Preliminary assessment of the geothermal potential of Rosario de la Frontera area (Salta, NW...

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Journal of South American Earth Sciences 54 (2014) 20e36

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Journal of South American Earth Sciences

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Preliminary assessment of the geothermal potential of Rosario de laFrontera area (Salta, NW Argentina): Insight from hydro-geological,hydro-geochemical and structural investigations

C. Invernizzi a,*, P.P. Pierantoni a, A. Chiodi b, R. Maffucci c, S. Corrado c, W. Baez b, F. Tassi d,G. Giordano c, J. Viramonte b

a Scuola di Scienze e Tecnologie, Sezione di Geologia, Università degli Studi di Camerino, via Gentile III da Varano, 62032 Camerino, MC, ItalybGEONORTE e INENCO (UNSa e CONICET), Av. Bolivia 5150, A4400FVY Salta, ArgentinacDipartimento di Scienze, Università degli Studi “Roma Tre”, 00146 Roma, ItalydDipartimento di Scienze della Terra, Università di Firenze, Italy

a r t i c l e i n f o

Article history:Received 21 December 2013Accepted 12 April 2014

Keywords:Andes retrowedgeGeothermal fluidsWater balanceStable isotopesNW ArgentinaReservoir

* Corresponding author. Tel.: þ39 0737 402621.E-mail address: [email protected] (C. Inv

http://dx.doi.org/10.1016/j.jsames.2014.04.0030895-9811/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

This work is part of a project aimed to the development and application of hydrogeological, hydro-geochemical and geological methodologies for the study of the geothermal system of Rosario de LaFrontera (NW Argentina).

The surface thermal manifestations of this area, whose temperatures range from 22.6 to 92.6 �C, aremainly located in the northern sector of Sierra de la Candelaria anticline. This regional structure crops outbetween the provinces of Salta and Tucuman (NWArgentina), at the foothills of the central Andean retro-wedge.

The present investigation focuses on hydrogeological and structural data, and isotopic compositions(18O, D and 3H) of thermal springs.

Preliminary results allowed to define: i) the meteoric origin of spring water and their long (more than50 years) residence time at depth, ii) a positive water balance, ranging between 2 and 4 millions of m3/yr,and iii) a conservative geothermal reservoir volume of about 39 km3, iv) a geothermal potential withEr ¼ 5.6*1018 J and Ef ¼ 0.8*1018 J.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The Argentine territory shows numerous thermal springs alongthe frontal structures of the Andean retro-wedge (Pesce, 2000).

Unlike the geothermal systems of the Puna plateau (Pesce andMiranda, 2003; Giordano et al., 2013), these systems are locatedin populated areas and therefore they have a high potential forexploitation both for therapeutic and bathing purposes, as at pre-sent, and for direct use of heat. This second intended purpose in-volves the quantification of the resource for its sustainable use, inorder to assess whether it is possible to combine traditional useswith those related to energy production.

Rosario de la Frontera thermal complex (Salta Province, NWArgentina), opened on April 1, 1880, is the oldest in South America.The thermal buildings are still in use and financially supports are

ernizzi).

provided by the Salta Province government. In 1876 Dr. Max Sia-wert started studying water chemistry and geology in the area, fouryears later Dr. Antonio Palau began the exploitation of thermalsprings for commercial purposes. In 1888, Doctors Doering andSchikendantz and in 1895 Dr. Tagliabue proved the occurrence ofnine different kinds of waters, which turned the “Termas de Salta”(as it was known until the end of the nineteenth century) to be aprivileged place for therapeutic purposes (Rivas, 2012).

The Rosario de la Frontera geothermal system is located in thenorthern sector of Sierra de la Candelaria anticline (Fig. 1).

Geology and structure of this region were previously studied(Gebhard et al., 1974; Salfity and Marquillas, 1994; Cristallini et al.,1997; Iaffa et al., 2011, 2013) also for hydrocarbon exploration(Nesossi, 1947). Scientific investigations in the Rosario de la Fron-tera geothermal area were mainly focused on traditional strati-graphic characterization (Moreno Espelta et al., 1975), geochemicaldata set on selected springs of the Termas de Salta (Seggiaro et al.,1995; Pesce and Miranda, 2003), and structural control ofgeothermal fluids circulation (Seggiaro et al., 1997). Recently, the

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Fig. 1. Geographic location of the studied area (a,b e modified from Carrera and Muñoz, 2008) and DEM (c).

C. Invernizzi et al. / Journal of South American Earth Sciences 54 (2014) 20e36 21

presence of hot fluids in the subsurface was proved from audio-magnetotelluric investigation (Barcelona et al., 2013). Furthermore,a preliminary model of fractured reservoir was proposed on thebase of meso- and macro-structural field investigation (Maffucciet al., 2012a,b; 2013).

However, hydrogeological and water isotopic studies, as well asdetailed structural analyses aimed to constrain the circulationmodel of the geothermal system, are still little developed toquantitatively assess the geothermal potential of Rosario de laFrontera area.

The main objective of this work is to produce a new quantitativehydrogeological, hydrogeochemical and structural dataset toconstrain a plausiblemodel for the geothermal system of Rosario dela Frontera. A preliminary conceptual model of the geothermal fluidreservoir is also proposed, based on the integration of new andexisting dataset.

2. Geological setting

The thermal area of Rosario de La Frontera (25�50.0470S e

64�55.7150W) represents one of the most important geothermalsites in Salta province. It is located in the foothills of the CentralAndean retro-wedge, in NW Argentina (Fig. 1), dominated by abasement-involved thrust system. It evolved by an eastwardmigrating compressive deformation that occurred during Miocene-Quaternary times (Salfity et al., 1993; Allmendinger and Gubbels,1996; Kley and Monaldi, 1999; Reynolds et al., 2000). As a result,the Andean structure in this area is characterized by a stepped foldpattern generated by predominantly east verging high-angle thrustfaults (Kley andMonaldi, 2002) mainly due to the Andean inversionof pre-existing normal faults associated to a regional Cretaceousrifting event (Bianucci et al., 1981; Bianucci and Homovc, 1982;Grier, 1990; Grier et al., 1991; Salfity et al., 1993; Kress, 1995;Cristallini et al., 1997; Kley and Monaldi, 2002; Carrera et al.,2006; Carrera and Muñoz, 2008).

Sierra de la Candelaria represents one of these broad anticlines.Its structure is dominated by an NeS trending periclinal andasymmetric macro-anticline (Figs. 1 and 2), strongly plunging bothto the North (below the Metán alluvial plain) and to the South(Salfity andMonaldi, 2006). Along its eastern limb, it is uplifted by a

high-angle thrust fault, dipping to the west and with a top to theeast sense of transport (Moreno Espelta et al., 1975; Seggiaro et al.,1997; Iaffa et al., 2013). EeW trending extensional faults offset thestratigraphic succession in the central portion of the anticline(Salfity and Monaldi, 2006).

The stratigraphic succession deformed in Sierra de la Candelariaanticline consists of threemain sedimentary sequences (Fig. 3). Theoldest unit (Medina Formation), a low grade meta-sedimentaryPrecambrian basement, crops out at the core of the anticline(Bossi, 1969; Ramos, 2008). The basement is unconformably over-lain by a Cretaceous to Paleogene continental succession of red bedswith limestone interbeds (Salta Group) that mark the rift stage. TheSalta Group starts with the Early to Late Cretaceous Pirgua Sub-group that marks the syn-rift stage (Galliski and Viramonte, 1988;Salfity and Marquillas, 1994; Viramonte et al., 1999; Marquillaset al., 2005) whose deposition was structurally controlled (GomezOmil et al., 1989; Sabino, 2002). Sedimentary environments rep-resented by the Pirgua Subgroup evolved from alluvial-fan/lakes tofluvial (Marquillas et al., 2005). The Pirgua Subgroup basal unit (LaYesera Formation), is represented by conglomerates with clastsfrom Medina Formation, interbedded toward the top with basalticvolcanic rocks, and paleosoil, that may represent lateral facies ofthe Las Curtiembres Formation. Conglomeratic sandstones and ar-koses e of the Los Blanquitos Formation-overlie this unit in para-conformity (Moreno Espelta et al., 1975; Viramonte et al., 1999).

The post-rift thermal subsidence stage is represented by Bal-buena and Santa Barbara Subgroups (Bianucci et al., 1981, 1993;Cominguez and Ramos, 1995). The Balbuena Subgroup (LatestCretaceous e Early Paleocene) is about 180 m thick and is made ofsandstones and limestones. The Paleocene to Early Eocene SantaBarbara Subgroup, about 330m thick, is dominantly shaly with rarecarbonate intercalations.

The Cretaceous Salta rift was closed, since Eocene times, by theIncaic compressive phase, that caused the partial inversion of therift structures (Grier et al., 1991; Salfity and Marquillas, 1994).

Post-rift deposits are overlain by thick continental retrowedgeforeland basin fill of the Orán Group (Middle Miocene-Pleistocene)related to the Andean mountain building (Gebhard et al., 1974;Reynolds et al., 2000). The basin fill includes two Subgroups (Metánand Jujuy) whose outcrops border the anticline at lower elevation.

Fig. 2. a e Regional geological map with structural elements. Location of geological cross section of Fig. 2b are shown. Contour plots 1e2e3 (lower hemisphere) are referred to polesto fracture planes along the northern, eastern and western sector of the studied area respectively. Supposed faults are suggested by remote sensing data or by morphostructuralevidences. b e Geologic cross sections. Location is shown on the geological map of Fig. 2a.

C. Invernizzi et al. / Journal of South American Earth Sciences 54 (2014) 20e3622

Fig. 2. (continued).


3. Methods and materials

The evaluation of the reservoir water budget was performed bymeans of hydrogeological and geological classical techniques. Thisapproach allowed: 1) the identification of the catchment area, 2)the reconstruction of average annual rainfall and

evapotranspiration, 3) the evaluation of surface water runoff andspring water discharge, 4) the evaluation of the effective waterinfiltration, 5) the reservoir volume estimation.

Geological surveys were helpful to: a) check the position ofstratigraphic boundaries reported on existing maps, b) evaluate theattitude of the different lithological units that may contain or seal

Fig. 3. Stratigraphic column (redrawn by Viramonte et al., 1999) including field photos of Anta and Medina Formations, and Pirgua Subgroup. Basalts interbeds are also shown.


geothermal fluids, c) reconstruct the axial trend of the main foldstructures, and d) perform preliminary structural analyses at theoutcrops scale. A new 1:25,000 topographic base was constructedfrom SRTM V2 data as well.

The catchment area was identified by topographic maps1:25,000 and SRTMV2 data (NASA). The average annual rainfall andevapotranspiration were derived from literature and availabledatabase (INTA, 2014, Bianchi et al., 2012). The evaluation of surfacewater runoff and spring water discharge derives from measure-ments carried out in the field. Surface runoff measurements wereperformed along transverse sections to the outflow direction inselected points, using a hydrometric reel and reading the time witha stopwatch. For the spring discharge, flow measurements wereperformed in thirteen points using a container of known volume

and a stopwatch. Finally, the reservoir volume was calculated onthe basis of remote sensing and field qualitative structural analyses,geological map, compilation of geological cross sections, andhydrogeological data. Remote sensing analyses were performedusing SRTM V2 data at 30 m and they were elaborated by GlobalMapper software. From existing geological maps (González et al.,2000; Salfity and Monaldi, 2006), field recognitions and qualita-tive analyses of remote sensing data, we derived detailed infor-mation both on structural elements (faults and fractures) andlithotypes.

Primary porosity and permeability of Pirgua were performedanalysing hand samples respectively by a mercury porosimeter andby portable permeameter (TinyPerm II, New England Research ltd)at Parma University laboratory (Italy).


Thirteen water samples from hot springs were collected andanalysed for d18O and dD (Craig, 1961; Gonfiantini, 1978; Truesdelland Hulston, 1980; Nuti, 1992; Clark and Fritz, 1997). Temperature(�C), pH, Electrical Conductivity (EC,in mS/cm), and flow rates (FR)were measured in situ.

The 18O/16O and 2H/1H isotopic ratios (expressed as d18O anddD&V-SMOW, respectively) were determined at the Laboratorio deIsótopos Estables y de Tritio del Instituto de Geocronología y Geo-logía Isotópica (INGEIS-CONICET-UBA of Buenos Aires). Chemicalanalyses were carried out by Los Gatos Research Inc. laser spec-troscopy (OA-ICOS: Off-Axis Integrated Cavity Output Spectros-copy) (Lis et al., 2008). Analytical errors are �0.3& and �1.0& ford18O and dD, respectively. Tritium analyses in 4 selected watersamples were carried out at CNR Isotopic Laboratory of Pisa (Italy).The instrument used is the Quantulus produced by WALLAC groupPERKIN-ELMER (standard error of measurement: TU 0.0 � 0.4/TU10.0 � 1.2). Isotopic analysis of tritium in water were conducted bydistillation and concentration of the sample through electrolyticenrichment of water. Preparation of mixture for counting wasmadeusing 8 cc of sample and 12 cc of scintillation liquid. Sample mea-surement was done by liquid scintillation.

4. Results

4.1. Chemical and isotopic composition of waters

Outlet temperature, pH, Electrical Conductivity (EC, in mS/cm)andflow rates (FR) values of thermal springs located in the Rosario dela Frontera geothermal area (Fig. 4) are reported in Table 1. Watertemperatures range from 22.6 �C (Vicky) to 92.6 �C (Sulfurosa),whereas pH values range from 6.65 (Laxante) to 8.43 (Sulfurosa).Water samples shows relatively low EC (<2 mS/cm), with theexceptionof four sources (SVQ, Salada I, Laxante andVichy)which arecharacterized byECvalues ranging from3.29 tomore than20mS/cm.

Most springs are alkali-chloride waters, whereas Zarza, AguasChicas I, Agua de la Virgen, Aguas Chicas IB and Aguas Chicas IIS are

Fig. 4. Thermal springs location. 1: Ferruginosa, 2: SVQ, 3: Salada I, 4: Laxante, 5: Zarza, 6: ASilicosa, 12: Aguas Chicas IB and 13: Aguas Chicas I.

sodium bicarbonate waters (Seggiaro et al., 1995; Pesce andMiranda, 2003; Chiodi et al., 2012a, b).

Analytical data for d18O and dD vary in a relatively narrow range(Fig. 5), from �7.1 to �6.4, and from �40 to �36& V-SMOW,respectively (Table 2; Chiodi et al., 2012a,b). These isotopic datashow that hot springs plot adjacent to the Local Meteoric WaterLine (LMWL) proposed by Dapeña and Panarello (2011) for SaltaProvince, suggesting that these waters have a meteoric origin.

Four water samples were analysed for tritium content (Table 3).Two of the selected samples, Laxante and Sulfurosa, have alkali-chloride composition with low and high temperature, respec-tively (Table 1). The two remaining water samples, Aguas Chicas Iand Agua de la Virgen, have sodium bicarbonate composition andhigh temperature (Table 1). Measured values are all close to zero,and were compared for interpretation with data of the networkIAEA tritium (Station of Salta) from the international catalogue ofIAEA. Used data from the catalogue show values of 10.4,10.6 and 9.8TU referred to the years 1982, 1983 and 1986 respectively. From thiscomparison it was proven a relatively long residence time (>50years; Smedley et al., 2002) of water at depth.

These data are at the base of possible hypotheses on therecharge area of the geothermal system.

In a conservative hypothesis, and in agreement with Seggiaroet al. (1997), we addressed our hydrogeological reconstructionconsidering only the northern sector of Sierra de la Candelaria asthe main recharge area (Pierantoni et al., 2012). Nevertheless, othercontributions from areas outside this catchment area cannot beexcluded, and they can provide a further contribution to rechargethe system (Moreno Espelta et al., 1975; Bercheni, 2003).

4.2. Preliminary hydrogeological investigations for water balance

4.2.1. Lithological components of the geothermal systemThe role played by the main lithological groups in the

geothermal system was assigned by field observation and, in somecases, by laboratory data acquisition.

guas Chicas II (N), 7: Aguas Chicas II (S), 8: Agua de la Virgen, 9: Vichy, 10: Sulfurosa, 11:

Table

1Outlet

temperaturesT(in

� C),pH,T

otal

Dissolved

Solid

sTD

S(ing/L)

andflow

ratesFR

(inL/h)va

lues

ofthermal

andco

ldwater

samplesco

llected

intheRo

sariodela

Fron

tera

geothermal

area

.n.d.:not

determined

.

Sample

Sample

description

Latitude

Longitude

Altitude

TAp-12

pH

Ap-12

ECAp-12

TDec-12

pH

Dec-12

ECDec-12

FRDec-12

TAp-13

pH

Ap-13

ECAp-13

FRAp-13

Ferruginosa

Thermal

spring

25,838

5864

,931

7292

474

.08.19

1.52

69.8

8.41

1.68

w10

0079

.18.28

1.6

w54

0SV

QTh

ermal

spring

25,838

5364

,931

4990

581

.27.84

3.51

81.4

7.53

3.52

n.d.

82.8

7.47

3.29

w35

0Sa

ladaI

Thermal

spring

25,838

3964

,931

7991

871

.47.06

w20

72.3

7.10

>20

w72

070

7.27

>20

w80

0Laxa

nte

Thermal

spring

25,836

8664

,933

6186

524

.16.84

19.2

24.8

6.65

>20

w60

23.3

6.65

>20

w38

Zarza

Thermal

spring

25,838

8764

,930

6294

852

.78.13

0.92

57.0

7.66

1.03

w20

052

.98.11

0.91

w27

0Agu

asChicas

II(N

)Th

ermal

spring

25,841

4064

,932

9296

064

.88.13

1.42

61.3

7.64

1.45

w72

061

.77.97

1.41

w95

0Agu

asChicas

II(S)

Thermal

spring

25,842

0564

,932

7296

758

.27.95

0.68

57.1

7.73

0.67

w21

6055

.58.12

0.67

w13

00Agu

adela

Virge

nTh

ermal

spring

25,839

6864

,929

4695

474

.98.39

0.91

77.0

8.14

0.99

w14

4075

8.07

0.93

w25

00Vichy

Thermal

spring

25,837

7564

,931

9989

424

.27.36

8.75

26.8

7.14

9.7

w36

022

.67.30

8.75

w46

0Su

lfurosa

Thermal

spring

25,839

0664

,930

9992

890

.57.82

1.64

92.6

8.43

1.7

w36

088

.58.19

1.66

w40

0Silic

osa

Thermal

spring

25,839

2164

,931

1594

290

.48.23

1.39

91.8

8.25

1.46

w10

8089

.48.08

1.43

w12

00Agu

asChicas

IB

Thermal

spring

25,839

2164

,925

8586

745

.27.95

0.96

n.d.

n.d.

n.d.

0n.d.

n.d.

n.d.

0Agu

asChicas

ITh

ermal

spring

25,842

0264

,927

6491

872

.28.21

1.09

71.7

7.6

1.2

w10

0070

.67.91

1.11

w32

00

Fig. 5. d18O vs. dD binary diagram. The Global Meteoric Water Line (GMWL) and theLocal Meteoric Water Line (LMWL: dD& ¼ (8.25 � 0.18) d18O& þ (15.08 � 0.96);Dapeña and Panarello (2011)) are also shown.


Themain litotypes of theMedina Formation (phyllites with lowpermeability) and the presence of healed fractures (Fig. 3) suggestthat this subgroup acts as the impermeable basement of thegeothermal system, in agreement with the model of Seggiaro et al.(1995).

The Pirgua Subgroup, characterized by a prevalence of sand-stone and conglomeratic beds, and by a local thickness rangingbetween 450 and 1000 m (Reyes and Salfity, 1973; Moreno Espeltaet al., 1975; Bercheni, 2003; Salfity and Monaldi, 2006), representsthe main thermal fluid reservoir, in agreement with Seggiaro et al.(1995). Laboratory test measures on sandstone hand sample fromArroyo Bonete Puca high valley provide values of porosity andpermeability. The test provide density ¼ 2.24 g/cc, totalporosity ¼ 7.2%, pore diameter with a mean value ¼ 0.3 micron.Five consistent measures of permeability provide a mean value of81.2 mDarcy (min ¼ 24.1 mD, max ¼ 137.1 mD).

Despite Seggiaro’s el al. (1995) hypothesis, we consider that theBalbuena Subgroup, that includes abundant carbonatic lithologies,can also act as a potential reservoir when appears intensely frac-tured as well.

From a regional point of view, some of the units composing theSanta Barbara Subgroup and the Oran Group show a prevalence ofmarly and clay rich thick horizons, and, although fractured, theymaintain low permeability as fractures often show clayey, gypsumand calcite filling. Thus, these units behave as the main cap rock ofthe geothermal system (Corrado et al., submitted).

4.2.2. Geological and structural dataIn order to define the reservoir volume and its characteristics, a

geological map was compiled (Fig. 2a) on the basis of 1) existingmaps (Moreno Espelta et al., 1975; Seggiaro et al., 1997; Bercheni,2003; Salfity andMonaldi, 2006;Maffucci et al., 2013), 2) new fieldsurveys, and 3) qualitative analysis of lineaments and morpho-structures identified through satellite data SRTM V2.

Remote sensing qualitative analyses (satellite data SRTM V2)allowed to identify a network of main lineaments (Fig. 6) thatpartially correspond to the main faults detected in the field andreported on existing geological maps. Different families of linea-ments have been detected: NNEeSSW, represented by continuouslineaments mostly developed in the foothills both to the west and

Table 2d18O and dD (& V-SMOW) values of the thermal water samples collected in the Rosario de la Frontera geothermal system.

Sample Sample description Latitude Longitude Altitude Date of sampling d18O& dD&

Ferruginosa Thermal spring 25,83858 64,93172 924 Oct-11 �7.0 �38SVQ Thermal spring 25,83853 64,93149 905 Oct-11 �6.9 �38Salada I Thermal spring 25,83839 64,93179 918 Oct-11 �7.1 �40Laxante Thermal spring 25,83686 64,93361 865 Oct-11 �6.4 �38Zarza Thermal spring 25,83887 64,93062 948 Oct-11 �6.8 �37Aguas Chicas II (N) Thermal spring 25,84140 64,93292 960 Oct-11 �6.6 �36Aguas Chicas II (S) Thermal spring 25,84205 64,93272 967 Oct-11 �6.6 �36Agua de la Virgen Thermal spring 25,83968 64,92946 954 Oct-11 �6.6 �37Vichy Thermal spring 25,83775 64,93199 894 Oct-11 �6.6 �38Sulfurosa Thermal spring 25,83906 64,93099 928 Oct-11 �6.9 �39Silicosa Thermal spring 25,83921 64,93115 942 Oct-11 �6.5 �36Aguas Chicas I B Thermal spring 25,83921 64,92585 867 Oct-11 �6.6 �36Aguas Chicas I Thermal spring 25,84202 64,92764 918 Oct-11 �6.5 �37


to the east of the Sierra de la Candelaria relief; WNWeESE and,subordinately, NWeSE mainly diffused along Sierra de la Cande-laria and in the western foothills; NEeSW lineaments concentratedin the northern part and in the middle portion of the Sierra de laCandelaria and both on the western and eastern foothills. In aconservative assumption, we have chosen the continuous longitu-dinal lineaments closer to the positive structure (interpreted asback-thrust in Fig. 2b) and emphasising the boundary betweenrelief and flood plain, as the western edge of the reservoir (Fig. 6).From the revised geological map of Fig. 2a, some geological sec-tions, three perpendicular and one longitudinal to the anticline,were drawn (Fig. 2b). The geological sections highlight the peri-clinal plunge of the macroanticline towards the north (Fig. 2b,section DeD0). The Sierra de la Candelaria structure is bounded tothe east by a NeS high-angle fault, probably a Cretaceous normalfault inverted during compression (Iaffa et al., 2013). Furthermore,the main anticline is offset in its northern sector by a NNWeSSEhigh-angle fault (Fig. 2a). A strike-slip component along the faultssub-parallel to the axial trend is suggested by their geometries, byoffset of stratigraphic boundaries, and kinematic indicatorsobserved in the field. A general NeS extension is accommodatedalong the about EeW oriented faults as shown by the offset ofstratigraphic boundaries.

Structural surveys at the outcrop scale were also performed toobtain information about the presence and relevance of the mainfracture systems. Qualitative and quantitative analysis were carriedout collecting data about typology, orientation and frequency of thedifferent structural discontinuities. In particular, generic fractures(e.g. with small aperture not sufficient to exclude a shear compo-nent during the opening; isolatedweathered fracture), gypsum andcalcite veins, shear fractures and shear bands were recognised(Maffucci et al., 2012a,b; Maffucci et al., 2013).

According to orientation data, fractures at the outcrop scaleshow six main trends (contour plots in Fig. 2a): NeS, NNEeSSW,

Table 3Results of isotopic analyses of Tritium on four springs’ water samples.

n. Sample Date Tritium Error

(T.U.) � (T.U.)

Tritium analysis e 23/07/11 Aguas Chicas1 22/05/13 0.0 0.42 Sulforosa 22/05/13 0.5 0.43 Laxante 22/05/13 0.4 0.54 Agua de la Virgen 22/05/13 0.1 0.3

Tritium analisys e 31/07/131 Aguas Chicas1 22/05/13 0.0 0.32 Sulforosa 22/05/13 0.0 0.33 Laxante 22/05/13 0.1 0.34 Agua de la Virgen 22/05/13 0.0 0.3

NNWeSSE, WNWeESE, EeW and ENEeWSW. In particular, incorrespondence with the northern plunge of the anticline wherecap rock crops out, generic and shear fractures occur with aWNWeESE, NNWeSSE and EeW orientation along the westernand eastern limbs of the anticline, and with a NNWeSSE, NeSand NNEeSSW trend in the periclinal closure area. In the sameportion of the anticline, on the western limb, gypsum veins witha NNEeSSW trend were recognized. Shear fractures also occur inthe central and southern portion of the study area wherereservoir rock crops out. They occur with a NNWeSSEorientation in correspondence with the periclinal closure areaand along the eastern limb of the anticline, and with a NeS,NNEeSSW and EeW trends on the western one. Detailed analysison spacing of the fracture systems characterizing the PirguaSubgroup, acquired by scan-lines and scan-areas methods, showlow values of fractures frequency (Maffucci et al., 2012a,b;Maffucci et al., 2013).

4.2.3. Hydrographic network and recharge areaThe northern sector of Sierra de la Candelaria, located to the

south-east of the Rosario de la Frontera town (Figs. 1c and 2a),belongs to the Rio Rosario hydrogeologic basin (Bercheni, 2003). Inparticular, the northwestern side of the ridge forms a sub-basintogether with San Martin and Aguas Negras streams locatedimmediately northwestward of the studied area. The morpholog-ical characteristics of the area and the surface water flow suggestthat rainwater, possibly recharging the thermal reservoir, mainlyinfiltrates in the northern sector of Sierra de la Candelaria. Withinthis basin, the hydrographic network map was drawn and thecatching area delimited (Fig. 7a).

For the catching area boundaries, we considered the MedinaFormation as an impermeable basement, and the depositsbelonging toMetán Subgroup as the cap rock of the thermal aquifer.Therefore, the recharge area can be limited to the boundaries of theoutcrop area of Pirgua Subgroup. We also considered that part ofthe surface runoff can infiltrate the Pirgua Subgroup coming fromMedina runoff where the surface hydrography allows it. To the SW,the catching area was delimited by a NWeSE structural lineament.As a whole an effective catching area of 51,539 km2 was calculated(Fig. 7a). A more extended catching area cannot be excluded if weconsider the Balbuena Subgroup outcrops as possible infiltrationzones. Taking into account this second hypothesis(Pirgua þ Balbuena as infiltration units), the catching area can beextended to 62,957 km2 (Fig. 7a).

4.2.4. Precipitation and evapotranspirationData available from INTA (2014) provide a record of rainfall for

more than 50 years (1934e1990, Bianchi et al., 2012). These data

Fig. 6. Structural lineaments detected by qualitative analysis of satellite data (SRTM V2) are drown on a DEM representation of the studied area and surroundings.


coupled with the dataset reported by Bercheni (2003) allowed areliable estimation of the average annual rainfall on the northernsector of Sierra de la Candelaria. The map in Fig. 7a also shows theisohyet distribution where iso-lines from 1000e700 mm/yr crossthe catching area with the highest rainfall values along the easternflank of Sierra de la Candelaria. Thus a value of 900 mm/yr may beadopted for further calculations.

Because the involved area is subject to strong evapotranspira-tion, the rainfall does not immediately translate into potentialinfiltration. Data available from official statistics obtained bytermo-pluviometric stations located close to the study area (INTA,2014) allowed to establish a value of effective precipitation (Pe).It derives from the difference between precipitation and evapo-transpiration (Fig. 7b) and represents the availablewater. A Pe valueof 400 mm/yr is adopted for this area, considering a mean evapo-transpiration value of 500 mm/yr.

The amount of water actually available on the area of infiltrationcan be evaluated according to two different hypotheses:

Hypothesis (1). Pirgua Subgroup catching area:

51:539 km2*4� 10�4 km ¼ 0:0206 km3

¼ 20:6*106 m3ðavailable water volumeÞ

Hypothesis (2). Pirgua þ Balbuena Subgroups catching area:

62:957 km2 � 4*10�4 km ¼ 0:025 km3

¼ 25*106 m3ðavailable water volumeÞTherefore, available water in the catching area can range from

20.6 to 25 millions of m3/yr.

4.2.5. Water dischargeThe flow data of streams were collected in April 2013 (austral

autumn) and were measured along the flanks of Sierra de la

Fig. 7. a e Hydrographic net (thin dashed bluelines), isohyet lines and infiltration (catching) area. Two hypotheses can be evaluated for the catching area: the area of Pirgua outcrops(light blue, 51,539 km2) and the Pirgua þ Balbuena outcrops (light þ dark blue, 62,957 km2). Yellow spots (BP, BY, AS) represent field flowmeasurements along streams (see Table 4).Thermal springs are shown by blue dots in the proximity of Hotel Termas. b e Map of evapotranspiration values: each range of value is shown by numbers and colours. Thin blackline shows the Pirgua infiltration area perimeter; thick black line shows the Pirgua þ Balbuena infiltration area perimeter (see also Fig. 7a). (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)


Fig. 7. (continued).


Table 4Stream flowmeasurements on: a) Bonete Puca, b) Barba Yaco, c) Rio Salado streams.

Detection nr. Partial area (m2) Velocity (m/s) Flow (m3/s)

a) Bonete Puca (BP 1 in Fig. 7a)1 0.052 0.380 0.0192 0.052 0.421 0.0213 0.052 0.481 0.0254 0.052 0.533 0.0275 0.052 0.485 0.0256 0.052 0.476 0.0247 0.052 0.444 0.0238 0.052 0.507 0.0269 0.052 0.430 0.022Mean value 0.052 0.461 0.023Bonete Puca (BP 2 in Fig. 7a)1 0.056 0.404 0.0222 0.056 0.431 0.0243 0.056 0.429 0.0244 0.056 0.444 0.0245 0.056 0.364 0.0206 0.056 0.405 0.022Mean value 0.412 0.022

b) Barba Yaco (BY 1 in Fig. 7a)1 0.030 0.75 0.0222 0.030 0.76 0.0223 0.030 0.73 0.0214 0.030 0.67 0.0205 0.030 0.81 0.024Mean value 0.74 0.021Barba Yaco (BY 2 in Fig. 7a)1 0.098 0.35 0.0342 0.098 0.44 0.0433 0.098 0.43 0.0424 0.098 0.47 0.0465 0.098 0.45 0.0446 0.098 0.45 0.044Mean value 0.43 0.042Barba Yaco (BY 3 in Fig. 7a)1 0.094 0.53 0.0492 0.094 0.55 0.0513 0.094 0.59 0.0554 0.094 0.97 0.0915 0.094 0.69 0.064Mean value 0.66 0.062

c) Arroyo Salado (AS 1 in Fig. 7a)1 0.056 0.84 0.0472 0.056 0.80 0.0443 0.056 0.68 0.0384 0.056 0.69 0.0385 0.056 0.77 0.039Mean value 0.75 0.041Arroyo Salado (AS 2 in Fig. 7a)1 0.224 0.349 0.0782 0.224 0.468 0.1043 0.224 0.470 0.1054 0.224 0.482 0.1075 0.224 0.350 0.078Mean value 0.423 0.094


Candelaria where the river flow on the bedrock before entering thealluvial deposits in the foothills. Some measures in alluvial areaswere also carried out for comparison.

Along the eastern flank, direct flow measurements were takenin the Bonete Puca stream (yellow spots BP 1e2 in Fig. 7a). Themeasures were repeated systematically in two different sites andcan be summarized as follows (Table 4):

site BP 1� Section ¼ 0:052 m2; Velocity

¼ 0:461 m=s; Flow ¼ 0:023 m3.s

site BP 2� Section ¼ 0:056 m2; Velocity

¼ 0:412 m=s; Flow ¼ 0:022 m3.s

Furthermore, there are five streams similar and sub parallel toBonete Puca stream and among them that fall in the catchment areaof Fig. 7a and flow along the eastern flank of the main structure.They are altogether comparable in length, flow and bedrock fea-tures. Therefore, we hypothesize an approximate runoff along ofthis side in 0.023 m3/sec � 6 ¼ 0.138 m3/s.

We excluded from this calculation streams running through thecap rock and that collect water of the hot springs, because thesprings’ outflow has been calculated independently.

Along the western flank of the structure, the direct flow mea-surements were taken along the Barba Yaco stream. Threedifferent sites flowing onto the bedrock were monitored (yellowspots BY in Fig. 7a), and they were compared to a trait of the riverthat flows onto the alluvial deposits. Numerous measures weretaken in each site and they are summarized in Table 4. The averageof the three sites flowing onto the bedrock gave a mean valueequal to 0.041 m3/s. The site onto the alluvial deposits clearlyreflected the presence at the stream talweg of permeable alluvialQuaternary deposits that caused an apparent decrease of flow atthe surface due to a possible sliding of the flap in sub-riverbed. Sixstreams flow along the western flank of the structure. Thesestreams are comparable to Barba Yaco stream and among eachother for length, flow and bedrock. Furthermore they flow withinthe catching area of the thermal reservoir. Therefore the seasonalrunoff along the western flank is assumed equal to 0.041 m3/s � 7 ¼ 0.287 m3/s.

Flow rate measurements were also carried out along the ArroyoSalado (Fig. 7a) which, presumably, collects the runoff from thenorth-western portion of the periclinal closure of the anticline. Twomeasurement sites were performed (AS in Fig. 7a). Data record ascarce flow showing a remarkable infiltration of water into sub-riverbed, and do not allow the evaluation of the contribution ofthe numerous short streams that flow into the main Arroyo Salado.Table 4 summarises the field flow measures along Arroyo Salado,and they give a mean flow value of 0.067 m3/s.

Additional surveys of flow were carried out at the output ofother streams in flood zone. These are dry at surface even after theperiods of maximum rainfall, and confirm the remarkable waterinfiltrationwithin Quaternary alluvial deposits. Nevertheless, theseparts of the rivers do not flowwithin the catching area and they areof negligible interest for the evaluation of the water available foreffective infiltration. Considering the sum of the contribution to therunoff along the two sides of the anticline, and adding the contri-bution of the Arroyo Salado, the total surface flow can be calculatedas follows:

0:138 m3.sþ 0:287 m3

.sþ 0:067 m3

.s ¼ 0:492 m3

.s

This means a loss due to streams’ surface runoff of less than0.5 m3/s which corresponds to 43.536 m3/day, and about16*106 m3/yr.

As a part of the calculations for water balance, the losses due tosurface runoff should take into account for flow variationsthroughout the year. Bercheni (2003) showed that the highest flowfor Rio Rosario occurs in January, February and March, whereasintermediate values are recorded in December and April. This cor-responds to the main rainfall during the year. For the rest of theyear, the river flow is much lower and sometimes almost absent.Our assessment considers only direct measures recorded in April2013, and extended over the year. So the surface runoff could beslightly overestimated.

Fig. 8. Areal extension of the Pirgua Subgroup at depth, corresponding to 101.6 km2. It represents the base of the reservoir.


In addition, thirteen thermal springs were monitored tocalculate the water discharge rate. All the springs are located inthe area of Hotel Termas (see Figs. 4 and 7a). Data were collectedin December 2012 and April 2013. Springs show some seasonalvariation (FR in Table 1). Data must be considered with caution,because some of the springs are captured. Furthermore, hotsprings generally do not show regular variations between winterand summer. Total discharge in December was around 9.100 l/h on11 measured springs. In April it was around 12.100 l/h on 12

measured springs. Two hot thermal springs utilized for industrialpurposes (Palau mineral water), could not be measured, but wecollected some informations from the staff which indicate a flowof 2200e1800 l/h and of 1000e600 l/h (seasonal variation 400 l/h)for these springs (Empresa Palau, personal communication).Calculating the annual average discharge between the 12measured springs, we obtained an approximate total value of 3e4 l/s. This means a total discharge of approximately 94,600 to126,000 m3/yr from springs. Because the real discharge is under-

Fig. 9. Schematic conceptual model for the reservoir characteristics and fluid circulation referred to section BeB0 and DeD0 of Fig. 2b. Cap rock (light grey-Mt), bedrock (dark grey-Md) and reservoir rocks (spotted-Pr) are sketched. Water table is shown with a horizontal line and saturated reservoir is evident (green colour). The orange spotted area indicatesthe geothermal reservoir with T � 60 �C. Part of the reservoir rock is at greater depths than the 100 �C isotherm, at least on the western side of the structure. Arrows are indicative ofthe convective mixing of fluids. Dashed lines in proximity of Hotel Termas indicate intense fracturing within Metan subgroup and thermal water upwelling. (For interpretation ofthe references to colour in this figure legend, the reader is referred to the web version of this article.)


estimated due to spring captures and to the Palau springs notincluded in our monitoring, we considered to increase of 50% thisvalue, and we calculated about 189,000 m3/yr of springs dischargein our estimation of water balance.

4.2.6. Potential infiltrationThe potential infiltration (Ip) over an area depends on the

structural and lithological features of the ground. In particular,lithologic characteristics such as porosity, permeability, state offracturation, as well as information about vegetation, soil andarea slope are needed to define the Ip. Homogeneous sectors atsurface, with almost homogeneous characteristics should beconsidered to understand the runoff rainfall. The potential infil-tration is given by:

Ip ¼ Pe*P:i:c:

where P.i.c. is the potential infiltration coefficient and representsthe percentage of the water able to infiltrate into the underground.The P.i.c. has values ranging between 0 and 1, and each lithotypecan be associated to a P.i.c. value (Celico, 1988).

In the studyarea, because of the difficulties in accurately definingsome factors (such as slope, occurrence and type of vegetation,degree of weathering), and because data on surface runoff are not

yet carefully monitored, the potential infiltration coefficient (P.i.c.)was used to estimate the volume of infiltrating waters.

Generally, a typical P.i.c. value for sandstone ranges between 0.3and 0.5 (Celico, 1988). In order to evaluate P.i.c. value for our area,we considered:

- a comparison with lithologies similar to the Pirgua reservoirsuch as “Flysch del Cilento” (P.i.c. ¼ 0.2; Celico et al., 1994) andMarnoso-Arenacea Formation (P.i.c. ¼ 0.15; CAVET, 2001);

- direct measures of primary porosity of Pirgua sandstones onhand samples with a low-to-medium value (7.2%)

- presence of diffuse vegetation and soil on the infiltration areaand low value of fracture frequency (Maffucci et al., 2013) thatcharacterize the reservoir outcrops in the catchment area.

According to these evidences we adopted a conservative value ofP.i.c. in the range of 0.1/0.2 (maximum and minimum P.i.c. values).This can give an acceptable evaluation of the water volumeinvolved in the infiltrating process, and allowed to obtain a Po-tential infiltration (Ip) of:

1) Ip ¼ 20.6*106 m3*0.1 ¼ 2,060,000 m3 or20.6*106 m3*0.2 ¼ 4,120,000 m3 (Pirgua Subgroup)

2) Ip ¼ 25*106 m3*0.1 ¼ 2,500,000 m3 or25*106 m3*0.2 ¼ 5,000,000 m3 (Pirgua þ Balbuena Subgroups)


4.3. Reservoir volume and geothermal potential

A possible confinement for the reservoir continuity at depth,in particular on the western side of the main anticline, is pro-posed, and a conservative volume of Pirgua Subgroup, as reser-voir formation, is here considered. It is delimited to the east andto the north by the main high angle thrust and longitudinaltranspressive planes, to the west by a continuous, almost NeSoriented, lineament well visible on remote sensing images(Fig. 6) and to the south by the outcrops of Pirgua and by a NWe

SE structural elements. The reconstruction of the reservoir sur-face at depth is shown in Fig. 8, and corresponds to 101.6 km2.However, this can be considered an underestimation of the realsurface of the reservoir, because, due to the fold geometry, it is asector of a cone with a gentle immersion of about 10e15� to theN and a mean value of dip along the flanks of 25e30�. Consid-ering the projection of this curved surface on a plane, we obtain areduction of about 5e10% of the area.

An average thickness for Pirgua subgroup in the study areacan be evaluated in 700 m considering a minimum thickness of450 m along the western limb of the anticline (Fig. 2a and sectionCeC0 in Fig. 2b) and a gradually increasing thickness toward thenorth reaching a maximum of 1000 m (Moreno Espelta et al.,1975, Fig. 2b). A volume of Pirgua corresponding to 71,12 km3

is derived from calculations (using the average thickness of700 m and the reservoir surface of Pirgua at depth of 101,6 km2

as shown in Fig. 8). The minimum saturated volume (reservoir) ofPirgua corresponds to 60.35 km3. It was calculated consideringthe elevation of the piezometric surface (Fig. 9), and subtractingthe volume of Pirgua lying above the water table. For a pre-liminary evaluation of the geothermal potential, we consideredthe volume of Pirgua where fluid temperature exceedes 60 �C asan interesting volume of the reservoir for geothermal purposes.This corresponds to an effective volume of the geothermalreservoir of about 39 km3, based on the depth of the 60 �Cisotherm (Fig. 9). With a 40�/km geothermal gradient (as sug-gested by Seggiaro et al., 1995), and a surface temperature of20 �C, the meteoric water can reach 60 �C at 1000 m depth andcan exceed temperatures of 100 �C at the base of the reservoirlocated deeper than 2000 m (Fig. 9). So, a convective circulationcan homogenize the temperature to an average value of about90 �C within the system (as suggested also by the maximumtemperatures of thermal spring water).

The definition of the first order volume of the reservoir allowsthe application of the volume method (Muffler and Cataldi, 1978)for the calculation of the geothermal potential at Rosario de laFrontera. We use the following equation E ¼ Er þ Ef ¼ [(1 � 4) rrcr V (T � To)]þ[(4) rf cf V (T � To)], where Er is the heat stored inthe solid rocks and Ef in the fluid phase. Given a total volume forthe reservoir of 39 � 109 m3, a rock density of 2200 kg/m3 for thePirgua reservoir (rr) with an average effective porosity of 0.07(4), 1000 J/kg K for the rock specific heat (cr), 4200 J/kg Kfor water specific heat (cf), an average T of 90 �C in the reservoirand a T of 20 �C as air temperature, we obtain the followingresults:

Er ¼ 5:6*1018 J

Ef ¼ 0:8*1018 J

Further calculations can be made considering an alternativehypothesis with a reservoir made up by Pirgua and BalbuenaSubgroups, total thickness of about 900m, and a total reservoir rockvolume corresponding to about 91,44 km3.

5. Discussion and conclusions

In order to define a semi-quantitative hydrological conceptualmodel for the geothermal system of Rosario de La Frontera, weaddressed the following main topics: origin of water, reservoircharacteristics, water balance (recharge and discharge, water vol-umes, water infiltration), and water temperature.

The main results concerning these topic are summarized asfollows.

1. Isotopic data summarized in the dD vs. d18O diagram (Fig. 5),show that hot springs plot adjacent to the Local Meteoric WaterLine (LMWL) proposed by Dapeña and Panarello (2011) for SaltaProvince, suggesting that these waters have a meteoric originwithout any magmatic contribution and fractionation processfor watererock interaction. This allowed us to constrain arecharge area close to the area of the thermal baths, located inthe northern sector of Sierra de la Candelaria (Fig. 7a).

2. The main thermal fluid reservoir is represented by the PirguaSubgroup with high degree of certainty, because of its litho-logical, petrophysical and structural features. It consists ofconglomerates and sandstones as main lithotypes. Their pri-mary permeability that may be enhanced by natural fracturing,ranges between 24 to about 137 mD. Whereas, Balbuena Sub-group may contribute to the reservoir volume in case of naturalfracturing that may increase its secondary permeability. Thuswe propose two hypotheses for reservoir rock volume: i) avolume corresponding to 71,12 km3 based on the averagethickness of Pirgua within the studied area; ii) a volume corre-sponding to about 91,44 km3 based on the total thickness ofPirgua and Balbuena Subgroups in the same area.

3. Based on a reconstruction of surface runoff (streams andsprings) and a potential infiltration coefficient (P.i.c.) rangingbetween 0.1 and 0.2, we suggest a preliminary hydrogeologicwater balance where:(i) total amount of effectivewater reaching the infiltration area

ranges between 20 and 25 million of m3/yr;(ii) a high surface water runoff (about 16 millions m3/yr) and a

spring discharge of about 189,000 m3/yr are calculated;(iii) an effective infiltration at depth of 2e4 million of m3/yr

within the proposed recharge area is calculated.

As a result, the total amount of available effective water fits withthe sum of surface runoff and effective infiltrated water. Thisstrengthens the calculation on a positive water balance at depth inthe range of 2e4 million of m3/yr. Then, the input water stronglyexceeds the outgoing one, discharged from the springs suggesting ahigh probability of the reservoir to contain large amounts of water.

On the base of 3H data that describe a long residence time ofwater at depth (over 50 years), we suggest a long timing for theaquifer recharge. Furthermore, the exiguity of spring dischargesupports a gradual filling and a maintenance of thermal conditionswithin the reservoir. Thus, we hypothesize that the effective watercan reach the base of the reservoir at depth, and gradually fills it,assisted by the reservoir permeability.

4. Temperature acquisition can be achieved at depth where theinfiltrated water reaches the base of the reservoir as assumedabove. This latter is located up toe 2500 m below the surface inthe western side of La Sierra de la Candelaria (geological sec-tions in Fig. 2b). A long residence time of water at depth, arelatively high geothermal gradient (about 40�/km, as suggestedby Seggiaro et al., 1995), a surface temperature of 20 �C can allowmeteoric water to reach temperatures over 100 �C. So, thetemperature of 130 �C in the reservoir reported by Seggiaro et al.


(1995), on the base of geothermometers is consistent with ourreconstruction, and with paleothermometric data on seal andreservoir rocks provided by Di Paolo et al. (2012). As a conse-quence, a convective circulation can be triggered at depthwithin the system (Fig. 9).

This determines a maintenance of the temperature around 90e100 �C also at shallower depths.

A preliminary approach to the evaluation of the geothermalpotential in the study area includes the application of a conductivegradient (about 40 �C/km), a surface temperature of 20 �C, and anestimate of the reservoir volume containing fluids of interest atleast for the direct use of heat. In our opinion this could be aT � 60 �C, and it is reached 1000 m below the surface with aconductive geothermal gradient. This means a geothermal reser-voir volume of Pirgua of about 39 km3. Considering an averagetemperature of 90 �C in the reservoir, due to convection, ageothermal potential of Er ¼ 5.6*1018 J and Ef ¼ 0.8*1018 J wasestimated.

Finally, an upward flow of water along fractures fast enough tominimize the role of conduction, complete the picture of thisgeothermal reservoir that turns out to be highly perspective. A fastwater ascent can be driven by fracture systems well developed incorrespondence of the northern end of the plunging structure(Figs. 2a and 9). So far, no quantitative evaluationsweremade aboutthe influence of fracture systems in enhancing the capacity of thegeothermal reservoir and in conditioning the fluid flow. Furthergeochemical studies are also needed to better assess the mixtureprocesses between the geothermal reservoir and shallower aquifers(within Metán Subgroup), as was suggested by Barcelona et al.(2013). Nevertheless, it has been highlighted from remote sensinglineaments, geological maps, field relationships between fracturesystems and literature, that NNWeSSE oriented faults have beenpossibly reactivated during compression and can drive the fluidflow (Seggiaro et al., 1997).

These preliminary results, if confirmed by further on-goingquantitative studies, may shed new light on the effective poten-tial and sustainability of Rosario de la Frontera thermal area as ageothermal field for low and medium enthalpy energy production.

Acknowledgements

This work focuses on the preliminary results achieved by acollaboration among italian and argentinian universities. It is partof the project entitled “Exploration and utilization of geothermalresources of medium and low enthalpy in sub-Andean area forsustainable energy development of the cities in the provinces ofJujuy and Salta” financially supported by C.U.I.A. (Consorzio Uni-versitario Italiano per l’Argentina), IV� Programma di ricerca 2010e2012 for the development of applied researches on the Argentinaterritory. The project belongs to the research line devoted to the“Sustainable development of future towns”.

Wewarmly thank, Lea di Paolo fromRoma Tre University, SabinaBigi and Luca Aldega from Sapienza University who were alsoinvolved to develop other topics of the same Project, and PabloCaffe from Jujuy University, Giorgio Stangalino from GeotermiaAndina and Carlo Ungarelli from Schlumberger for exciting dis-cussions on Andean geology and geophysical methods for explo-ration. Jose G. Viramonte, Agostina Chiodi andWalter Baez thank toINENCO e UNSa-CONICET for the financial and logistic support tothis project.

Mario Mussi (CNR e Pisa, Italy) is kindly thanked for Tritiumanalyses and useful discussions. Fabrizio Balsamo and FabrizioStorti (University of Parma) are kindly thanked for providingporosity and permeability laboratory test analyses on Pirgua

sandstone. Chiara Invernizzi and Pietro Paolo Pierantoni alsowarmly thank Domenico Aringoli (University of Camerino) forsome hydrogeological input.

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