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The use of isotopes in evolving circulation models of regional continental : The case of the Guarani System

Article in Hydrological Processes · April 2019 DOI: 10.1002/hyp.13476

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The user has requested enhancement of the downloaded file. Received: 8 December 2017 Accepted: 5 April 2019 DOI: 10.1002/hyp.13476

SI STABLE ISOTOPES IN HYDROLOGICAL STUDIES

The use of isotopes in evolving groundwater circulation models of regional continental aquifers: The case of the Guarani Aquifer System

Roberto Eduardo Kirchheim1 | Didier Gastmans2 | Hung Kiang Chang3 | Troy E. Gilmore4

1 Hydrology and Territorial Management Directory (DHT), The Geological Survey of Abstract Brazil (CPRM‐SGB), São Paulo (SP), Brazil The Guarani Aquifer System (GAS) has been studied since the 1970s, a time frame 2 Environmental Studies Center (CEA), São that coincides with the advent of isotopic techniques in Brazil. The GAS isotope data Paulo State University (UNESP), Rio Claro (SP), Brazil from many studies are organized in different phases: (a) the advent of isotope tech- 3 Laboratory for Hydrogeology and Basin niques, (b) consolidation and new applications, (c) isotope assessments and Studies (LEBAC), São Paulo State University hydrochemistry evolution, and (d) a roadmap to a new conceptual model. The reasons (UNESP), Rio Claro (SP), Brazil 4 Conservation and Survey Division, School of behind the phases, their methodological approaches, and impacts on the regional flow Natural Resources, University of Nebraska‐ conceptual models are examined. Starting with local δ2H and δ18O assessments of Lincoln, Lincoln, (NE), USA values for fingerprinting and estimates of recharge palaeoclimate scenarios, Correspondence studies evolved to more integrated approaches based on multiple tracers. Stable iso- Roberto Eduardo Kirchheim, The Geological Survey of Brazil (CPRM‐SGB). Rua Costa 55, tope application techniques were consolidated during the 1980s, when new dating CEP 01304‐010, São Paulo (SP), Brazil. approaches dealing with radiogenic and heavy isotopes were introduced. Through Email: [email protected] the execution of an international transboundary project, the GAS was studied and extensively sampled for isotopes. These results have triggered wider application of isotope techniques, reflecting also world research trends. Presently, hydrochemical evolution models along flow lines from recharge to discharge areas, across large‐ scale tectonic features within the entire sedimentary basin, are being combined with residence time estimates at GAS outcrop areas and deep confined units. In a complex system, it is normal that many, and even contradictory hypotheses are proposed, but isotope techniques provide a unique chance to test them. Stable isotope assessments are still needed near recharge areas, and they can be combined with groundwater classical dating procedures, complemented by newer techniques (3H‐3He, CFCs,

and SF6). Recent noble gas sampling and world pioneer analytical efforts focused on the confined units in the GAS will certainly led to new findings on the overall GAS circulation. The objective of this article is to discuss how isotope information can contribute to the evolution of conceptual groundwater flow models for regional continental aquifers, such as the GAS.

KEYWORDS

groundwater management, Guarani Aquifer System, stable isotopes

Hydrological Processes. 2019;1–13. wileyonlinelibrary.com/journal/hyp © 2019 John Wiley & Sons, Ltd. 1 2 KIRCHHEIM ET AL.

1 | INTRODUCTION The Mesozoic aged continental clastic units from the Paraná and Chacoparanaense sedimentary basins were grouped as GAS units, In the last 60 years, abundant information on isotopes in groundwater named to recognize the indigenous people of the Great Guarani has been obtained, and widespread application to hydrogeological Nation who used to live in this same region (Organization of American research has recently increased because analytical techniques have States (2009). become faster, more sensitive, and less expensive (Wassenaar, Coplen, Collectively, GAS resources are shared by more than 90 million & Aggarwal, 2014). The sustainable use and management of aquifers people from , Brazil, , and , for whom it require an understanding of aquifer hydrogeology and groundwater represents the most important source for public water supply, agricul- dynamics. This understanding can be gained over a period of decades ture, and industrial uses as well (Organization of American States by observations and measurements of precipitation, river flows, and (2009)). At a regional scale, Vives, Rodriguez, and Goméz (2008) have groundwater levels. Isotope techniques and particularly those that identified a water deficit condition, where GAS annual extraction esti- can be applied to estimate the residence time of groundwater help mates (1.04 km3/year) are higher than recharge (0.8 to 1.4 km3/year). to cost‐effectively build a conceptual framework of aquifer hydrogeol- According to Gastmans, Veroslavsky, Chang, Caetano‐Chang, and ogy and flow system. Pressinotti (2012), despite the calculation uncertainties, this balance The use of the radioactivity of 14C and 3H isotopes for estimating scenario should be strongly considered for GAS management planning. groundwater age, aquifer storage, rate of groundwater renewal, and In some areas of the State of São Paulo, GAS drawdowns are already flow velocity was identified in the 1940s (Libby, 1946). Groundwater well known leading to economical externalities. According to Manzano age provides unmatched advantages for improving numerical models and Guimaraens (2012), who conducted an extensive hydrochemical of groundwater flow in large, regional aquifers where water level data assessment covering the entire GAS body, the great majority of the are normally scarce (Aggarwal, 2013), such as the Nubian Aquifer, water was found to be of very good quality and suitable for domestic whose model calibration was based on 36Cl and 81Kr residence times consumption and agricultural use. Anthropogenic contamination has estimates (Sturchio et al, 2004; Patterson et al., 2003. been detected only at some GAS outcrops and adjacent semiconfined The use of environmental isotopes in hydrological studies in Brazil areas. Sound management of such a transboundary aquifer demands started at the end of the 1960s and early 1970s, with focus on the strategic action mechanisms against overexploitation at confined and semi‐arid northeast region, followed by the Amazonia region, storage‐controlled areas and protection of recharge areas where basal and, finally, the southeast regions, lying within the Paraná flows are being produced through hydraulic connection to the surficial intracratonic basin (Silveira & Silva Junior, 2002). These early studies drainage network. were motivated by the need to understand the groundwater saliniza- The purpose of this article is to present the continuous evolution tion processes, aquifer recharge mechanisms, and groundwater ages of the isotope knowledge of the GAS, pointing out how determinant in semi‐arid and densely populated areas (Gat, Mazor, & Mercado, these studies were in promoting enhanced insights over its regional 1968). Salati et al. (1974) offer an overall synthesis of isotope studies flow and hydrochemical evolution dynamics. Isotope assessments, during this period. In the Amazonia region, on the other hand, daily conducted locally and across the entire GAS, helped to shape the δ18O and δ2H data for moisture and rain water allowed researchers actual GAS hydrogeological knowledge and enhanced the evolving to recognize the relevance of water recycling within the continental conceptual models. The isotope application novel in the GAS is coher- hydrographic basin and the importance of the forest evapotranspira- ent with the evolution of the technical developments worldwide. At tion in determining rain patterns (Salati, Dall'Olio, Matsui, & Gat, the other hand, it offers valuable lessons learned with possible replica- 1979). Finally, isotope initiatives at the southeast region began with tion in other regional aquifer systems, such as the Congo, Kalahari, isotopic fingerprinting for rain and groundwater, including samples Karoo, Nubian, Taoudeni‐Tanezrouft, Senegalo‐Mauritanian, Yakut, from the Botucatu and Pirambóia Formations (actually belonging to Angara‐Lena, High Plains, Great Plains, Floridian, Edwards, Gulf the Guarani Aquifer System [GAS]), considered to be the most promi- Coastal Plain, and (Aggarwal et al., 2014). nent aquifer units of the Paraná Basin (Matsui et al., 1971). The large aquifer potential of the sandstone sequences was already well recognized (Rebouças, 1976). Gallo and Sinelli (1980) 2 | HYDROGEOLOGICAL SETTING developed a hydrochemical and isotopic study in an aquifer area with high groundwater demand. The analytical effort deployed in this study The GAS, lying on the eastern edge of South America (lat. 16° to 32° was considered quite emblematic in Brazil, whereby samples for 14C S; long. 47° to 60° W), extends for 1,088,000 km2 and covers consid- were processed at the University of São Paulo, 13C was determined erable areas of Brazil (8%), Argentina (8%), Paraguay (21%), and at the Center for Nuclear Research in Agriculture, 3H at the new facil- Uruguay (21%). It is the most important aquifer within the continent ities of the Center for Development of Nuclear Technology, and 2H and one of the largest transboundary aquifers in the world. Between and 18O were processed at the International Atomic Energy Agency 2003 and 2009, it was the target of a Global Environmental Facility lab facilities in Vienna, Austria. International support at that moment (GEF) supported project with the World Bank and the Organization was a key factor because national research groups were in their early of American States. During the execution of this GEF‐funded research, development stages. many hydrogeological aspects of the GAS were studied, in part KIRCHHEIM ET AL. 3 through isotope assessments. Figure 1 shows the location of the GAS, A synthesis for the main characteristics of the regional and the main features constituting its geological framework. hydrogeological systems (POST‐GAS, GAS, and PRE‐GAS) are pre- GAS is described as a thick package of clastic detrital sedimentary sented by the Table 1. sequences of Mesozoic age, whose geometry and structural frame- At the northern compartment, GAS includes sandy rocks of Trias- work is controlled by large structural features, mainly arcs and fault sic age from the Pirambóia Formation and aeolian sandstones from zones that not only imposed physical boundaries and sediment source Botucatu Formation, which is Jurassic‐Neo in age. This areas but also were responsible for the significant erosion of previ- sequence is bounded at the base by an aquidard unit, the Passa Dois ously deposited sediments (Gastmans et al., 2017). Group, and at top by the basaltic lava from Serra Geral Formation. GAS geometry, framework, and groundwater flow are directly con- The basalt flows are covered by the sedimentary red beds of the trolled by these large‐scale structural features of these sedimentary Bauru Group, which represents another important aquifer unit for basins: to the North, the Canastra and São Vicente arches; to the this region of Brazil. The Pirambóia Formation in São Paulo state is West, the Asunción and Pampeano/Puma arches; and to the South, composed mainly of fine sandstones that are regular to well sorted the Rio de la Plata Arch; likewise, other important geological struc- and are intercalated with clay layers and silty to sandy mudstones. tures have been recognized: the Ponta Grossa and Rio Grande arches The sandy units were deposited in a desert environment, as dune and the Torres synclinal (Laboratory for Basin Studies (LEBAC), 2008a, deposits, with interdune facies of silty sandstones, which contain 2008b). Figure 1 shows the most important structures of the Paraná aeolian ripples and bioturbation. The presence of sediments depos- Basin, and cross‐section A‐A′ shows their importance in GAS geome- ited under wetter conditions is generally associated with fluvial facies try and stratigraphy. The GAS units are associated with the Mesozoic (Caetano‐Chang, 1997). A widespread desertification of Gondwana, continental clastic sedimentary sequences from Parana and during the Mesozoic Atlantic pre‐breakup time, was responsible for Chacoparananense sedimentary basins, bounded in its base by a the appearance of large aeolian deposits of Botucatu Formation in Permo‐Eotriassic regional unconformity and at the top by lava flows the Parana Basin (Milani et al., 1998). This unit is composed of of the Serra Geral Formation. The lava flow covers (POST‐GAS) may well‐sorted very fine to fine‐grained aeolian sandstones, with cross reach a total thickness of 1,500 m at the basin depocentre, and the stratifications of medium to great thickness. Locally, there are con- Permean pelite sequences (PRE‐GAS) may reach up to 4,000 m thick- glomeratic sandstones deposited in wadis and, rarely, muddy sand- ness. Figure 2 presents the GAS stratigraphy and its differences stones and mudstones deposited in lakes. In general, the rocks were between the northern and southern parts of the basin. deposited by aeolian processes in a desert environment, with a

FIGURE 1 The GAS location within South America and its geological framework. (Modified from Foster, 2009) 4 KIRCHHEIM ET AL.

FIGURE 2 The stratigraphy units that constitute the GAS, where original national stratigraphic nomenclature was correlated. Units above and below GAS are also identified. (Modified from Consorcio Guaraní, 2009)

predominance of aeolian dune foresets, wet interdunes, and sand based on full groundwater flow continuity across the main structures sheets (Caetano‐Chang, 1997; Soares, 1973, 1975). (Gastmans et al., 2012), according to the model developed by Vives Based on De Santa Ana and Veroslavsky (2003), the southern et al. (2008) and Mira, Vives, Rodríguez, and Veroslavsky (2018), compartment of the GAS has greater stratigraphic and lithological whose results for the water balance at each one of the GAS domains complexity. Since Rebouças (1994) and Araújo, França, and Poter are summarized in Table 2. (1995), the sedimentary Triassic sequence (see Caturrita, Santa Maria, According to the model outputs (Vives et al., 2008), water inflows and Sanga do Cabral Formations in the Figure 2), studied by Faccini vary between 0.2 and 1.6 km3/year, close to the values estimated by (1989, 2000), was included as being GAS units, and they represent water balance, whereas discharge reach values from 0.18 to the final consolidation of the Pangea, coincident with the progressive 0.8 km3/year. These differences may indicate ascending GAS flow into continentalization of the basin prior to the final Gondwana break out the basalt cover (Vives et al., 2008). Once pumping volumes are con- (Turner et al., 1994). sidered, it turns out that there are water deficits in all four GAS Aquifer recharge occurs in outcrop areas located at eastern, domains, meaning that overall water extractions are larger than northern, and southern borders of GAS uplifted by the Gondwana recharge. Despite the many model uncertainties, the results are critical reactivation, giving rise to intermediate to local flow lines, with marked for the development of groundwater management plans. The total different hydrochemical evolution once compared with the old deep amount of water stored within the GAS lies between flow paths. 29,550 ± 4,000 km3 and 32,830 ± 4,400 km3. Despite this apparently The GAS hydrogeological knowledge has evolved from a large huge water volume, it must be emphasized that not all of it is available homogeneous groundwater structure (Gilboa et al., 1976) to a more for use. Considering a drawdown limit of 400 m, available GAS volume complex and heterogeneous system, where regional fluxes are con- decreases to 2,000km3 (OAS, 2009). trolled by its geological structural framework according to the central axis of the large Paraná Basin (Araújo et al., 1995; Araújo, França, & Potter, 1999). Nevertheless, as follows, the most recent modelling 3 | EVOLUTION OF ISOTOPE attempts are still derived from conceptual hydrogeological model APPLICATIONS

Studies dealing with isotopic data in the GAS started in the 1970s TABLE 1 Synthesis for the hydrogeological characteristics and have evolved from local to regional sampling campaigns to Aquifer systems T (m) K (m/s) Ө Function TABLE 2 Water balance at the GAS domains POST‐GAS Unconfined − Bauru Group 200 1 × 10 6 to 0.07–0.15 aquifer system. Σ extracted Water balance (km3/year) − 1×10 5 Regional aquitard. GAS Σ extraction volume − Serra Geral 1,500 5 × 10 7 to 0.01–0.05 Up to 200 m, an domain wells (km3/year) Min Max Formation 8×10−3 unconfined NE 1,165 −0.83 −0.75 −0.14 fractured aquifer − E 267 −0.06 −0.05 0.13 GAS 700 1.5 × 10 4 0.1–0.15 Regional aquifer − W 346 −0.07 −0.21 −0.27 PRE‐GAS 4,000 8 × 10 8 0.1 Regional aquitard S 276 −0.08 −0.07 0.06 Note. Modified from Sracek and Hirata (2002). Abbreviations: Ө, porosity (a dimensional); GAS, Guarani Aquifer System; Note. Modified from Gastmans et al. (2012) with Vives et al. (2008). K, hydraulic conductivity (m/s); T, thickness (m). Abbreviation: GAS, Guarani Aquifer System. KIRCHHEIM ET AL. 5

FIGURE 3 Evolution of the isotope knowledge applied to the GAS

better define the flow circulation model. Figure 3 summarizes 3.1 | The advent of isotope techniques—Phase I the temporal evolution of the GAS isotopic knowledge leading to a roadmap for new conceptual models and identifies four Isotope studies within the first phase were incipient and representa- different knowledge phases. The chart is complemented by tive of small spatial scales. According to Gilboa et al. (1976), the GAS Table 3, where the most relevant isotope references are organized recharge sources were derived from precipitation at the outcropping chronologically. areas from where divergent flow lines towards the basin axis would Figure 4 shows the different conceptual models, which were occur (Figure 4—Phase I). Gilboa et al.'s (1976) conceptual flow considered representative for the first three knowledge phases, understanding, based on a large homogenous reservoir, biased iso- respectively: The homogeneous reservoir GAS model by Gilboa tope studies during the 1980s up to 1990s. Water balance scenarios et al. (1976) for the Phase I (the advent of the technique); the were not known, and management was still founded by water myths model conceived by Araújo et al. (1995) based on two large domin- (Foster, Tuinhof, Kemper, Garduño, & Nanni, 2003). A first regional ions for the Phase II (consolidation and new applications); and hydrochemistry assessment was carried out in São Paulo state by the model developed after the GEF‐funded research imitative da Silva (1983); Kimmelmann, de Cunha, Reboucas, and Santiago (OAS, 2009), based on four distinctive domains, considered repre- (1989); and Kimmelmann, Forster, and Coelho (1995) using a sentative for the Phase III (isotopes assessment and hydrochemistry multitracer approach with analytical determinations of 18O, 2H, 3H, evolution). There are pending questions related to the role played 13C, and 14C. This study was considered quite innovative not only by the large structures and how far they act as flow barriers to due to the integrated approaches but also due to the regional scale regional fluxes. Depending on the scale, important particularities within the GAS. Rain water isotope monitoring established correla- regarding vertical flow gradients, water mixtures processes, and tions with modern water from young recharge events. Study outputs hydrochemical differences are expected demanding more appropri- corroborated previous conclusions stated by Gallo and Sinelli (1980), ate model constructions, which are then expected at the Phase IV who indicated isotopic and hydrochemical evolution evidence along (the actual trends), where GAS research is standing now. GAS issues flow lines from the recharge areas through the aquifer matrix. The such as degree of confinement, the role of the aquitards, horizontal increasing residence time correlates with the increase of GAS tem- versus vertical flow, location of recharge and discharge areas, perature and salinity towards southwest. Bicarbonate and calcium– and water balance scenarios are tackled within the subsequent magnesium water were likely to evolve to sodium‐bicarbonate and research efforts, which were grouped in the different knowledge chloride sulfate sodic after residence times of about phases. 30,000 years, time frame considered to be close to the detection 6 KIRCHHEIM ET AL.

TABLE 3 Isotope references considered representative to GAS knowledge evolution

Author (Date) Isotopes Main contribution

Matsui et al. (1971) 18O and 2H Isotope composition of the rain and GAS groundwater Gallo and Sinelli (1980) 18O, 2H, 14C, and 13CE–W GAS transect with isotope composition of GAS water and residence time estimates da Silva (1983) 18O, 2H, 3H, 13C, and 14C Integrated GAS hydrochemical assessment with isotope component for the state of São Paulo; residence time estimates, mean velocities, and recharge origin Kimmelmann et al. (1986); Kimmelmann 18O, 2H, 3H, 13C, and 14C GAS zoning based on δ18O and δ2H; pioneer studies with 3H, He et al. (1989); and Kimmelmann et al. isotopes, and 13C and 18O from carbonate minerals present in aquifer (1995) cement Bonotto et al. (1996); Bonotto et al. (2002); 40K, 238U, 234U, 226Ra, 222Rn, Starts research area with heavy isotopes and their mobilization Bonotto et al. (2006); and Bonotto et al. 210Po, 210Pb, 232Th, 228Th, mechanisms in water rock interaction scenarios. Residence time (2008) and 228Ra estimates using U isotopes; GAS α and β radioactivity determination and natural background for dissolved radionuclides Sracek and Hirata (2002) and Hirata et al. 18O, 2H, 3H, 13C, and 14C Reinterpretation of the data from da Silva (1983); establishment of a (2011) zoning based on isotope concentration spatial variability Gastmans (2007); Gastmans et al. (2010a); 18O, 2H, 3H, 13C, and 14C Extensive sampling efforts in the northern and western GAS boarders; Gastmans et al. (2010b); and Gastmans palaeo recharge events under different climate scenarios; GAS zoning et al. (2012) proposal based on isotope and hydrochemical processes; residence time estimates Aravena (2008) 18O, 2H, 3H, 13C, and 14C Sampling campaign over the entire GAS body; regional isotope results for the GAS Cresswell and Bonotto (2008) 36Cl New sampling campaign at the same section previously evaluated by da Silva (1983) and Sracek and Hirata (2002). Pioneer residence time estimates using 36Cl Chang et al. (2011) 18O, 2H, 3H, 13C, and 14C Incorporation of isotope results into a regional flow conceptual model for the GAS Bonotto (2012) 234U and 238U Residence time estimates with 234U/238U ration and comprehension of water mixing processes Gastmans et al. (2013) 81Kr, 18O (sulfate), and 14C Determination of age residence time with the 81Kr method and calibration of He age estimates Aggarwal et al. (2014) 14C, 13C, 4He, 3He, 39Ar, 40Ar, Large number of confined GAS samples results for 81Kr and noble gases; and 81Kr GAS being the transferring media for He back into the atmosphere Soleri et al. (2015) 2H, 18O, δ18O (sulfate), and Presents new insights over GAS hydrochemical evolution along NW–SE δ34S transect Gastmans et al. (2017) 2H, 18O, 3He, and 4He Field sampling campaign for stable isotopes and He across large tectonic structure Bonotto and Elliot (2017) 87Sr/86Sr and δ11B Transect in São Paulo State, Brazil, and the analysis of trace elements, REEs, and stable isotopes (B, Sr) in both rainwater and groundwater samples Elliot and Bonotto (2017) 87Sr/86Sr, δ11B, δ13C, 14C, δ18O, New insights over recharge from flood basalts directly overlying the and δ34S GAS

Abbreviation: GAS, Guarani Aquifer System.

limit for the radiocarbon dating. The approaches focused on the the eastern border towards the deeper confined parts at the western determinations of δ18O and δ2H and comparisons of data sets side. The spatial distribution of the GAS samples used for isotope against the global meteoric water line. Age estimates (from studies discussed so far are shown in Figure 4—Phase I (grey‐shaded 30,000 ± 1,900 to 800 ± 160 years) were used to calibrate hydraulic areas). These age and mean conductivities were not used for man- gradients and mean conductivities along the flow lines cross agement purposes at that time. The legal framework dealing with sections, respectively, 0.0015 to 0.0014 and 2 × 10−4 to groundwater management at federal and state levels, establishing 2×10−5 m/s. One of the most important findings from these assess- state ownership over the groundwater, water use permits, and ments dealt with isotope coherence against the hydrochemical groundwater quality protection tools had appeared later, in the late evolution along flow lines, from well‐known GAS recharge areas in 1990s (Brazilian Federal Law 9.433, 1997). KIRCHHEIM ET AL. 7

FIGURE 4 The evolving conceptual models for GAS. Phase I model, conceived by Gilboa et al. (1976), represented the Botucatu Formation and correlated unit in the neighbouring countries as single homogeneous and aquifer body. Phase II model (Araújo et al., 1995) presented the Jurassic sandstones sequences and their spatial potenciometric level distribution through a transboundary aquifer divided into two large dominions by the Ponta Grossa Arch. Phase III model (Organization of American States (2009)) was developed during the GEF Research Project after collection and systematization of new data sets. It consisted of four domains due to the influence of the large tectonic structures of the Paraná Basin

3.2 | Consolidation and new applications—Phase II values. A first group circumscribed to the confined GAS portion in the state of São Paulo was composed of more depleted values (δ18O Important attempts to improve regional circulation models of GAS and δ2H ranging from −8.1‰ to −9.8‰ and −56‰ to −67‰, respec- dynamics were conducted, and for example, the first hydrogeological tively) and a second group with water originating at the GAS recharge map for the entire aquifer body and flow model was developed outcropping areas on the west at the state of Mato Grosso do Sul (Campos, 1994). It was complemented by a potentiometric flow net- hosted more enriched values (δ18O and δ2H ranging from −5.7‰ to works and the identification of major recharge and discharge. Isotope −6.8‰ and −32‰ to −47‰, respectively). data available so far were not considered in this model design or Similar isotopic values were found by Calf and Habermehl (1984) calibration. Almost simultaneously, the GAS geometry, expressed by in the Great Artesian Basin. This spatial pattern was credited to the GAS and POST‐GAS regional isopachs, followed by supplementary altitude effect on isotopic signature of rain, latitude differences, and regional cross sections, allowed Araújo et al. (1995, 1999) to launch climatic variations (Kimmelmann E Silva, Silva, Rebouças, & Santiago, a new GAS conceptual model. This model consisted of two large flow 1986). In deep confined areas, age estimates using 14C methods domains, which were separated by the Ponta Grossa Arch, as seen in reached magnitudes close to 40,000 years. It was clear from this Figure 4—Phase II. The GAS limits at the southern border were still moment on that more suitable age tracers for older groundwater were not known, and the stratigraphic coherence of the aquifer units that needed to estimate groundwater age in confined portions of the GAS. later on would be grouped as a single aquifer system was not yet fully 13C and 14C compositions of all dissolved inorganic carbon (DIC) understood. The hydraulic gradients were higher close to the recharge samples ranged from −5.2‰ to −18.9‰ and between ~8 and areas than along the deep confined parts. Regional discharge was >100 pmC, respectively. This wide variation of 13C compositions is hypothesized to be coincident with the main rivers Paraná and most likely the reflection of geochemical processes in the groundwa- Uruguay. Prior to publication of the GEF project results, isotopic data ter system. Both the dissolution of calcite and exchange processes, were reinterpreted after this new conceptual model. Most of the which are possibly represented by dissolution–precipitation reactions, research sampling efforts were addressed over previous recognized can dominate the carbon isotope evolution (Kimmelmann et al., recharge areas on the western and southern GAS borders. New data 1989). Helium samples were collected at few deep wells and 3He/ provided by Kimmelmann et al. (1995) showed δ2H and δ18O data 4He ratio results were in the range of 10−7 to 10−8. Thus, a deep‐ along the global meteoric water line, an evidence for its meteoric seated mantle component was not recognized at that moment. source, even for deeper groundwater with age scenarios confirming However, the distribution of U–Th in the aquifers was very poorly initial attempts carried out by da Silva (1983). 14C age estimates were known, and therefore, no detailed discussion of the data was deliv- used to calculate mean velocities (2.6 × 10−5 m/s), which seemed to be ered (Kimmelmann et al., 1995). coherent to GAS hydraulic parameters and aquifer geometry (between GAS fluoride anomalies seemed to correlate with 14C ages ventilat- 2.4 × 10−6 and 4.5 × 10−5 m/s). Despite the sparse spatial sampling ing questions regarding mixing processes with younger water from the distribution, a zoning was proposed based on the δ18O and δ2H basalts above (POST‐GAS) and/or older ones from Permean aquitards 8 KIRCHHEIM ET AL. below (PRE‐GAS; Kimmelmann et al., 1989; Roisemberg, Marimon, & GAS (Cresswell & Bonotto, 2008). Their estimated ages of 320,000 Viero, 2006). However, a question regarding the open or closed condi- and 1,150,000 years were considered consistent with previous age tion of GAS with possible mixing due to transport mechanisms estimates from U isotopes. through the neighbouring aquitards was still a matter of scientific However, PRE‐GAS and deeply confined GAS water presented debate (Lehmann et al., 1995; Solomon, Hunt, & Poreda, 1996). markedly different isotopic fingerprints. The main reasoning behind Age models based on 234U/238U isotope ratios were first evaluated each single GAS knowledge evolution step promoted by different in GAS samples from the recharge to the central confined areas authors was to understand how GAS water circulated and evolved (Bonotto, 2000). Age estimates throughout this method ranged from across the entire aquifer body and further, how did local GAS charac- 106,750 to 32,726 years, in clear discrepancy to the ones previously teristics fit into regional conceptual models and be explained by them. estimated. New management challenges arose when GAS deep confined por- At the end of knowledge Phase II, strong evidence led to postula- tions begun to be tapped intensively. National legal framework and tion of a large heterogeneous aquifer tectonically divided into tight management tools did not reflect the GAS zoning suggestions so far. compartments (Machado, 2005; Portela Filho, Ferreira, da Rosa Within this context, the isotopic fingerprints combined with geochem- Filho, & Rostirolla, 2005; Rosa Filho, Hindi, Rostirolla, Ferreira, & ical processes encouraged authors to establish three different zones, Bittencourt, 2003). Deep normal faulting and diabase intrusions as which at the end reflected the degree of confinement in the GAS. dykes and sills near the large tectonic structures of the Paraná Basin Waters evolved from younger waters that were calcium bicarbonate would be, according the nominated authors, responsible for condition- rich, to chloride, sulfate, and sodium rich, influenced by saline ground- ing GAS water flow and its recharge. The variations in GAS hydraulic water from aquitard units below (PRE‐GAS). properties and depths to the top of the aquifer system, registered at The execution of an international research initiative for the GAS, water wells relatively close to each other (distances less than 20 km) comprising the four countries (Argentina, Brazil, Paraguay, and Uruguay) and marked hydrochemistry characteristics, are indicative of the and financed by the GEF allowed new insights over the GAS and finally existence of distinct GAS compartments. raised political and diplomatic compromises (Organization of American Most of the isotope findings covered by this time frame had fit well States, 2009). Within the technical project components, an overall into the regional flow model conceived by Araújo et al. (1995), but the regional isotope sampling campaign was carried out, and results were limited spatial scale of studies and issues with sampling procedures such published in 2008 (Aravena, 2008). One of the most important advances as the careful selection of well‐known production wells extracting GAS regarding this initiative was the selection of fully documented represen- representative water were considered to be limiting factors. tative wells, the set‐up of an integrated data set, and the delineation of transects along theoretical flow lines. The interpretation of the isotopic results was still based on the conceptual model brought by Araújo et al. 3.3 | Isotopes assessment and hydrochemistry (1995), with two large dominions, separated by the Ponta Grossa Arch. evolution—Phase III In the northern part of the GAS, a wide variation of isotopic δ18O and δ2H values was observed, suggesting that no evaporation processes During Phase III, the isotope and hydrochemistry applications reached had taken place prior to recharge (Aravena, 2008). Later, Batista et al. a consolidated status as efficient tools for evaluating GAS flow and (2018), in a minor scale study in a GAS recharge area, revealed isotopes hydrochemistry hypotheses. Sracek and Hirata (2002), after a reinter- data showing secondary evaporation processes during GAS recharge pretation effort of the data collected by da Silva (1983), corroborated events. In the southern part, isotopic composition was slightly different, the statement of Kimmelmann et al. (1989) regarding the role of the being correlated more directly to the air masses responsible for precip- cation exchange and carbonate dissolution mechanisms as GAS flow itation in that region. 14C patterns were coherent with previous assess- evolved downgradient. Nevertheless, according to the authors, advec- ments, with values being greater at recharge areas and lower in tive flow velocities were considered higher than the cation exchange confined areas. Activities for 14C around 80 pmC were found for mod- front. δ2H and δ18O values seemed to get more depleted along flow ern recharge water, and they abruptly decreased to about 1 pmC after lines, and the enrichment of (DIC) δ13C suggested calcite dissolution 50 km along the transect flow lines. In general terms, old GAS ground- under closed system conditions. Deep thermal GAS water showed water was found relatively near to respective recharge zones. These high δ13C DIC values (>−6.0‰), indicating isotope exchanges with conclusions have important consequences for GAS management carbonates. The authors identified mechanisms of vertical flow inside (Aravena, 2008). Despite the achievement of a reliable regional data the GAS system responsible for promoting water mixtures. bank for the whole GAS reservoir, some limitations were pointed out: New age interpretations using U isotopes reached 45,000 and (a) Sample spatial distribution was considered to be poor; (b) sampling 61,000 years, similar to 14C age estimates from sampling confined units campaigns took place at production wells, and water from distinct (Bonotto, 2006). However, these new estimates might be best viewed as GAS depths might be pumped; (c) a clear recognition of the limitations minimum ages, because sensitivity analyses using the same data of the radiocarbon method for GAS ages >30,000 years. could suggest residence times 600,000 years or greater (Bonotto, 2006). A whole new set of unconfined and confined GAS, river, and Through an end‐member analysis, 36Cl age determinations could springs water samples, collected by Gastmans, Chang, and Hutcheon be depicted for two samples from deep confined portions of the (2010b) in the northern part of the GAS body, revealed a strong KIRCHHEIM ET AL. 9 relationship with the meteoric water from the closest Global Network Changes of recharge temperatures throughout time could be evalu- of Isotopes in Precipitation (GNIP) station in Cuiabá. A new isotopic ated from the variation in isotopic ratios for δ18O, along four transects seasonal pattern was clearly identified, where summer rains were representing the isotopic evolution from the outcropping areas along more depleted and GAS δ18O and δ2H signatures were correlated with the main flow lines (Gastmans et al., 2010b). The isotopic evolution the summer season. along these transects suggested an increase in the temperature in These data were used to evaluate the hydrochemical evolution and the recharge zone through time, up to the average present‐day tem- to construct a hydrogeological conceptual model for this portion of the peratures in recharge areas (±20°C). Groundwater samples with more GAS. Gastmans, Chang, and Hutcheon (2010a) and Hirata et al. (2011) negative δ18O ratios were recharged under more humid conditions revealed that the GAS hydrochemistry evolution is consistent with the and with temperatures 10°C below the present average temperatures, observed diagenetic features, with the presence of siliceous cement in reflecting the climate at the end of the Last Glacial Maximum, ca. the outcrop areas and carbonate cement towards the centre of Paraná 25,000 years. With increasing temperature after the glacial period, Basin. The mechanisms responsible for this evolution are dissolution of the GAS seemed to become more enriched in δ18O, with similar values feldspars and removal of the carbonate cement from the sandstone to those observed in the present‐day recharge waters. mineral framework, followed by ion exchange, responsible for the Based on the spatial variations of the isotopic rates and increase in the Na concentration and decrease of Ca, and, finally, hydrochemistry evolution found in Hirata et al. (2011) and Gastmans enrichment in Cl and SO4 derived from underlying aquifer units. et al. (2010), different GAS flow zones were identified: (a) Zone A:

Hydration and dissolution reactions of silicates between aquifer modern recharge Ca‐HCO3 water from GAS outcrop areas bearing 18 matrix and recharge water occurred in GAS low‐temperature water– δ O>−7‰; (b) Zone B: confined and more alkaline Na‐HCO3 waters rock interaction close to outcrop regions might acted as an additional characterized by δ18O>−7‰, related to older residence times 18 isotopic fractionation factor resulting in δ O‐depleted waters. (>10,000 years); (c) Zone C: very deep confined Na‐HCO3/CO3‐Cl‐ 18 Gastmans et al. (2010b) suggested that these reactions were the SO4 waters with δ O>−7‰ with residence times ≫10,000 years. first ones to happen during the GAS hydrochemistry evolution. Accord- Despite increased understanding of regional flow models within the ing to them, in these reactions, cations—mainly calcium, potassium, GAS, management of GAS resources did not change. Nevertheless, magnesium, and sodium, would dissolve silica and smaller amounts of Brazilian authorities begun to execute some of the activities considered bicarbonate anions—were introduced and also might produce a lower to be priorities in the strategic action plan of the GAS (Organization of enrichment in δ18O. There might be also water extraction from PRE‐ American States, 2009), such as the regional GAS vulnerability mapping GAS aquifers (Meng & Maynard, 2001; Sracek & Hirata, 2002; Bonotto, and the inclusion of better GAS diagnostics within water plans at state 2005) given discrepancies among depleted water and high total dis- and river basin scale. solved solid content. δ13C values measured in the carbonate cement According to the current conceptual model (Chang et al., 2013), the of the aquifer (done for the first time in the GAS) suggested that the GAS was a continuous aquifer, despite lithological heterogeneities water responsible for the cement precipitation was about −5‰ Vienna between the northern and southern parts of the Paraná Basin. The main PeeDee Belemnite (VPDB) and that δ13C rates become higher towards flow direction was defined as being from north to south, strongly the centre of the basin, as consequence of the progressive carbonate affected by large tectonically structures where four distinct compart- cement dissolution along the main groundwater flow directions. If cal- ments could be distinguished as follows (Figure 4—Phase III): (a) the cite saturation was not achieved, this process did continue, and it could NE domain, located in the northeastern portion of the GAS, in São be explained by Ca‐Na exchange reactions in a CO2 closed system. Paulo and Minas Gerais states (BR); groundwater flows towards the Flow velocities based on the age estimation are similar to the ones Paraná River from the recharge area located to the east. The largest determined at previous studies by Araújo et al. (1995) and Sracek and hydraulic gradients are found near the outcrop zone (3 to 5 m/km), with Hirata (2002). Mean velocities were about 3 × 10−7 m/s and along the lower gradients in the confined zone (0.1 m/km); (b) the E domain, proposed flow lines they had implied residence times >30,000 years, located in Paraná, Santa Catarina, and northern part of Rio Grande do consistent with da Silva (1983) and Bonotto (2006). Sul States (BR), is separated from NE domain by the Ponta Grossa Arch, Extensive areas of the northern part of the GAS seemed to have where basaltic dikes function as a regional hydraulic barrier, condition- very old recharge according to their isotopic fingerprint. Multivariate ing a preferential groundwater flow direction from E to W; (c) the W statistical and geochemistry modelling approaches confirmed the pro- domain is an isolated system, with recharge and discharge associated posed zoning. The marked gradient in δ18O concentration from to the outcrop zone, located in Goiás, Mato Grosso, and Mato Grosso recharge to the confined areas evidenced recharge events occurred do Sul States (BR) and in Paraguay; (d) in the S domain, the groundwater across a range of climatic periods, from recent Holocenic glaciation flows from east to west, with recharge areas associated to outcrop records back to 50,000 years in the past (da Silva, 1983). zones, oriented N–S, towards the Uruguayan territory. In Argentina, The δ18O spatial distribution showed that heavier isotopic ratios, despite the limited amount of wells and information, it was inferred a close in composition to the present‐day rainwater, are found in the recharge zone associated to the Mercedes High in Corrientes. outcrop recharge zones. In the central zone of the study area, due to This model however left some open uncertainties regarding their larger residence time, the GAS groundwater samples turned to recharge magnitudes and discharge location. Some tributaries of the be the most δ18O depleted showing higher total dissolved solid values. Paraná and Uruguay rivers and the Iberá wetlands in the southern part 10 KIRCHHEIM ET AL. of Argentina were identified as discharge fronts. δ18O, δ2H, 14C, δ13C, 65,830 to 145,900 years (Nittmann, 2014). Much older water was and 3H results of Manzano et al. (2011) revealed that superficial water expected at this southwest GAS borders, and reservations regarding might consist of contributions from GAS and even PRE‐GAS aquifers. the regional flow model and transport parameters are still common. It is The whole GAS system was considered to be storage dependent well accepted that regional flow is controlled by large tectonic structures (i.e., mostly “nonrenewable;” Foster et al., 2009), a scenario imposing of the Paraná Basin, which divide the aquifer into compartments. Still, relevant constraints on resource management. It has turned out clear there are remaining questions concerning the way the GAS regional flow that previous 14C age estimates for the confined GAS were not cor- dynamics and hydrogeochemical evolution is affected. Does the struc- rect, and new approaches based on cosmogenic radionuclides 36Cl tural framework represent flow barriers? Are there recharge and dis- and 81Kr must be used. charge areas not considered yet? Preliminary results by Gastmans et al. (2017) have shown that hydrochemical evolution encountered across 3.4 | Roadmap to a new conceptual model—Phase IV the Rio Grande‐Asuncion Arch is incompatible with premodelled regional flow lines. A mixture including more recent recharge seems to be hap- The comprehension of the residence time distribution across flow pening in the GAS southern part. It means that, at least in that particular lines is a key information for the GAS conceptual and numerical region across the Arch, the conceptual model should be updated. GAS calibration with strong implications for management purposes and 18Oand2H isotopic composition of samples lying in both sides of the scenario simulations. The last numerical attempt, based on the exis- Arch did not show relevant differences. This approach may also be tence of the four distinctive GAS domains (Vives et al., 2008), needs applied to the other regional tectonic structures of the Paraná Basin. to be reconsidered. Extraction rates were higher, and recharge Another important research trend deals with the reinterpretation patterns might have changed. of stable isotope concentration in rain events and young groundwater. Recent research has been focused in the northern part of the GAS, Recently, Gastmans et al. (2017) suggested the strong influence of the and new samples from deep confined wells were collected according South Atlantic convergence zone in defining the regional rain isotope to the multitracer approaches. Recent advances in the atom trap trace fingerprints and, therefore, recharge water also. Recent 18O and 2H analysis technique (Lu et al., 2014) turned possible the use of 81Kr for data on the GAS monitoring wells from the RIMAS (Groundwater Inte- dating very old groundwater beyond the 14C age range (Lehmann grated Monitoring Network Project) operated by the Geological Sur- et al., 2003; Sturchio et al., 2004; Aeschbach‐Hertig, 2014 and vey of Brazil revealed marked spatial gradients for stable isotopes Matsumoto et al., 2018). The 81Kr is used to constrain model parame- coherent with the positioning of the South Atlantic convergence zone ters to convert 4He concentrations in groundwater into ages (unpublished). What are the rain events in terms of frequency, inten- (Aggarwal et al., 2014). This approach was conducted in the GAS, sity, duration, and water moisture provenance that promote expres- and in other regional aquifers around the world as well, as part of a sive recharge inputs at different GAS regions? pioneer research effort (Jiang, Yang, Gu, Hu, & Lu, 2017). A number Because most of the GAS demands happen along a fringe zone of five representative deep GAS wells were sampled for 81Kr, 4He, sta- near the recharge areas, mostly in the São Paulo state, a better under- ble isotopes, and 14C. Results have shown residence times varying standing of the residence times using new tracer approaches should from modern water up to about 834 ± 91 ka, based on 81Kr (Aggarwal be used. Applications with 39Ar have provided valuable insights in sim- et al., 2014). Mean velocities reached about 2.12 × 10−8 m/s. ilar areas (Ritterbusch et al., 2014). In the deep confined GAS, 4He concentrations were three times higher than those at the recharge areas considered to be in equilibrium 4 | CONCLUSIONS with atmospheric He. Ne/He and 3He/4He rates indicated that the He built in the GAS is a mixture of atmospheric and crustal origin and that Recognized as one the most important worldwide transboundary the mantellic build‐up of He is considered negligible. According to regional aquifers, the GAS has been the target of extensive research Aggarwal et al. (2014), regional aquifers, such as the GAS, are respon- efforts. The aquifer potential to supply large amounts of water for sible for transferring radiogenic 4He produced at the intermediate and domestic, agriculture, and industrial use is well known and served as upper crust back to the atmosphere. a key motivation factor to understand its circulation model. However, Present groundwater recharge occurs in outcrop areas, as indicated despite the huge volumes of groundwater stored in GAS, its recharge by the presence of 3Hand14C, indicating modern recharge. The abun- rates are considered low. Overall demands are rising significantly, dance of 81Kr in samples free of 14C decreases from 0.81 ± 0.11 mainly at confined areas associated with agribusiness and recreation expressed as (81Kr/Kr) sample/(81Kr/Kr)airattheeastto0.18±0.03in thermal plants, and large drawdowns and pressure losses have been the western GAS boarders. Groundwater ages in these samples are in already registered. It is a similar scenario experienced by other the order of 566 ± 60 ka. Measured 4He‐excess is far above that regional continental aquifers around the world. expected from in situ production rates in sandstone aquifers and overes- Isotope approaches started early in the 1970s, thanks to interna- timate ages by several orders of magnitude. Thus, 81Kr ages are used to tional cooperation, coinciding with the advent of isotopic techniques calibrate the 4He geochronometer. Similar work is being conducted in in Brazil. The application of isotopes in the enhancement of the GAS Argentina in the southern portion of the GAS. New 4He age estimates hydrogeological knowledge covered different phases. Early assessments at the western boarder in Argentina were produced, ranging from dealing with stable isotopes determination focused on very restricted KIRCHHEIM ET AL. 11 areas near recharge outcropping zones. Later, studies evolved to ACKNOWLEDGMENTS address preliminary questions regarding GAS regional flow dynamics The authors wish to express their gratitude to the anonymous from recharge to confined areas. Recharge areas were first recognized reviewers, whose contributions and insights were considered essential through piezometric head measurements followed by the development for the final version of this paper. This study is being done under the of regional flow nets. Early isotope data revealed different financial support of the AIEA, under the CRP (Characterization of fos- palaeoclimatic recharge conditions. Regional flow occurred from the sil groundwater systems using long‐lived radionuclides—F30063), to northern recharge areas towards the south along the main basin axis, whom we extend our acknowledgement. Finally but not less important coinciding with the Paraná River. Sampling campaigns started to reach we thank all the support given by the São Paulo State University confined areas, far from those studied during the first phases (as GAS (UNESP) and the Geological Survey of Brazil (CPRM‐SGB). confined areas begun to be tapped by deep wells in other Brazilian states) and recharge areas in the west and southern aquifer borders. ORCID Hydrogeochemistry tools were used to interpret isotope data and Roberto Eduardo Kirchheim https://orcid.org/0000-0002-2660- reduce uncertainties on flow directions and age estimations through 9565 14 C method, allowing the proposal of distinctive conceptual models Didier Gastmans https://orcid.org/0000-0002-1340-3373 for the GAS. Initial age estimates allowed the reconnaissance of large ‐ flow lines from well known recharge areas through a relatively homo- REFERENCES geneous aquifer body, divided into a northern block and a southern Aeschbach‐Hertig, W. (2014). 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