Influence of Hydrogeochemistry on Tunnel Drainage in Evaporitic Formations: El Regajal Tunnel Case Study (Aranjuez, Spain)

sustainability

Article Influence of Hydrogeochemistry on Tunnel Drainage in Evaporitic Formations: El Regajal Tunnel Case Study (Aranjuez, Spain)

Ignacio Menéndez Pidal 1,* , Jose Antonio Mancebo Piqueras 2, Eugenio Sanz Pérez 1 and Clemente Sáenz Sanz 3

1 Laboratorio de Geología, Departamento de Ingeniería y Morfología del Terreno, Universidad Politécnica de Madrid, 28040 Madrid, Spain; [email protected] 2 Departamento de Ingeniería Mecánica, Química y Diseño Industrial, Universidad Politécnica de Madrid, 28012 Madrid, Spain; [email protected] 3 Geotechnics Department, Ferrovial, Houston, TX 77039, USA; [email protected] * Correspondence: [email protected]

Abstract: Many of the large number of underground works constructed or under construction in recent years are in unfavorable terrains facing unusual situations and construction conditions. This is the case of the subject under study in this paper: a tunnel excavated in evaporitic rocks that experienced significant karstification problems very quickly over time. As a result of this situation, the causes that may underlie this rapid karstification are investigated and a novel methodology is presented in civil engineering where the use of saturation indices for the different mineral specimens present has been crucial. The drainage of the rock massif of El Regajal (Madrid-Toledo, Spain, in the Madrid-Valencia high-speed train line) was studied and permitted the in-situ study of the hydrogeochemical evolution of water flow in the Miocene evaporitic materials of the Tajo Basin as a full-scale testing laboratory, that are conforms as a whole, a single aquifer. The work provides a novel method- Citation: Menéndez Pidal, I.; ology based on the calculation of activities through the hydrogeochemical study of water samples in Mancebo Piqueras, J.A.; Sanz Pérez, different piezometers, estimating the saturation index of different saline materials and the dissolution E.; Sáenz Sanz, C. Influence of capacity of the brine, which is surprisingly very high despite the high electrical conductivity. The Hydrogeochemistry on Tunnel Drainage in Evaporitic Formations: El circulating brine appears unsaturated with respect to thenardite, mirabilite, epsomite, glauberite, Regajal Tunnel Case Study (Aranjuez, and halite. The alteration of the underground flow and the consequent renewal of the water of the Spain). Sustainability 2021, 13, 1505. aquifer by the infiltration water of rain and irrigation is the cause of the hydrogeochemical imbalance https://doi.org/10.3390/su13031505 and the modification of the characteristics of the massif. These modifications include very important loss of material by dissolution, altering the resistance of the terrain and the increase of the porosity. Academic Editors: Rubén Galindo, Simultaneously, different expansive and recrystallization processes that decrease the porosity of the Zhigang Cao, Nihat Sinan I¸sıkand massif were identified in the present work. The hydrogeochemical study allows the evolution of Antonio Lara-Galera these phenomena to be followed over time, and this, in turn, may facilitate the implementation of Received: 12 December 2020 preventive works in civil engineering. Accepted: 27 January 2021 Published: 1 February 2021 Keywords: tunneling; geotechnical risk; evaporitic karstification; saturation index

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional afﬁl- 1. Introduction and Objectives iations. Karst in evaporitic rocks presents a large variety of risks associated with any type of infrastructure. The risk component is inevitable. It cannot be totally eliminated, although it can be reduced to an acceptable level if rigorous studies are performed in the initial phases of geotechnical and geological research. Thus, risk can persist not only during the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. construction phase, but also throughout the useful life of the infrastructure, since detecting This article is an open access article in advance all problems of this nature is practically impossible [1]. Since evaporitic rocks distributed under the terms and can be karstiﬁed in months and/or years, continuous monitoring and surveillance of the conditions of the Creative Commons hydrogeological behavior of the massif near a tunnel is crucial during its useful life. Attribution (CC BY) license (https:// Regarding the risk reduction strategy during the previous phases of the investigation, creativecommons.org/licenses/by/ the geological predisposition constitutes the most important factor in determining the karst 4.0/). conditions that could occur on site during construction. Of the set of factors that explains the

Sustainability 2021, 13, 1505. https://doi.org/10.3390/su13031505 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 1505 2 of 25

development of a karst system, the lithological factor is the most important and necessary, since karst can be considered as intrinsically linked to certain lithologies. In evaporitic rocks, the ease of karstification will depend mainly on the solubility of the mineral specimens present. The prevention and prediction of risks related to karst in evaporites is based on a good knowledge of geological formations with minerals susceptible to dissolution, as well as their location. Secondly, it is important to know the hydrogeological conceptual model prior to the infrastructure construction, as well as the factors that are expected to become involved in the evolution of the massif karstification as a consequence of these anthropic changes. Regarding the importance of hydrogeology, gypsum and evaporite rocks in general have not aroused the interest of hydrogeologists, since they constitute low-quality aquifers for human water supply and use. The hydrogeological interest lies more in the field of applied geology and geotechnics, as they are related to problematic geological formations in the infrastructures construction and its interaction with water [2]. Among these problems the risk of karstification near tunnels is found. Gypsiferous massifs are not excessively permeable if they are not karstified. However, natural karstification originated in geological times, or was produced in an accelerated way by the alteration of the flow induced by a tunnel construction, for example, which can create a new anisotropic aquifer with a much higher porosity and permeability [3,4]. The knowledge of the mechanisms that operate in karstification and other processes of interaction with water in gypsum and other evaporitic rocks has a clear scientific interest and relevant engineering importance. These mechanisms condition the stability of any civil engineering work located in this type of terrain. Given the large extent that gypsum and evaporite soils reach in general in Spain [5], this becomes one of the problems that most frequently concerns infrastructure construction. The consequent problems of geological, applied geology, and hydrogeology character are important. Their practical consequences can become even more complex and important if a good geological and hydrogeological knowledge is not available. This also implies knowing the mechanisms that operate between the terrain and the water. In the case of underground excavations, it is also a problem that is occurring with increasing frequency in the tertiary sedimentary basins of Spain and Europe, as the depth of these new infrastructures reaches more saline units below, where besides gypsum, there may be other more expansive minerals (case of anhydrite) or much more soluble minerals, such as halite, epsomite, glauberite, thenardite, etc. Example cases are tunnels in the French Jura, Wagenburg North in Baden–Württemberg (Germany) [6,7], Lilla tunnel (Lérida, Spain) [8,9], Fabares (Asturias, Spain), Albertia (Alava, Spain) and Regajal (Madrid, Spain) [10–13]. Many infrastructures modify substantially the natural flow of groundwater during their construction, and even during their operation. This is the case in dam foundations on soluble materials, where the speed increase of the seepage can lead to their dissolution. In tunnels dug below the water table, groundwater drainage and karst reactivation of the massif can occur. The processes and chemical reactions that occur due to interaction of water with the minerals of evaporitic rocks are usually very slow under natural conditions, where hydraulic gradients are usually small, and there is a hydrochemical balance between groundwater and rock. However, the alteration of the underground flow due to the construction of tunnels can accelerate these processes and increase the hydraulic gradient and, consequently, the speed and the water renewal. This can imply loss of tunnel pe- ripheral support mainly by the generation of dissolution voids, which involves important consequences for the stability of the infrastructure and the area around it. Some of the processes that can occur in the evaporitic massifs that contain the abovementioned minerals are related to mineral expansiveness and to rock porosity changes. The main processes are dissolution, transformation from glauberite to gypsum, physical erosion, expansiveness of anhydrite, and salt crystallization [10,11]. Among all, it is Sustainability 2021, 13, 1505 3 of 25

worth mentioning dissolution, which greatly affects halite, thenardite, and mirabilite, and moderately affects gypsum and anhydrite. These modifying processes in evaporitic rock massifs are accelerated near dams, tunnels, and other structures and could compromise their stability due to the alteration of the initial hydrogeological conditions. During tunnel construction, water can significantly accelerate karstification phenomena. Expansion phenomena are also frequent due to the change in anhydrite volume, creeping, corrosion, etc. They are phenomena that, in some way, are interrelated with each other, but are not the subject of this work. This paper deals exclusively with the first generation of karstification. All these phenomena influence, when planning technical solutions in the excavation system to limit the active zone [10] as much as possible, the waterproofing of the tunnel and the support-lining. The chemistry, flow, and groundwater temperature play fundamental roles in the processes of dissolution, precipitation, and swelling [14]. In addition to mineralogy, the hydrogeological context knowledge and the conceptual model of hydrogeological functioning is essential, as has been stated already. The water mobility is key to estimating the stability of the rock through which it circulates or is stored. In this sense, to identify supersaturated watertight zones without dissolution, it may be convenient to use isotopes to determine the water age, such as tritium. It is convenient, however, to carry out water stability diagrams with respect to calcium and sodium sulphates that allow one to know if such water can cause dissolution in different mineral species existing in the massif. To determine and predict the progress of karstification, it is important to verify if all the ions are active, since their activity is conditioned by the electrical charges of other ions. For this reason, it is necessary to obtain the activity coefficients and subsequently determine the chemical equilibrium of the reactions of the different materials existing and involved in the dissolution process (saturation index), which will allow the dissolution reaction direction for each material to be deduced. It is about knowing the residual dissolution capacity and the tendency to saturation. Additionally, in a particular way, the solid mass loss and, consequently, the karstification increase due to the drainage to the tunnel or to the pumping wells, as occurred in the case here studied. The karst processes and the dissolution of gypsum and evaporitic rocks were discussed by [15–21], but the saturation index is a novel methodological technique in the tunneling and civil engineering field and has not been used before, although it has been applied from a speleogenetic point of view in geomorphology [17]. The need for a saturation index calculation with respect to several mineral species is justified. It helps to determine the water capacity to continue dissolving materials and therefore allows the increase in karst progress to be predicted. Thus, the main objective of this work is to contribute to a greater knowledge in the gypsum and evaporitic rocks karstification field in those massifs crossed by tunnels by applying the saturation index of the mineral specimens present in them. To reach this main final objective, we address, in an orderly manner, the following specific objectives: (i) the definition of the geological and hydrogeochemical conceptual model of the site, characterizing the local petrology and mineralogy; (ii) the chemical characterization of the groundwater and its evolution over time; (iii) the study of the groundwater chemical stratification and the potential relationship between the stratification and the geology and mineralogy of the area; (iv) the determination of equilibrium states of the saturation reactions of the groundwater regarding the different evaporitic minerals; and (v) the identification of the modifying processes of mineral transformation as a consequence of the flow alteration caused by the drainage operations.

2. Description of the Case Study The case study in this work is the south portal of the El Regajal Tunnel, on the high- speed railway line from Madrid to Valencia (Figure1). The area is located between the provinces of Madrid and Toledo, in its rise from the Tagus river plain to the Ontígola Sustainability 2021, 13, x FOR PEER REVIEW 4 of 26

2. Description of the Case Study Sustainability 2021, 13, 1505 4 of 25 The case study in this work is the south portal of the El Regajal Tunnel, on the high- speed railway line from Madrid to Valencia (Figure 1). The area is located between the provinces of Madrid and Toledo, in its rise from the Tagus river plain to the Ontígola plateau. The drainageplateau. networkThe drainage in this network plain is in quasi-endorheic, this plain is quasi-endorheic, and at its head and there at its are head there are (and were) damp(and areas were) prone damp to areas ﬂooding. prone Onto flooding. the eastern On the edge eastern of these edge highlands of these highlands are are the the so-called Ontso-calledígola springs. Ontígola Their springs. ﬂow Their drains flow to drains the north to the through north through the Arroyo the deArroyo la de la Vega Vega to the Aranjuezto the Aranjuez and Tagus and River. Tagus The River. tunnel The is tunnel part of is the part Aranjuez–Ocaña of the Aranjuez–Ocaña section section of the of the LevanteLevante high speed high railway speed railway access. Workaccess. on Work the tunnelon the lastedtunnel fromlasted 2008 from to 2008 2010. to 2010. It is a It is a double trackdouble tunnel track withtunnel an with approximate an approximate inner diameter inner diameter of 8.50 of m. 8.50 The m. useful The useful section section interiorinterior is 85.7 m is2 ,85.7 but m the2, excavationbut the excavation section reachessection reaches 180 m2. 180 The m maximum2. The maximum cap is cap is 65 m 65 m with an averagewith an cap average of 40 m.cap The of 40 average m. The slope average is 25 slop thousandths.e is 25 thousandths. The total length,The total length, in- including falsecluding tunnels, false is 2445 tunnels, m. is 2445 m.

Figure 1.Figure High-speed 1. High-speed train line trainand El line Regajal and El tunnel Regajal location tunnel in location the municipality in the municipality of Aranjuez of city, Aranjuez Madrid city, province. Madrid province. The tunnel was built with robust sections, preventing decompression of the massif, The tunneland was avoiding built with all robustpossible sections, water contributions, preventing decompression both from the very of the excavation massif, (which was and avoiding allcarried possible out waterwith the contributions, least amount both of water from possible), the very excavationand from all (which types of was water flow into carried out withthe the tunnel. leastamount of water possible), and from all types of water ﬂow into the tunnel. For this, the excavation was performed mainly in phases by a roadheader in saline For this, thesoils excavation and hydraulic was performedhammers in mainly clay soils. in phases The lining by a was roadheader designed in as saline a full section, very soils and hydraulicclose to hammers the excavation in clay front, soils. to Theachieve lining a ra waspid closure designed of the as section a full section,[22], avoiding move- very close to thements excavation and decompressions front, to achieve that a would rapid closurepromote of incoming the section water. [22], In avoiding the project, a series of movements andstandard decompressions sections were that proposed, would promote which in incoming the construction water. Inphase the project,were simplified a to four series of standardvery sections similar were designs proposed, (Figure which 2), varying in the constructiononly in the inverted phase were vault simpliﬁed thickness and its rein- to four very similarforcement. designs These (Figure sections2), varying are (1) only the in ex thecavation inverted in clay vault and thickness clayey andmarls its (I–M1), (2) the reinforcement.section These sectionsof glauberite are (1)and the halite excavation without inwater clay (I–R andN clayey), (3) the marls anhydrite (I–M 1and), (2) expansive ma- the section of glauberiteterials section and halite(I–MEXP without), and (4) water the sections (I–RN), (3)with the risk anhydrite of dissolution and expansive (I–MPIL) [22]. materials section (I–MEXP), and (4) the sections with risk of dissolution (I–MPIL)[22]. SustainabilitySustainability2021 2021, 13, 13, 1505, x FOR PEER REVIEW 55 of of 25 26

FigureFigure 2. 2.El El Regajal Regajal tunnel tunnel ﬁnal final cross cross sections. sections.

TheThe tunnel tunnel was was executed executed asas aa falsefalse tunneltunnel forfor thethelast last334 334m m of of the the southern southern mouth.mouth. TheThe southern southern portal portal was was carried carried out out by by a a trench trench excavation excavation 527 527 m m long long with with a a maximum maximum depthdepth of of 15 15 m m and and an an intermediate intermediate berm, berm, a a width width that that varies varies from from 80 80 m m on on the the surface surface to to 4040 m m at at the the bottom bottom of of the the excavation, excavation, and and slopes slopes of of a a 1:1 1:1 ratio. ratio. InIn this this area, area, this this underground underground work work crosses crosses evaporitic evaporitic Miocene Miocene materials. materials. The The final final stretchstretch of of the the tunnel tunnel ends ends in ain false a false tunnel, tunnel, as mentioned, as mentioned, and a and large a trenchlarge (Figurestrench (Figures3 and4), 3 bothand in4), an both ascending in an ascending slope. This slope. track This intersects track intersects a subhorizontal a subhorizontal geological geological contact between contact abetween lower layer a lower of saline layer materials of saline (in materials which halite (in andwhich glauberite halite and predominate, glauberite sometimespredominate, in asometimes massive form in a andmassive others form interbedded and others with interbedded clay) and with an upperclay) and horizon an upper rich horizon in gypsum rich andin gypsum clay (Figure and 4clay). During (Figure the 4). construction During the construction of the southern of the portal, southern it was portal, observed it was that ob- thisserved stratigraphic that this stratigraphic discontinuity discontinuity acts as a water acts exudation as a water zone, exudation and waterproofing zone, and water- and otherproofing measures and other were taken,measures such were as waterproofing taken, such as of waterproofing concrete modules of concrete joints, injections, modules etc.joints, [22 ,injections,23]. etc. [22,23]. In order to avoid problems during construction and exploitation of the tunnel, a drainage well was drilled to the northeast of the trench, near the tunnel portal, to drain the massif. A small wall of cut-off piles was built parallel to the tunnel axis to stop or reduce water infiltration into the tunnel from the ground. The groundwater, pumped since 2011 from the well, was first poured into a decanter before joining the drainage waters from the trench located in the last stretch. Analysis of the drained groundwater indicated an electrical conductivity of about 56,000 µS/cm due to the dissolved salts. The drainage well was drilled during the construction of the excavation of the false tunnel to work in dry conditions, and it is still (when this work was being delivered) in use to avoid infiltration to the tunnel. Nevertheless, the groundwater extraction, heavily loaded with salts, caused material loss in the tunnel environment due to dissolution in the evaporitic massif (Figure4D). Sustainability 2021, 13, x FOR PEER REVIEW 6 of 26 Sustainability 2021, 13, 1505 6 of 25 Sustainability 2021, 13, x FOR PEER REVIEW 6 of 26

FigureFigureFigure 3. Detail3. Detail 3. Detail of of the theof souththe south south portal portal portal of of theof the the tunneltunnel tunnel andand and trench. trench. The tren trenchch isis is shownshown shown on on on the the the right right right side side side indicating indicating indicating its itsexcavation itsexcavation excavation slopes of the abovementioned portal, which are shown in photos A and B in Figure 4. Piezometers are shown as red circles. slopesslopes of theof the abovementioned abovementioned portal, portal, which which are are shown shown inin photos A and and B B in in Figure Figure 4.4 .Piezometers Piezometers are are shown shown as asred red circles. circles.

Figure 4. Pictures of El Regajal tunnel and trench. (A). View of the trench from the south portal. Picture taken from the false tunnel looking south. The slopes of the excavation and part of the railway line are visible. (B). Picture of the slope FigureFiguretrench 4. Pictures4. Picturesshowing of ofthe El El Regajalevaporitic Regajal tunnel tunnel formation and and trench.stratification.trench. ((A).). ViewThe arrow of the points trench trench to from fromthe contact the the south south between portal. portal. the Picture upper Picture gypsum-richtaken taken from from the the falsematerials tunnel lookingand the lowersouth. saline The slopesmaterials of the(halite- excava andtion glauberite-rich) and part of thatthe railwayact as a water line are exudation visible. plane (B). Picture (C). Detail of the of theslope false tunnel looking south. The slopes of the excavation and part of the railway line are visible. (B). Picture of the slope trench showing the evaporitic formation stratification. The arrow points to the contact between the upper gypsum-rich trenchmaterials showing and the lower evaporitic saline formation materials (halite- stratiﬁcation. and glauberite-rich) The arrow points that act to as the a water contact exudation between plane the upper (C). Detail gypsum-rich of the materials and the lower saline materials (halite- and glauberite-rich) that act as a water exudation plane (C). Detail of the water exudation plane. Below it, whitish marks are precipitates of halite, mirabilite, and thenardite. (D). After pumping, water from the trench and drainage well were directed to the El Salobral basin. The picture shows salts that precipitated from the drained groundwater. Sustainability 2021, 13, 1505 7 of 25

To study the system as a whole, a hydrogeological investigation of the tunnel and the trench environment was carried out. The main objective was to determine the hydrogeological development after the construction of the tunnel and inauguration of the high speed Madrid–Valencia line. In a retrospective way, the hydrogeological process was inferred before and during the construction of the tunnel [24,25]. This hydrogeological study addressed two fundamental questions: the analysis of the drained groundwater origin, and geotechnical risk mitigation. Subsequently, the saturation indices research presented here was developed. This work also aims to reﬂect on the need to carry out prior hydrogeological studies during the project phase of any tunnel with a regional character without being restricted to the narrow strip deﬁned around the tunnel axis [26].

3. Methodology To achieve the objectives proposed above and given the hydrogeochemical nature of the research, the following methodology was followed: 1. Compilation of background and historical data available (construction project and data that were collected at the time of construction of the tunnel). 2. Mineralogy identification of the evaporite massif in order to define potentially karsti- fiable mineralogical species. 3. Analysis of the hydrogeological situation before and especially shortly after the execution of the tunnel works. The features of the existing aquifers in the area and the natural hydrological behavior of the streams were defined in order to identify the piezometric changes and the influence of the water flow in the massif. 4. Modeling of the behavior of the waters of the massif in relation to its dissolution capacity. This model was made based on the calculation of solute content activities in those waters. The objective was to fix the saturation index of each salt at the time of its sample collection. This index established a specific state in the dissolution process of each salt and, therefore, allowed the karstification degree at that moment to be estimated and, where appropriate and with more data, the evolution over time to be determined. This methodology entailed, therefore, some field work and cabinet and laboratory work as well.

3.1. Field Work and Laboratory Tests Firstly, a site geological study (geological cartography) and in depth geological cross section analysis were performed, which involved a detailed stratigraphy of the different geological formations. For that, intense field work was carried out, taking advantage of the outcrops of trench clearings, etc., and the information provided from ten railway tunnel project boreholes and twenty other boreholes fitted with piezometers (open-pipe type) specifically carried out later for this specific current work (Figure3). The borehole depths ranged between 20 and 45 m. Since it was not possible to visually determine all the mineralogical species in the sounding recognition obtained from these boreholes, careful sampling was carried out, identifying and making comparisons with the mineralogical analysis collected in the preliminary geotechnical tunnel project studies. The field work included an inventory of forty-five existing water-points in the wider environment of the study area, which included springs, wells, other boreholes, and streams. Complementing this information and to determine in detail the hydrogeology of the site, a piezometric network of 20 boreholes/piezometers was designed. This network of piezometers helped, among other things, to measure water levels and draw isopieze maps, thus determining the induced underground flow system. Among the water points, in addition to the piezometers, the following should be mentioned: the abovementioned pumping well, the drainage pump of the ditch, and the Ontígola springs, which are located at 2725 m in the center of the abovementioned village at the public park of Los Manantiales Sustainability 2021, 13, x FOR PEER REVIEW 8 of 26

the outcrops of trench clearings, etc., and the information provided from ten railway tunnel project boreholes and twenty other boreholes fitted with piezometers (open-pipe type) specifically carried out later for this specific current work (Figure 3). The borehole depths ranged between 20 and 45 m. Since it was not possible to visually determine all the mineralogical species in the sounding recognition obtained from these boreholes, careful sampling was carried out, identifying and making comparisons with the mineralogical analysis collected in the preliminary geotechnical tunnel project studies. The field work included an inventory of forty-five existing water-points in the wider environment of the study area, which included springs, wells, other boreholes, and streams. Complementing this information and to determine in detail the hydrogeology of the site, a piezometric network of 20 boreholes/piezometers was designed. This network of piezometers helped, among other things, to measure water levels and draw isopieze maps, thus determining the induced underground flow system. Among the water points, Sustainability 2021, 13, 1505 in addition to the piezometers, the following should be mentioned: the8of abovementioned 25 pumping well, the drainage pump of the ditch, and the Ontígola springs, which are located at 2725 m in the center of the abovementioned village at the public park of Los (Los Manantiales),Manantiales (Figures (Los1 and Manantiales),5). A speciﬁc explanation(Figures 1 and and 5). its A results specific were explanation studied in and its results depth in the Sectionwere studied 4.2. Hydrogeological in depth in the SynthesisSection 4.2. of Hydrogeological this work. Synthesis of this work.

Figure 5. Geological map of the area of study showing the location of the Ontígola springs. Figure 5. Geological map of the area of study showing the location of the Ontígola springs. The evolution of hydrochemistry was monitored by taking samples at different depths in the most representativeThe evolution piezometers of hydrochemistry in the network (PZ1,was moni PZ2,tored PZ3, PZ5, by taking PZ6, PZ7, samples PZ8, at different PZ10, and PZ11).depths In somein the piezometers,most representative only one piezometers sample was in performed. the network These (PZ1, samples PZ2, PZ3, PZ5, PZ6, were distributedPZ7, in PZ8, each PZ10, piezometer and PZ11). covering In some the piezomet entire studieders, only area one in sample order towas collect performed. These the ion concentrationssamples were in the distributed water. These in each samples piezometer provided covering results the of entire hydrochemical studied area in order to processes in thecollect field. the ion concentrations in the water. These samples provided results of hydrochem- Likewise,ical and processes given that inthe the mineralogical field. quality of the ground varies depending on the stratification, atLikewise, different and depths given in somethat the piezometers mineralogical (number quality 6—PZ6, of the forground example) varies depending water sampleson were the stratification, also carefully at taken. different Some depths of the in some samples piezometers were taken (number during 6—PZ6, the for exam- excavation ofple) the boreholewater samples or piezometer. were also carefully taken. Some of the samples were taken during the Water samplesexcavation were of alsothe bore takenhole from or piezometer. the water-points around the tunnel, in the drainage well, and at the pump exits. Samples were analyzed, characterizing their composition, activity, saturation, and dissolution capacity in the laboratory (Table1). The field works were performed in 2011–12, including sampling. In winter 2012, mineralogical studies were carried out, and, finally, modelling was fulfilled in 2015.

3.2. Modeling Based on the sample analyses, the evaluation of the chemical activity of the salts was estimated in order to determine the different saturation indices and, therefore, water capacity to continue dissolving saline minerals. In this technique, a calculation model of the ionic activities, the saturation degree, and the dissolution capacity of materials was obtained. For that, a specific and simple model was developed by using an Excel spreadsheet, which allowed us to achieve all calculations. By using this model, the theoretical development of which is described below, mineral transformation of modifying processes in the field were identified. It was then possible to explain them by the flow alteration caused by the tunnel drainage operations. Sustainability 2021, 13, 1505 9 of 25

Table 1. Summary of analysis/tests of water samples.

Subject Analysis/Test Description Ammonium NESSLER Method Nessler Reagent. Water quality. Determination of the water hardness. Complexometric method Bicarbonates UNE 77040:2002 with EDTA. Water quality. Determination of the sum of calcium and magnesium. EDTA Calcium UNE-ISO 6059:2014 titrimetric method. Water quality. Determination of the water hardness. Complexometric method Carbonates UNE 77040:2002 with EDTA. Conductivity 20 ◦C UNE-EN 27888:1994 Water quality. Determination of electrical conductivity. (ISO 7888:1985). Water quality. Determination of the water hardness. Complexometric method Hydroxides UNE 77040:2002 with EDTA. Water quality. Determination of the sum of calcium and magnesium. EDTA Magnesium UNE-ISO 6059:2014 titrimetric method. pH UNE-EN ISO 10523:2012 Water quality—Determination of pH (ISO 10523:2008). Potassium UNE-EN ISO 16995 Determination of the water soluble chloride, sodium, and potassium content. Sodium UNE-EN ISO 16995 Determination of the water soluble chloride, sodium, and potassium content. Chlorides UNE 77042:2015 Water quality. Chloride determination. Potentiometric method. Sulphates UNE 77048:2019 Water quality. Sulphate determination. Gravimetric method.

3.3. Saturation Index Calculation In saline waters, not all ions are active, but their activity is conditioned by the electrical charges of other ions. The activity coefﬁcients are calculated applying the Debye–Hückel formula [27], and the activity is determined in mol/L. Subsequently, the chemical equilibrium of the reactions with the different salts are determined, and then the saturation index is obtained in order to determine the direction of each reaction. The process followed to determine the saturation index (SI) is shown below. The activity of a species or material according to its concentration is deﬁned as

(Xi) = mi·γi (1)

where

(Xi): activity of the species or material under study (mol/L), mi: concentration (mol/L), and γi: ionic activity coefficient (values are between 0 and 1) where the activity coefficient depends on • the radius and the ion charge, • the dissolution temperature, and • the water salinity (the higher the salinity, the lower the active proportion of the element). The activity coefficient is obtained using the Debye–Hückel formula: √ 2 −A·zi · I log γi = √ (2) 1 + B·ai· I where

γi: ionic activity coefﬁcient of the ion (i), ai: constant for each ion (it depends on its radius and its charge) (Table2), and zi: valence of the element i. Sustainability 2021, 13, 1505 10 of 25

Table 2. Values of constant ai in the Debye–Hückel formula.

2+ 2+ + + − 2− − Ion Ca Mg Na K CO3H SO4 Cl

Constant ai 5.0 5.5 4.0 3.0 5.4 5.0 3.5

A and B are constants that depend on the temperature. They are tabulated, although they can also be obtained in the following way, expressing T in ◦C: A = 0.4883 + 8.074·10−4·T mol−0.5·kg0.5 (3)

B = 0.3241 + 1.600·10−4·T mol−0.5·kg0.5 (4)

The influence of temperature on the activity coefficient is irrelevant for the usual range of temperatures, so that the variation of the saturation index against temperature was not significant in our case of study. This point was checked and validated in piezometer number 6 (Figure3) for the range of 10 to 19 ◦C. Parameter I using the Debye–Hückel formula expresses the ionic force, which also indicates the water salinity: 1 I = m z2 (5) 2 ∑ i i where

mi: element I concentration (mol/L), and zi: valence of the element i. The saturation index is related to the chemical equilibrium of the reaction, and it is expressed using the following reaction:

3A + 2B ↔ 4C + D (6)

When the reaction is in equilibrium, by the law of mass action, the following is obtained: γ4 · γ = C D K 3 2 (7) γA · γB where K is a constant (theoretical) that characterizes a reaction at a given temperature (25 ◦C, if no other is indicated); for example, for gypsum, K = 10−4.58 (Table3). K is also called the dissociation constant (equation in equilibrium).

Table 3. Values of the dissolution constant (K) of the evaporitic materials.

Material Dissolution Reaction K ++ = −4.36 Anhydrite CaSO4 ↔ Ca + SO4 10 = −0.179 Thenardite Na2SO4 ↔ 2Na + SO4 10 + = −1.14 Mirabilite Na2SO410H2O ↔ 2Na + SO4 + 10H2O 10 + ++ = −5.245 Glauberite Na2Ca(SO4)2 ↔ 2Na + Ca + 2SO4 10 ++ = −2.14 Epsomite MgSO4 7H2O ↔ Mg + SO4 + 7H2O 10 Halite NaCl ↔ Na+ + Cl− 10+1.582 + = −4.58 Gypsum CaSO4 2H2O ↔ Ca + SO4 + 2H2O 10

To evaluate the imbalance (no equilibrium) of a dissolution sample and, therefore, the direction of a reaction in such an unsaturated dissolution sample, the ion activity product (IAP) is used. Note that in Equation (7), K expresses equilibrium in the sample:

γ4 · γ = C D IAP 3 2 (8) γA · γB Sustainability 2021, 13, 1505 11 of 25

Saturation index (SI) is a function of the IAP/K ratio, where K is the dissociation constant. IAP SI = log (9) K Regarding the SI, the following cases in relation to a speciﬁc material dissolution can occur: SI > 0, reaction to the left: supersaturated water, therefore precipitation, SI = 0, equilibrium reaction, and SI < 0, reaction to the right: subsaturated water, therefore dissolution.

4. Geology and Hydrogeology 4.1. Stratigraphic Synthesis and Mineralogy of the Site The area of study is located in the Tajo basin that occupies the central part of the Iberian Peninsula. This basin was formed and filled in the Cenozoic by several basement uplifts [28]. Saline rocks in this basin are abundant [29] and have been dated to the Neogene (Figure5). Miocene deposits of the Tajo basin were divided by [ 30] into three main units: the lower or saline unit, the intermediate or middle unit, and the Upper Miocene units. The lower unit, where the tunnel is located, is Lower Miocene in age (23.2–16.2 Ma), and it was formed in a wide saline lake system that occupied the basin center [31]. The saline deposits, a succession up to 500 m thick, are composed of alternating anhydrite, halite, and glauberite beds with some thin layers of interbedded fine detritic sediments [29]. At the top of the lower unit, there is a 10–20 m thick alternation of thin layers (tens of cm thick each) composed of secondary gypsum, both alabastrine and macro-crystalline, interbedded with shales and marls (Figure4B). This unit was interpreted as a weathering cover, a product of the replacement of glauberite and anhydrite by gypsum [30]. The top of the lower unit is established at a paleokarstic surface [32]. Higher up in the sequence, the Miocene middle unit is mostly composed of primary gypsum forming tabular beds up to 1 m thick (Figure4B). This unit passes upwards into abundant carbonate bodies and is characterized by the absence of evaporitic minerals such as halite or glauberite. During this period, there was a significant progradation of siliciclastic sediments that reached the central part of the basin. Sedimentation during the Miocene upper unit was dominated by carbonates and marls [31]. In the area of the south exit of the tunnel, the materials found in the boreholes are at the base a minimum of 10 m of massive layers of halite and glauberite covered by a 2 m thick brecciated layer of glauberite, anhydrite, and gypsum in a clayey matrix that gradually passes to about 15 m of massive beds of glauberite and thenardite with minor contents of mirabilite, polyhalite, and epsomite. This sodium sulphate-rich layer passes upwards to a 10 m thick layer of interbedded clay and secondary gypsum after glauberite and anhydrite. The contact between the lower Na-rich and the upper gypsum-rich layers is made by a 1 to 4 m thick transition layer where glauberite and anhydrite is being transformed into secondary gypsum. According to the classification defined by [30], the Na-rich facies at the base belong to the lower unit, while the upper part, rich in secondary gypsum, corresponds to the weathering cover. A representative geological cross section is included in Figure6 across the section between the origin of the false tunnel and the trench, covering the most affected draining area of the study. Sustainability 2021, 13, x FOR PEER REVIEW 12 of 26

thick brecciated layer of glauberite, anhydrite, and gypsum in a clayey matrix that gradually passes to about 15 m of massive beds of glauberite and thenardite with minor contents of mirabilite, polyhalite, and epsomite. This sodium sulphate-rich layer passes upwards to a 10 m thick layer of interbedded clay and secondary gypsum after glauberite and anhydrite. The contact between the lower Na-rich and the upper gypsum-rich layers is made by a 1 to 4 m thick transition layer where glauberite and anhydrite is being transformed into secondary gypsum. According to the classification defined by [30], the Na- rich facies at the base belong to the lower unit, while the upper part, rich in secondary gypsum, corresponds to the weathering cover. A representative geological cross section Sustainability 2021, 13, 1505 12 of 25 is included in Figure 6 across the section between the origin of the false tunnel and the trench, covering the most affected draining area of the study.

FigureFigure 6. 6. GeologicalGeological cross section between the origin ofof thethe falsefalse tunneltunnel andand thethelast lasttrench trench placed placed in inthe the south south portal. portal. My Mya:a gypsum: gypsum with with gray gray clay; clay; MyMybb:: gypsum with brown brown marl marl clay; clay; R: R: anthropic anthropic filler;ﬁller; QV: QV: alluvial alluvial (lime, (lime, clay, clay, pebbles); pebbles); MG: MG: massive massive glauberite, glauberite, occasionally occasionally with with gray gray clay; clay; MH: MH: massivemassive and and crystalline crystalline halite; halite; CL: CL: contact contact layer layer between between tertiary tertiary units. units. Weathered Weathered and and fractured; fractured; PZ-3:PZ-3: piezometer piezometer (in (in black, black, piezom piezometereter projection; projection; in inblue, blue, piezomet piezometerer in inthe the cross cross section); section); SH2: SH2: borehole;borehole; (600.9): ground ground le level;vel; 20.30: boring boring depth. depth.

4.2.4.2. Hydrogeological Hydrogeological Synthesis Synthesis 4.2.1.4.2.1. Evaporite Evaporite Aquifers Aquifers before before the the Tunnel Tunnel Construction UnderUnder natural natural conditions, conditions, without without any any human human action action or or alteration, in in this area the followingfollowing sectors sectors (Figures (Figures 11 andand5 5)) within within the the evaporite evaporite massif massif were were defined defined [ 25[25].]. 1. OntígolaOntígola springs springs sector. AboutAbout 33 kmkm to to the the east east of of the the southern southern mouth mouth of theof the El Regajal El Re- gajaltunnel, tunnel, there there are the are so-called the so-called Ontí golaOntígola springs, springs, as mentioned, as mentioned, with with an appreciable an appre- ciableflow (4flow L/s (4 in L/s the in middle the middle of 2012, of 2012, lowwater), low water), and originatingand originating the stream the stream that runsthat runsnorthward northward to Aranjuez to Aranjuez city city and and the Tagusthe Tagus rivers rivers (Figure (Figure5). These 5). These springs springs drain drain an anaquifer aquifer comprised comprised of karstified of karstified gypsum gypsum of the of evaporitic the evaporitic massif massif upper upper part, as part, well as wellof the as alluvialof the alluvial subalve subalve drainage. drainage. This aquifer This aquifer is a good is a examplegood example of the of aquifer the aquifer types typesthat exist that inexist the in area, the area, associated associated with thewith massif the massif upper upper gypsum, gypsum, calcium calcium sulfate sulfate facies faciesgroundwater, groundwater, etc. etc. 2. Intermediate sector. Includes the highlands or intermediate plain between the abovementioned springs and the southern El Regajal tunnel portal of the tunnel and the trench. The consequent drainage difficulty of this plain explains the existence of a water table close to the surface. Regajal is a Spanish place name that means mud, pond, swamp, or quagmire. 3. The aquifer in the area of the El Regajal tunnel portal. The existence of old lagoons that were drained and desiccated to about 4 m deep during the construction of the N-IV highway (Figure1) is known. These lagoons drained a shallow gypsiferous aquifer that included the upper part of the evaporitic massif and perhaps spots of terraced deposits located in the surroundings. Sustainability 2021, 13, x FOR PEER REVIEW 13 of 26

2. Intermediate sector. Includes the highlands or intermediate plain between the abovementioned springs and the southern El Regajal tunnel portal of the tunnel and the trench. The consequent drainage difficulty of this plain explains the existence of a water table close to the surface. Regajal is a Spanish place name that means mud, pond, swamp, or quagmire. 3. The aquifer in the area of the El Regajal tunnel portal. The existence of old lagoons that were drained and desiccated to about 4 m deep during the construction of the N-IV highway (Figure 1) is known. These lagoons drained a shallow gypsiferous aquifer that included the upper part of the evaporitic massif and perhaps spots of terraced deposits located in the surroundings.

4.2.2. Conceptual Model of Induced Flow after the Construction of the Tunnel in 2012 Sustainability 2021, 13, 1505 According to [25], when southern El Regajal tunnel mouth excavation began, the13 ofap- 25 proximate water table level was about 4 m deep, which coincides with the drainage of the old lagoon. In this first stage, water was pumped directly from the excavation. Subse- quently, the abovementioned drainage well was drilled. It is clear that the trench excava- 4.2.2. Conceptual Model of Induced Flow after the Construction of the Tunnel in 2012 tion and the drainage well pumping caused water table lowering in this entire aquifer sector.According This was tothe [ 25situation], when in southern 2012 [25], El as Regajal seen in tunnel the isopieze mouth excavationmap in Figure began, 7. The the piezometricapproximate contour water tablemap was level obtained was about with 4 mthe deep, information which coincidesof the piezometric with the drainagenetwork andof the shows old the lagoon. flow situation, In this ﬁrst alreadystage, altered water by was the pumped drainage directly operations. from the excavation. Subsequently,Therefore, theFigure abovementioned 7 shows the potentiometric drainage well surface was drilled. in which It is the clear drawdown that the trench cone excavation and the drainage well pumping caused water table lowering in this entire aquifer produced by the pumping and the trench catchment creating an influence radius of about sector. This was the situation in 2012 [25], as seen in the isopieze map in Figure7. The 1000 m can be easily seen. The continuity of the piezometric contours and the existence of piezometric contour map was obtained with the information of the piezometric network the macrocone evidence the behavior of the massif as a single aquifer, with neither having and shows the ﬂow situation, already altered by the drainage operations. separated compartments nor hung levels.

FigureFigure 7. 7. MacroconeMacrocone of of depression depression showing showing the the radial radial flow ﬂow to to a a dr drainageainage pumping pumping well well in in an an unconfined unconﬁned evaporitic evaporitic aquifer aquifer and to a drainage trench at the south tunnel portal. and to a drainage trench at the south tunnel portal.

Therefore, Figure7 shows the potentiometric surface in which the drawdown cone produced by the pumping and the trench catchment creating an inﬂuence radius of about 1000 m can be easily seen. The continuity of the piezometric contours and the existence of the macrocone evidence the behavior of the massif as a single aquifer, with neither having separated compartments nor hung levels.

5. Results 5.1. Hydrogeochemistry: General Characteristics and Ion Distribution The characteristics and chemical facies of the groundwater reﬂect the stratigraphy and mineralogy of the site rocks, with an increase of the salinity in depth, and a groundwater stratiﬁcation according to their salt concentration. According to the results of the chemical analyses, the groundwater can be assigned to sulphated calcic facies down to approximately 5 m depth. At greater depths and in the catchment areas, the dominant water facies is sulphated sodic. The results of the chemical analyses of the water are shown in Table4. The last lines of the table show the results of the water taken from the two existing catchments in the Sustainability 2021, 13, 1505 14 of 25

area of study, the drainage well and the pump room, as well as the union of both of them at the exit.

Table 4. Chemical parameters (mg/L) of water from different sampling points (except for PZ 6, which is shown in Table5).

Sample + ++ SO = − ++ + − CO = − Conductivity Depth Na Ca 4 Cl Mg K CO3H 3 NO3 (µS/cm) PZ-1 13 m 53.086 686 101.682 5.141 1.072 145 270 0 13 94.900 PZ-2 14 m 53.462 698 103.967 584 339 67 199 35 5 91.100 PZ-3 15 m 9.650 554 21.234 346 215 14 215 0 15 29.300 PZ-3 17.6 m 46.476 658 87.620 2.850 631 102 176 24 5 84.500 PZ-5 8.5 m 1.812 511 5.144 29 837 3 162 0 0 8.270 PZ-7 25 m 53.720 590 103.650 9.178 1.792 119 597 0 5 98.000 PZ-7 30 m 76.308 590 136.729 20.138 2.005 648 306 113 5 120.600 PZ-8 15 m 59.717 570 100.489 15.278 1.074 101 665 0 7 105.100 PZ-10 5 m 298 499 1.952 34 103 3 195 0 72 3.060 PZ-11 12 m 1.473 416 5.190 36 361 2 128 0 0 7.770 Final discharge basin n.a. 43.741 654 106.187 414 559 27 264 38 5 87.700 Trench pumping n.a. 4.520 622 84.349 484 213 145 188 18 7 81.200 Drainage well n.a. 10.447 503 21.897 558 382 18 278 0 6 31.200 pumping

Table 5. Distribution of the water chemical concentration in PZ-6.

Concentration (mg/L) Sampling Depth 5.00 m 10.50 m 18.00 m Na+ 428 5.971 49.610 Ca++ 570 519 682 = SO4 2.374 13.073 100.593 Cl− 122 154 1.820 Mg++ 133 249 837 K+ 1.9 5.6 102 − CO3H 202 213 304 = CO3 0.001 0.001 0.001 − NO3 77 48 5 Conductivity (µS/cm) 30.830 18.680 89.900

Table5 shows the data recorded at different depths (5.00 m, 10.50 m, and 18.00 m) in PZ-6 (Figure3). This piezometer is located in one of the less disturbed sectors of the aquifer by the pumping operations. A prevalent increase of concentration of ions against depth can be clearly observed. This trend depends on each ion, which represents a speciﬁc predominant mineral. The electrical conductivity of the groundwater was found to be consistently high, which gives an idea of the high concentration of salts. The measured values ranged between 3.000 µS/cm in shallow groundwater (up to 5 m) and 120.000 µS/cm in the deepest parts. Figure8 shows a plan view of the distribution of the conductivities in the depth range 5 to 15 m. Figure9 shows the conductivity distribution with depth in a vertical section parallel to the false tunnel. The sulphates in water have their origin in the dissolution of gypsum, anhydrite, thenardite, glauberite, mirabilite, polyhalite, and epsomite. The depth distribution (Figure 10) shows that sulphates increased up to 30 m, where the gypsum, glauberite, and thenardite layers are located. At greater depths, halite predominates. Figure 11 also shows the depth distribution of the chloride concentration (mg/L). It can be seen that the concentration and dispersion of the values increased with depth, as it approached the lower halite layer. The highest ionic concentrations were found in the deepest part of the drawdown cone. The origin of the sodium concentration in water is related to the presence of thenardite, glauberite, halite, and mirabilite. High sodium concentrations were found mostly in the lower part (Table4), probably due to the proximity to the halite layer. It is relevant that the highest values of sodium concentration were associated with the depression cone near the drainage well, similar to what happens with chlorides. Sustainability 2021, 13, x FOR PEER REVIEW 15 of 26

Sustainability 2021, 13, x FOR PEER REVIEW 15 of 26

The electrical conductivity of the groundwater was found to be consistently high, The electricalwhich conductivity gives an idea of of the the ground high concentrationwater was found of salts. to be The consistently measured high,values ranged be- which gives antween idea 3.000 of the μ S/cmhigh inconcentration shallow groundwater of salts. The (up measured to 5 m) and values 120.000 ranged μS/cm be- in the deepest tween 3.000 μparts.S/cm inFigure shallow 8 shows groundwater a plan view (up to of 5 the m) distandribution 120.000 μofS/cm the conductivitiesin the deepest in the depth parts. Figure range8 shows 5 to a 15plan m. view Figure of the9 shows distribution the conductivi of the conductivitiesty distribution in with the depthdepth in a vertical Sustainability 2021, 13, 1505 15 of 25 range 5 to 15section m. Figure parallel 9 shows to the the false conductivi tunnel. ty distribution with depth in a vertical section parallel to the false tunnel.

Figure 8. ConductivityFigure 8. at Conductivity depth between at depth 5 m and between 15 m. 5 m and 15 m. Figure 8. Conductivity at depth between 5 m and 15 m.

Sustainability 2021, 13, x FOR PEER REVIEW 16 of 26

Figure 9. ConductivityFigure ( µ9.S/m) Conductivity proﬁle along (μS/m) the profile railway along track. the railway track. Figure 9. Conductivity (μS/m) profile along the railway track. The sulphates in water have their origin in the dissolution of gypsum, anhydrite, The sulphatesthenardite, in water glauberite, have their mirabilite, origin polyhalite,in the dissolution and epsomite. of gypsum, The depth anhydrite, distribution (Figure thenardite, glauberite,10) shows mirabilite, that sulphates polyhalite, increased and epsomite. up to 30 m,The where depth thedistribution gypsum, (Figureglauberite, and the- 10) shows thatnardite sulphates layers increased are located. up toAt 30greater m, where depths, the halite gypsum, predominates. glauberite, and thenardite layers are located. At greater depths, halite predominates.

Figure 10. Vertical proﬁle of sulphate content (mg/L) in the evaporitic aquifer along the false tunnel Figure 10. Vertical showingprofile of drainage sulphate well content pump (mg/L) (left side in the of theevaporitic draw) and aquifer trench along (right the side false of tunnel the draw). showing drainage well pump (left side of the draw) and trench (right side of the draw).

Figure 11 also shows the depth distribution of the chloride concentration (mg/L). It can be seen that the concentration and dispersion of the values increased with depth, as it approached the lower halite layer. The highest ionic concentrations were found in the deepest part of the drawdown cone.

Figure 11. Relation between depth and chloride and sulphate content in the evaporitic aquifer.

The origin of the sodium concentration in water is related to the presence of thenardite, glauberite, halite, and mirabilite. High sodium concentrations were found mostly in the lower part (Table 4), probably due to the proximity to the halite layer. It is relevant that the highest values of sodium concentration were associated with the depression cone near the drainage well, similar to what happens with chlorides. The presence of magnesium in water is probably due to the dissolution of epsomite. The values of magnesium concentration varies from 100 mg/L to more than 2000 mg/L. Magnesium concentration increased with depth, which seems logical due to the higher abundance of this mineral at depth. Finally, the potassium content could be explained by Sustainability 2021, 13, x FOR PEER REVIEW 16 of 26

Figure 10. Vertical profile of sulphate content (mg/L) in the evaporitic aquifer along the false tunnel showing drainage well pump (left side of the draw) and trench (right side of the draw).

Figure 11 also shows the depth distribution of the chloride concentration (mg/L). It can be seen that the concentration and dispersion of the values increased with depth, as it Sustainability 2021, 13, 1505 16 of 25 approached the lower halite layer. The highest ionic concentrations were found in the deepest part of the drawdown cone.

FigureFigure 11. 11. RelationRelation between between depth depth and and chloride chloride and and su sulphatelphate content content in in the the evaporitic evaporitic aquifer. aquifer.

TheThe origin presence of the of sodium magnesium concentration in water isin probably water is related due to to the the dissolution presence of thenard- epsomite. ite,The glauberite, values of halite, magnesium and mirabilite. concentration High varies sodium from concentrations 100 mg/L to were more found than 2000 mostly mg/L. in theMagnesium lower part concentration (Table 4), probably increased due with to the depth, proximity which to seems the halite logical layer. due It to is the relevant higher abundance of this mineral at depth. Finally, the potassium content could be explained by that the highest values of sodium concentration were associated with the depression cone the dissolution of polyhalite. Values of potassium in water ranged from a few mg/L to near the drainage well, similar to what happens with chlorides. several hundred mg/L, increasing with depth. The presence of magnesium in water is probably due to the dissolution of epsomite. The5.2. values Geochemistry of magnesium Behavior. concentration Saturation Index varies from 100 mg/L to more than 2000 mg/L. Magnesium concentration increased with depth, which seems logical due to the higher Tables6 and7 show the calculated activity of the different sample ions at the drainage abundance of this mineral at depth. Finally, the potassium content could be explained by pump exit.

Table 6. Ion activity in water at the drainage pump exit, where C is concentration (mg/L); M is molecular mass; Cm is molar concentration (mol/L); z is valence; I is ionic strength; a is constant for each ion, which depends on its radius and its charge (Table2); Υ is activity coefﬁcient, and Υ’ is activity in mol/L.

0 C (mg/L) M Cm (mol/L) z I a log ΥΥΥ (mol/L) Na+ 10.447 23 0.45422 1 0.75030 4.0 −0.20426 0.6248 0.283792 Ca++ 503 40 0.01258 4 0.75030 5.0 −0.72121 0.1900 0.002389 = SO4 21.897 96 0.22809 4 0.75030 5.0 −0.72121 0.1900 0.043341 Cl− 558 35.5 0.01572 1 0.75030 3.5 −0.21880 0.6042 0.009497 Mg++ 382 24.3 0.01572 4 0.75030 5.5 −0.68126 0.2083 0.003275 K+ 18 39.1 0.00046 1 0.75030 3.0 −0.23557 0.5813 0.000268 − CO3H 278 61.11 0.00455 1 0.75030 5.4 −0.17222 0.6726 0.003060 = CO3 0.001 60 0.00000 4 0.75030 4.5 −0.76615 0.1713 0.000000 − NO3 6 62 0.00010 1 0.75030 3.0 −0.23557 0.5813 0.000056 Sustainability 2021, 13, 1505 17 of 25

Table 7. Chemical equilibrium and water saturation at the drainage well according to different evaporitic minerals, where K is dissociation constant, IAP is ionic activity product, and SI is saturation index.

Mineral K IAP SI Reaction Direction Gypsum 0.00003 0.00010 0.5952 saturated Anhydrite 0.00004 0.00010 0.3752 saturated Thenardite 0.66222 0.00349 −2.2781 dissolves Mirabilite 0.07244 0.00349 −1.3171 dissolves Glauberite 0.00001 0.00000 −1.1969 dissolves Epsomite 0.00724 0.00014 −1.7079 dissolves Halite 38.19443 0.00270 −4.1514 dissolves

Table7 shows the chemical equilibrium and the water saturation index (SI) for the different evaporitic minerals. Table8 shows the conductivity, the total salt concentration, and the volume of salt dissolved per month in the massif. Water was slightly saturated in gypsum and anhydrite, with a very high electrical conductivity of 31.200 µS/cm. However, the saturation point of gypsum was far from what it would have been in the case of containing only calcium sulphate (whose average saturation concentration is 2.6 g/L). This is due to the presence of the other ions, and in particular the Cl−, which together determine the chemical activity of sulphate ions. The coexistence of dissolved gypsum and Cl−, modiﬁes the chemical activity of sulphate ions. In fact, the incipient gypsum saturation in water samples of PZ-11 (SI = 0.3) and anhydrite (SI = 0.08) with a calcium sulphate concentration of 5.600 g/L and an electrical conductivity of 7.770 µS/cm was observed. In these conditions, the water was still dissolving the gypsum despite the apparent saturation evidenced by such a high conductivity.

Table 8. Electrical conductivity and concentration of total salts. Material dissolved in the drainage well.

Electrical Conductivity Dissolved Salts Salts Dissolved Volume (µS/cm) Concentration (g/L) (m3/Month) 31.200 33.8 88.6

Likewise, the subsaturation of water should be highlighted with regard to the rest of the evaporite minerals (thenardite, mirabilite, glauberite, epsomite, and halite), which accentuates the water dissolution capacity and, therefore, the potential loss of material in the massif by dissolution (karstiﬁcation). The monthly volume of dissolved material is 88.600 m3, increasing progressively the volume of pores and caves in the massif and therefore modifying its mechanical characteristics. Table9 shows the saturation index (SI) of water from different sampling points. The progress of the saturation index with depth for the different minerals is shown in Figure 12. Water is slightly saturated (>0) for gypsum and anhydrite at all depths, for epsomite and mirabilite only below 16 m, and for the rest of the minerals, saturation is never reached. The lines corresponding to thenardite and mirabilite, and also to gypsum and anhydrite, are parallel, since they have the same chemical composition except for their respective water contents. Sustainability 2021, 13, 1505 18 of 25

Table 9. Saturation indexes (SIs) of the existing materials at scheduled representative sampling points. SI < 0, subsaturated water in each mineral; SI > 0, supersaturated water.

Sample Location Gypsum Anhydrite Thenardite Mirabilite Glauberite Epsomite Halite Ontígola 1 Spring 0.24 0.02 −6.78 −5.82 −6.06 −2.45 −6.53 Ontígola 2 Spring 0.23 0.01 −6.33 −5.37 −5.62 −2.41 −6.23 Ontígola 3 Spring 0.22 0 −5.8 −4.83 −5.09 −2.64 −5.58 SustainabilityPZ-5 8.5 m2021, 13, x FOR PEER0.32 REVIEW 0.1 −4.15 −3.19 −3.35 −1.67 27 of 27 −6.08 PZ-6 5.0 m 0.28 0.06 −5.54 −4.58 −4.77 −2.56 −6.01 PZ-6 10.5 m 0.54 0.32 −2.86 −1.9 −1.84 −1.97 −4.9 PZ-10 5.0 m 0.2 −0.02 −5.89 −4.93 −5.21 −2.7 −6.71 PZ-11 12.0 m 0.3 0.08 −4.27 −3.31 −3.48 −1.96 −6.06 Final Discharge Basin 0.28 0.06 −4.46 −3.5 −3.69 −2.31 −4.78 Tunnel Discharge Basin 1 0.78 −0.68 0.28 0.81 −1.23 −3.8 Trench Drainage 1.01 0.79 −2.64 −1.68 −1.14 −1.62 −4.67 Drainage Well 0.6 0.38 −2.28 −1.32 −1.2 −1.71 −4.15

Figure 12. Progress of saturation (saturation index) against depth in piezometers of water intake. Sustainability 2021, 13, 1505 19 of 25

It can be deducted that gypsum, when accompanied by other evaporitic minerals (anhydrite, mirabilite, halite, etc.), shows the combined action of the rest of the ions, which manifests itself in the different ionic forces and ionic activities produced. Therefore, the saturation index (SI) is critical to determine the direction of the reaction and to determine whether the groundwater is saturated in different saline minerals or if it will continue dissolving minerals and creating or enlarging pores. The combined inﬂuence of ions directly affects the water saturation in relation to each of the minerals. Taking as an example the gypsum’s solubility, it is remarkably modiﬁed; therefore, waters with a high electrical conductivity and a high saline concentration remain unsaturated in gypsum, having the capacity of dissolving this mineral. The described phenomenon can be observed in Table8; an electrical conductivity of 31.200 µS/cm was measured in the water, which is slightly high and, nevertheless, the water remains unsaturated in relation to the thenardite, mirabilite, glauberite, epsomite, and halite, while in the absence of these other minerals, the gypsum saturation is reached at only 2.600 µs/cm [21].

6. Discussion From the results obtained and considering the evolution observed in groundwater (piezometers PZ-1 to PZ-17) and in the surface waters analyzed, some aspects can be highlighted. The water flowing through the evaporitic deposits to the drainage well has two origins: (i) rain infiltration water (vertical flow), and (ii) lateral incoming flow, mostly from the aquifer of the Ontígola springs, located outside the piezometric contour map (Figure7). In its circulation, the infiltrated water quickly reaches equilibrium or becomes slightly saturated in sulphated calcium salts in the upper part of the aquifer. For the rest of the evaporitic minerals, the waters remain unsaturated, and rapid dissolution takes place at depth. Even so, water is only saturated in glauberite, anhydrite, and thenardite at levels below 16–20 m, and it remains unsaturated in epsomite and halite. This process is showed in Figure 12, where the prevalence of linear trends shows this phenomenon. The incoming water to the system affected by the pumping should originally have the average chemical characteristics of the Ontígola springs. The characteristics of the outgoing water are different than those of the Ontígola spring due to the pumping operations. The chemistry variation from the original water (Ontígola springs) to the outgoing water (average of the two main catchment points, the drainage well, and drainage of the trench) is shown in Table 10. The resulting increase in salt concentration is 58.8 g/L.

Table 10. Concentration variations between surface waters of the Ontígola springs and drainage points.

Concentration of Concentration at Ontígola Springs Trench and Well Difference (%) over Ions Difference (mg/L) (Initial Water Drainage Water Ontígola Parameters Chemistry) (mg/L) (mg/L) Na+ 213.74 7.48350 7.26976 3.40122% Ca++ 581.67 562.50 −19.17 −3.30% SO4= 1.99994 53.12300 51.12306 2.55623% Cl− 235.59 521.00 285.41 121.15% Mg++ 178.99 297.50 118.51 66.21% K+ 22.03 81.50 59.47 269.95% CO3H− 276.10 233.00 −43.10 −15.61% CO3= 0.00 9.00 9.00 NO3− 115.93 6.50 −109.43 −94.39% Conductivity (µS/cm) 3.58667 56.20000 52.61333 1.46691% Salts content 3.210 6.1990 58.780 1.83115%

Table 11 shows the variation of the saturation index of the different salts before and after the water inﬁltration process of the Ontígola springs and its subsequent pumping and draining through the massif. Figure 13 shows the clear trend of the subsaturated waters Sustainability 2021, 13, 1505 20 of 25

towards saturation in several salts. However, water can only be considered saturated in gypsum and anhydrite with saturation indexes close to zero, being at equilibrium reactions. The presence of ions of different saline compounds in dissolution leads to a general subsaturation state for the rest of the salts at the drainage points, although less

Sustainability 2021, 13, x FOR PEER REVIEWobvious than in the original water. 21 of 26

Table 11. Saturation index between surface waters of Ontígola springs and catchment points.

Table 11. Saturation index between surface waters of Ontígola springs and catchment points.Variation of Saturation Saturation Index (SI) Saturation Index (SI) in Material Index (%) Over Saturation Index (SI) in Ontígolain OntSaturationígola Springs Index (SI)Outcoming in Outcoming Drainages Variation of Saturation Index (%) Material Ontígola One Springs Drainages Over Ontígola One Gypsum 0.23 0.80 247.83% Gypsum 0.23Anhydrite 0.01 0.80 0.58 247.83% 5.70000% Anhydrite Thenardite0.01 −6.30 0.58 −2.46 5.70000%−60.95% Thenardite −Mirabilite6.30 −5.34 −2.46 −1.50 −60.95%−71.91% Mirabilite −Glauberite5.34 −5.59 −1.50 −1.17 −71.91%−79.07% Glauberite −5.59Epsomite −2.50 −1.17 −1.67 −79.07%−33.20% Epsomite −2.50Halite −6.12 −1.67 −4.41 −33.20%−27.94% Halite −6.12 −4.41 −27.94%

Figure 13. SaturationSaturation index index evolution evolution across across lo locations:cations: Ontígola Ontígola springs springs and and water water drained drained and and pumped pumped from the ground.

6.1. Porosity Increase in the Evaporitic Massif Due to the Drainage from the Well and the Trench The variation of saline concentration between the incoming water to the system and the water collected from the catchment points allows the total amount of dissolved evaporitic minerals and the volume that it occupied in the terrain to be estimated. The input flow data between the two-catchment points averages 4 L/s (drainage well: 2.5 L/s, trench: 1.5 L/s). The average dissolution is 58.800 g/L with 90.188 g/L in the trench and 33.787 g/L in the drainage well. Sustainability 2021, 13, x FOR PEER REVIEW 22 of 26 Considering an average density of the evaporitic minerals of 2.47 t/m3, the yearly volume of salts dissolved is about 2.772 m3. This figure is obtained from the monthly dissolution estimated in the trench area of 142 m3/month (350.7 t/month) and from the drainage6.2. Porosity well Increase (88.6 m in3 /month,the Evaporitic 218.94 Massif t/month). Due to the Transformation of Glauberite to Gyp- sum 6.2. Porosity Increase in the Evaporitic Massif Due to the Transformation of Glauberite to Gypsum The water presence or simply the moisture can produce the replacement of glauberite into Thegypsum water (solid) presence andor mirabilite simply the (dissolved) moisture canby incongruent produce the dissolution. replacement The of glauberite transfor- intomation gypsum of glauberite (solid) and into mirabilite gypsum is (dissolved) a prototype by of incongruent an incongruous dissolution. dissolution The transfor-reaction, mationthe result of glauberiteof which is into the gypsum neoformation is a prototype of amorphous of an incongruous gypsum and dissolution the enrichment reaction, of thegroundwater result of whichin sulfate is the anion neoformation and sodium of cation. amorphous This transformation gypsum and the entails enrichment a 27% de- of groundwatercrease in volume in sulfate and favors anion the and generation sodium cation. of moderate This transformation voids in the ground, entails agenerally 27% de- creasefrom pre-existing in volume andjoints favors and cracks the generation [33]. The diameter of moderate of the voids created in the voids ground, can range generally from froma few pre-existingmillimeters to joints several and decimeters. cracks [33]. Thes Thee diameterprocesses ofcan the be created increased voids by canthe massif range frompermeability a few millimeters achievement, to severalwhich will decimeters. tend to promote These processes the renewed can groundwater be increased circula- by the massiftion and, permeability therefore, karstification. achievement, This which transf willormation tend to promote process has the slow renewed kinetics groundwater in natural circulationconditions, and,but it therefore, can be fast karstification. (days or months This) transformation in the presence process of renewed has slow “fresh” kinetics waters in naturalwith high conditions, capacity butto dissolve it can be and fast transfor (days orm months) the glauberite. in the presenceThe following of renewed possible “fresh” pat- waters with high capacity to dissolve and transform the glauberite. The following possi- terns have been inferred from the behavior of each mineral observed in this study. Figure ble patterns have been inferred from the behavior of each mineral observed in this study. 14 represents a vertical section through the complex CaSO4Na SO4–NaCl–H2O units. The Figure 14 represents a vertical section through the complex CaSO Na SO –NaCl–H O units. water turns glauberite into gypsum by these ways: (1) through 4the upper4 gypsum2 aquifer The water turns glauberite into gypsum by these ways: (1) through the upper gypsum with the formation of an horizon of secondary gypsum after glauberite, (2) through karstic aquifer with the formation of an horizon of secondary gypsum after glauberite, (2) through formations caused by the volume reduction in the transformation process of the glauberite karstic formations caused by the volume reduction in the transformation process of the into gypsum, (3) by karst caused by the gypsum dissolution, (4) through faults, (5) by the glauberite into gypsum, (3) by karst caused by the gypsum dissolution, (4) through faults, “water call” process caused by the excavation of the tunnel, and (6) through non-closed (5) by the “water call” process caused by the excavation of the tunnel, and (6) through drill holes, which reduce the phreatic level. non-closed drill holes, which reduce the phreatic level.

Figure 14. Vertical section through the complex CaSO Na SO –NaCl–H O units and ways that water Figure 14. Vertical section through the complex CaSO4 4Na SO4 4–NaCl–H2 2O units and ways that transformswater transforms glauberite glauberite into gypsum. into gypsum. Both the well and the trench lowered the phreatic level below the contact level between Both the well and the trench lowered the phreatic level below the contact level be- gypsum and glauberite at 590 m a.s.l. in a considerable area. Therefore, during the tween gypsum and glauberite at 590 m a.s.l. in a considerable area. Therefore, during the pumping process, the transformation of glauberite into gypsum was produced, which pumping process, the transformation of glauberite into gypsum was produced, which implied a reduction of 28% of the initial volume. The effects of this process are difficult to quantify, but certainly it contributed hastily to the formation of a new aquifer, in the form of a semi-circular sinkhole around the well, where the porosity and permeability were significantly increased. The new drainage level of the trench increased the underground flow in the saline layers down to 20 to 25 m deep, a zone where water previously barely circulated. This is the reason why waters collected at the drainage points are so saline. The large volume of additional terrain involved is large enough to maintain the current salinity levels for years. Sustainability 2021, 13, 1505 22 of 25

implied a reduction of 28% of the initial volume. The effects of this process are difficult to quantify, but certainly it contributed hastily to the formation of a new aquifer, in the form of a semi-circular sinkhole around the well, where the porosity and permeability were significantly increased. Sustainability 2021, 13, x FOR PEER REVIEW The new drainage level of the trench increased the underground flow23 in of the 26 saline layers down to 20 to 25 m deep, a zone where water previously barely circulated. This is the reason why waters collected at the drainage points are so saline. The large volume of additional terrain involved is large enough to maintain the current salinity levels for years. Water comes out with a higher saline concentration in the trench than in the drainage Water comes out with a higher saline concentration in the trench than in the drainage well because the matrix flow is more important in the trench (the catchment is linear and well because the matrix flow is more important in the trench (the catchment is linear and not punctual, as in the well). Additionally, the saturated zone of the source basin of the trenchnot is punctual, entirely located as in the in the well). glauberite Additionally, layer. the saturated zone of the source basin of the trench is entirely located in the glauberite layer. 6.3. Distribution and Development of Porosity and Karstification in Space and Time 6.3. Distribution and Development of Porosity and Karstification in Space and Time The most likely consequence of drainage by pumping is the development of a conduit The most likely consequence of drainage by pumping is the development of a conduit network near the well caused by direct dissolution and erosion. It remains unknown how thisnetwork porosity nearincrease the is well distributed, caused by although direct dissolution it is plausible and that erosion. the voids It remainsare concentrated unknown how aroundthis porositythe pumping. increase In the is distributed,initial stages, although a conduit it network is plausible would that be the established voids are in concentrated the preferredaround direction the pumping. oriented In (i) theto the initial maximu stages,m hydraulic a conduit slope, network and (ii) would along be the established hori- in zontalthe stratification preferred direction planes, where oriented the (i) flow to theresistance maximum is lower, hydraulic taking slope,advantage and (ii)of the along the levelshorizontal of halite stratification and sodium planes,sulphate where with thehigher flow solubility. resistance Figures is lower, 7–11 taking support advantage these of the observations.levels of halite and sodium sulphate with higher solubility. Figures7–11 support these observations.In the first phase, the stimulated water circulation caused the decrease of the phreatic level, theIn evacuation the first phase, of the theunderground stimulated water water stored circulation in the causedupper part the decreaseof the evaporitic of the phreatic aquifer,level, and the in evacuation the overlying of the terrace, underground radially propagating water stored the in development the upper part of the of theporos- evaporitic ity. aquifer, and in the overlying terrace, radially propagating the development of the porosity. TheThe analyzed analyzed karst karst system system has a hassingle a single outgoing outgoing point and point multiple and multiple incoming incoming points points alongalong the theaquifer’s aquifer’s feeding feeding area (Figure area (Figure 15). Hydrochemistry 15). Hydrochemistry also plays also an plays important an important role, role, becausebecause from from the thesubsaturated subsaturated water water from from the rain the or rain irrigation or irrigation recharge, recharge, with the with capac- the capacity ity to dissolve dissolve the the saline saline minerals, minerals, it becomes it becomes saturated saturated in sulphates in sulphates when wheninteracting interacting with with gypsum.gypsum. However, However, water water remains remains subsaturated subsaturated and with and the with capacity the capacity to dissolve to dissolve halite, halite, thenardite,thenardite, mirabilite, mirabilite, and andepsomite, epsomite, up to up the to pumping the pumping point, point,inclusive. inclusive. It was verified It was verified thatthat the themain main factor factor in karstification in karstification is the is presence the presence of hyper-soluble of hyper-soluble materials, materials, in which in which the thewater water is predominantly is predominantly subsaturated subsaturated and the and dissolution the dissolution process process continues continues due to the due to the changechange of saturation of saturation point, point, in some in some cases cases extraordinarily extraordinarily (Figures (Figures 11 and 11 12). and 12).

Figure 15. Possible development of karstic networks from the drainage well and the trench. Figure 15. Possible development of karstic networks from the drainage well and the trench.

7. Conclusions The study and monitoring of the evaporitic massif drainage of El Regajal (Madrid– Toledo), in the works of the trench and the false tunnel of the Madrid–Valencia high-speed railway line, has allowed the hydrochemical evolution of the water in evaporitic materials to be determined. The piezometric contours map, obtained from the piezometric control network, shows a macrocone of depression around the pumping and at the catchment of the trench, with an influence radius of at least 1000 m. The topographic height of these two drainage points determines the new hydrogeological base level, which is lower than Ontígola springs, located 2.5 km eastwards, and with whose aquifer they are hydraulically and laterally connected. The continuity of the potentiometric surface and the existence of the macrocone of depression show that the evaporitic massif behaves hydrogeologically as a single aquifer, without separate compartments or hanging levels in vertical. This does not imply that, on a small scale, the groundwater circulates preferably through the horizontal stratification planes, karstic conduits and fractures [33]. The minerals composing this evaporitic massif are arranged in horizontal layers with gypsum dominating in the upper and thenardite, glauberite, halite, mirabilite, and polyhalite in the lower. The characteristics and chemical facies of the groundwater re- flect the stratigraphy and mineralogy of the area, with an increase of salinity at depth, and groundwater stratification. The sulphated sodic water facies are dominant in the catchment areas. The hydrochemical study of the water samples in the different piezometers at different depths, including the calculation of their activities, has allowed the saturation index for different saline materials to be identified, and the water residual dissolution capacity was largely surprising despite electrical hyperconductivity, as is shown in Figures7–11. Except for gypsum and anhydrite, which appear saturated in most of the piezometers, especially at the points located at low depth, all the present minerals of evaporitic origin in this massif are susceptible to be dissolved according to the groundwater chemistry, at least to 16 m depth. Below this depth, the water remains subsaturated in relation to thenardite, mirabilite, epsomite, glauberite, and halite. The alteration of the underground flow and the consequent water renewal of the aquifer by the infiltration of rainwater and irrigation are the cause of the hydrogeochemical imbalance and of the modification of the massif characteristics. These changes, in general, cause an important loss of material, changing the strength of the terrain and the increase of voids index. These processes have been quantified, highlighting the dissolution of halite, thenardite, and mirabilite to an extreme degree, and to the gypsum and anhydrite to a moderate degree. They also include different expansive and recrystallization processes that decrease the massif porosity. Likewise, the transformation of glauberite into gypsum by incongruent dissolution, the physical erosion by water circulation, the anhydrite swelling by hydration, and the crystallization of new salts were identified. It was estimated that the evaporitic material lost in the area from the ground by dissolution is about 2.700 m3/year, extrapolating the results of study as constant over time. All these processes are originating a new aquifer by karstification and a new evaporitic massif by mineralogical transformation. It is estimated that the porosity near the tunnel increased by 1% during the construction period. The processes occurring in an evaporitic massif, such as the one under study, are identifiable and in any way quantifiable, in relation to the contents of the different evaporitic minerals (gypsum, halite, thenardite, mirabilite, glauberite, epsomite, and anhydrite). These processes are the increase of voids by erosion, dissolution, and by mineralogical transformations and, on the other hand, recrystallization processes, some of them expansive, which would decrease the rock porosity, neutralizing the increase of the porosity, but with consequences in the geomechanical properties [34]. Sustainability 2021, 13, 1505 24 of 25

Quantifying the magnitude of these processes separately is complex, but generally, the processes increasing the porosity will be dominant over time, since as the circulation of the renewed water increases, the processes of dissolution and mineral transformation are being accelerated. A measurement campaign of the ions existing in the water at different depths would offer more clarity about the evolution of these phenomena over time, which would facilitate the implementation of preventive and corrective measures in Civil Engineering.

Author Contributions: Conceptualization, J.A.M.P., I.M.P., and E.S.P.; methodology, J.A.M.P.; soft- ware, J.A.M.P. and I.M.P.; validation, J.A.M.P. and I.M.P.; formal analysis, C.S.S. and E.S.P.; investigation, E.S.P.; resources, C.S.S.; data curation, C.S.S.; writing—original draft preparation, I.M.P.; writing—review and editing, I.M.P. and E.S.P.; visualization, I.M.P.; supervision, I.M.P. and E.S.P.; project administration, I.M.P.; funding acquisition, C.S.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Fundación Agustín de Betancourt (FAB 2011). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors sincerely thank J.I. Escavy Fernández for advice and support in stratigraphic synthesis and mineralogy of the site, Eng. Mariano López Serrano for his contribution to the manuscript editing, and three anonymous reviewers who improved the original manuscript with their advice and suggestions. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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