(ERT) Applied to Karst Carbonate Aquifers: Case Study from Amdoun, Northwestern Tunisia

Pure Appl. Geophys. Ó 2015 Springer Basel DOI 10.1007/s00024-015-1173-z Pure and Applied Geophysics

Electrical Resistivity Tomography (ERT) Applied to Karst Carbonate Aquifers: Case Study from Amdoun, Northwestern Tunisia

1 2 1 1,3 1 BELGACEM REDHAOUNIA, BATOBO OUNTSCHE ILONDO, HAKIM GABTNI, KHOMSI SAMI, and MOURAD BE´ DIR

Abstract—The Amdoun region is characterized by a high Key words: Electrical resistivity tomography (ERT), Karst, degree of karstification due to the climate impact Cavities, Perched aquifer, Amdoun (NW Tunisia). (±1500 mm year-1) and the development of fracture network. Survey using electrical resistivity tomography (ERT) is deployed to provide a cost-effective characterization of the subsurface karst environments. A total of seven ERT profiles with lengths of 315 m were evaluated at the Be´ja governorate (NW Tunisia). The area 1. Introduction represents a small syncline of Boudabbous limestone rocks (Lower Eocene), which is covered by a thin layer of clay. In this study, an ERT survey was conducted to examine the spatial distribution and Karst aquifers are known to be highly heteroge- shape of underground cavities in the karst area in Jebel Sabah neous, formed by a complex conduit system that is anticline and Aı¨n Sallem-Zahret Medien syncline. In this study, generally impossible to locate. Moreover, the geological, hydro-geological and electrical resistivity tomography (ERT) methods were applied to determine the geometry of the resources are very hard to exploit, with permeability perched aquifer in the Amdoun region (NW Tunisia). The area is coefficients ranging from 10-9 to 10-1 ms-1, and characterized by fractured and karstic limestone aquifer of Late flow velocities ranging from a few millimeters a day Cretaceous (Abiod Fm.) and Lower Eocene (Boudabbous Fm.). The aquifers have a karstic functioning and drain aquifers of eco- to hundreds of meters an hour (BAKALOWICZ 2005). nomical interest, despite some wells exploiting them. Seven Carbonate rock is soluble in the presence of acid resistivity profiles were conducted along the survey area at three dissolved in water, most commonly resulting from sites. The orientation, extension and the degree of inclination of CO solution. The process that combines solution of those profiles are shown in the location map. The correct resistivity 2 data were interpreted using Earth Imager 2D software. The results carbonate rock and flow in openings is named of the interpreted geo-electrical sections showed that the resistivity ‘‘karstification’’ (FORD and WILLIAMS 1989). Karst of the carbonate aquifer varied between 2.5 to over 5794 Xm. The aquifers constitute 12 % of the global land surface thickness of the perched aquifer ranged from 15 to 50 m, while its depth from the surface lies between 10 and 60 m. The ERT not and store water on which approximately 20–30 % of only provided precise near surface information, but was also very the Tunisia population depends, as stated in classical useful for establishing the 3D geometry and the position of several textbooks on karst. These carbonate reservoirs (Abiod potential cavities and karts. The results show the presence of small to large isolated cavities at various depths. The low resistivity of and Boudabbous/El Gueria formations) in the study cavities in the Boudabbous Formation has been explained by the area constitute approximately less than 1 % of all groundwater saturation. The ERT technique could be effectively limestone outcrops in Tunisia. Hydro-chemical used for 3D detection of underground limestone cavities. analyses aid in the identification of the mechanisms causing the accumulation of carbonates in groundwater and the water type. The stable isotopes of water have been analyzed in investigations of karst aquifers to understand the recharge processes, reservoir mixing and the inter- 1 Water Researches and Technologies Center Borj-Cedria actions between groundwater and surface water and (CERTE), BP 273, Soliman 8020, Tunisia. E-mail: belgacem.red- [email protected]; [email protected]; [email protected] to identify water sources in karst springs (KATZ et al. 2 Advanced Geosciences Europe-Spain, Madrid, Spain. 1997;VANDENSCHRICK et al. 2002;MALOSZEWSKI et al. E-mail: [email protected] 2002;PERRIN et al. 2003;ANDREO et al. 2004;LONG 3 King Abdulaziz University, Jeddah, Saudi Arabia. E-mail: [email protected] and PUTNAM 2004;SCHWARZ et al. 2009;HAMED et al. B. Redhaounia et al. Pure Appl. Geophys.

2014…). Geophysical methods are very useful for et al. 2010;ARIDHI et al. 2011;REDHAOUNIA et al. studying karst terrains because of the intrinsic 2012, 2013a, b;ANENE et al. 2013). heterogeneity of the medium (SACKS and TIHANSKY From the structural point of view, the Meso– 1996;Sˇ UMANOVAC and WEISSER 2001;VAN SCHOOR Cenozoic formations in northwestern Tunisia are 2002;VOUILLAMOZ et al. 2003;SMITH 2005;JARDANI intensely folded and faulted. The tertiary formations et al. 2006;NEUKUM et al. 2010). However, the are structured in a series of anticlines and synclines in electrical resistivity tomography (ERT) imaging the NE–SW direction. The main structural features provided significant information of the structural are anticlines and synclines, faults and fractures. The setting of the Amdoun region characterized by high structural and geomorphological features have groundwater potential, fractured and chalky lime- affected the limestone sedimentary rocks in the stone bodies due to the elevated density and Amdoun study area and have a great impact on the concentration of open fracture network within the karst processes. The oldest rocks are the E–W Tri- very brittle chalk of the lower Eocene (Boudabous/El assic diapiric outcrop of the Jebel Ezzouza, and Gueria Formations) and Upper Cretaceous (Abiod except for the lithological sequences in this study Formation). These two units are characterized by area it varies in age from the Upper Cretaceous to the open fractured limestones, interconnected drainage Quaternary (GOTTIS and SAINFELD 1956). The major paleochannels, shallow and deep karst and caves part of the Amdoun basin (35 % of the carbonate (REDHAOUNIA et al. 2014b, c;AYADI et al. 2014). units) is underlain by carbonate rocks with moderate The present study is an approach using the ERT to high degree of karstification. The densities of caves imaging technique to identify and characterize the and karstic springs are the hydro-geological features discontinuities, faults and water investigation of the used to check the calculated degree of karstification. fractured and karstified limestone aquifers in the These types of surface karst features are common in Amdoun Mounts. The conducted research demon- the Amdoun study area where relatively pure car- strated that the ERT method was an effective tool for bonate bedrock is present and precipitation levels are imaging the subsurface in the karst terrain. high (REDHAOUNIA et al. 2014c). Karsts developed in the Upper Cretaceous limestones as well as in the tertiary carbonate (Eocene Formations). The karst 2. Geology of the Study Area systems in the Upper Cretaceous limestones are independent of those in the carbonate limestones Tunisia is located at the intersection between the (Abiod Formation). The most extended caves systems African, and European and Mediterranean plates develop in the Upper Cretaceous and constitute the (Fig. 1a). Its structural framework is characterized by regional karst system which can be divided into a transition from the Saharan platform in southern several subsystems. Tunisia to Alpine folded structures (Atlasic domain and Tellian troughs) in the north (BOUAZIZ et al. 2002). The Amdoun region is divided both geologi- 3. Hydrogeological and Petrophysical cally and geographically into two zones roughly of Characteristics equal importance, characterized by highly faulted outcrops of Oligo-Miocene Numidian Flysch series The region where the aquifer is located presents a unconformably over folded and faulted para-au- typically Mediterranean climate, where cool and tochthonous to autochthonous Cretaceous to Eocene rainy seasons alternate with hot dry seasons. The units (Fig. 1b, c). The Nappes Zone (Alpine thrust average temperature is 16.5 °C and the potential belt) and the Imbricated Zone is immediately situated evapotranspiration estimates range between 1000 and -1 and affected by thrusts and reverse faulting (GOTTIS 1800 mm year . The mean precipitation is -1 and SAINFELD 1956;ROUVIER 1985;KHOMSI 1998;EL 1400 mm year and there are marked seasonal EUCHI et al. 1998, 2004;KHOMSI et al. 2009;RIAHI variations. Most rainfall takes place in the autumn. Electrical Resistivity Tomography (ERT) Applied to Karst Carbonate Aquifers

Figure 1 a Geographical location of the study area; b general geological map of the Amdoun study area; c geological map of the study area including ERT survey of the three sites, 1, 2, and 3; d positioning plan of ERT proﬁles in site 3

Amdoun carbonate aquifers are made up of Upper represented more than 200 9 106 m3 year-1 with Cretaceous limestones (Abiod Formation) of three more than 13 % of groundwater resources of Tunisia. members whose thickness ranges between 200 and These formations present as very karstifiable 250 m and presents anticlines structures in the axis geologic units in the Amdoun Mounts, because the direction NE–SW (Jebel Sabbah). The Boudabbous karstic rock masses are pervaded by a network of perched carbonate aquifer, about 4.7 km2, is com- open discontinuities and fractures with calcite filling pletely isolated from other aquifers (Abiod and epikarst forms (Lapiaz, Karren and Kamenitza) Formation) and presents a synclinal structure in the (Figs. 2a, b, 3c, d). These discontinuities are the axis direction NE–SW. The hydro-geological stocks primary flow paths and infiltration of meteoric water of the perched carbonate aquifers are estimated by for groundwater and the development of the karsti- ENNABLI (1980) and ENNABLI and DRAS (1981) and fication process. B. Redhaounia et al. Pure Appl. Geophys.

Figure 2 a Interconnecting fissures and open fractures of limestone (upper Abiod member), b epikarst forms include the solutional hollows (Kamenitza) and open structures (Lapiez); c photo shows a view of fractured and karstified limestone with a large cavern in the left developed in the Jebel Sabah anticline (SW flank); d, e, f photos show the front view of three caverns in the Abiod limestone formation (Upper Cretaceous) of the Jebel Sabbah anticline (SW flank), which developed at the intersection of the open fracture networks

The investigations of cavern networks generally Both the upper and lower members of the Abiod agree that conduits are related to discontinuities and Formation (Campanian–Maastrichtian lower) are major faults within the rock mass of Abiod Formation composed of fractured limestones with a high matrix (Fig. 2c). It is unknown to this point how large these porosity of 8–20 % and a permeability of caverns are (Fig. 2d–f), but these caverns were about 100–120 mD and average thickness of 250 m (EL 1–4 m in diameter and about 5 m long. The system is EUCHI et al. 1998). These limestones have a good drained by six springs (Aı¨n El Goussa, Aı¨n Menzel groundwater potentiality, conﬁrmed by several Gourchi, Aı¨n El Kbira, Aı¨n Sabbah, Aı¨n El Hadj and springs: Aı¨n El Goussa (10 l/s), Aı¨n Menzel Gourchi Aı¨n Sallem) (Fig. 1). (17–25 l/s), Aı¨n El Kbira (5 l/s) and Aı¨n Sabbah According to the results of the ERT approaches, the (4 l/s); petrophysical analyses of carbonates limestones of Fractured limestones of the Ypresian are repre- Abiod and Boudabbous/El Gueria Formations shows a sented by Boudabbous Formations (Globigerina)/El signiﬁcant potential aquifer with a high porosity per- Gueria (nummulites), and these bioclastic limestones meability EL EUCHI et al. (1998) and ETAP (2006). were deposited in a carbonate platform type of ramp, However, the hydro-geological potential of carbonate with a porosity of 5–12 % and improved permeability aquifers is largely the main reservoirs of the Amdoun in the presence of fractures from 100 to 700 mD and region which is divided into two separate aquifers: average thickness of 250 m (EL EUCHI et al. 1998). Electrical Resistivity Tomography (ERT) Applied to Karst Carbonate Aquifers

Figure 3 a Panoramic view of the mountain shows the abandoned quarry in the Zaouiet Medien-Aı¨n Sallem syncline (site 2); b thick layers of fractured limestone (Boudabbous Formation). c The Karren epikarst forms can vary between 0.5 and 1.5 m in length, developed in the Boudabbous Formation; d photos show the alignment of Kamenitza structures with major fractures

These limestones are of great hydro-geological infiltration of the groundwater (Figs. 2a, c, 3b). In potential due to the high number of sources that addition, the minor and major faults affecting the spring to the surface through faults and fractures as Abiod and Boudabbous/El Gueria influence karsti- the source of Aı¨n El Hadj and Aı¨n Sallem (15–20 l/s). fication development in the study area. During the Groundwater flow is predominantly through frac- rainy period, the karst system evacuates part of its tures, which are often solutionally enlarged to form waters by some major sources (Aı¨n El Goussa, Aı¨n fissures or small karst conduits. Sabbah, Aı¨n Sallem…), but also as multiple local Karst developed in the Upper Cretaceous car- seeps into contact marly levels. The low thickness bonate as well as in the Lower Eocene carbonate of the limestone nummulitic did not allow the formation. The karst systems in the Upper Cretaceous evolution of a deep network and lapiaz which carbonate at anticlines are local and independent developed from the joints or larger fractures from those developed in the Lower Eocene limestone (Fig. 3). synclines. Upper Cretaceous and early Eocene Limestones outcrops are extremely cracked and present a similar 4. Geo-Electrical Resistivity Imaging geomorphological index in the e´pikarst zone; while networks fracture of joints and faults explain surface Different geophysical prospecting techniques are hydrology and especially to determine the under- used to detect underground karst and cavities ground hydrology. Karstification develops including seismic methods (RECHTIEN and STEWART intensively, mainly in the first 30 m of the upper 1975;BENDERITTER 1997;GRANDJEAN 2006), part of the Late Cretaceous limestones and Lower gravimetry (CHICO 1964;BERES et al. 2001;JACOB Eocene. Groundwater flow is predominantly through et al. 2010), GPR (ROBERT and DE BOSSET 1994; fractures, which are often solutionally enlarged to BENSON 1995;MCMECHAN et al. 1998) and ERT (LOKE form fissures or small karst conduits. These various 2000;LOKE et al. 2003;MARTIŃEZ-PAGA´ N et al. 2013; lineaments have great interest in the recharge and GE´ LIS et al. 2010). B. Redhaounia et al. Pure Appl. Geophys.

Figure 4 a Photo shows the protocol ERT survey of EP2 in site 3; b details of the geophysical instrumentation that was used in the ERT survey; c, d photos show a top view of these sinkholes in the limestones karst terrain of Boudabbous Formation (Ypresian)

The geophysical survey was complemented with a profile after inversion and topographic correction field investigation to gather information on the using the EarthImager 2D software. Outliers were underground cavities of the prospected area. The removed and topographic corrections were carried investigation is based on collecting information from out with measurements of elevation and geographic the inhabitants who had a relatively accurate knowl- coordinates for each electrode position through a edge of known cavities. The ERT 2-D data acquired in Trimble GPS. After completing the field survey, the February 2014 (EP1 and EP 2) and December 2015 resistance measurements were regularly converted (EP3 to EP 7) by the ‘‘Centre of Water Research and to apparent resistivity values. The data were devel- Technologies (CERTE)’’, using the SAS-4000 ABEM oped to produce two-dimensional resistivity earth Terrameter with the 32 or 64 electrodes configuration model of the subsurface. An inversion program to characterize the shape and depth of the groundwater, converts the array of apparent resistivity data into a provides a resistivity distribution section of a subsur- model of the geology that would yield the observed face portion of the earth from potential measurements distribution of apparent resistivity values. The data made on the surface (Fig. 4b). The data acquisition was inversion software used in this study was AGI performed using a Wenner electrode array, which EarthImager 2D with a maximally smooth least provides good sensitivity for the detection of vertical squares algorithm (CONSTABLE et al. 1987;LOKE and heterogeneities and a good signal-to-noise ratio than BARKER 1996). The modeling was done by an other arrays (DAHLIN and ZHOU 2004;LOKE 2011). This inversion process based on an iterative method that facilitates studying sub-horizontal structures consid- attempts to minimize the difference between the ered to be probable in the study area. measured pseudo-section and a pseudo-section Measuring the resistivity with ERT 2-D enables recalculated from a model of electrical resistivity us to interpret the data in pseudo-section resistivity theory. This modeling process is modified with each Electrical Resistivity Tomography (ERT) Applied to Karst Carbonate Aquifers

Figure 5 Results of electrical imaging by Wenner array along the proﬁle EP1 in the Jebel Sabbah NW ﬂank (site 1)

iteration until the calculated and measured data of carbonates from Abiod and Boudabbous/El Gueria reach an acceptable agreement or until no further limestones at different depths along each profile line, improvement is possible. The deviation between the starting from a low value of 2.5 Xm to a higher value theoretical model and the true resistivity model is of 5794 Xm. expressed as root mean square ‘‘RMS.’’ While this is The electrical profile EP1 on the NE–SW direc- a good indication of the quality of a cross section, it tion was located in the west flank of Jebel Sabbah is not always the iteration which has the lowest RMS anticline (site 1). In the inverted resistivity section error profile that gives the most geologically correct (final model), an RMS of 2.73 % was reached in the one. In general, the most reliable model is located fourth iteration. The reliability of ERT detection of just after the iteration where the RMS error does not potential caves was verified by geological exploration significantly change (\0.5 % improvement), which of four newly discovered caves at the Jebel Sabbah usually happens between four and six iterations anticline before the ERT campaign. The EP1 line is (NOUIOUA et al. 2015). run near large caverns about 15 m south of the We used for the continuous dataset, inversions to entrance (Fig. 2c, d). Wenner array electrode con- earth model resistivity values. Between two and four figuration was used to provide a good horizontal and iterations were run for the December 2013 vertical resolution for a clear image of the cave. The (EP1 ? EP2) and April 2015 (EP3 to EP7) surveys, resistivity model obtained is seen at a distance respectively, with best-fit model RMS errors between between -95 and -52 m and at 10–20 m of depth. 1.24 and 5.29 %. The RMS error statistics enumer- The presence of an important zone of high-resistivity ated the distribution of the percentage differentiation values with a concentric resistivity anomaly body between the logarithms of the calculated resistivity (1000–5794 Xm) has been interpreted as the presence values and those calculated from the true resistivity of an ovular cave (Fig. 5). The low resistivity may be model. This allows us to judge the quality of inver- due to resistivity contrasts between the ambient car- sion images achieved from field data inversion. bonates rocks of the Abiod Formation outside of the The distribution of resistivity in the subsurface cave. The unsaturated zone of the upper part of the soil of the study shows a wide variation of resistivity Abiod Formation shown in the EP1 contains fissures B. Redhaounia et al. Pure Appl. Geophys.

Figure 6 Results of electrical imaging by Wenner array along the proﬁle EP2 (site 2) in the abandoned quarry. Measured apparent resistivity pseudo- section (a), calculated apparent resistivity pseudo-section (b) and inverted resistivity section of the proﬁle

and solution channels that provide conduits for the Fig. 1, Structurally, the area is an elongated syncline vertical and horizontal flow direction of surface water that extends in the NE–SW direction and for about into the underlying aquifer (saturated zone) 4 km (Fig. 3a). The apparent resistivity pseudosec- (50–100 Xm). The difference in resistance between tion uses a Wenner array configuration of 32 the cavity and the surrounding limestone of Upper electrodes, and the length of the profile is 155 m with Cretaceous may be the most outstanding physical an electrode spacing of 5 m. In the inverted resistivity feature of a cave. However, the section shows a rel- section, an RMS of 2.98 % was reached in the second atively higher resistivity layer (marly limestones) iteration (Fig. 6). The interpretation of the EP2 overlaying a very low-resistivity layer of strongly deduced from the joint inversion of both arrays, using karstic limestones (14.8–30 Xm). the two-inversion techniques, shows the presence of The ERT measurements reveal an increase of high three cavities with heterogeneous and complex aspect resistivity (5794 Xm) of the topmost layer from the and two major vertical faults affecting the sub-hori- external to the center of EP1. This can also lead to the zontal limestone of the Boudabbous Formation. We formation of a larger cave, as observed in the EP1 at observe also a very resistant block (*280 Xm) with depths of about 10 m. Resistivity varying from 1000 a length of approximately 15 m, limited by resistant to 5794 Xm of the karstic void is indicative of the horizontal structure of cavity on the east and west of karstified limestone bedrock (Abiod Fm.). The cave the inverted resistivity section. The resistivity con- has a diameter of several meters and is partly filled trast in the inverse image between the cavities and with sediments and/or void from the upper strati- surrounding rock environment is very important graphic unit of the Abiod limestone formation (about *200 to *280 Xm) (Fig. 6). (Fig. 5). Profiles 3–7 were compiled in site 3 located in the Contrary to the NE–SW profile EP1, the electrical central portion of the ‘‘Zaouiet Madien-Aı¨n Sallem’’ profile EP2 in the NE–SW direction located in the syncline (Figs. 1d, 4a). These inverted resistivity abandoned quarry of ‘‘Zaouiet Madien-Aı¨n Sallem models of profiles were obtained using a Wenner syncline’’ (site 2) is shown in the location map, in configuration of 64 electrodes, and the length of each Electrical Resistivity Tomography (ERT) Applied to Karst Carbonate Aquifers

Figure 7 3D view for inverted ERT sections along the different proﬁles 3, 4, 5, 6 and 7 in site 3 in the carbonate limestone of Boudabbous Formation with high-resistivity zone. 5 m electrode spacing, 64 electrodes, EarthImager 2D software inversion in two and three iterations

profile was 315 m with electrode spacing of 5 m cube (Fig. 7); this low-resistivity (4–16 Xm) is (Fig. 7). interpreted as due to the weathered zone of clays and marls (Souar Formation). • Under this zone, an anomaly of moderate 4.1. The Interpretation of Results resistivity (4–80 Xm) is present; this epikarst Figure 7 shows the interpretation of five profiles zone shows a concentration of large fractures with 3D inverted resistivity image. A 3D picture of the and karstic features clearly visible on the fracture system was drawn showing a geo-syncline outcrop. structure of carboante limestone bedrock (Boudabbous • Under the epikarst zone, a higher resistivity Fm.). Geo-electrical data were interpreted with anomaly (80–1670 Xm) was associated with the Zond2Dinv software, leading to geometric construc- endokarst zone of the Boudabbous Formation. tion of Boudabbous fractured aquifer and to 3D The aim was to test the efficiency of ERT in modeling of subsurface architecture. The geo-electical detecting secondary faults and sinkholes (S1 and S2) cube illustrated in Fig. 7 was obtained by the intersec- observed in situ in the Boudabbous Formation and tion of five 2D ERT sections, allowing detecting, illustrated in the picture of the landscape (Fig. 4a, b). delineating important fractures and also characterizing The use of ERT and the additional information in this the hydro-geology of the underground karst. The site coupled with the 2D and 3D visualization and interpretation of 3D ERT images is based on the analysis softwares has led to a far better understand- distribution and contrasts in electrical resistivity. ing of the geometric construction and the subsurface • A small zone of lower resistivity in the EP7 is architecture of Abiod and Boudabbous fractured imaged on top of the southeastern corner of the aquifers. B. Redhaounia et al. Pure Appl. Geophys.

5. Results and Discussion Groundwater input by springs can promote carbonate dissolution (carbonate-rich rocks) along the Aı¨n El Goussa Aı¨n Menzel Gourchi and Aı¨n Sab- fringes of alluvial fans and directly into surface water bah Sallem springs discharge is the output of a karst (dams: Kasseb and/or drainage network: Majerda). groundwater system and its changes should correspond The decrease in bicarbonate concentration causes the with those of the input to the karst groundwater system water to be undersaturated with respect to dolomite (precipitation), although there is often a lag in its and promotes dissolution of dolomite (HAMED et al. response to precipitation. As a result of the effect of 2014). Ca–Mg–HCO3 water is the predominant type human activity and other uncertain factors, a great deal in the carbonate-rock aquifers in this study area, and of noise is overlapped in the response and the internal the lithology of rapid weathering and erosion rate relations between spring discharge and precipitation suggests that dissolution of carbonate minerals will are concealed or even distorted (HAMED et al. 2014; add significant amounts of Ca and Mg to the reser- HAMED and DHAHRI 2013). voir. This is generally produced by dissolution of the The geochemical analyses of the groundwater from carbonate minerals (calcite and dolomite) ‘‘karstifi- these two reservoirs of Abiod limestone formations cation processes’’. The reaction of these minerals (Campanian–Maastrichtian) and Boudabbous (Ypre- with water and carbon dioxide can be written as sian) show that they belong to calcium bicarbonate follows: facies, indicating a good quality of water (REDHAOUNIA 2013;REDHAOUNIA et al. 2014a). CaCO3ðcalciteÞþH2O þ CO2 2þ À The natural effect is not the only factor that , Ca þ 2HCO3 ; ð1Þ threatens the karstic aquifers; thus, the anthropogenic effect has a significant impact: the three deep wells CaMgðCO3Þ2ðdolomiteÞþ 2H2O þ 2CO2 2þ 2þ À drilled in Zahret Medien-Aı¨n Sallem syncline [Sidi , Ca þ Mg þ 4HCO3 ; ð2Þ Saadoun (109 m), Aı¨n Sallem deep wells 1 (105 m) Ca2Mg5Si8O22ðOHÞ þ 14CO2 þ 22H2O and 2 (50 m)] and Aı¨n Sallem spring are excessively 2 , 2Ca2þ þ 5Mg2þ þ 14HCOÀ þ 8SiðOHÞ : exploited, especially by the domestic sector (drinking 3 4 3 water) (DGRE 1992, 1997, 1998; O.T 2008). The ð Þ overexploitation of this aquifer could change the Other types of rocks (e.g. calcium sulfate depos- water quality along with the climate variability which its) may additionally contribute to hydrochemical is observed in the TDS difference between the winter properties. However, the composition of groundwa- (TDS = 0.3 g/l) and summer (TDS = 0.6 g/l). ters and the formation of other precipitates such as Groundwater salinity in the study area varies both in calcite/dolomite, gypsum or Mg clays depends not the vertical and lateral directions. Higher values of only on the hydrology, but also on the tectonic, cli- these parameters characterize wells located in the mate and/or the presence of source rocks containing central and the southern parts of the basin Generally, evaporites (Triassic sediments) from Tunisian and TDS increases from the mountainous regions (the Algerian territories. piedmont zone of the study area, which characterizes Groundwaters from Cretaceous aquifers (Abiod the recharge areas) toward the discharge area and Boudabbous Formations) and karst waters in the (southern part), as a result of the scarcity of recharge NW basin of Tunisia and around the springs [Aı¨n in these regions due to the relatively long-term Sabbah, Aı¨n Sallem, Aı¨n Menzel Gourchi (Fig. 8c, water–rock interaction. The pCO2 in the groundwater d)…] have been abstracted for water supply and for -3 -3 ranging between 1.2 9 10 and 0.6 9 10 atm the agricultural sector at the Amdoun region. The -3.5 compared to the precipitation pCO2 of 10 atm impact of karst water use on the spring discharge suggested that the water gained CO2 from root res- should be the most intensive, but the extent of use is piration and the decay of soil organic matter. quite small. The so-called restored spring discharge Subsequently, an increase in pCO2 caused a drop in was calculated by adding the observed spring dis- pH (Fig. 8a). charge with the volume of exploited karst water. Electrical Resistivity Tomography (ERT) Applied to Karst Carbonate Aquifers

Figure 8 a Conceptual hydrogeological model of Aı¨n El Goussa (S1) and Aı¨n Menzel Gourchi springs (S2); b fractured and karstiﬁed limestone of Abiod Formation in the Jebel Sabbah Anticline (NW Flank); c, d PHOTOS show the panoramic view of the Aı¨n Menzel Gourchi spring (S2)

Stable isotopes of water are ideal environmental at higher elevations, which is known as the altitude tracers; they are constituents of water molecules, which effect (HAMED and DHAHRI 2013). However, the present allow them to be used as tracers of water molecules recharge is high due to the humid conditions within the through various segments of the hydrological cycle and catchment and depends mostly on infrequent storm are excellent tracers for determining the origin of events. The carbonate aquifers of the study area are groundwater and widely used for studying the ground- recharged by the recent meteoric water by vertical water recharge, migration pathways and mixing of infiltration from the fractures and/or the permeable waters from different sources (FAURE 1986;SACKS and faults. TIHANSKY 1996;SUBYANI 2004). Moreover, the isotopic The isotopic compositions of the groundwater composition of monthly recent meteoric waters is in the samples from the NW aquifer range from -6.37 to range -8.45 to -4.12 % d18Oand-55.4 to -22.6 % -0.82 % for d18O and from -40.8 to -7.7 % for 18 2 dD(Fig.8a). The average values of d OanddDinthe d H(HAMED and DHAHRI 2013). These samples are precipitation are -6.28 and -39 %,respectively(JOU- taken from different depths and altitudes of the study ZEL et al. 1991;CLARK and FRITZ 1997). This variation is area. This groundwater is interpreted as contempo- controlled by local climatic parameters, including the raneous recharge at the high-altitude surrounding origin of the vapor mass, re-evaporation during rainfall mountains (Atlas Monts) and a mixing groundwater and the seasonality and monthly rate of precipitation resulting from the dominant upward leakage from the (HAMED and DHAHRI 2013). The isotopic composition of deep artesian water table. precipitation is strongly influenced by temperature, Seven 2D geo-electrical measurements were car- which is reflected as a seasonal effect (as a result of the ried out in three sites aiming to characterize the near lower condensation temperature in winter than in sum- surface problems of the fractured and karst lime- mer) and the depletion of heavier isotopes (2Hand18O) stones aquifers (Abiod and Boudabbous Formations) B. Redhaounia et al. Pure Appl. Geophys. in the Amdoun study area. The results of tomography 6. Conclusions ERT profiles, the most representative ones, are described below to present the characteristics of the This study presents the results of a multi-method cavities existing in the three sites. The cross sections approach of characterizing of the carbonates lime- of the true resistivity distribution are displayed. The stone aquifers in northwestern Tunisia. The high-resistivity zones interpreted as karstic galleries, groundwater was affected by the climate impact and which represent the level that is the fractured lime- human activities (mostly in the form of groundwater stones of the upper member of the Abiod Formation, overabstraction). A dual-porosity model that includes were noted in the indices of the epikarst (fractures, a fissure-porous aquifer (baseflow) and karstic con- alteration, kamenitza, caverns…) to the outcrop. The duits (rapid infiltration) best described the analyzed presence of faults can be deduced from the abrupt systems of Amdoun carbonate aquifers. This study change between the lower and upper range limits of revealed that the recharge was dominated by winter resistivity. precipitation; the dual-porosity system dominated by The ERT imaging provided significant informa- the baseflow of the individual springs has different tion about the sub-surface structural setting in the degrees of karstification. Joint interpretation of geo- Amdoun region. This area is characterized by high physical (ERT), hydro-geological, structural and groundwater potential fractured and chalky lime- synthetic simulation data allowed identifying the stones bodies, showing elevated density and carbonate aquifer of the Amdoun region. concentration of open fracture network within the The application of geophysical method of ERT very brittle chalk of the Lower Eocene (Boudabous/ measurement reveals three distinct three sites in the El Gueria Fms.) and Upper Cretaceous (Abiod Fm.) subsurface below strong fracturation zones. ERT (Fig. 8a, b). The results proved that ERT was capable survey supports the theory of significant shifts in the of detecting karst, sinkholes or caves of variable resistivity parameters of the carbonate limestones dimensions. Several other Wenner profiles with 5 m with height resistivity in the Jebel Sabbah spacing were measured in the area of study and the ([5000 Xm—site 1) and along the syncline of presence of other sinkholes and cavities developed in ‘‘Zaouiet Medien-Aı¨n Sallem’’ with low resistivity the Abiod and Boudabbous Formations was detected (\350 Xm—sites 1 and 2). The 3D model of these (Fig. 4c, d). Finally, the interpretation process favors cavities and sinkholes in site 3 resulted from the smooth contouring over the abrupt resistivity con- application of geo-electrical methods in the fractured trasts that cave passages show in seven electrical carbonate limestone of the Boudabbous Formation. profiles (Figs. 5, 6 and 7). Because of this fact, the Geo-electrical measurements with ERT method resistivity values that the model applies to the cave were carried out in the studied area with seven geo- passages depend on the surrounding rocks resistivity. electrical profiles to obtain a better characterization The karst system in the Amdoun region is a of the geological anomalies associated with perched unique and complex system. The genesis and the carbonate aquifers. This work also introduces a new original structure of karst are known through various tool for the identification of karstic, sinkholes, frac- works (MANGIN 1975;BAKALOWICZ 1979;MARSAUD tures and voids, since cave detection may be the first 1997;AL-FARES et al. 2002). The karstic aquifer itself application of this ERT approach in the study area. has three main areas: the epikarst, the infiltration zone Moreover, this approach gives an easier interpretation (vadose zone) and flooded area (saturated zone) of its size and position of the subsurface anomaly (MANGIN 1975;BAKALOWICZ 1979). In the nearest part caves, sinkhole, faults, etc. of the surface, water infiltration can be temporarily In future works, it will be necessary to contem- retained by forming a small temporary tank perch, plate other approaches, such as for example, ‘‘the epikarst’’. This part is characterized by a high integration of ERT imaging and ground penetrating degree of fracturing and alteration, giving it a sec- radar (GPR) to investigate and characterize the ondary porosity in the limestone of the Upper hydro-geological perched groundwater bodies with Cretaceous and Lower Eocene. more accuracy. Electrical Resistivity Tomography (ERT) Applied to Karst Carbonate Aquifers

Acknowledgments BERES, M., LUETSCHER, M., OLIVIER, R., (2001), Integration of ground-penetrating radar and microgravimetric methods to map shallow caves. Journal of Applied Geophysics 46, 249–262. This research was supported by Water Researches BOUAZIZ, S., BARRIER, E., SOUSSI, M., TURKI, M.M., ZOUARI, H., and Technologies Center Borj-Cedria (CERTE), (2002), Tectonic evolution of the northern African margin in research program MESRS/CERTE 2010-2013: Tunisia from paleo-stress data and sedimentary record. Tectonophysics 357, 227–253. « Caracte´risation et valorisation locale des micro- CHICO, R.J., (1964), Detection of caves by gravimetry. International aquife`res de montagnes », research project : « Iden- Journal of Speleology, 1: 101–108. tification et caracte´risation des aquife`res perche´set CLARK, I.D., FRITZ, P., (1997), Environmental Isotopes in Hydro- des re´servoirs fracture´s associe´s de la Tunisie Nord- geology. Lewis Publishers, New York, p. 328. CONSTABLE, S., PARKER, R.L., and CONSTABLE, C.G., (1987), Oc- occidentale ». We would like to express our gratitude cam’s inversion: A practical algorithm for generating smooth to the AGI EarthImager 2D software Office in models from electromagnetic sounding data. Geophysics 52, no. Madrid-Spain and the Water, Environmental and 3: 289–300. DAHLIN, T., and ZHOU, B., (2004), A numerical comparison of 2D Energy Laboratory at the University of Sfax for their resistivity imaging with 10 electrode arrays. Geophys Prospect collaboration and valuable comments. 52, 379–398. DGRE (DIRECTION GE´ NE´ RALE DES RESSOURCES EN EAU)., (1992), compte rendu de fin de travaux, forage d’Aı¨n Sallem IRH 8948. DGRE (DIRECTION GE´ NE´ RALE DES RESSOURCES EN EAU)., (1997), REFERENCES compte rendu de fin de travaux, forage d’Aı¨n Sabbah IRH 8802. DGRE (DIRECTION GE´ NE´ RALE DES RESSOURCES EN EAU), (1998), AL-FARES, W., BAKALOWICZ, M., GUE´ RIN, R., DUKHAN, M., (2002), compte rendu de fin de travaux, forage Sidi Saadoun IRH 9014. Analysis of the karst aquifer structure of the Lamalou area EL EUCHI, H., FOURATI L., GHENIMA, R., HAMOUDA, F., SAIDI, M., (He´rault, France) with ground penetrating radar, Journal of (1998), Structural style and hydrocarbon habitat in Northern Applied Geophysics 51 97–106. Tunisia. Field trip guide book. ETAP. Mem. N°13. The 6th ANDREO, B., LIN˜ A´ N, C., CARRASCO, F., JIMENEZ DE CISNEROS, C., Tunisian Petroleum Exploration and Production Conference. CABALLERO, E., MUDRY, J., (2004), Influence of rainfall quantity EL EUCHI, H., SAIDI, M., FOURATI, L., EL MAHERSI, C., (2004), on the isotopic composition (18O and 2H) of water in moun- Northern Tunisia thrust belt. Deformation models and hydro- tainous areas. Application for groundwater research in the carbon systems. In. Swennen R., Roure F. and Granath, J. W. Yunquera-Nieves karst aquifers (S Spain). Appl. Geochem. 19, (Ed.), Deformation, fluid flow, and reservoir appraisal in fore- 561–574. land fold and thrust belts. American Association of Petroleum ANENE, I., GABTNI, H., REDHAOUNIA, B., KHOMSI, S., BEDIR, M., Geologists Hedberg Series (1), 371–390. (2013), Contribution de l’imagerie e´lectrique haute re´solution a` ENNABLI, M., (1980), Etude Hydrogeólogique des aquife`res du Nord la prospection des micro-aquife`res Eoce`ne karstiques fracture´s du Nord-Est de la Tunisie. Pour une gestion inte´greé des res- du Nord-ouest de la Tunisie. cas du site de Aı¨n Sallem (Zahret sources en eau. The`se doctorat e`s Sciences Nice. Median), The 5th Tunisian Days off Applied Geology-JTGA and ENNABLI, M., DRAS, R., (1981), Hydrogeólogie karstique dans le International Conference on Natural Hazards and Geomatics- N-E de la Tunisie, actes du premier congre`s national des sci- ICNHG, Hammamet- Tunisia, 17–20 May. ences de la terre (Tunis, septembre), 349–358. ´ ´ ` ARIDHI,K,ARIDHI, S., OULD BAGGA, M A., ABDELJAOUAD, S., ZAR- ETAP (ENTREPRISE TUNISIENNE D’ACTIVITES PETROLIERES)., (2006), GOUNI, F., MERCIER, E., (2011), Paleogeographical restoration and Petroleum geology of Tunisia. A renewed synthesis, ETAP ramp tectonic evidence in Tunisian Tellian domain. Aı¨n El Bey- me´moire n°22. 233 p. Bou Awen area. FAURE, G., (1986), Principles of Isotope Geology. Wiley, New AYADI, Y., REDHAOUNIA, B., HAMED, Y., MANAAI, K., (2014), York. Response of carbonate aquifer to climate change in Northern FORD, D., WILLIAMS, P., (1989), Karst Geomorphology and Tunisia. A case study at the Bled Abida-El Kef karst springs. Hydrology. Unwin Hyman, Winchester, Massachusetts, p. 320. Colloque International Hydrogeólogie et Geóthermie 11–12 GE´ LIS, C., REVIL, A., CUSHING, M. E., JOUGNOT, D., LEMEILLE, F., Novembre, Guelma-Alge´rie. CABRERA, J., DE HOYOS, A., and ROCHER, M., (2010), Potential of BAKALOWICZ, M. (1979), Contribution de la geóchimie des eaux a` la Electrical Resistivity Tomography to Detect Fault Zones in connaissance de l’aquife`re karstique et de la karstification. The`se Limestone and Argillaceous Formations in the Experimental Docf. Sci., Univ. P. et M. Curie, Paris- VI, Geól. dyn. et Lab. Platform of Tournemire, France. Pure Appl. Geophys. 167, souterrain CNRS. 269 p. 1405–1418. doi:10.1007/s00024-010-0097-x. BAKALOWICZ, M., (2005), Karst groundwater. a challenge for new GOTTIS, CH., SAINFELD, P., (1956), Notice explicative de la Carte resources, Hydrogeology journal. 13 148–160. geólogique 1/50.000 de Zaouiet Madien au 1/50.000 feuille n° BENDERITTER, Y., (1997), Karst et investigations geóphysiques 14, Office National des Mines (ONM) Tunis. 31 p. (Karst and geophysical investigations). Hydrogeólogie 3:19–30. GRANDJEAN, G., (2006), Imaging sub-surface objects by seismic BENSON, A.K., (1995), Applications of ground penetrating radar in P-wave tomography: numerical and experimental validations. assessing some geological hazards. examples of groundwater Near Surf Geophys 4: 279–287. contamination, faults, cavities. Journal of Applied Geophysics HAMED, Y., DHAHRI, F., (2013), Hydro-geochemical and isotopic 33, 177–193. composition of groundwater, with emphasis on sources of B. Redhaounia et al. Pure Appl. Geophys.

salinity, in the aquifer system in Northwestern Tunisia. Journal of system in the Ellenburger dolomite, central Texas. Journal of African Earth Sciences, 83, 10–24. Applied Geophysics 39, 1–10. HAMED, Y., AHMADI, R., DEMDOUM, A., BOURI, S., GARGOURI, I., BEN NEUKUM, C., GRU¨ TZNER, C., AZZAM, R., REICHERTER, K., (2010), DHIA, H., AL-GAMAL, S., LAOUAR, R., CHOURA, A., (2014), Use of Mapping buried karst features with capacitive-coupled resistivity geochemical, isotopic, and age tracer data to develop models of system (CCR) and ground penetrating radar (GPR). In. Andreo, groundwater flow: A case study of Gafsa mining basin-Southern B., et al. (Ed.), Advances in Research in Karst Media. Environ. Tunisia. Journal of African Earth Sciences 100 (2014) 418–436. Earth Sci., 3, pp. 429–434. JARDANI, A., REVIL, A., DUPONT, J.P., (2006), Self-potential NOUIOUA, I., FEHDI, CH., BOUBAYA, D., SERHANE, B., DJELLALI, A., tomography applied to the determination of cavities. Geophysi- (2015), Mapping underground cracks using 2D electrical resis- cal Research Letters 33, L13401. tivity tomography: the case of the landslide of Kef Essenoun JOUZEL, J., KOSTER, R.D., SUOZZO, R.J., RUSSELL, G.L., WHITE, phosphate deposit, Djebel Onk (northeast of Algeria). Arab J J.W.C., BROECKER, W.S., (1991), Simulations of the HDO and Geosci doi:10.1007/s12517-014-1769-0. 18 H2 O atmospheric cycles using the NASNGISS general circula- OFFICE DE THERMALISME (O.T)., (2008), Principales manifestations tion model. sensitivity experiments for present-day conditions. thermo-mine´rales en Tunisie, 220 p. Journal of Geophysical Research 96, 7495–7507. PERRIN, J., JEANNIN, P.Y., ZWAHLEN, F., (2003), Epikarst storage in a KATZ, B.G., COPLEN, T.B., BULLEN, T.D., DAVIS, J.H., (1997), Use of karst aquifer. a conceptual model based on isotopic data, chemical and isotopic tracers and geochemical modeling to Milandre test site, Switzerland. J. Hydrol. 279, 106–124. characterize the interactions between ground water and surface RECHTIEN, R.D., STEWART, D.M., (1975), A seismic investigation water in mantled karst. Ground Water 35 (6), 1014–1028. over a near-surface cavern. Geoexploration, 13(1–4): 235–245. KHOMSI, S., (1998), De´formations ante´-mioce`ne moyen et struc- REDHAOUNIA, B., REZGUI, A., GABTNI, H., BEDIR, M., (2012), Iden- turation tardive dans la re´gion de Zahret Medien (Tunisie tification et caracte´risation geólogique, geóphysique et septentrionale).Me´moire de D.E.A, Universite´ Tunis El Manar, hydrogeólogique des micro-aquife`res de montagnes et des Tunis, 75p. re´servoirs fracture´sdelare´gion de Amdoun (Tunisie Nord). KHOMSI, S., SOUSSI, M., MAHERSI, C., BE´ DIR, M., FAKHFAKH-BEN Forum de l’eau 3, Jerba 26-27-28 Mars. JEMIA, H., RIAHI, S., BOU KHALFA, K., (2009), New insights on the REDHAOUNIA, B., (2013), Geophysical study and hydrogeological structural style of the subsurface of the Tell units in north- characterization Amdoun region mountain aquifer (NO de la western Tunisia issued from seismic imaging. Geodynamic Tunisie). Master’s thesis, Tunis El Manar University, Tunis, implications. C. R. Geoscience. 107 p. LOKE M.H., (2000), Electric imaging surveys for environmental REDHAOUNIA, B., GABTNI, H., ANENE I., KHOMSI, S., BE´ DIR, M., and engineering studies. aparactical guide to 2D and 3D survey, (2013a), Hydrogeophysical characterization for groundwater 63 p. resources potential of fractured limestone reservoirs in Amdoun LOKE, M.H., (2011), Electrical resistivity surveys and data inter- mountains (North-western Tunisia). The 5th Tunisian Days of pretation. In: Gupta, H. (Ed.), Solid Earth Geophysics Applied Geology-JTGA and international conference on Natural Encyclopedia, 2nd edition. Electrical & Electromagnetic. Hazards and Geomatics-ICNHG, Hammamet- Tunisia, 17–20 Springer-Verlag, pp. 276–283. May, 4 p. LOKE, M.H., and BARKER, R.D., (1996), Practical techniques for 3D REDHAOUNIA, B., GABTNI, H., BEDIR M., MOKADDEM, N., (2013b), resistivity surveys and data inversion. Geophysical Prospecting Identification et caracte´risation geólogique, geóphysique et 44, no. 3: 499–523. hydrogeólogique des aquife`res karstiques de la re´gion de e`me LOKE, M.H., ACWORTH, I., DAHLIN, T., (2003), A comparison of Amdoun (NW tunisien). 3 colloque international « les geós- smooth and blocky inversion method in 2D electrical tomography ciences au service du de´veloppement durable » , Universite´ de surveys. Exploration Geophysics 34, 182–187. Te´bessa, 18–19 Novembre, 4 p. LONG, A.J., PUTNAM, L.D., (2004), Linear model describing three REDHAOUNIA B., MOKADDEM N., GABTNI H., BE´ DIR M., HAMED, Y., components of flow in karst aquifers using 18O data. J. Hydrol. (2014a), Contribution of gravity to the study of fractured car- 296, 254–270. bonate reservoirs in the karst region of Be´ja (NW Tunisia). MALOSZEWSKI, P., STICHLER, W., ZUBER, A., RANK, D., (2002), Congre`s. Watmed7. 7th International Conference 10–12 October, Identifying the flow systems in a karstic-fissured-porous aquifer, Morocco, 4 p. 18 the Schneealpe, Austria, by modelling of environmental O and REDHAOUNIA, B., GABTNI, H., ANANE, I., BEDIR, M., MOKADDEM, N., 3H isotopes. J. Hydrol. 256, 48–59. (2014b), Apport de la geóphysique a` la caracte´risation geólogi- MANGIN, A., (1975), Contribution a` l’e´tude hydrodynamique des que et hydrogeólogique des aquife`res karstiques de la re´gion de aquife`res karstiques. Annales de spe´leólogie, vol. 29,no4, Amdoun/Be´ja (NW Tunisien). Congre`s. 1er Colloque Interna- p 495–601. tional Sol, Eau et Environnement (CISEE’2014) 10 et 11 MARSAUD, B., (1997), Structure et fonctionnement de la zone noyeé Novembre, Annaba-Alge´rie, 4 p. des karsts a` partir des re´sultants expe´rimentaux.Me´moire de REDHAOUNIA B., MOKADDEM N., ANANE I., GABTNI H., BE´ DIR M., the`se, Orsay-Paris XI, 301 p. HAMED, Y., (2014c), Hydro-geophysical characterization for MARTIŃEZ-PAGA´ N, P., GO´ MEZ-ORTIZ, D., MARTIŃ-CRESPO, T., MAN- groundwater limestones in Amdoun area (North-Western Tuni- er TECA, J.I., ROSIQUE, M., (2013), The electrical resistivity sia) Congre`s. 1 appel a` communication, Colloque International tomography method in the detection of shallow mining cavities. A Hydrogeólogie et Geóthermie 11–12 Novembre, Guelma- case study on the Victoria Cave, Cartagena (SE Spain). Engi- Alge´rie, 4 p. neering Geology, 156 (0): 1–10. RIAHI, S., SOUSSI, M., BOUKHALFA, K., BEN ISMAIL LATTRACHE, K., MCMECHAN, G.A., LOUCKS, R.G., ZENG, X., MESCHER, P., (1998), DORRIK, S., KHOMSI, S., BE´ DIR M., (2010), Stratigraphy, sedi- Ground penetrating radar imaging of a collapsed paleocave mentology and structure of the Numidian Flysch thrust belt in Electrical Resistivity Tomography (ERT) Applied to Karst Carbonate Aquifers

northern Tunisia. Journal of African Earth Sciences, 57, South Dakota, September 12–15, 2005. U.S. Geological Survey 109–126. Scientific Investigations Report 2005–5160. 296 p. ROBERT, A., DE BOSSET, C., (1994), Application du geóradar a`la SUBYANI, A.M., (2004), Use of chloride mass balance and envi- location de cavite´s, de nids de gravier et de zones karstiques. ronmental isotopes for evaluation of groundwater recharge in Journal of Applied Geophysics 31, 197–204. the alluvial aquifer, Qadi Tharad, western Saudi Arabia. Envi- ROUVIER, H., (1985), Geology of Northern Tunisia. Tectoniques et ronmental Geology 46 (6), 741–749. paleógeógraphies superpose´s a` l’extre´mite´ de la chaıˆne nord Sˇ UMANOVAC, F., WEISSER, M., (2001), Evaluation of resistivity and maghre´bine, Univ. P. and M. Curie, 703 p. seismic methods for hydrogeological mapping in karst terrains. SACKS, L.A., TIHANSKY, A.B., (1996), Geochemical and Isotopic Journal of Applied Geophysics 47, 13–28. Composition of Ground Water, with Emphasis on Sources of VAN SCHOOR, M., (2002), Detection of sinkholes using 2D electrical Sulfate, in the Upper Floridan Aquifer and Intermediate Aquifer resistivity imaging. Journal of Applied Geophysics 50, 393–399. System in Southwest Florida. US Geol. Surv. Water-Resour. VANDENSCHRICK, G., VAN WESEMAEL, B., FROT, E., PULIDO-BOSCH, Invest. Rep. 96–4146. A., MOLINA, L., STIEVENARD, M., SOUCHEZ, R., (2002), Using 18 SCHWARZ, K., BARTH, J.A.C., POSTIGO-REBOLLO, C., GRATHWOHL, P., stable isotope analysis (dD–d O) to characterize the regional (2009), Mixing and transport of water in a karst catchment. a hydrology of the Sierra de Gador, south east Spain. J. Hydrol. case study from precipitation via seepage to the spring. Hydrol. 265, 43–55. Earth Syst. Sci. 13, 285–292. VOUILLAMOZ, J.M., LEGCHENKO, A., ALBOUY, Y., BAKALOWICZ, M., SMITH, D.V., (2005), The state of the art of geophysics and karst. a BALTASSAT, J.M., AL-FARES, W., (2003), Localization of saturated general literature review. In. Kuniansky, E.L. (Ed.), U.S. Geo- karst aquifer with magnetic resonance sounding and resistivity logical Survey Karst Interest Group proceedings, Rapid City, imagery. Ground Water 41 (5), 578–586.

(Received May 8, 2015, revised August 24, 2015, accepted September 2, 2015)