Fluoride in the Serra Geral Aquifer System: Source Evaluation Using Stable Isotopes and Principal Component Analysis

Hindawi Publishing Corporation Journal of Geological Research Volume 2013, Article ID 309638, 9 pages http://dx.doi.org/10.1155/2013/309638 Research Article Fluoride in the Serra Geral Aquifer System: Source Evaluation Using Stable Isotopes and Principal Component Analysis Arthur Schmidt Nanni,1 Ari Roisenberg,2 Maria Helena Bezerra Maia de Hollanda,3 Maria Paula Casagrande Marimon,4 Antonio Pedro Viero,2 and Luiz Fernando Scheibe1 1 Universidade Federal de Santa Catarina (UFSC), Campus Universitario,´ Trindade, 88.010-970 Florianopolis,´ SC, Brazil 2 Universidade Federal do Rio Grande do Sul (UFRGS), Avenida Bento Gonc¸alves 9500, predio´ 43126/103, 91501-970 Porto Alegre, RS, Brazil 3 Centro de Pesquisas Geocronologicas´ (CPGeo), Instituto de Geocienciasˆ (USP), Universidade de Sao˜ Paulo (USP), RuadoLago562,CidadeUniversitaria,´ 05508-080 Sao˜ Paulo, SP, Brazil 4 Universidade do Estado de Santa Catarina, UDESC-FAED, 88.035-001 Florianopolis,´ SC, Brazil Correspondence should be addressed to Arthur Schmidt Nanni; [email protected] Received 30 January 2013; Revised 25 April 2013; Accepted 29 April 2013 Academic Editor: Umberta Tinivella Copyright © 2013 Arthur Schmidt Nanni et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Groundwater with anomalous fluoride content and water mixture patterns were studied in the fractured Serra Geral Aquifer System, a basaltic to rhyolitic geological unit, using a principal component analysis interpretation of groundwater chemical data from 309 deep wells distributed in the Rio Grande do Sul State, Southern Brazil. A four-component model that explains 81% of the total 18 2 variance in the Principal Component Analysis is suggested. Six hydrochemical groups were identified. Oand H were analyzed in 28 Serra Geral Aquifer System samples in order to identify stable isotopes patterns and make comparisons with data from the Guarani Aquifer System and meteoric waters. The results demonstrated a complex water mixture between the Serra Geral Aquifer System and the Guarani Aquifer System, with meteoric recharge and ascending water infiltration through an intensive tectonic fracturing. 1. Introduction is 1.5 mg/L [6]. In the study area, the SGAS fluoride average concentrations are around 0.24 mg/L, with a minimum value In the last several decades, the global water consumption of 0.02 m/L and the highest at 3.03 mg/L. has dramatically increased, especially for agriculture, water The SGAS overlies the Guarani Aquifer System (GAS; supply, and industrial uses. This paper examines the fluoride [7]), which has been the focus of several recent studies due content in water from the southernmost region of the frac- to its spatial extent and storage potential as a transboundary tured subcontinental Serra Geral Aquifer System (SGAS), an aquifer [8]. The area covered by the SGAS in Brazil, Uruguay, 2 important aquifer that supplies a large amount of water for an Argentina, and Paraguay is equivalent to 1,200,000 km economic developed region in Southern Brazil. (Figure 1). In Brazil, these groundwater resources should be Fluoride content in water is beneficial to human health efficiently managed to protect its water potential and quality. and in a moderate concentration (0.7–1.2 mg/L) prevents The climate in the study area ranges from subtropical to dental cavities. When used in excess can be toxic causing temperate, with precipitation average of 1,550 mm/year. human and animal dental and skeletal fluorosis, which has The SGAS reaches its maximum thickness of about been detected in China [1, 2], India [3], Kenya [4], and Israel 1,800malongthecentralaxisoftheParanaBasin,locatedin [5], among other countries. The drinking water limit rec- theSaoPauloState.IntheRioGrandedoSulState,thisaqui- ommended by the World Health Organization for fluoride fer has a wedge shape, ranging from 1,000 m in the eastern 2 Journal of Geological Research 58∘ 54∘ 50∘ 46∘ 70∘ 50∘ 0∘ 18∘ 18∘ 30∘ 22∘ 22∘ ∘ 26∘ 26 32∘ 32∘ ∘ 34∘ 34 58∘ 54∘ 50∘ 46∘ (km) 0 50 150 250 Bauru Group SAG outcrop Caiua´ Group Study area Serra Geral formation Figure 1: Location map showing the Serra Geral Formation (SGF) in South America (modified from [9]). area bordering the Santa Catarina State to thickness of 200 m systems from the Parana Basin, including the GAS, through in the western area adjacent to Argentina Border. ascending groundwater infiltration11 [ –16]. The aim of this study is to understand the spatial dis- Distinct tectonic fracturing occurs in the study area, con- tribution and geologic control of fluoride content in the trolling geomorphological features and groundwater circula- SGAS, identifying hydrochemical groups with high fluoride tion. These structures are directly related to the opening of Physicochemical analysis, geostatistics, and stable isotopes the South Atlantic Ocean, which is reflected in the major approaches were used along with a Geographic Information NE and NW fault and fracture directions. Neotectonics is System (GIS) in order to identify the spatial distribution of considered to play an important hydrogeological role in the different water types. the eastern region. Major tectonic systems define individual hydrochemical sectors, which are considered as individual hydrogeological blocks [15, 17, 18]. The tectonic block lim- 1.1. Geologic and Hydrogeologic Framework. The study area ited by the Terra de Areia-Posadas and Mata-Jaguari Fault islocatedintheRioGrandedoSulState,SouthernBrazil, 2 ∘ ∘ Systems (Figure 2) is uplifted with respect to the neighboring and underlies 164,200 km , extending from 27 Sto31Sand ∘ ∘ blocks. In the north, the adjacent block is divided into smaller from 50 Wto57W. The volcanic sequence has an average units with a gradual terrain lowering from east to west, thicknessofabout550mandconsistsofbasaltictorhyolitic influenced by NE normal faults, parallel to the Leao and units of Mesozoic age; the last ones occur on the Serra Geral Perimpo Fault Systems. Formation (SGF) top sequence. The clayish soil thickness in The identification of hydrochemical groups in the SGAS the SGF ranges from few decimeters in the eastern area to is an important tool for the definition of tectonic struc- several meters in the west and northwest. This soil controls tures, supporting the fluoride ascending circulation hypoth- water infiltration and, as a consequence, the SGAS recharge esis in the aquifer [22]. The meteoric recharge pattern 2+ − by meteoric waters [10]. as Ca HCO3 waters and also ascending infiltrations by + − 2− − Studies have suggested groundwater mixing processes Na HCO3 waters with SO4 and Cl in the GAS, resulting between the SGAS and older confined sedimentary aquifer in a complex water mixture [15]. Several authors have pointed Journal of Geological Research 3 27∘ 57∘ 56∘ 55∘ 54∘ 53∘ 52∘ 51∘ 50∘ N 3 28∘ 2 1 4 29∘ 30∘ 31∘ (km) Hydrochemical groups: 0 50 100 CaMg NaF Meteoric F SO4F Na SO4Na Figure 2: Sample points, hydrochemical groups distribution on SGAS, and major tectonic fault systems: (1), Mata-Jaguari, (2) Terra de Areia-Posadas, (3) Perimpo, and (4) Leao. 80 80 60 I 60 40 40 20 20 Mg SO4 II 80 80 60 60 40 40 20 20 − Ca80604020 Na HCO3 20 40 60 80 Cl Water groups: CaMg NaF F SO4F Na SO4Na Figure 3: Hydrochemical groups distribution in the piper diagram, showing two fluoride hydrochemical types (I—sulfated and II— bicarbonated, [19]). outthathighfluoridecontentingroundwatercanberelated the 309 physicochemical analyses of the SGAS in order to + − to Na HCO3 ascending infiltrations waters [11, 15, 22–24]. systematize the interpretation of a large number of variables Such mixing and hydrochemical groups can both result from through the maximization of differences between them26 [ ]. 2+ − + − Ca HCO3 and Na HCO3 waters [25]. The PCA was performed to 2, 3, and 4 principal components and normalized to a value of “1,” minimum eigenvalue accepted [27]. Subsequently, the outliers were cut by visual 2. Materials and Methods − + analysis in boxplots and dispersion charts for F ,Na, 2+ 2+ − 2− − 2.1. Multivariate Statistics. Principal components analysis Ca ,Mg ,HCO3 ,SO4 ,andCl, considering water (PCA) was used to discriminate hydrochemical groups in classification. This statistical procedure restricts the analysis 4 Journal of Geological Research Condition 1 Condition 2 + + Ca HCO3 Soil Ca HCO3 Well Well Soil SGAS SGAS Flow interaction Flow interaction Flow + Na HCO3 − + ±SO4/F GAS GAS Na HCO3 Figure 4: Water flow interactions to different geological conditions. −20 Table 1: Rotated matrix with four components and characteristic parameters. Data from [21]. −22 Phreatic Components LMWL Parameters −24 1234 2+ −26 Mg .874 .063 .072 .139 Ca2+ .811 .231 −.045 −.043 H −28 + 2 Na −.143 .188 .870 .271 − −30 SGAS Cl .353 .707 .206 −.051 − HCO3 .572 .095 .699 −.091 −32 2− SO4 .030 .872 .074 .237 − −34 GAS F .067 .145 .150 .956 Explained variance 37.433 20.260 12.694 10.770 −36 Cumulative % of variance 37.433 57.693 70.386 81.156 −38 −5.5 −5 −4.5 The hydrogen was analyzed through the reaction of water 18 ∘ O with pure zinc at 500 C for liberation of the H2 gas. Local meteoric water line (LMWL) Twenty seven samples were selected in order to establish Porto Alegre/BR =10.1652 + 7.54699 stable isotope patterns in the SGAS. These results were Corrientes/AR =9.86491 + 7.79688 considered together with eight additional ones belonging to GAS Meteoric SO4F the GAS (seven data from [28] and one from this study) and Na Phreatic SO4Na four from phreatic waters from an adjacent area [28]. This SGAS NaF Average values region procedure is appropriate for understanding water mixture CaMg features between aquifers and to estimate their relative ages. F Unfiltered water samples were collected and stored in 18 2 Figure 5: O(‰)versus H (‰) in GAS, SGAS, and phreatic 100 mL polypropylene bottles to provide adequate volume for 18 2 aquifer.

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