Geochemical evolution of groundwater in a basaltic aquifer based on chemical and stable isotopic data: Case study from the Northeastern portion of Serra Geral Aquifer, São Paulo state (Brazil)

doi:10.1016/j.jhydrol.2016.02.016

Journal of Hydrology

Volume 535, April 2016, Pages 598-611

https://doi.org/10.1016/j.jhydrol.2016.02.016 Get rights and content

Highlights

•
Chemical and isotopic composition of basaltic aquifer groundwater were analyzed.
•
Hydrodynamic data allows to deduce main flow paths in basaltic aquifer.
•
Geochemical modeling is carried out to deduce water–rock interaction process.
•
Groundwater composition reflects the geochemical processes along path ways.

Summary

Groundwater from the fractured basalt Serra Geral Aquifer (SGA) represents an important source for water supply in Northeastern São Paulo state (Brazil). Groundwater flow conditions in fractured aquifers hosted in basaltic rocks are difficult to define because flow occurs through rock discontinuities. The evaluation of hydrodynamic information associated with hydrochemical data has identified geochemical processes related to groundwater evolution, observed in regional flowpaths. SGA groundwaters are characterized by low TDS with pH varying from neutral to alkaline. Two main hydrochemical facies are recognized: Ca–Mg–HCO₃, and Na–HCO₃ types. Primarily, the geochemical evolution of SGA groundwater occurs under CO₂ open conditions, and the continuous uptake of CO₂ is responsible for mineral dissolution, producing bicarbonate as the main anion, and calcium and magnesium in groundwater. Ion exchange between smectites (Na and Ca-beidelites) seems to be responsible for the occurrence of Na–HCO₃ groundwater. Toward the Rio Grande, in the northern portion of the study area, there is mixing between SGA groundwater and water from the sandstones of the Guarani Aquifer System, as evidenced by the chemical and isotopic composition of the groundwater. Inverse mass balance modeling performed using NETPATH XL produces results in agreement with the dissolution of minerals in basalt (feldspars and pyroxenes) associated with the uptake of atmospheric CO₂, as well as the dissolution of clay minerals present in the soil. Kaolinite precipitation occurs due to the incongruent dissolution of feldspars, while Si remains almost constant due to the precipitation of silica. The continuous uptake of CO₂ under open conditions leads to calcite precipitation, which in addition to ion exchange are responsible by Ca removal from groundwater and an increase in Na concentrations. Down the flow gradientCO₂ is subject to closed conditions where the basalts are covered by the sediments of Bauru Group or associated with deeper isolated discontinuities. A decrease in the amount of dissolution of labradorite and augite is observed, associated with precipitation of carbonates and kaolinite. Stable isotope ratios of SGA groundwater vary from −37.8‰ to −61.3‰ VSMOW for δ²H VSMOW, and −5.7‰ to −8.9‰ VSMOW for δ¹⁸O, indicating temporal variations in climatic conditions during recharge.

Introduction

Basaltic rock aquifers represent an important groundwater resource, hosting water supply in several parts of the world. Basalts are good aquifers because they store water of excellent quality, generally characterized by low salinity, and the thickness and spatial extension of basaltic lava flows provide high storage capacity. Basaltic lava flows typically have geologic discontinuities responsible for groundwater storage and flow in these units (Deolankar, 1980, Léonardi et al., 1996, Domenico and Schwartz, 1998, Bourlier et al., 2005, Dafny et al., 2003, Dafny et al., 2006, Lastoria et al., 2006; among others). Basaltic provinces around the world constitute excellent aquifer units including: Deccan Plateau in India (Deolankar, 1980, Kulkarni et al., 2000), Columbia River Plateau in USA (Deutsch et al., 1982), Golan Heights in Israel (Dafny et al., 2006), and the Atherton Tabelands in North Queensland, Australia (Locsey and Cox, 2003), among others.

Evaluation of groundwater hydrochemistry and isotopic composition is being increasingly used to complement studies focused on understanding the flow condition and origin of groundwater in fractured and heterogeneous aquifers units, such as basaltic aquifers. Hydrochemistry can be an aid to define chemical reactions produced by water–rock interaction and stable isotope data (δ¹⁸O and δ²H) can be used as tracer of groundwater origin, mixing of waters of different origins, as well as to interpret paleoclimate recharge conditions for groundwater (Aggarwal et al., 2005). Combining these analytical data allows the construction of geochemical models, which can be used to determine the evolution of groundwater along flow paths in aquifers, based on the interpreted reactions and processes related to water–rock interactions or anthropogenic sources (Plummer et al., 1990, Rosenthal et al., 1998, Bouhlassa and Aiachi, 2002, Bretzler et al., 2011; among others).

Located in the Southeastern portion of South America, the Paraná Sedimentary Basin is comprised of a vulcano-sedimentary sequence up to 8000 m thick, with a generally elliptical shape that has the major axis trending NE–SW. Among several sedimentary units, the Serra Geral Aquifer (SGA) is an important Cretaceous volcanic stratigraphic unit that can reach up to 1500 m thick in the center of the sedimentary basin, and is one of the most important aquifers located in the region (Fig. 1). The SGA represents an important water source for public supply, irrigation and industrial purposes, largely in the states of Paraná, Santa Catarina, Rio Grande do Sul and Mato Grosso do Sul, Brazil, as well as in Argentina, Paraguay and Uruguay. Hydrochemical studies of groundwater in the SGA have been conducted in the southern portion of Brazil, allowing groundwater chemical characterization and the establishment of the hydraulic relationship with the underlying unit, the Guarani Aquifer (Bittencourt, 1996, Bittencourt et al., 2003, Boff et al., 2006, Buchmann Filho et al., 2002, Lastoria, 2002, Lastoria et al., 2006, Machado et al., 2002, Nanni, 2008, among others). Aquifers in Mesozoic rocks of the Paraná Sedimentary Basin (Bauru, Serra Geral and Guarani Aquifers) provide public water supply in the West portion of São Paulo state. Due to the importance of these units in the region, the hydrodynamics and hydrochemistry of the Guarani and Bauru aquifers have been studied since the 1970s, and several conceptual models for groundwater flow and hydrochemical evolution have been formulated (eg. Gallo and Sinelli, 1980, da Silva, 1983, Kimmelmann e Silva et al., 1986, Rebouças, 1994, Campos, 1987, Campos, 1993, Meng and Maynard, 2001, Sracek and Hirata, 2002, Barison, 2003, Paula e Silva et al., 2005, Gastmans et al., 2010; among others). However, the hydrochemical and hydrogeological characteristics of the SGA have not been studied extensively.

This study has three main objectives. First, to recognize and characterize the water types that exist in the basaltic aquifer, based on their chemical composition. Second, to examine the geochemical evolution and stable isotope composition of SGA groundwater along selected flow paths, thereby defining a set of possible reactions based on changes in groundwater composition and related to the observed mineralogy of the aquifer. Based on these possible reactions, using NETPATH XL (Plummer et al., 1994, Parkhurst and Charlton, 2008), mass transfer along a number of flow paths is tested, as well as the possibility of mixing with groundwater from the underlying unit (Guarani Aquifer System). The third objective is to evaluate the isotopic data to determine the effect of variations in climatic conditions over SGA groundwater stable isotope ratios.

Section snippets

Geological and hydrogeological settings

Basalts of the Serra Geral Formation are present in the Paraná Magmatic Province (PMP), which constitutes one of the largest volcanic manifestations of basic rocks in a continental area. This magmatic province includes lava flows and intrusive basic rocks (sills and dykes) representing, according to Milani et al. (1994), an important contribution to generation of continental crust during the Mesozoic. The radiometric ages indicate that volcanic activity began between 133 and 132 Ma, starting in

Methods

Thirty-one groundwater samples were collected directly from water supply wells. The location of sampled wells, drilled in basalts of the SGA, is presented in Fig. 2. The pH, electrical conductivity, dissolved oxygen and temperature of each water sample was measured in the field. The sample was filtered (0.45 μm) and part was acidified using HNO₃ to pH < 2 for cation analysis and part preserved refrigerated (approximately 4 °C) for anion analysis. Results of field measured data and chemical analysis

Groundwater chemical composition

The amount of total dissolved solids of SGA groundwater varies from 0.76 to 6.75 mmol L⁻¹, and electric conductivity (EC) varies from 27.2 μS cm⁻¹ to 370 μS cm⁻¹ (arithmetic mean of 145 μS cm⁻¹), reflecting low content of solutes. Values of pH vary from 5.7 to 9.9 (arithmetic mean of 7.8), indicating the existence of acid and alkaline water in the aquifer. Table 3 summarizes the correlation between measured physico-chemical parameters. EC and pH have a strong correlation (r = 0.80) (Fig. 3), mainly

Conclusions

The evaluation of new hydrochemical and stable isotopic data for groundwaters of the Serra Geral Aquifer in the northern portion of São Paulo state (BR), using isotopic and geochemical methods, has contributed to the recognition of the main geochemical process and controls related to groundwater evolution within this basaltic reservoir. Groundwater flow in the Serra Geral aquifer is driven by the topography, and the main rivers crossing the area represent local areas of discharge. High pressure

Acknowledgment

This project was funded by a grant from the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) under the process 2012/00241-5.

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Geochemical evolution of groundwater in a basaltic aquifer based on chemical and stable isotopic data: Case study from the Northeastern portion of Serra Geral Aquifer, São Paulo state (Brazil)

Highlights

Summary

Introduction

Section snippets

Geological and hydrogeological settings

Methods

Groundwater chemical composition

Conclusions

Acknowledgment

J. Hydrol.

Appl. Radiat. Isotopes

C. R. Geosci.

J. Hydrol.

Chem. Geol.

J. Hydrol.

Chem. Geol.

J. Geodyn.

Geothermics

Appl. Geochem.

J. Hydrol.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Appl. Geochem.

Geochemistry, Groundwater and Pollution

Petrogenetic aspects of acid and basaltic lavas from the Paraná Plateau (Brazil): geological, mineralogical and petrochemical relationships

J. Petrol.

Sobre o controle do quimismo de águas termais da Bacia do Paraná

Boletim Paranaense de Geociências

A influência dos basaltos e de misturas com águas de aquíferos sotopostos nas águas subterrâneas do Sistema Aquífero Serra Geral na Bacia do Rio Piquiri, Paraná – BR

Revista Águas Subterrâneas

Mineral weathering in acid saprolites from Subtropical, Southern Brazil

Sci. Agricola (Piracicaba, Brazil)

The hydrogeolgy of the Golan Basalt aquifer, Israel

Israel J. Earth Sci.

The Deccan Basalts of Maharashtra, India – their potential as aquifers

Ground Water

Physical and Chemical Hydrogeology

The Geochemistry of Natural Waters: Surface and Groundwater Environments