal (left) and vertical OO (right) sections showing the groundwater temperature trend for Salento aquifer. The trace of OO section is in the horizontal map. 119

8 CAPTACIÓN, GESTIÓN, CONTROL Y SEGUIMIENTO ter demand, low thickness of fresh groundwater, salinisation processes recognised in progress, etc.) must be the main element that determines the location of the monitoring stations of the network. The technical characteristics of these stations must allow the direct measurement of the previously mentioned data. Therefore, a good monitoring network should be constituted by a number of observation wells, penetrating into the underground salt water and completely screened in the saturated zone. A piezometer positioned inside the well into salt water and filled with this water, allows the recording of the equivalent salt water potential (figure 5). Figure 5 - Design of an observation-well. Periodical thermo resistivity logs have to be performed in order to check the variation in time of the parameters. MANAGEMENT CRITERIA In a general sense, aquifer management stands for the combination of political, legal, administrative and technical actions that have to be carried out in order to ensure a correct use of territory and groundwater resources. It means that the final target of the management is to protect groundwater resources from pollution or impoverishment phenomena. Here, speaking about coastal karstic aquifers, only the aspects of management strictly linked to the prevention of salinisation processes will be considered. The management of a coastal karstic aquifer understands that the peculiar features of this kind of aquifer have to be taken carefully into account: in particular, the high velocity of sea water intrusion under overexploitation and also, as a positive aspect, the presence along the coast of big springs that can be included in the hydraulic scheme of a region as non conventional water resources have to be considered. The ideal starting point of a correct management should be the planning of the human activities requiring water in relationship with the available water resources. Consequently, the important preliminary step should be the definition of these resources, which include treated wastewaters and brackish spring waters as well. The attribution of the resources to the different demands also should be planned. Of paramount importance is the right selection of capture-works and their design. Among the different types of capture works, those that prevent automatically strong depression of piezometric surface (trenches, galleries, tunnels) are recommended. When drilled wells are used for the exploitation, the adoption of a constant and continuous pumping is preferable. The following flow chart summarise the criteria that should be adopted by the Decision Makers who have the final responsibility of the management. 120

9 OVEREXPLOITATION CONSEQUENCES AND MANAGEMENT CRITERIA IN COASTAL KARSTIC AQUIFERS THE SALENTO AQUIFER CASE-STUDY The Salento Peninsula (Apulia - Southern Italy) represents a good example of karstic coastal aquifer subject to over-exploitation. There, heavy problems of salinisation of groundwater arose already in the fifties. In the following decades salinisation got on relentlessly, due to the increasing water demand, mainly for irrigation purposes. In the sixties, the Italian National Council of Researches financed a research program for the study of seawater intrusion in the Salento aquifer. As main tool, a few "observation-wells" were drilled. All drillings penetrated no less than 20 m into underground salt water; after the recognition of the absence of vertical currents, the forages were totally cased with screens. These wells were surveyed for almost fifteen years and last regular measurements date back to the middle of eighties. Only two other surveys were carried out in 1995 and The evaluation of collected data led to the characterisation of the seawater intrusion in the aquifer, to the outline of the best investigation methods (Cotecchia, 1977, Cotecchia et al., 1974, Cotecchia et al., 1981, Tadolini and Tulipano, 1970) and to the proposal of a few parameters for the description of the equilibrium conditions (Tadolini and Tulipano, 1977, 1979b, Cotecchia et al., 1986). In order to understand the dynamic of seawater intrusion, the data obtained from the direct investigation in the observation wells were framed into the regional hydrogeological scheme. The attempt to reconstruct the conceptual and hydrogeological models of the regional aquifer was pursued using different methodologies, among which worth mentioning are those based on the hydrogeochemical and isotopic characteristics of fresh and salt ground waters and temperature data (Tulipano et al., 1990, Fidelibus & Tulipano, 1996). Especially the spatial reconstruction of groundwater temperature demonstrated its value in distinguishing between lateral intrusion and upconing phenomena (Tulipano and Fidelibus, 1988). Aquifer characterisation Limestone and dolomitic limestone of Cretaceous make up the geological basement of the Salento Peninsula (figure 6). The carbonate for- 121

10 CAPTACIÓN, GESTIÓN, CONTROL Y SEGUIMIENTO mation outcrops in large areas of the peninsula, while in the remaining parts it is covered by different formations whose age ranges between Miocene and Pleistocene. At sea level the Cretaceous formation is extensively present and constitutes the main (coastal karstic) aquifer of the region. Figure 7 - Water table elevation (m a.s.l.) for the Salento Aquifer. Figure 6 Geological map of Salento Peninsula. 1) Calcarenites, sands and clays (Plio-Quaternary), 2) Calcarenites, marly calcarenites (Miocene), 3) Limestones, dolomitic limestones (Cretaceous). The hydraulic gradient generally is of the order of / 00. The aquifer discharges into the sea through numerous subaerial or submarine coastal springs, generally scattered on larg e fronts. The salinity of spring waters varies between 3.5 and 20 g/l. Figure 8 shows the TDS content distribution for groundwater. The setting of karstified levels is directly connected with the sea level changes in the geological times: karstification normally took place along horizontal levels corresponding to those levels occupied by the transition zone. A f i s s u r- ing system of tectonic origin connects the horizontal karstified levels, ensuring a high permeability degree almost homogeneously distributed (at large scale). Autumn-winter precipitation recharges the aquifer, which receives also a notable groundwater flow coming from the adjacent Murgia aquifer. The local recharge is 28 m 3 /s, while the contribution from the Murg i a aquifer is no less than 10 m 3 /s (Tulipano & Fidelibus, 1995). The piezometric surface reaches a maximum of 4 m a.s.l. in the central part of the peninsula, where the maximum thickness of the fresh water lens is about 120 m (figure 7). Figure 8 - T.D.S. (g/l) contour lines for Salento Aquifer. Trend in the time of the calculated parameters Since the methodological aim, the following discussion concerns the interpretation of the salinity logs related to only two observation wells, identified as LR and SR (location in figure 9). 122

11 OVEREXPLOITATION CONSEQUENCES AND MANAGEMENT CRITERIA IN COASTAL KARSTIC AQUIFERS the period compared with the evolution of the environmental head (t m ), the variation of the fresh water thickness ( H s i, calculated as d i fference of each Hsi from the first datum) and the equivalent groundwater head (t s ). It is worthy to note, in 24 years, the loss of about 15 m of fresh water. This lost thickness of fresh water is evidently replaced by an equal thickness of saltwater: in the real case this means that groundwater salinity is increased. This increased salinity justifies the increase of the equivalent groundwater head. Figure 9 Location of Salento aquifer and observation wells. Observation-well LR Figure 10 shows that the rise of the transition zone and of the theoric sharp interface occurred between the first (1973) and the last survey (1996). For the entire considered period the environmental head (t m ) is higher than the theoric head (t t ): consequently, Dt maintains negative values, while the equivalent groundwater head (t s ) is always positive. A p p a r e n t l y, this seems a good situation (equilibrium condition). However, looking at the thickness of the transition zone, having a mean value of about 65 m, it is possible to hypothesise that this equilibrium has been reached owing to a serious salinisation of groundwater. The natural thickness of the transition zone in undisturbed conditions for this zone, in fact, should be no more than 30 m because of the variation of tm less than 0.5 m. The above hypothesis is supported by the trends of the other parameters shown in figure 11. The figure shows the trend of total precipitation related to the recharge period ( O c t o- ber March) in the nearest rain gauge station for Observation-well SR For the observation-well SR (figure 10), the rise of the theoric sharp interface is accompanied by an important expansion of the transition zone, especially due to the deepening of its bottom. The t m values are always lower than the t t ones; consequently the t is always positive. The equivalent groundwater head maintains negative values: this represents a condition of depression of groundwater with respect to the sea. Also in the case of well SR, the decrease of the fresh water thickness is evident (figure 11). The whole trend of the parameters indicates that the aquifer surrounding the observation well is under strong overexploitation; salinisation is in progress. The recovery of a positive t s value will be obtained only with the increase of the weight (salinisation) of g r o u n d w a t e r. Effects of negative t s value on coastal discharge Thanks to the co-operation with the Italian Navy, in September 1999 an aerial infrared survey was carried out along a stretch of the Ionian coast of Salento Peninsula. The IRimages obtained in the 9 12 µm band (far infrared) were compared with IR-images taken in September Figure 12 clearly shows the strong decrease of groundwater discharg e into the sea. Field recognition confirmed the disappearance of many coastal springs. It means that, in the 123

12 CAPTACIÓN, GESTIÓN, CONTROL Y SEGUIMIENTO Figure 10 - Evolution in the time, respectively for observation well LR and SR, of (a) elevation of the top of transition zone, elevation of the theoric sharp interface, elevation of the bottom of transition zone, (b) measured hydraulic head (environmental head), theoric hydraulic head, (c) equivalent groundwater head and disequilibrium index. concerned coastal zone interested by the loss of equivalent groundwater head (negative values of t s ), the discharge of groundwater is replaced by seawater intruding inland through the same karstic channels that before were feeding the springs. CONCLUSIONS On the base of the experience carried out on the Salento Peninsula aquifer, some general conclusions related to the possible salinisation mechanism can be outlined. According to the scheme of figure 13, initial not-disturbed conditions of the aquifer (Step 1) should be marked by negative values of t and positive values of t s : transition zone has the thickness due to natural water table fluctuations and groundwater can flow regularly toward the coast. When over-exploitation intervenes, initial conditions of positive t values and negative t s values occur. Salinisation of groundwater takes place owing to the upward migration of underground saltwater (salt transport); in the meantime the groundwater flow toward the coast is prevented in that part of the aquifer (Step 2, example of SR). When the weight of the fresh water column reaches a value able to balance the saltwater pressure, the system reaches new equilibrium condi- 124

13 OVEREXPLOITATION CONSEQUENCES AND MANAGEMENT CRITERIA IN COASTAL KARSTIC AQUIFERS Figure 12 - Far-infrared images (density slicing) of Torre di Castiglione coast (Ionian Sea side Salento Peninsula) respectively of September 1972 and September Figure 11 - Variation in the time, respectively for observation wells LR and SR, of total precipitation of the recharge period (October-March), environmental head, variation of the thickness of freshwater calculated from the first datum, equivalent groundwater head tions: t becomes negative and t s positive and groundwater starts again flowing towards the coast (Step 3, example of LR). The last two steps should happen by turns under continuous overexploitation conditions. O b v i o u s l y, maintaining conditions of overexploitation, groundwater will be completely substituted by seawater: aquifer is died! Fig Sketch of the salinisation mechanism in a coastal aquifer. 125

14 CAPTACIÓN, GESTIÓN, CONTROL Y SEGUIMIENTO BIBLIOGRAPHIC REFERENCES Cotecchia, V., Studies and investigation on apulian groundwaters and intruding seawaters (Salento Peninsula). Quaderni IRSA- CNR. No. 20. Roma. pp Cotecchia, V., Fidelibus, M. D.,Tulipano, L Phenomenologies connected with the variation of equilibria between fresh and saltwater in the coastal karst carbonate aquifer of the Salento Peninsula (Southern Italy). Proc. 9th Salt water Intrusion Meeting, Delft (The Netherlands), 1986, Boekelman R.H.et al.(eds). Reprinted as Selected Paper in W. De Breuck (Ed), Hydrogeology of Salt water Intrusion, A Selection of SWIM Papers, A Report of The Commission of Hydrogeology of Salt water Intrusion, IAH, International Contributions to Hydrogeology. Hannover: Heise, 1991, Vol. 11: Cotecchia, V., Tavolini, T., Tulipano, L The results of researches carried out on diffusion zone between fresh and sea water intruding the land mass of Salento Peninsula (Southern Italy). Int. Symp. on Hydrology of Volcanic Rocks, Lanzarote, Spain. Cotecchia, V., Tavolini, T., Tulipano, L Saline contamination phenomena in the karstic and fissured carbonatic aquifer of Salentine Peninsula (Southern Italy) and their evolution. 7th SWIM. Uppsala (Sweden). Custodio, E Aquifer overexploitation: what does it means? Hydrogeology Journal, Vol. 10, No. 2. pp Fidelibus, M. D., Tulipano, L Regional flow of intruding sea water in the carbonate aquifers of Apulia (Southern Italy). 14th SWIM, Malmo, Sweden, 1996, in Rapporter och meddelanden, n. 87, Geological Survey of Sweden, Uppsala. Tavolini, T., Tulipano, L Primi risultati delle ricerche sulla zona di diffusione della falda profonda della Penisola Salentina (Puglia). I C o n v. Int. sulle Acque Sotterranee, Palermo. Tadolini, T., Tulipano, L. 1977a. ldentificatíon by means of discharge tests of water-bearing layers in fractured and karstic aquifers through the analysis of the chemico-physical properties of pumped waters. Symposium on Hydrodinamic diffusion and dispersion in porous media, Pavia. Tavolini, T., Tulipano, L. 1977b. The conditions of the dynamic equilibrium of ground water as related to encroaching sea water. Symp. on Hydrodynamic diffusion and dispersion in porous media, Pavia, pp Tavolini, T., Tulipano, L The evolution of fresh water-salt water equilibrium in connection with drafts from the coastal carbonate and karst aquifer of the Salentine Peninsula (Southern Italy). 6th Salt water Intrusion Meeting, Hannover (Germany). Tulipano, L Modalità di deflusso a mare delle acque sotterranee degli acquiferi carbonatici costieri della Puglia. Atti del III Conv. di Speleologia Pugliese. Castellana Grotte. Tulipano, L., Cotecchia, V., Fidelibus, M. D An example of multitracing approach in the studies of karstic and coastal aquifers. Int. Symp. and Field Seminar on Hydrogeologic Processes in Karst Terranes. A n t a l y a. Tu r k e y. I.A.H.S. Publ. No pp Tulipano, L., Fidelibus, M. D Temperature of groundwaters in coastal aquifers: some aspects concerning saltwater intrusion. Proc. 10th Salt water Intrusion Meeting. Gent (Belgium). W. De Breuck (Ed.), Natuurwetenschappelijk Tijdschrift. Vol. 70. pp Tulipano, L., Fidelibus, M. D Metodologie per la valutazione degli effetti del rilascio di reflui urbani sulla distribuzione dei nitrati nelle acque sotterranee delle unità della Murgia e del Salento (Italia Meridionale). 2nd C o n v. Int. di Geoidrologia "La cooperazione nella ricerca con i paesi in via di sviluppo e quelli dell Est Europa" Firenze. A.Aureli (Ed.). Quad. di Tecniche di Protezione Ambientale. Vol. 49. pp Tulipano, L., Fidelibus, M. D Mechanisms of groundwater salinisation in a coastal karstic aquifer subject to over-exploitation. Proc. 17th SWIM, Delft (The Netherlands), in press. 126