Water-Rock Interactions in the Bauru Aquifer System - ScienceDirect

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ScienceDirect Procedia Earth and Planetary Science 17 (2017) 388 – 391

15th Water-Rock Interaction International Symposium, WRI-15

Water-Rock Interactions in the Bauru Aquifer System – São Paulo State, Brazil Marcia Regina Stradiotoa,1, Elias Hideo Teramotoa, Maria Rita Caetano-Changa, Hung Kiang Changa a

Laboratório de Estudos de Bacias do Instituto de Geociências e Ciências Exatas da Universidade Estadual Paulista, Rio Claro(SP), Brasil

Abstract The study focuses on the assessment of mineral saturation indexes using data of physicochemical analysis of groundwater samples from different units of the Bauru Aquifer System, in the State of São Paulo, Brazil. The analysis revealed that calcite (CaCO3) undersaturated waters dominate in the aquifer, but eventually groundwater reaches saturation with respect to this mineral. The majority of water samples are supersaturated with quartz, SiO2, and undersaturated with amorphous silica, SiO2(am). In the equilibrium diagrams, the samples are predominantly concentrated in the stability fields of the kaolinite and montmorillonite, which indicates that these mineral phases are the most probable sinks for Si and Al derived from weathered aluminosilicates. 2017The TheAuthors. Authors. Published by Elsevier © 2017 Published by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15. Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Bauru Aquifer System; water-rock interaction, PHREEQC

1. Introduction The Bauru Aquifer System (BAS) is one of the most extensively used aquifers in the São Paulo state, largely due to its shallow depth, reasonable well yields and lateral extends. This study focuses on the understanding of waterrock interactions in the aquifer units of the BAS, contributing to the hydrogeochemical knowledge of the system. The Bauru Group formation extends on an area approximately 117,000 km 2 within the Sao Paulo state, and the aquifer within this formation is a water source for >300 cities with population above 5,5 million inhabitants 1. The Bauru Group overlies the basalts of the Serra Geral Formation, above an erosive discordance 2-5, and locally above sandstones of the Botucatu and Piramboia formations6-8. The formation is 100 m thick on average but may locally * Corresponding author. Tel.: +5519-35269479. E-mail address: [email protected]

1878-5220 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2016.12.098

Marcia Regina Stradioto et al. / Procedia Earth and Planetary Science 17 (2017) 388 – 391

exceed 300 m.. The first formal stratigraphic subdivision of Bauru Group was performed in 1980 based on the surface geology data, and distinguished the Caiuá, Santo Anastácio, Adamantina and Marília formations. They form separate hydrostratigraphic units3. Recent studies based on subsurface data maintained such subdivision, and added Araçatuba, Pirapozinho and Birigui formations to the hydrostatigraphic subdivision of BAS 9,10. BAS is a hydrogeologically continuous, unconfined, semi-confined to confined unit of a regional extent; its large outcropping area allows direct recharge of about 1,300 mm of annual rainfall 11. The hydrostratigraphic units are recognizable by using geophysical well logs. These units largely coincide with the lithostratigraphy established for the Bauru Group; according to their hydraulic properties the hydrostratigraphic units are classified in Caiuá, Santo Anastácio, Birigui, Adamantina and Marília aquifers, and Pirapozinho and Araçatuba aquitards 9,10. Hydrogeochemical studies of the BAS in São Paulo state12 concluded that HCO3-, Ca+2 and Mg+2 are the main ions; these waters have low salinity ranging generally from 100 mg/L to 200mg/L. According to other studies the BAS waters are predominantly calcium bicarbonate and calcium-magnesium in the meridional portion of São Paulo state13; calcium bicarbonate waters occur all over BAS extension, with a slight predominance in the central portion of the state, while the sodium bicarbonate occurs in the eastern, northeast and central portions of southwestern region of São Paulo state14. 2. Material and methods Groundwater samples from 345 wells screened within the BAS in the São Paulo state were used in this study; in addition, physicochemical data of13 and14 were also included in the assessment, 517 samples in total: 56 from the Marília Aquifer, 273 from the Adamantina Aquifer, 84 from the Santo Anastacio Aquifer, 42 the from Caiuá Aquifer and 62 samples of mixed water from wells with filter sections into two or more units of the BAS. The selection of wells for sampling was based following a survey of geological profiles; the identification of the exploited aquifer unit was done based on the position of the filter section in each well. Field measurements including temperature, electrical conductivity and pH, and the geographical position of the wells were taking at the time of groundwater sampling. Also in the field, the water samples collected for analysis of cations and anions were filtered using Millipore System of 0.45 μm porosity filters. Samples for cation analysis were acidified with concentrated nitric acid (7.2 molar), two drops for each 50 mL sample; the bottles were kept under refrigeration for later physical and chemical analysis. The method of Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) was used for determining the elements Ca, Fe, Mg, Mn, Sr, Ba, Zn, Pb, Si, Al, Ni, Cr, P, Cu, Co and Cd. The alkalinity (HCO3, CO3 e OH) was obtained by titration (HCO3, CO3 e OH); the anions (F, Cl, Br, NO3, NO2, SO4, PO4, C2O4, CH3COO e ClO2) and the cations (Na, K, Li, NH4) were determined by ion chromatography (IC). Chemical analysis were performed according to the methods recommended by Standard Methods for the Examination of Water and Wastewater 15. Estimate of speciation ions and mineral saturation indexes were accomplished by PHREEQC16. The water chemistry data were used as input data to Phreeqc assuming pressure equal to 1 bar and the temperatures measured in the field. The saturation indexes were calculated for calcite, quartz and amorphous silica. 3. Results and discussion 3.1 Carbonate equilibrium Groundwater from the BAS is predominantly undersaturated with respect to calcite (Figure 1a). The origin of bicarbonate ion in the BAS waters is derived from carbon dioxide from rainfall, soil and also dissolution of carbonate minerals (calcite) and feldspars. The geochemical evolution of the BAS groundwater typically represents gradual increases in Ca and HCO3 ions until the saturation with calcite is reached. The evolution is associated with the increase of electrical conductivity value (Figure 1a). The subsaturation of water samples with respect to calcite would normally be characteristic of open system aquifers, in which slightly acidic rainwater of low electrical conductivity percolates through shallow rock formations, infiltrates the aquifer and react with detrital framework minerals, such as feldspars. These results are typical of wells that do not exceed 300 m in depth.

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Higher concentration of bicarbonate ions is associated with waters of higher electrical conductivity. Calcite supersaturated water samples are mainly found in groundwater samples from the Santo Anastacio Aquifer or mixed groundwaters which have longer filter sections within the Santo Anastácio unit. The groundwaters with calcite saturation index above 0.5 come mainly from wells located in the southwest region of the study area and deeper portions.

Fig. 1. (a) Calcite saturation index (log IAP/K) vs. electrical conductivity of BAS water samples. (b) Quartz saturation index (log IAP/K) vs. electrical conductivity of BAS water samples.

3.2 Quartz and amorphous silica equilibrium The BAS waters are normally supersaturated with respect to quartz (Figure 1b), indicating a system dominated by silicate weathering. The same samples are undersaturated with respect to amorphous silica. Some samples with higher salinities, however, are undersaturated for quartz: they are mainly from Santo Anastacio Aquifer at the southwest portion of São Paulo state, from deeper wells, and classified as sodium bicarbonate waters. It was found that he supersaturation occurs in waters close to the ground surface, however, not necessarily silica precipitation takes place due to kinetic and hydrologic constraints (continuous flow of groundwater). In other words the supersaturation conditions of the samples do not lead to silica minerals precipitation16. 3.3 Clay minerals Quartz and feldspar dominate the detrital framework of the BAS, whereas smectite and kaolinite to lesser extent are the clay minerals that prevail in the matrix and/or cement of the reservoir rocks. According to equilibrium diagrams NaO-Al2O3-SiO2-H2O and CaO-Al2O3-SiO2-H2O the BAS waters pertain to the kaolinite stability field; the total or partial hydrolysis liberates calcium, sodium, potassium and silicic acid, suggesting that groundwater is stable with respect to kaolinite. Many samples, mainly from Santo Anastacio and Adamantina aquifers and mixed waters which have higher calcium concentrations are associated with the montmorillonite stability field; as the calcium content increases, the waters are likely to develop into a balanced position with calcic montmorillonites. Some water samples mainly from the Santo Anastácio Aquifer and mixed water from the southwest area of São Paulo state remain within the gibbsite stability field; these samples have high pH, most over 8, and low silica concentration (undersaturated in relation to quartz), which implies low content of H 4SiO4. In the stability diagram K2O-Al2O3-SiO2-H2O the BAS waters remain mainly in the stability field of kaolinite, which may indicate hydrolysis process of the potassium feldspar total alteration. Some samples stay within the gibbsite stability field and others in the muscovite stability field; these are more alkaline water samples with pH higher than 9. 4. Conclusions The groundwaters with calcite undersaturation prevail in the BAS, due to the conditions of open system with


large quantity of rainwater percolating. Samples that show calcite supersaturation are the most salinized ones. Regarding the silica minerals, a majority of samples is supersaturated for quartz and undersaturated for amorphous silica. Strong undersaturation for silica minerals prevails in water samples from the Santo Anastácio Aquifer in the southwest portion of São Paulo. The water samples of BAS are mainly concentrated in the stability field of kaolinite; some samples with higher calcium content stay in the stability field of montmorillonite suggesting that these minerals likely sinks for Si and Al derived from weathering of aluminosilicates. References 1 Instituto Brasileiro de Geografia e Estatística – IBGE. Available at: http://www.ibge.gov.br/home/mapa_site/mapa_site.php#populacao. Accessed October 21, 2015 2 Brandt Neto M, Yamamoto JK, Tachibana JE, Mato, LF. Quaternary sediments associated with the lower valley of the Tiete River. In: Simpósio Regional de Geologia, 1, 1977, São Paulo. Atas... São Paulo: Sociedade Brasileira de Geologia, 1977. p.248-266. 3 Soares PC, Landim PMB, Fúlfaro VJ, Sobreiro Neto AF. Stratigraphic characterization test of the Cretaceous in São Paulo State: Bauru Group. Revista Brasileira de Geociências, São Paulo, v.10 (n.3), p.177-185, 1980. 4 Almeida MA, Fernandes LA, Dantas ASL, Sakate MT, Gimenez AF, Teixeira AL, Bistrichi CAE, Almeida FFM. Considerations on the stratigraphy of Bauru Group in the Pontal do Paranapanema region in São Paulo State. In: Simpósio Regional de Geologia, 3., 1981, Curitiba. Atas... Curitiba Sociedade Brasileira de Geologia, 1981. v.1, p.77-89. 5 Ricomini C, Gimenez Filho A, Stein DP, Almeida FFM, Pires Neto AG, Dehira LK, Melo MS de, Braga T de O, Ponçano WL. Features of the basal portion of Caiuá Formation in Paraná Northwest. In: Simpósio Regional de Geologia, 3.. 1981, Curitiba. Atas... Curitiba: Sociedade Brasileira de Geologia, 1981. v.2, p.34-48. 6 Suguio K, Fulfaro VJ, Amaral GE, Guidorzi LA. stratigraphic and structural behavior of Bauru Formation in administrative regions 7 (Bauru), 8 (São José do Rio Preto) e 9 (Araçatuba) in São Paulo State. In: Simpósio Regional de Geologia, 1, 1977, São Paulo. Atas... p. 231-247. 7 Paula e Silva F, Cavaguti N. New stratigraphic and tectonics characterization of Mesozoic in Bauru city - SP. In: Simpósio sobre Bacias Cretácicas Brasileiras, 2., 1992, Rio Claro, Atas... Rio Claro: p.141-144. 8 Paula e Silva F, Cavaguti N. New stratigraphic and tectonics characterization of Mesozoic in Bauru city – SP. Geociências, UNESP, p.83-99, 1994. 9 Paula e Silva F. Subsurface geology and hydrostratigraphic of Bauru Group in São Paulo State. 2003. 166f. Tese (Doutorado em Geociências) – Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, Rio Claro, 2003. 10 Paula e Silva F, Chang HK, Caetano-Chang MR. Stratigraphy of the subsurface of the Bauru Group (K) in São Paulo State. Revista Brasileira de Geociências, v. 35, n. 1, p.77-88, 2005. 11 Departamento de Águas e Energia Elétrica – DAEE. Information system for management of water resources in São Paulo State, São Paulo SP, 2000. 12 Campos, HCS. Contribution to the hydrochemical study of the Bauru Group in São Paulo State. 1987. 158f. Dissertação (Mestrado em Geologia Geral e de Aplicação) – Instituto de Geociências, Universidade de São Paulo, São Paulo, 1987. 13 Barison, MR. hydrogeochemical study of the meridional portion of the Bauru Aquifer System in São Paulo State. 2003, 153f. Tese (Doutorado em Geociências e Meio Ambiente) - Instituto de Geociência e Ciências Exatas, Universidade Estadual Paulista, Rio Claro - SP, 2003. 14 Stradioto MR. Hydrochemical and diagenetic aspects of Bauru Aquifer System in the southwestern of the São Paulo State. 2007. 103f. Dissertação (Mestrado em Geociências e Meio Ambiente) – Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista. Rio Claro, 2007. 15 Methods for the Examination of Water and Wastewater (2012) Standard Methods for the Examination of Water and Wastewater is a joint publication of the American Public Health Association (APHA), the American Water Works Association (AWWA), and the Water Environment Federation (WEF). 1368 pages. 2012 16 Stradioto MR.; Chang HK, Caetano-Chang MR. Petrography and diagenetic aspects of the Bauru Group sandstones in the southwestern region of the São Paulo State. Revista Escola de Minas-REM, Ouro Preto, v.61 (n.4), p.433-441, 2008.

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