COBISS: 1.01

Hydrogeological characterization of the Salinas-Los Hoyos evaporitic karst (Malaga province, S Spain) using topographic, hydrodynamic, hydrochemical and isotopic methods
Hidrogeološka opredelitev evaporitnega krasa na obmoèju Salinas-Los Hoyos (provinca Malaga na jugu Španije) z uporabo topografskih, hidrodinamiènih, hidrokemiènih in izotopskih metod
José Manuel Gil-Márquez1,*, Matías Mudarra1, Bartolomé Andreo1, Luis Linares2, 
Francisco Carrasco1 & José Benavente3

Abstract	UDC  551.444(460.356)
José Manuel Gil-Márquez, Matías Mudarra, Bartolomé An­dreo, Luis Linares, Francisco Carrasco & José Benavente: Hydrogeological characterization of the Salinas-Los Hoyos evaporitic karst (Malaga province, S Spain) using topo­graphic, hydrodynamic, hydrochemical and isotopic meth­ods
The Salinas-Los Hoyos karst system is a geological diapiric structure formed by materials of diverse nature (clays, sand­stones, evaporites, volcanic rocks, dolostones, etc.) placed be­tween Malaga and Granada provinces (S Spain). The abundance of evaporite rocks (gypsum, anhydrite and halite) and their high solubility contribute to the development of exokarstic fea­tures (dolines, uvalas, sinkholes). Grande and Chica lakes are dolines located in the western border of the diapir that are in­tersected by the piezometric level. Close to the first wetland is the Aguileras spring, which is the main discharge point of the west sector of the system. To assess the wetland-spring relation and the general functioning of the system, the geomorphologic framework has been analyzed and hydrogeological controls have been performed, consisting in limnimetric and discharge logging and in situ measurements of physico-chemical param­eters (EC and water temperature). Furthermore, spring, wet­land and rain water samples have been taken for subsequent chemical and isotopic analysis. Preliminary results show that wetland water level and spring discharge follow a similar trend, consistently with the inertia of the system. However, their hy­drochemical evolution and isotopic values differ, thus wetland-groundwater interaction has not been fully determined. Nev­ertheless, present research suggests that the hydrogeological connection would be more likely during wet periods, when the water table is at higher altitude. 
Key words: Evaporitic (karst) aquifer, Hydrological and hydro­geological behaviours, Natural responses, South Spain, Wet­lands.
Izvleèek	UDK  551.444(460.356)
José Manuel Gil-Márquez, Matías Mudarra, Bartolomé An­dreo, Luis Linares, Francisco Carrasco & José Benavente: Hidrogeološka opredelitev evaporitnega krasa na obmoèju Salinas-Los Hoyos (provinca Malaga na jugu Španije) z upo­rabo topografskih, hidrodinamiènih, hidrokemiènih in izo­topskih metod
Kraški sistem Salinas-Los Hoyos je geološka struktura v ob­liki diapirja, ki ima pestro sestavo (gline, pešèenjaki, evapo­ritne, magmatske in dolomitne kamnine idr.) ter se nahaja med provincama Malaga in Granada na jugu Španije. Prisot­nost evaporitnih kamnin (sadra, anhidrit in halit) in njihova visoka topnost so omogoèili nastanek kraških pojavov (vrtaèe, uvale, ponori). Jezeri Grande in Chica sta vrtaèi na zahodni meji diapirja, ki ju èleni piezometrièni nivo. V bližini prvega mokrišèa najdemo izvir Aguileras, ki predstavlja pretoèno toèko v zahodnem sektorju sistema. Da bi bilo mogoèe oceniti razmerje med mokrišèem in izvirom ter splošne znaèilnosti delovanja tega sistema, je bila opravljena analiza oblikovanosti površja, vzpostavljene pa so bile hidrogeološke meritve. Te so obsegale meritve višine in pretoka vode ter terenske meritve fizikalno-kemijskih znaèilnosti (temperatura vode in specifièna elektrièna prevodnost). Za namene kemijske in izotopske analize so bili vzeti tudi vzorci izvira, mokrišèa in padavinske vode. Predhodni rezultati so pokazali, da nivo vode v mokrišèu in pretok izvira sledita podobnemu trendu, kar nakazuje na povezanost tega sistema. Njihov hidrokemièni razvoj in izo­topske vrednosti pa se razlikujejo, zato odnosa med mokrišèem in podzemno vodo ni bilo mogoèe v celoti doloèiti. Kljub temu prièujoèa raziskava nakazuje, da bi hidrogeološka povezavo lahko potrdili v namoèenem obdobju, ko je gladina vode na veèji nadmorski višini.
Kljuène besede: evaporitni (kraški) vodonosnik, hidrološke in hidrogeološke lastnosti, naravni odziv, južna Španija, mokrišèa.

1 Department of Geology and Centre of Hydrogeology of the University of Malaga (CEHIUMA), 29071 Malaga, Spain, 
e-mail: josemgil@uma.es, mmudarra@uma.es, andreo@uma.es, fcarrasco@uma.es
2 Academy of Sciences of Malaga, C/Moratín 4 (1o-2), 29015 Malaga, Spain, e-mail: luislinares@telefonica.net
3 Department of Geodynamics and Water Research Institute, University of Granada. Av. Fuente Nueva s/n, 18071 Granada, Spain, e-mail: jbenaven@ugr.es
* Corresponding author
Received/Prejeto: 17.03.2016

ACTA CARSOLOGICA 45/1, 147–160, POSTOJNA 2016

José Manuel Gil-Márquez, Matías Mudarra, Bartolomé Andreo, Luis Linares, Francisco Carrasco & José Benavente

Introduction

Carbonate karst aquifers have been broadly studied dur­ing the last decades, leading to significant advances in the knowledge of Karst Hydrogeology and related processes (White 1988; European Comission 1995; Drew & Goldsc­heider 2007; Ford & Williams 2007; Andreo et al. 2015). On the contrary, research on evaporite karst areas have mainly been focused on aspects related to geomorphol­ogy, karstology, natural impacts or human-induced geo­hazards (Forti & Sauro 1996; Calaforra & Pulido-Bosch 1999; Klimchouk et al. 1999; Nicod 2006; Gutiérrez et al. 2008, 2014; Iovine et al. 2010; Cooper & Gutiérrez 2013), rather than on its hydrogeological functioning, since wa­ter resources stored in these media are normally low and of poor quality (characterized by high salinity).
At the southern sector of the Subbetic Domain of the Betic Cordillera (S Spain), there is a large outcrop made up by Upper Triassic (Keuper) clays and evaporite rocks (gypsum, anhydrite and halite), as well as blocks from oth­er lithologies (limestones, dolostones, sandstones, etc.) of Triassic to Miocene ages (Pérez-López & Sanz de Galdeano 1994). All these materials, termed as Chaotic Subbetic Complexes –CSC– Unit (Vera & Martín-Algarra 2004), appear as a chaotic mega-breccia highly deformed due to the northwestwards movement of the so-called Internal Zone of the Betic Codillera and its collision with the Sub­betic Domain during Miocene age (Rodríguez-Fernández et al. 2013). Low permeability and aquitard behavior have been traditionally attributed to the Triassic rocks of the CSC Unit (e.g. López-Chicano et al. 2001; Martos-Rosillo et al. 2013). However, dissolution/karstification processes affecting evaporite rocks provoke an increase in the devel­opment of secondary porosity and permeability. Thereby, unstable karst conduits and cavities are originated, whose collapses give place to subsidence phenomena and to a rapid geomorphological evolution of exokarst features: sinkholes and surface depressions (Calaforra & Pulido-Bosch 1999; Gutiérrez et al. 2008). Similar processes have been described in other evaporitic karst areas of Europe (Parise & Trocino 2005; Liguori et al. 2008; Parise et al. 2008; Iovine et al. 2010), given the abundant outcropping of evaporitic rocks in the Mediterranean basin.
The high solubility of evaporitic materials plays a notable influence on the hydrology and hydrogeology of CSC outcrops. In general, most of the exokarst features are placed in areas of gentle relief where a poorly defined drainage network exists, frequently found in watersheds dividing the main river basins. In these areas, the de­velopment of surface depressions has been enhanced by recent uplifting movements linked to diapiric or ha­lokinetic processes (Pezzi 1977; Rodríguez-Estrella 1983; Linares 2008). Some of these endorheic depressions can be flooded temporary or even permanently, originating wetland areas and ephemeral lakes of variable size but of great environmental value (some of them are listed in the Ramsar Convention of Wetlands). In this context, groundwater flow influences the hydrological function­ing of wetlands, especially when it is in connection with the piezometric level. Thus, wetlands located in relative elevated topographic positions are flooded in high water conditions whenever the water table reaches their bot­tom. On the contrary, during dry periods, the water table may move below the bottom of wetlands, resulting on them being dried up (Andreo et al. 2016). Consequent­ly, surface depressions can contribute to the recharge of subjacent aquifers, even more considering the usual presence of karst swallow holes, which can become ac­tive during rainstorm. In any case, wetland areas locat­ed in these relatively higher positions do not constitute the last destination of groundwater flow, but rather the flow path is towards other wetlands or springs situated at lower altitude coinciding with the base levels for CSC outcrops.
Despite a clear dependence on the water table variations has been found in the hydrological regime of many wetlands (Almécija 1997; Rodríguez-Rodríguez et al. 2006; Gutiérrez et al. 2008; Linares 2008; Andreo et al. 2016), the hydrogeological heterogeneity detected in CSC outcrops provokes that, in several cases, it is not possible to define accurately the hydrogeological con­nection and the flow paths between wetlands and nearby springs. This occurs in the case-study exemplified in this work: Los Hoyos-Salinas system, a diapiric structure be­longing to the CSC Unit and located between Malaga and Granada provinces (Southern Spain, Fig. 1), where a singular evaporite-karst landscape developed. At the NW border of study area, two permanent wetlands (known as Grande and Chica lakes) occupy two endor­heic depressions. Groundwater drainage in this sector takes place visibly towards the western border of the sys­tem, mainly through a spring located 250 m westward of Grande Lake. Although the hydrological dependence of both wetlands with groundwater seems possible (Linares 2004; Rodríguez-Rodríguez et al. 2007), the hydrogeo­logical connection between them and the spring has not been fully determined, in spite of their proximity. 
The aims of this work are to progress in the char­acterization of the general hydrogeological function­ing of the evaporite karst systems existing within CSC materials and to advance in the determination of the wetland-spring relationship by jointly applying differ­ent approaches: topographic, hydrodynamic, hydro­chemical and isotopic. With these goals, high resolu­tion digital elevation data from Airborne LIDAR (Laser Imaging Detection and Ranging) and hydrological in­formation from continuous record data on variations in water level and spring discharge, as well as hydrochem­ical and isotopic observations compiled in the pilot site (sector of Los Hoyos-Salinas, Fig. 1) during two years, have been used. This will help to better understand the hydrogeological functioning of evaporitic karst areas, taking into account their geological and geomorpho­logical particularities, for protection, management and, if necessary, a hydrological restoration of the wetlands located in this context.

Hydrogeological characterization of the Salinas-Los Hoyos evaporitic karst (Malaga province, S Spain) ...


Fig. 1 : Geographical and geological setting. Modified from Carrasco et al., 2007 (above) and Pineda Velasco, 1990 (below).

José Manuel Gil-Márquez, Matías Mudarra, Bartolomé Andreo, Luis Linares, Francisco Carrasco & José Benavente

Site description

The Salinas-Los Hoyos diapirc structure is the east­ernmost of an extensive and elongated outcrop of CSC materials, known as “Trias of Antequera” (pink colour in Fig. 1; Peyre 1974; Calaforra & Pulido-Bosch 1999; Sanz de Galdeano et al. 2008), with ENE-WSW direction in the northern part of the province of Malaga. The study area, of approximately 20 km2, presents an altitudinal range from 700 to 900 m a.s.l. Despite its low orogra­phy in comparison with the nearby reliefs, the pilot site is situated in the watershed divide between two signifi­cant rivers in Andalusia (Fig. 1): Guadalhorce River (W) and Guadalquivir River (E). The prevailing climate in the region is temperate Mediterranean, with a marked seasonal pattern in the annual distribution of precipita­tion. Rainfall mainly occurs in autumn, winter and, to a lesser extent, in spring time, associated with wet winds coming from the Atlantic Ocean (mean value of 90 mm for November and December, whereas is 3 mm for July). The mean historic annual precipitation is 563 mm (Consejería de Medio Ambiente 2005). The research pe­riod (January 2014 to January 2016) could be considered slightly dry, with an average annual precipitation of 409 mm. Mean annual temperature is close to 16 oC (Con­sejería de Medio Ambiente 2005), with relatively warm summers (26.2 oC in July) and mild winters (7.7 oC in January). The annual potential evapotranspiration cal­culated by the Thornthwaite method (Thornthwaite 1948) is 861 mm, with maximum values during summer months (166 mm in July). 
Geologically, CSC Unit is characterized in this area by the predominance of multi-coloured clay, sandstones and evaporitic materials (gypsum and halite). Gypsum occupies the center of the diapiric structure, showing typical massive outcrops, or as a polygenic breccia made up by gypsum fragments and little parts of clays, lime­stones and dolostones (Calaforra & Pulido-Bosch, 1993). Although halite is not present at the surface, due to its high solubility, its existence in depth can be inferred from groundwater hydrochemistry of a nearby saline spring (Carrasco 1986; Calaforra 1998; Figs. 1 & 2). The CSC rocks are highly deformed, in a chaotic way, thus it is difficult to appreciate their original stratigraphic rela­tions. Furthermore, the halokinetic processes related to the presence of gypsum and salt at depth have caused a ground uplifting, resulting on a higher elevation of the study area (about 50 m) with respect to the surrounding Plio-Quaternary materials (placed between 700 –E bor­der- to 750 m a.s.l. –W border-, Fig. 2).
Karst landscape in Salinas-Los Hoyos area is char­acterized by the existence of a large number of dolines (of different typology), sinkholes, small surface depres­sions, etc. (Fig. 1). In the core of the diapir, where uplift­ing processes are most active, collapses, conical and cy­lindrical dolines (someone with small sinkholes) appear, whose bottoms are placed around 850–840 m a.s.l. To­wards the edges of the diapiric structure, the size of do­lines increases, with flat-floored bottoms, topographical­ly placed at lower altitude (from 830 to 810 m a.s.l.), even lower than 800 m a.s.l. to northwestwards, where Grande (Fig. 3a & b) and Chica lakes are situated (Consejería de Medio Ambiente 2005). The main axes of these large de­pressions tend to follow the direction of the perimeter of the diapir, reflecting its notable structural influence (Calaforra & Pulido-Bosch, 1999). The high concentra­tions and the diversity of karst morphologies found in a relatively small area, made the Salinas-Los Hoyos system a place of particular interest from a geomorphology and landscaping point of view.
In hydrogeological terms, the Salinas-Los Hoyos di­apir constitutes a small and interesting system, well-de­fined by its borders (Linares 2008). Recharge takes place by diffuse infiltration of rainwater through dolines and gypsum blocks, but also by the direct infiltration of sur­face water into swallow holes. Discharge occurs through springs located at the borders of the diapiric structure (Figs. 1 & 2) and by water evaporation from the wetland surfaces. The most remarkable outflow point is Aguileras spring (15 l/s annual mean flow, Rodríguez-Rodríguez et al. 2007), placed at 787 m a.s.l. in the western sector of the system (Figs. 3c & 4). In fact, this spring corre­sponds to a gallery constructed in order to use ground­water in a neighboring old flour mill. Moreover, there are also other outflow points in the system, although with low discharge rates (less than 2 l/s), highlighting the hyper-saline spring of Fuente Camacho, placed at 705 m a.s.l. in the eastern border of the diapiric structure (Figs. 1 & 2). Grande and Chica lakes, each with an ap­proximate average surface area of 7 ha, are the most sig­nificant wetlands existing in the study area. Grande Lake is permanently flooded and reaches a maximum depth of 13.2 m, whereas the greatest depth in Chica Lake is 8.3 m (Rodríguez-Rodríguez et al. 2007). The latter can occasionally become dry during some drought periods, when the phreatic level is below its bottom. Both wet­lands are principally fed by groundwater and outputs are produced by evaporation, but also via infiltration to­wards adjacent aquifer located to the West (Andreo et al. 2016). Additionally, there are other surface depressions in the area that can be considered as seasonal wetlands, since the water table intersects periodically their bot­toms. Finally, other dolines placed at higher altitude can be flooded for a short period of time after heavy rainfall events.

Hydrogeological characterization of the Salinas-Los Hoyos evaporitic karst (Malaga province, S Spain) ...


Fig. 2: Cross section of Salinas-Los Hoyos diapiric structure. 


Fig. 3: Panoramic views of Grande Lake in Feb­ruary 2014 (a) and August 2015 (b). Gaug­ing section in Aguileras spring, equipped with 90o V-notch weir and water capacitance level logger (c). Installation of water capacitance level logger in Grande Lake (d). 

José Manuel Gil-Márquez, Matías Mudarra, Bartolomé Andreo, Luis Linares, Francisco Carrasco & José Benavente

Methods

From February 2014 to January 2016, hourly records of water level variations in Aguileras spring and Grande Lake (from April 2014) were acquired using Odyssey® capacitance water level probes (Figs. 3c and d), with a resolution of approximately 0.8 mm. Both data-loggers were in situ calibrated before their installation, following manufacturer specifications. Water level variations were converted to continuous measurements of water surface altitude (in m a.s.l.), adopting identifiable points in a high resolution digital elevation model (DEM) as stable ref­erence marks. DEM was created from Airborne LiDAR cover dataset (using GIS ArcMap© v.10.2 software), per­formed over the study area in the summer of 2008 (Con­sejería de Medio Ambiente 2008). The vertical accuracy of the delivered LiDAR data was within 8.0 cm root mean square error (RMSE) and the LiDAR point cloud cover was acquired at a nominal point spacing of 0.5 m. On the other hand, discharge rate evolution was obtained from continuous water level measurements applying manual discharge measurements at the spring (starting from Jan­uary 2014, OTT® C2 Flow Meter). A gauging station with a 90o V-notch weir was equipped in Aguileras spring for continuous discharge record. 
During the study period, field measurements of electrical conductivity (EC) and temperature (WTW® Cond 3310) were made at the spring water (fortnightly) and at Grande and Chica lakes (monthly) from January 2014 to January 2016 and from August 2014 to January 2016, respectively. Simultaneously to in situ measure­ments, water samples (totally 47 from Aguileras spring and 13 from Grande and Chica Lakes) were collected in 150-ml amber glass bottles for subsequent chemical and isotopic analysis in the laboratory. Water sampling in both lakes was carried out at approximately 0.5 m depth, using a 3-m extensible stick. Moreover, in No­vember 2015, 9 vertical profiles of temperature and EC (measurements every 1 m) were made along the S-N and E-W axes of Grande Lake (Fig. 4), using a water quality multiprobe (SEBA® KLL-Q). At the same time, a water sample was collected at the surface and another one at the maximum depth of each profile (for their chemical and isotopic analysis) by means of a submersible pump (Waterra® WSP-12V-3 Tempest). Finally, precipitation and air temperature were hourly recorded at a meteoro­logical station located 10 km to the west (530 m a.s.l., Fig. 1). A rain water capture system was emplaced near­by Grande Lake for hydrochemistry and isotopic rainfall characterizations (Fig. 4).
Chemical and isotopic analyses were carried out at the Centre of Hydrogeology of the University of Mala­ga (CEHIUMA). Alkalinity (Alk) was determined by volumetric titration using 0.02 N H2SO4 to pH 4.45. The chemical analyses of the major components (Ca2+, Mg2+, Na+, K+, Cl–, SO42–, NO3–, F–, Br–) were performed using high pressure liquid chromatography (HPLC, Metrohm® 792 Basic IC and Metrohm® Compact 881 IC pro) with ±0.1 mg/l of accuracy. Samples were diluted to 1 mS/cm and filtered before being introduced in the system (filter in line and precolumn-filter). .18O and .2H, relative to the Vienna-Standard Mean Ocean Water (RVSMOW), were determined by a Picarro® L2130-I Isotopic Water Analyzer (Cavity ring-down laser spectrometer, Picarro Ltd.). 


Fig. 4: Digital Elevation Model from LiDAR point dataset (Con­sejería de Medio Ambiente 2008) of the western sector of Salinas-Los Hoyos diapir (left). Grande Lake surrounding area and sche­matic position of vertical profile measurements done in November 2015 (right).

Hydrogeological characterization of the Salinas-Los Hoyos evaporitic karst (Malaga province, S Spain) ...

Results

The DEM created from LiDAR point dataset (Consejería de Medio Ambiente 2008) has permitted to precisely lo­cate the topographic elevation of Aguileras spring emer­gence: 787.0 m a.s.l. (Fig. 4), as well as to identify the ge­ometry of all closed depression areas in the western sector of Salinas-Los Hoyos diapir. However, using this dataset it was not possible to establish the topographic altitude of the bottom of Grande Lake, since it was submerged when LiDAR data acquisition was performed (summer 2008). In that moment, water surface in Grande Lake was at 787.7 m a.s.l, 0.5 m below Chica. Grande Lake has an overflow channel at its northwest border whose base is located at 795.9 m a.s.l. (Fig. 4), determining its maxi­mum level of flooding. In comparison with this value, al­titude of water surface in Grande Lake was 793.8 m a.s.l. at the beginning of the recording period considered in the present work and 3.5 meters lower (790.3 m a.s.l.) at the end.
Tab. 1 presents the most significant statistical pa­rameters corresponding to selected chemical compo­nents and isotopic values of the water stored in Grande and Chica lakes as well as those of the groundwater drained by the Aguileras spring. All present calcium-sulphated hydrochemical facies (Fig. 5), according to the prevailing gypsiferous nature of the system rocks. EC values recorded range from 2.65 to 5.78 mS/cm. In gen­eral, wetlands waters (especially from Chica) have the highest mean values of all analysed components except for Alkalinity and NO3–. The greatest variations in most of the parameters are observed in the wetlands, although the spring water presents slightly higher coefficient of variation for Cl–, Na+ and K+. Water temperature varied from 7.1 oC to 30.5 oC at the wetlands and from 16.8 to 17.2 oC at Aguileras spring. Finally, mean .18O and .2H values range from –6.65 ‰ to 6.25 ‰ VSMOW and from –42.64 ‰ and 16.12 ‰, respectively. The average discharge value from Aguileras spring during the study period was 29.3 l/s. 
Fig. 6 shows the variations of water table in Grande Lake and the temporal evolution of discharge (single and continuous measurements) from Aguileras spring, to­gether with EC and water temperature values measured in both, wetland and spring, from January 2014 (from April in the case of Grande Lake) to January 2016; this figure also displays the temporal evolution of most of the major hydrochemical components listed in Tab. 1, to­gether with the rainfall and air temperature recorded in the area for the same time period. Fluctuations in altitude of water surface and in discharge rate were similar, show­ing a clear descending trend, although less accentuated or even without significant variations, which coincide with the winter months when rainfalls and minimum evaporation occurred. Discharge values ranged from 40.5 to 18.5 l/s, while altitude of water surface varied be­tween 793.8 and 790.3 m a.s.l. On the contrary, wetland and spring presented different hydrochemical and hy­drothermal responses during the study period, certainly opposite in the case of EC. Thus, in the water drained by Aguileras spring there was a general and gradual de­crease in EC values, from 3.03 to 2.66 mS/cm, similar to the falling trend observed for the flow rate (Fig. 6). On the other hand, an ascending pattern was found for EC in the water from Grande Lake, from values close to 3.25 mS/cm to more than 3.60 mS/cm, slightly buffered during recharge periods. These variations in EC, both in spring and wetland, are mainly caused by the corre­sponding changes in Cl– and Na+ contents and, to a less­er extent, in SO42– and Mg2+. Finally, Alkalinity in Grande Lake followed a general seasonal variation, so that the highest value was recorded at spring time and the lowest at the end of the summer. With respect to hydrothermal response, water from Aguileras spring presented small changes in temperature (0.4 °C), with recharge having little or no influence on values (Fig. 6), while water tem­perature at Grande Lake, with a greater range of varia­tion (2.15 °C), was clearly influenced by air temperature changes in the area.
The vertical temperature and EC profiles complet­ed in November 2015 along the N–S and W–E axes of Grande Lake (see location in Fig. 4) permitted to observe a common and exponential slight fall in water tempera­ture values, from the wetland surface to the deepest part of each profile (Fig. 7). Hydrothermal variations mainly take place in the first 3 m of depth, with a general water cooling between 1 and 1.5 oC. However, in the case of water mineralization, no significant changes can be dis­tinguished, only very slight increases of EC values (from 3.58 to 3.59 mS/cm), which are produced at the upper part of water columns. Chemical analysis of the water samples taken during the profiles did not show relevant differences between surface and depth water composi­tion. Figure 7 also reveals limited isotopic variations in the vertical, with values ranging from 3.23 ‰ to 3.56 ‰ for .18O and from 4.56 ‰ to 5.89 ‰ for .2H.
In Fig. 8 isotopic data (.18O vs .2H) collected from rainwater, the spring and Grande and Chica Lakes (in­cluding water samples collected at the bottom of Grande Lake during vertical profiles) are represented. From iso­topic determinations of rain water samples, a preliminary local meteoric water line (LMWL) has been calculated for the study area, which is plotted in Fig. 8 together with the global meteoric water line, GMWL (Craig & Gor­don 1965). The isotopic composition of surface water in Grande and Chica Lakes range from 1.06 ‰ to 6.25 ‰ VSMOW for .18O and from 15.58 ‰ to 16.12‰ for .2H. Likewise, deep water samples from Grande Lake taken during profile measurements (November 2015) show very slight differences compared to the surficial wa­ter samples collected the same day (Fig. 7 & 8). All of them present isotopic enrichment values and are notably aligned to the right of the GMWL and the LMWL, fol­lowing an equation (Fig. 8) whose slope (4.52) is lower than those for the meteoric lines. In general terms, the lower the altitude of water surface in lakes is during sam­pling, the higher isotopic enrichment in water exists, re­flecting isotopic fractioning processes by direct evapora­tion of water surface. This is, if possible, more evident in the water of Chica Lake, where greater isotopic enrich­ment has been detected. Meanwhile, the isotopic compo­sition of groundwater drained by Aguileras spring range from –6.65‰ to –3.00 ‰ VSMOW for .18O and from –42.65‰ to –33.54‰ for .2H (Tab. 1). Most of the sam­ples are fitted to the layout of the meteoric lines (GMWL and LMWL), although some of them have a clear enrich­ment in .18O in comparison with .2H, defining another evaporation line (Fig. 8) with a slope (2.41) lower than that for the surface water. Both evaporation lines con­verge with LMWL in the same point, which could be considered as an average value of the isotopic composi­tion of the rain water (Fig. 8). 


Fig. 5: Piper diagram showing the water samples taken during the study period in the western sector of the Salinas-Los Hoyos Diapir.

Tab. 1: Mean values (M), standard deviation (.) and coefficient of variation (CV = ./M) of spring discharge rate and EC, water tem­perature, major ion concentrations (mg/l) and .18O and .2H values (‰ VSMOW) from water samples.
 
 No of samples
  
 EC 
(mS/cm)
 T 
(oC)
 Discharge (l/s)
 Alk
 F-
 Cl-
 Br-
 NO3-
 SO42-
 Ca2+
 Mg2+
 Na+
 K+
 .18O
 .2H
 
Grande Lake
 13
 M
 3,43
 17,4
 -
 65,0
 0,4
 201,2
 1,8
 2,6
 2597,7
 782,8
 144,1
 139,1
 7,3
 2,19
 -1,42
 
.
 0,16
 7,48
 -
 16,81
 0,04
 19,78
 0,59
 1,18
 241,98
 39,73
 9,34
 12,01
 1,02
 0,95
 4,48
 
CV
 0,05
 0,43
 -
 0,26
 0,09
 0,10
 0,33
 0,46
 0,09
 0,05
 0,06
 0,09
 0,14
 0,43
 -3,16
 
Grande Lake deep samples
 5
 M
 3,59
 14,8
 -
 60,1
 0,4
 205,4
 2,2
 3,1
 2676,7
 788,0
 155,0
 138,4
 6,3
 3,45
 4,91
 
.
 0,01
 0,13
 -
 0,56
 0,01
 1,60
 0,01
 0,15
 23,61
 7,04
 1,76
 1,58
 0,08
 0,13
 0,58
 
CV
 0,00
 0,01
 -
 0,01
 0,01
 0,01
 0,00
 0,05
 0,01
 0,01
 0,01
 0,01
 0,01
 0,04
 0,13
 
Chica Lake
 13
 M
 4,96
 15,8
 -
 127,3
 0,4
 680,3
 2,6
 3,9
 2631,4
 729,3
 212,3
 431,2
 14,0
 3,77
 4,76
 
.
 0,54
 8,19
 -
 37,06
 0,04
 108,33
 0,71
 1,09
 288,20
 61,80
 35,68
 60,43
 1,27
 1,65
 7,42
 
CV
 0,11
 0,52
 -
 0,29
 0,09
 0,16
 0,28
 0,28
 0,11
 0,08
 0,17
 0,14
 0,09
 0,44
 1,56
 
Aguileras spring
 47
 M
 2,93
 16,9
 29,3
 274,7
 0,4
 192,4
 1,7
 15,5
 1808,4
 672,4
 77,0
 130,1
 3,5
 -5,70
 -40,36
 
.
 0,15
 0,09
 5,89
 6,03
 0,04
 50,36
 0,48
 1,08
 81,43
 40,32
 5,29
 35,44
 1,07
 1,00
 2,37
 
CV
 0,05
 0,01
 0,20
 0,02
 0,10
 0,26
 0,29
 0,07
 0,05
 0,06
 0,07
 0,27
 0,31
 -0,18
 -0,06
 



José Manuel Gil-Márquez, Matías Mudarra, Bartolomé Andreo, Luis Linares, Francisco Carrasco & José Benavente


Fig. 6: Temporal evolution of discharge rate, wetland surface altitude, water temperature, EC and major chemical components of the water from Aguileras spring (left) and from Grande Lake (right), respect to daily precipitation events and air temperature variations. 

Hydrogeological characterization of the Salinas-Los Hoyos evaporitic karst (Malaga province, S Spain) ...


Fig. 7: a) Water temperature (solid line) and EC (dashed line) vertical profiles in Grande Lake. b) Some examples of .18O and .2H iso­topic values, from samples taken at different depths and surface at distinct profile measurements (November 2015). The number of the profile in which each plotted samples was taken is indicated (location in Fig. 4).

José Manuel Gil-Márquez, Matías Mudarra, Bartolomé Andreo, Luis Linares, Francisco Carrasco & José Benavente

Fig. 8: Isotopic composition of lo­cal rain, spring and wetland wa­ters. Global meteoric water line (GMWL) and local meteoric wa­ter line (LMWL) are included.


Discussion

High resolution land elevation data, generated from LiDAR cloud cover, has helped to precisely define the altitude of the different hydrological elements exist­ing in the western sector of Los Hoyos-Salinas system. From this information, the magnitude of the temporal variations in the hydraulic gradient between wetland and spring could be specifically established. Thus, if water surface in the lake reaches 787 m a.s.l. (spring altitude) or less, no gradient and therefore neither groundwater flow from Grande Lake to Aguileras spring could exist. As it has been previously mentioned, this spring corre­sponds to a gallery whose precise emerging point is not accessible to define its exact elevation. Nevertheless, the real outflowing altitude, although slightly higher, should be very similar to this determined in the present work (787 m a.s.l.). 
The total 3.4 m fall that has been observed in the al­titude of wetland surface during the more than 18-month logging period (Fig. 6), between 793.8 and 790.3 m a.s.l., is caused by evaporation outputs but also by fluctuations in the water table, produced by a general decrease of pre­cipitation inputs, whose consequences affect the spring discharge as well. The hydrodynamic stability observed in both, the wetland water altitude and the spring dis­charge rate during rainy months could be indicative of a slowly and lagged system response to recharge events, which would produce an attenuation of general descend­ing trend in phreatic variations. Nevertheless, such sta­bilization could also be related to a decrease in evapo­ration outputs or, more likely, to a combination of both processes. In any case, during the study period, water surface of wetland was always located at higher position than the altitude of the spring and, therefore, groundwa­ter flows from the first towards the second could exist. In May 2006, after several years of drought, altitude of the water surface in Grande Lake was found several meters below the position of the Aguileras spring (Rodríguez-Rodríguez et al. 2007). 
The increment of EC values observed in wetland surface (Fig. 6) during the study period is mainly due to a general concentration of dissolved ions in water because of evaporation, simultaneously to the isotopic fractioning, which is accentuated at the surface of the water column during the summer months. Considering that stratification is produced in Grande Lake in sum­mer (Rodríguez-Rodríguez et al. 2007), surface mea­surements could not be representative of the water lake as whole. However, data from vertical EC and water temperature profiles (Fig. 7), carried out in November 2015, as well as the chemical and isotopic determina­tion of samples at different depths, do not reveal signifi­cant variations among surface and deep water, beyond temperature changes in the first few meters (due to the influence of air temperature). In fact, during sampling campaign, surficial water temperature decreased 0.5 oC from the first measurement (03:00 pm) to the last one (06:00 pm). This difference in an approximately 3 hours interval suggests that higher variations are possible throughout the day and, therefore, water at the surface could be cooled down to the same temperature as depth or even more, which would propitiate the mixing of the water column towards the end of the night or the early morning. This interpretation could explain the chemi­cal and isotopic homogeneity of the water column when the vertical profile measurements were carried out. Nev­ertheless, it would be necessary to repeat the procedure several times along the year, especially during the sum­mer, in order to check this hypothesis. In any case, water body as a whole could be considered hydrochemically homogenous (Rodríguez-Rodríguez et al. 2007).
Despite the increasing trend in water mineralization of Grande Lake, this does not seem to have any influence on the groundwater drained by Aguileras spring. On the contrary, EC of spring water fell during the study period (Fig. 6). Variations in salinity of spring water mostly de­rived from changes in the dissolution rate of the evapor­itic rocks (gypsum and halite), which constitutes the core of diapiric structure. During significant recharge events, such as that at the beginning of the control period, an in­crease in outflow rate was accompanied by a sympathetic rise in water mineralization and in the components that most affect it (SO42–, Cl– and Na+), which took place in a moderate and buffered way during few months. Never­theless, when the main recharge effects finished (toward dry conditions), water drained by the spring became less saline as discharge decreased, reflecting a general pi­ezometric recession into the system. In other words, if recharge events are sufficiently relevant, they are able to involve the entire system, causing a delayed piston effect that pushes water previously stored within the saturated zone towards the spring; this water is characterized by a bigger residence time within the aquifer, greater EC and higher contents of SO42–, Cl– and Na+. On the contrary, if piezometric level decreases, influence of piston effect gradually disappears and the more mineralized water sinks down due to its higher density, leading to an inter­nal stratification of groundwater into the system.
The increasing evaporation rate at the beginning of summer would produce an increment of most of the dis­solved ion concentrations in Grande Lake waters (Fig. 6). On the other hand, the descent of Alkalinity observed in the same period indicates that calcite precipitation was tak­ing place, not only due to evaporation but also to the effect of temperature in its solubility and to common ion effect (Calaforra & Pulido-Bosch 1993; Ford & Williams 2007). Thus, the higher presence of Ca2+ in the lake waters, as a re­sult of gypsum dissolution, would favor the oversaturation in calcite and, therefore, its precipitation. Additionally, the concentrations of NO3– in the waters of both wetlands, lower than in the groundwater drained through Aguileras spring (Tab. 1), may be a result of denitrification. 
The isotopic composition of Aguileras spring and of the surficial waters of Grande and Chica lakes show clear evidences of evaporation fractioning, although the isoto­pic values differ depending on the origin of the samples. In the case of wetlands, deviations from GMWL and LMWL define an evaporation line with a slope value between 4 and 6 (Fig. 8), typical of free surface waters directly exposed to the atmosphere (Mook 2001). The isotopic enrichment in Chica Lake is higher than the ob­served for Grande, since lower flooded area and water column exist at the first one and, thus, evaporation ef­fect is accentuated in its waters. Likewise, as the wetland flooding surface and consequently their water column were diminishing, the isotopic composition of their wa­ters became more enriched in heavy isotopes.
Although groundwater samples from the spring are closer to the meteoric water, they also present some isotopic fractioning (mainly .18O), defining a regression line with a slope smaller than 4, which could be associ­ated to evaporation processes within the subsoil (Geyh et al. 2001). However, the marked exo and epikarst de­velopment in Los Hoyos-Salinas diapir would be consis­tent with a relatively high infiltration rate and, therefore, with an important role of unsaturated zone in the hydro­geological functioning of the system. Under this scenar­io, most of recharge water (overall fast infiltration) does not undergo significant evaporation processes because it flows rapidly toward the saturated zone via karst conduits. In opposition, water inputs by slow diffuse infiltration through massive gypsum outcrops and polygenic breccia can be stored in the soil and epikarst, and within the frac­tures of the unsaturated zone (dissolving gypsum miner­als), where several cycles of evaporation processes could affect groundwater, before it is being pushed toward the saturated zone. Hence, isotopic information from Aguil­eras spring reveals a mixing between more rapidly infil­trated water (via sinkholes) and the recharge water stored in the soil and epikarst, which circulates slowly through the unsaturated zone, demonstrating the heterogeneous hydrogeological functioning of this system. 
Although wetland and spring waters have under­gone evaporation process, it seems probable that such processes would have occurred in different sectors of the system. Therefore, it is not possible to assume, with the available isotopic data, the contribution of wetlands water to spring flow. Rodríguez-Rodríguez et al. (2006, 2007) carried out several isotopic determinations of both water points between April 1992 and April 1994. Results already reflected clear differences between .2H and .18O values from wetland and spring. Nevertheless, no isotopic fractioning was observed for Aguileras water samples. Results of this work suggest that waters drained by the spring could also have been affected by evapora­tion at a certain time. Thus, given the hydrogeological complexity of the system, greater isotopic variability in groundwater could be expected, overall under different climatic conditions.
The analysis of all information here presented (top­ographic, hydrodynamic, hydrochemical and isotopic) does not permit to discern the degree of hydrogeologi­cal connection between the wetlands (especially Grande Lake) and the Aguileras outflow point. Isotopic and hy­drochemical responses do not suggest a clear hydrogeo­logical connection conversely to the temporal evolution of hydrodynamic response, as well as to the topographi­cal altitude. In any case, it is premature to assume any hypothesis if the hydrogeological functioning of the sys­tem is not fully understood in detail yet, and also without data from a longer study period. Nevertheless, prelimi­nary results reveal that the system is complex and has a high capacity of natural regulation, in which both unsat­urated and saturated zones might play relevant roles.

Hydrogeological characterization of the Salinas-Los Hoyos evaporitic karst (Malaga province, S Spain) ...

José Manuel Gil-Márquez, Matías Mudarra, Bartolomé Andreo, Luis Linares, Francisco Carrasco & José Benavente

Conclusion

Results obtained in this work allow deducing pre­liminary conclusions about the hydrological and hy­drogeological functioning of the western sector of the evaporitic system of Los Hoyos-Salinas diaper (Malaga province, South Spain), during a period of low precipi­tation, in which water levels underwent a general de­creasing trend. In this context, it has not been possible to confirm the existence of direct groundwater flows from Grande Lake towards Aguileras spring, from the hydraulic point of view. This hypothesis would make sense if the water table in the wetland is higher than the spring emerging altitude, which is common during high and intermediate water conditions. Thus, for bet­ter understanding of hydrogeological behavior, a longer monitoring period should be recorded, comprising maximum and minimum flooding episodes. This would permit to deduce the hydrogeological functioning of the system and to determine precisely the direction and evolution of groundwater flows. 
The wetland-groundwater interaction in Grande and Chica lakes is not an isolate case. On the contrary, it is fairly common in Andalusia region and especially in many wetlands related to CSC outcrops. The continu­ous limnimetric control of those areas, together with the analysis of the natural responses recorded in associated springs, provide valuable information about their hy­drological and hydrogeological contexts. Furthermore, LiDAR cover datasets are useful tools to precise wetland depressions shapes, which combined with limnimetric measurement may allow to calculate, in further studies, the variation of water volume stored in wetlands and, therefore, to accurately determine their water budgets. 
Due to its peculiarities (from a geomorphological, hydrogeological, ecological and landscaping point of view), the Los Hoyos-Salinas diapiric structure is an ex­ceptional example of evaporitic karst system. Therefore, special protection and correction actions (beyond the ecological ones), should be applied in order to reduce anthropogenic impacts and pressures, especially tak­ing into consideration the high vulnerability of gypsum karst. Furthermore, the inclusion of Los Hoyos-Salinas diapir in the Spanish List of Geosites must be consid­ered, as an example of evaporite karst system where dif­ferent geomorphological and structural elements can be observed (diapirism, dolines, sinkholes, swallow holes, etc).

Hydrogeological characterization of the Salinas-Los Hoyos evaporitic karst (Malaga province, S Spain) ...

Acknowledgment

This work is a contribution to IGCP-598 project of UNESCO, to the Excellence Projects RNM-8087 and RNM-6895-R of Junta de Andalucía and to the Research Group RNM-308 of the Junta de Andalucía.

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José Manuel Gil-Márquez, Matías Mudarra, Bartolomé Andreo, Luis Linares, Francisco Carrasco & José Benavente