Journal of Environmental Radioactivity 132 (2014) 21e30
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222
Rn, Brazil
220
Rn and other dissolved gases in mineral waters of southeast
Daniel Marcos Bonotto* Departamento de Petrologia e Metalogenia, IGCE e Instituto de Geociências e Ciências Exatas, UNESP e Universidade Estadual Paulista Júlio de Mesquita Filho, Av. 24-A, No. 1515 e CP 178, Bela Vista CP 178, CEP 13506-900 Rio Claro, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2013 Received in revised form 8 January 2014 Accepted 10 January 2014 Available online 14 February 2014
This paper describes the natural radioactivity due to 222Rn and 220Rn in mineral waters occurring at São Paulo and Minas Gerais states, Brazil, that are extensively used for drinking in public places, bottling and bathing purposes, among other. The measurements of these alpha-emitting radionuclides were also accompanied by the monitoring of temperature and some dissolved gases (O2, CO2 and H2S) in 75 water sources located in 14 municipalities of those states. Eight water sources yielded 220Rn activity concentration values below the detection limit of 4 mBq/L. On other hand, 222Rn activity concentration values exceeding the WHO guidance level of 100 Bq/L in drinking-water for public water supplies were found in two springs, named Villela and Dona Beja, whose discharge occurs in areas characterized by the presence of enhanced levels of natural radioelements in rocks. The obtained results were compared with the guidelines of the Brazilian Code of Mineral Waters (BCMW) that was established in 1945 and is still in force in the country. The 222Rn and 220Rn activity concentration data allowed perform dose radiation calculations based on the potential alpha energy concentration (PAEC), whose implications for health risk have been also considered in this paper. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Radon and thoron Mineral waters Dissolved gases Radiation dose
1. Introduction In many countries, spring waters have been extensively used for consumption purposes as an option to tap water, as many people believe they are healthy and/or can be utilized for health cures. Additionally, economic reasons have also favored their use as bottled waters so that the commercialization of mineral waters has widely increased. The thermal and mineral waters use in Brazil is not recent due to arrival of European immigrants, mainly from Portugal. For instance, Gonsalves (1936) reported that the thermal waters occurrence in Caldas Velhas (Goiás State) was realized in 1722. The thermal spas were gradationally constructed in Brazil for therapeutic and leisure purposes, corresponding the period elapsed between the 1930s and 1950s to the most auspicious hydrothermal period in the country (Mourão, 1992). The Brazilian Code of Mineral Waters (BCMW) was established in this time, under French influence, by Register 7841 published on 8 August 1945 (DFPM, 1966). This rule is still in force without any actualization, focusing the mineral waters for spas and bottling uses, as well the potable waters for bottling (Serra, 2009). * Tel.: þ55 19 35269244; fax: þ55 19 35249644. E-mail addresses: [email protected], danielmarcosbonotto@gmail. com. 0265-931X/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2014.01.005
In Brazil, the production and commercialization of mineral waters is managed by the National Department of Mineral Production (DNPM). Traditionally, the mineral waters had been used or directly consumed in the springs, where touristic centers developed around. However, in the present days, the mineral water for consumption is distributed in vessels for ingestion distant from the springs, whereas hydrothermal spas for therapeutic baths and leisure exhibit infrastructure with hotels and many facilities for users. The first available information about the production of bottled mineral waters in Brazil dates back 1911 (CPRM, 2012). Only Minas Gerais and Rio de Janeiro States had industries by that time, whose annual production was 1,420,000 L (Minas Gerais- 1,220,000 L; Rio de Janeiro- 200,000 L) (CPRM, 2012). However, an accentuated expansion of the national production occurred in the period between 1996 and 2007, i.e. North region (þ386%), West Central region (þ287%), South region (þ207%), Northeastern region (þ130%) and Southeastern region (þ127%) (CPRM, 2012). The Southeastern region in 2007 was responsible by about 48% of the national production of mineral and potable water, the highest in the country (2.08 billion liters) (CPRM, 2012). São Paulo State presented a production superior to 1.5 billion liters in 2007, corresponding to 34% of the total in the country, followed by the states of Rio de Janeiro (7%) and Minas Gerais (6%) (CPRM, 2012). In general terms, circa 20 million consumers are involved (SEBRAE, 2012).
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D.M. Bonotto / Journal of Environmental Radioactivity 132 (2014) 21e30
Many spring waters in Brazil do not contain high concentrations of dissolved constituents, but the waters are considered mineral due to the radioactivity in them, chiefly due to the presence of dissolved radon (222Rn) and thoron (220Rn) (DFPM, 1966). 222Rn (half-life 3.82 days) and 220Rn (half-life 55.6 s) are noble gas isotopes belonging to the 238U (4n þ 2) and 232Th (4n) decay series, respectively. 222Rn is produced by a-decaying 226Ra and reaches the stable 206Pb, whereas 220Rn is generated by a-emitting 224Ra, forming the stable 208Pb (Clayton, 1983; Chu et al., 1999). Some 222 Rn and 220Rn fraction escapes from the rocks and minerals to the surrounding fluid phase, such as groundwater and air, whose proportion depends on factors such as total surface area of solids and the concentration (and distribution) of 238U (226Ra) and 232Th (224Ra) in them (Flügge and Zimens, 1939). Potential health hazards from natural radionuclides in consuming water have been considered worldwide, with many countries adopting the guideline activity concentration for drinking water quality recommended by WHO (2011). In general, the recommendations apply to routine operational conditions of watersupply systems, however, special attention must be also given to mineral waters frequently ingested by population. Thus, the approach for radioactivity in the BCMW (DFPM, 1966) is contradictory to that focused by WHO (2011). Another relevant aspect in this situation is the lack of confident data on the 222Rn and 220Rn activity concentration in Brazilian spring waters, mainly for 220Rn. The available 222Rn data in some cases is partially a consequence of the timescale of its half-life that is longer than that of the 220Rn. This paper describes a comparative study of well-known Brazilian mineral waters occurring at São Paulo and Minas Gerais states that was held for evaluating the presence of the following dissolved gases: 222Rn, 220Rn, O2, CO2 and H2S. With exception of oxygen, all other gases are taken into account in the BCMW (DFPM, 1966) that has been utilized on the data evaluation. The radiation dose due to the presence of dissolved 222 Rn and 220Rn has been calculated as both radionuclides are health threat when ingested in water in activity concentrations exceeding the guideline reference values. 2. Study area The groundwater samples (75) for dissolved gases analyses were taken from springs and pumped tubular wells drilled at different aquifer systems that are inserted in Paraná and Southeastern Shield hydrogeological provinces (Fig. 1 and Table 1). Águas de São Pedro, Águas de Santa Bárbara and Termas de Ibirá cities are inserted in the intercratonic Paraná sedimentary basin, in which the sedimentary sequence covers since the SilurianDevonian up to the Cretaceous periods (IPT, 1981). The sampling point at Águas de Santa Bárbara city corresponded to a 120 m-deep tubular well that cut the Serra Geral and Botucatu formations of the Paraná sedimentary basin. The waters of Águas de São Pedro city provided from tubular wells drilled at the Tubarão Group (Paraná basin) in 1936 by DNPM (National Department of Mineral Production) for petroleum exploration (Kimmelmann et al., 1987). However, such water sources have been popularly referred to Gioconda (Tubarão Group depth ¼ 275e625 m), Juventude (Tubarão Group depth ¼ 240e469 m) and Almeida Salles (Tubarão Group depth ¼ 207e329 m) “springs” (Kimmelmann et al., 1987). Águas de Lindóia, Serra Negra and Lindóia cities are located in a region characterized by the occurrence of several phases and cycles, involving different aspects of metamorphism, deformation and magmatism that acted from the Archean to the Upper Proterozoic times and affected rocks characterized by high metamorphic grade, generally of granulite and amphibolite facies (Ebert, 1955; Almeida and Hasui, 1984). The water circulation at Águas de Lindóia city
Fig. 1. Sketch map of the research region in Brazil and location of the groundwater sampling points in the following cities of São Paulo and Minas Gerais states: ASP ¼ Águas de São Pedro, ADL ¼ Águas de Lindóia, SEN ¼ Serra Negra, LIN ¼ Lindóia, TEI ¼ Termas de Ibirá, ASB ¼ Águas de Santa Bárbara, ADP ¼ Águas da Prata, PDC ¼ Poços de Caldas, PRV ¼ Pocinhos do Rio Verde, LAM ¼ Lambari, SLO ¼ São Lourenço, CAM ¼ Cambuquira, CAX ¼ Caxambu, AXA ¼ Araxá.
generally realizes through fractures and the flow is from the higher to lower altitudes (del Rey, 1989), leaching migmatite (Lindália and Santa Isabel), quartzite (Comexim, Curie, Filomena and Beleza) and milonite/quartzite (São Roque). Águas da Prata, Poços de Caldas and Pocinhos do Rio Verde cities are geologically situated in the Poços de Caldas alkaline complex that is a ring structure of Mesozoic age, comprising a suite of alkaline volcanic and plutonic rocks, mainly phonolites and nepheline syenites (Schorscher and Shea, 1992). Seven springs were sampled at Águas da Prata city: Villela (discharges into a well silicified and lightly folded sandstone), Vitória (discharges through fissures in diabase), Platina (discharges in outcropping phonolites), Prata (discharges in diabase), Boi (discharges in a silicified and recrystallized sandstone), Paiol and Padre (discharges through volcanic tuffs, phonolites and eudialite-bearing nepheline syenites) (Szikszay, 1981). Groundwater at Poços de Caldas area has been mainly exploited in crystalline fractured rocks due to the better yielding of the wells, but it also occurs in diffuse and punctual thermal and non-thermal springs that discharge at the depression in Poços de Caldas city (Cruz, 1987; Cruz and Peixoto, 1989). The geological substrate of Lambari, São Lourenço, Cambuquira and Caxambu cities in south of Minas Gerais State comprises rocks of the Proterozoic times like ophthalmic biotite gneisses, migmatized granitoids, protomilonites and milonite gneisses, migmatized garnet-biotite-gneisses, metabasites intercalations secondarily cut by pegmatoids veins and schists. There is also the presence of very weathered quartzites and Quaternary alluvial deposits (CPRM, 1999). The hydrogeological model for mineral waters occurring in São Lourenço, Lambari and Caxambu cities involves the rainwater infiltration in weathered horizons of gneissic rocks at the more elevated topographic areas close to the springs (CPRM, 1999). Then, this is succeeded by percolation through milonitized zones (São Lourenço, Caxambu, and Lambari) and fractures partially filled by pegmatoids
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Table 1 Description of the groundwater samples analyzed in this paper. City (Statea)
Spring (well) name/Sample code
Hydrogeological Provinceb
Dominant flow
Major rock types
Geological context/age
Águas de São Pedro (SP)
Almeida Salles/ALS Gioconda/GIO Juventude/JUV
Paraná
Porous
Sandstones
Botucatu Fm. (Jurassic) Pirambóia Fm. (Triassic) Itararé Fm. (Permian) Irati Fm. (Permian)
Águas da Prata (SP)
Platina/PLA Paiol/POL
Paraná
Fractures
Diabases Phonolites
Botucatu Fm. (Jurassic) Serra Geral Fm. (Jurassic-Cretaceous) Poços de Caldas intrusive complex (Cretaceous)
Águas de Lindóia (SP)
Serra Negra (SP)
Lindóia (SP) Termas de Ibirá (SP)
Vitória/VIT
Alkaline rocks
Boi/BOI Prata/PTA Villela/VIL Padre/PDE
Silicified sandstones
Santa Isabel/SIL
Yes
Yes
Paraná
Fractures
Filomena/FIL Beleza/BEL São Roque/SRE Comexim/COM Lindália/LIN Curie/CUR São Jorge/SJO São Carlos/SCA Italianos/ITA Santa Luzia/SLU Santo Agostinho/SAT Brunhara/BRU Laudo Natel/LAN Sant’Anna/SAA São Benedito/SBE Bioleve/BIO Jorrante/JOR Ademar de Barros/ADB
Commercializationc
Granites
Amparo Gp. (Lower Proterozoic)
Gneisses Migmatites Schists Quartzites Limestones Dolomites
Yes
Yes Yes
Yes Yes Paraná
Porous and Fractures
Sandstones Basalts
Bauru Gp. (Cretaceous) Serra Geral Fm. (Jurassic-Cretaceous)
Bauru Gp. (Cretaceous) Serra Geral Fm. (Jurassic-Cretaceous) Paraíba do Sul Gp. (Proterozoic) Barbacena Gp. (Proterozoic) São João dél Rei Gp. (Proterozoic) Andrelândia Gp. (Proterozoic) magmatic plutonic series (Brasiliano)
Carlos Gomes/CGO Saracura/SRC Seixas/SEI Águas de Santa Bárbara (SP)
Balneário Municipal/BMU
Paraná
Porous and Fractures
Sandstones Basalts
Lambari (MG)
No. 1/LA1
Southeastern shield
Porous and fractures
Ortogneisses
No. 2/LA2
Granulites
No. 3/LA3
Migmatites
No. 4/LA4
Metassedimentary seq. Metavulcanossedimentary seq.
No. 5/LA5
São Lourenço (MG)
Cambuquira (MG)
Caxambu (MG)
No. 6/LA6 No. 7-Bis/SL7 No. 5-Alcalina/SL5 No. 6-Sulfurosa/SL6 No. 3-Vichy/SL3 No. 4-Ferruginosa/SL4 No. 1-Oriente/SL1 No. 10-Primavera/SL10 No. 9-Carbogasosa/SL9 Roxo Rodrigues/ROR Regina Werneck/REW Com. Augusto Ferreira/CAF Fernandes Pinheiro/FEP Marimbeiro/MAR Souza Lima/SLI Geiser Floriano de Lemos/GFL Venâncio/VEN Mayrink/MAY (continued on next page)
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D.M. Bonotto / Journal of Environmental Radioactivity 132 (2014) 21e30
Table 1 (continued ) City (Statea)
Spring (well) name/Sample code
Hydrogeological Provinceb
Dominant flow
Major rock types
Geological context/age
Southeastern shield
Fractures
Alkaline rocks
Poços de Caldas intrusive complex (Cretaceous)
Commercializationc
Ernestina Guedes/EGU Viotti/VIO D. Pedro II/DPE Beleza/BZA Duque de Saxe/DXE Da. Leopoldina/LEO Da. Isabel/Conde dÉu/ISA Poços de Caldas (MG)
Pocinhos do Rio Verde (MG)
Araxá (MG) a b c
Quisisana/QUI XV de Novembro/NOV Macacos/MAC Sinhazinha/SIN Frayha/FRA Pedro Botelho/PEB Rio Verde/RIV Samaritana/SMA São José/SJO Amorosa/AMO Dona Beja/DBJ Andrade Júnior/AJU
Nepheline syenites Phonolites Pyroclastics Volcanic tuffs
Southeastern shield
Porous and fractures
Quartzites and schists Alkaline-carbonatitic rocks
Cretaceous PreCambrian
SP ¼ São Paulo, MG ¼ Minas Gerais. According to Mente (2008). Water commercialized by private company in addition to consumption in taps accessible to population in public areas.
dykes or alkaline breccias (Caxambu) (CPRM, 1999). The local conditions favor the periodical eruption (up to 5-m height) of a nongeothermal geyser at the Caxambu waterpark due to the build-up of pressure from dissolved carbon dioxide in the water. Araxá city is geologically located at Alto Paranaíba Igneous Province that includes the renowned carbonatite intrusion of Araxá, which covers approximately 16 km2 and is in general related to a NW-trending linear structure bordering the São Francisco cratonic area that is thought to be in evidence since late Precambrian times (Traversa et al., 2001). Two springs were sampled at Araxá city: 1) Dona Beja, associated to an aquifer system classified as granular, free and semi-confined, mainly in the intrusive body domain (Beato et al., 2000); 2) Andrade Júnior, related to a deep fractured aquifer, unconfined to semi-confined, mainly occurring in rocks surrounding the carbonatite complex (Beato et al., 2000). 3. Sampling and analytical methods The sampling points (75) were chosen according to the hydrochemical information from previous studies and to the easy access for collection. The groundwater chemical composition is attained due to processes occurring at the liquidesolid interface when different rock matrices are extensively leached. The sampling sites were (Fig. 1 and Table 1): Águas de São Pedro (3 water sources), Águas da Prata (7 water sources), Águas de Lindóia (7 water sources), Serra Negra (8 water sources), Lindóia (2 water sources), Termas de Ibirá (5 water sources), Águas de Santa Bárbara (1 water source), Lambari (6 water sources), São Lourenço (8 water sources), Cambuquira (6 water sources), Caxambu (10 water sources), Poços de Caldas (6 water sources), Pocinhos do Rio Verde (4 water sources) and Araxá (2 water sources). All water sources (except geyser “Floriano de Lemos”, Caxambu city) have been used for drinking purposes in thermal and non-thermal spas and some of them are commercialized by private companies (Table 1). The sampling campaign occurred between March and June 2012 (autumn), when the local air temperature was often lower than that of the summer months (average in January ¼ 30e32 C). The rainfall was lower than that of the wet season (summer, DecembereJanuary) but higher than that of the dry season (winter, JulyeAugust).
About 500 mL of groundwater sample was directly taken from taps/pipes installed in each spring/well, which was stored in glass/ plastic flasks of variable volume as required by each gas (222Rn, 220 Rn, O2, CO2 and H2S) analysis. Temperature was also measured by the use of a digital portable meter in addition to the dissolved gases readings. All data acquisition was in situ performed for avoiding gases losses and modification of other physical and chemical parameters of the waters. The dissolved oxygen (DO) concentration was determined in a digital portable meter recording the potential values generated by an O2 sensible electrode consisting on a metallic wire covered by a thin gold layer (detection limit ¼ 0.1 mg/L). The electrode was inserted into a 60-mL Winkler bottle filled with the water sample, then, the final value was recorded after stabilization of the reading. All DO data were corrected due to differences in altitude of the sampling points, using as reference the value corresponding to the sea level. The carbon dioxide was evaluated by the buret titration method on using water volumes of 25 or 50 mL (Hach, 1992). This amount was inserted in a clean erlenmeyer flask, then, one powder pillow of a phenolphthalein indicator solution (5 g/L) (Hach, 1992) was added and the solution mixed gently. A 25-mL buret was filled to the zero mark with 0.0227 N NaOH solution and the prepared sample was titrated while gently swirling the flask until a light pink color formed and persisted for 30 s. The reaction of NaOH with CO2 (as carbonic acid) occurs in two steps, first a reaction from carbonic acid to bicarbonate and then to carbonate. Because the conversion of CO2 to bicarbonate is complete at pH 8.3, phenolphthalein was used as a color indicator for the titration (Hach, 1992). The dissolved CO2 (in mg/L) was calculated on multiplying the volume (in mL) of the titrant used by the factor 20 (sample volume ¼ 50 mL; detection limit ¼ 2 mg/L) or 40 (sample volume ¼ 25 mL; detection limit ¼ 4 mg/L) (Hach, 1992). The pink color in some samples was immediately formed with the addition of the phenolphthalein indicator. The dissolved CO2 corresponded to 0 mg/L in these cases as it was not necessary the addition of the titrant (0.0227 N NaOH solution) for the color change. The dissolved sulfide gas was determined by colorimetry (wavelength 665 nm) using a program stored in the Hach DR/2000 spectrophotometer (Hach, 1992). The methylene blue method was adopted in which the hydrogen sulfide and acid-soluble metal
D.M. Bonotto / Journal of Environmental Radioactivity 132 (2014) 21e30
sulfides react with N,N-dimethyl-p-phenylenediamine oxalate to form methylene blue (Hach, 1992). One glass cell (the prepared sample) was filled with 25 mL of sample, whereas another (the blank) was filled with 25 mL of deionized water. Then, 1 mL of Sulfide 1 Reagent (N,N-dimethyl-p-phenylenediamine oxalate) was added to each cell and, after mixing, 1 mL of Sulfide 2 Reagent (potassium dichromate) was put into both cells. After mixing and a 5-minute reaction time, the blank was inserted into the cell holder, the spectrophotometer was set to zero reading, the prepared sample was placed into the cell holder and the equipment displayed the result in mg/L sulfide (S2) (Hach, 1992). The intensity of the blue color was proportional to the sulfide concentration and proper dilution was used to determine high sulfide levels is some samples. The detection limit of the method corresponded to 1 mg/L sulfide. The radon and thoron dissolved in water were analyzed on site using RAD7 alpha particles detector coupled to accessory RADH2O from Durridge Co. The RAD7 utilizes a solid state alpha detector, comprising a Si semiconductor material that converts the energy of the alpha particles into an electrical signal. The accumulation of many signals results in a spectrum that is presented on a scale of alpha energy in the range of 0e10 MeV. Such energy interval is suitable for 222Rn and 220Rn readings as data acquisition takes place between 6 and 9 MeV (Durridge, 2009a). The spectrum is displayed in a set of 200 channels grouped into eight windows of different energy range. A, B, C and D are the main windows, whereas E, F, G and H are the diagnostic of main windows. Windows A and C provide 222Rn activity concentration data from 218Po and 214Po decays, representing “new” (218Po) and “old” (214Po) 222Rn, respectively (Durridge, 2009a). Windows B and D provide 220Rn activity concentration data from 216Po and 212Po decays, representing “new” (216Po) and “old” (212Po) 220Rn, respectively (Durridge, 2009a). Thus, the RAD7 separates the radon and thoron signals by energy of the alpha particles from their progeny, becoming possible to measure both Rn isotopes simultaneously (Durridge, 2009a). The equipment was factory calibrated by way of inter-comparison with radon chambers run by the U.S. EPA and the U.S. Department of Energy (Durridge, 2009a). The groundwater samples were collected in 100-mL flasks, avoiding as much as possible the exposure to atmospheric air. The RADH2O accessory unit employed an aeration system in a closed circuit connecting the sample, RAD7 and a desiccant tube containing Drierite to humidity absorption (Durridge, 2009b). Then, the air circulated through the sample during about 10 min, extracting radon and thoron until reaching an equilibrium state (for readings in windows A and B). The radon and thoron extracted were pumped into the RAD7, where their progeny were detected. The instrument display recorded the dissolved 222Rn and 220Rn activity concentration data in pCi/L that were converted to the SI unit (Bq/L for 222Rn and mBq/L for 220Rn). The Detection Limit (DL) corresponded to 4 mBq/L (Durridge, 2009a), whereas the uncertainties given of the results were one standard deviations, resulting from propagation of all statistical uncertainties in the entire measurement process. The low activity concentration in some samples implied on high analytical uncertainty as reported elsewhere (for instance, Jia et al., 2002). 4. Results and discussion 4.1. Temperature and dissolved oxygen, carbon dioxide and sulfide in the waters The groundwater temperature ranged from 20.4 to 35.7 C (mean ¼ 24.8 C) (Table 2). According to the BCMW guidelines for temperature (DFPM, 1966), three categories can be defined for the
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analyzed waters: cold (values lower than 25 C ¼ 41 samples), hypothermal (values ranging from 25 to 33 C ¼ 33 samples) and mesothermal (values ranging from 33 to 36 C ¼ 1 sample). The mean dissolved oxygen (DO) concentration was 3.5 mg/L (Table 2). The lowest DO content was 0.8 mg/L (Macacos spring, Poços de Caldas city, Minas Gerais State), whereas the highest DO concentration was 9 mg/L (São Benedito water source, Lindóia city, São Paulo State) that corresponds to the solubility of oxygen at 100% air saturation at 21 C and 760 mmHg mount air barometric pressure (EIFAC, 1986). The gas content (in solution) of a mass of water can be evaluated considering the Henry’s law and Dalton’s law of partial pressure. From both gas laws, it has been widely recognized that the solubility of oxygen in water varies with temperature, the DO content reducing sharply with increase in temperature (Cole, 1983; APHA, 1985; EIFAC, 1986). Treusdale et al. (1955) reported the solubility value for oxygen at various temperature, which was re-estimated using improved Henry coefficients (Benson and Krause, 1980) as discussed by Cole (1983). The values of solubility of oxygen in water at various temperatures, from moist air at 760 mmHg at 0.0 ppt salinity, taken from EIFAC (1986) is 9.1 mg/L at 21 C and 6.9 mg/L at 35 C. There is a significant inverse relationship between temperature and dissolved O2 in the analyzed groundwater (Fig. 2a), confirming that the increase in temperature causes a decrease in dissolved O2. The two-tailed P value estimated by GraphPad software from the Pearson correlation coefficient (r ¼ 0.41, n ¼ 75; Fig. 2a) equals 0.0003 by conventional criteria, suggesting this difference is extremely statistically significant. The mean dissolved CO2 concentration was 590 mg/L (Table 2). The dissolved CO2 was absent in nine samples, from which five were collected at Termas de Ibirá city in São Paulo State. Degassing favored by cracks, faults and fissures in basalts of the Serra Geral Formation may be a possible reason for the null CO2 values at Termas de Ibirá city. The dissolved CO2 ranged from 52 to 1840 mg/ L in the remaining samples (Table 2) and its presence in waters is mainly related to the carbonates dissolution in the rock matrices. The groundwater temperature range is from 25 to 35.7 C in the hypothermal and mesothermal waters (Table 2) and such large difference (10.7 C) favors the significant inverse relationship between temperature and dissolved carbon dioxide (Fig. 2b), indicating that its solubility decreases accompanying the temperature raising as also verified for oxygen. The two-tailed P value estimated by GraphPad software from the Pearson correlation coefficient (r ¼ 0.49, n ¼ 34; Fig. 2b) equals 0.0033 by conventional criteria, suggesting this difference is very statistically significant. According to the BCMW guidelines, the carbogaseous waters contain dissolved carbon dioxide gas corresponding to a minimum of 200 mL per liter of water (DFPM, 1966). The CO2 concentration data in Table 2 can be used for estimating the CO2 volume (in mL) per liter of water from the ideal gas law that is the state equation of a hypothetical (ideal) gas. Although it has limitations, it is a rough approximation to the behavior of some gases like CO2, under some conditions. It is often written as PV ¼ nRT where P is the gas pressure, V is the gas volume, n is the gas number of moles, T is the gas temperature and R is the ideal, or universal, gas constant. If T is expressed in Kelvin, then, R ¼ 0.0821 L.atm/K.mol. Remembering that 1 mol CO2 corresponds to approximatelly 44 g, then, it is possible estimate V values (in mL) varying from 0 to 1025 mL (Table 2). The comparison of the V values (in mL) for CO2 in Table 2 with the BCMW guideline of 200 mL per liter of water (DFPM, 1966) indicates that 33 water sources are carbogaseous. However, some of them are popularly recognized by other chemical attributes rather than dissolved CO2, for instance: No. 2 (Lambari city) and No. 5 (São Lourenço city) are said alkaline waters; No. 3 (Lambari city) and Com. Augusto Ferreira (Cambuquira city) are said magnesian
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D.M. Bonotto / Journal of Environmental Radioactivity 132 (2014) 21e30
Table 2 Temperature, dissolved gases, radon and thoron data in groundwater samples analyzed in this paper. Sample codea
Temp. ( C)
O2 (mg/L)
CO2 (mg/L)
CO2 (mL)b
H2S (mg/L)
222
220
ALS GIO JUV PLA POL VIT BOI PTA VIL PDE SIL FIL BEL SRE COM LIN CUR SJO SCA ITA SLU SAT BRU LAN SAA SBE BIO JOR ADB CGO SRC SEI BMU LA1 LA2 LA3 LA4 LA5 LA6 SL7 SL5 SL6 SL3 SL4 SL1 SL10 SL9 ROR REW CAF FEP MAR SLI GFL VEN MAY EGU VIO DPE BZA DXE LEO ISA QUI NOV MAC SIN FRA PEB RIV SMA SJO AMO DBJ
27.1 27.4 26.7 25.4 25.8 24.5 24.2 25.1 24.3 23.7 22.5 26.9 26.4 27.3 23.6 24.6 26.2 22.0 21.7 20.4 21.5 21.8 22.1 23.1 21.9 24.9 23.6 26.8 27.2 28.4 28.7 26.4 24.6 23.0 23.0 23.1 24.0 25.4 24.0 24.7 23.6 25.2 26.7 26.6 25.6 25.2 23.9 24.0 24.0 26.3 25.0 25.5 23.2 23.8 25.0 25.5 25.8 25.3 24.7 23.9 25.4 24.5 24.2 22.4 25.7 32.1 25.7 23.2 35.7 23.3 23.9 22.0 21.7 22.1
3.13 5.08 2.81 1.33 3.06 1.33 3.77 7.14 5.71 5.10 5.35 4.65 3.84 4.44 6.06 5.66 4.65 6.16 5.66 5.66 2.83 3.23 4.85 3.64 4.65 9.05 6.45 2.05 2.59 1.30 0.97 2.38 5.99 5.00 4.69 4.49 4.69 6.73 5.51 1.53 1.73 1.22 3.67 1.94 1.33 3.06 1.02 3.74 3.03 4.34 2.63 3.33 3.13 4.69 2.86 4.18 1.73 1.84 1.94 1.84 1.84 1.02 1.53 2.25 1.37 0.78 2.94 3.82 1.18 1.30 1.40 1.30 5.80 5.00
120 100 88 400 800 1032 128 180 140 256 96 108 88 164 100 136 128 132 236 212 188 236 160 212 200 180 132 0 0 0 0 0 252 1840 1600 1480 600 1840 1480 800 1560 1720 1320 920 1420 1200 1480 800 1380 1000 1760 1200 1080 1100 1680 800 1600 800 880 1440 720 680 920 104 0 0 140 52 0 180 80 60 112 252
67.2 56.1 49.2 222.8 446.2 573.2 71.0 100.2 77.7 141.8 53.0 60.5 49.2 91.4 55.4 75.6 71.5 72.7 129.8 116.1 103.4 129.9 88.1 117.2 110.1 100.1 73.1 0 0 0 0 0 140 1016.8 884.1 818.1 332.7 1025.0 820.6 444.6 863.8 957.5 738.5 514.6 791.6 668.0 820.3 443.6 765.1 558.7 979.1 668.7 597.2 609.5 934.6 445.8 892.5 445.5 489.1 798.1 401.1 377.7 510.4 57.4 0 0 78.1 28.8 0 99.6 44.3 33.0 61.6 138.8
0.003 0.006 3.064 0.005 0.012 0.002 0.004 0.004 0.003 0.004 0.001 0.002 0.002 0.002 0.001 0.004 0.005 0.002
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
222
Rn, Brazil
220
Rn and other dissolved gases in mineral waters of southeast
Daniel Marcos Bonotto* Departamento de Petrologia e Metalogenia, IGCE e Instituto de Geociências e Ciências Exatas, UNESP e Universidade Estadual Paulista Júlio de Mesquita Filho, Av. 24-A, No. 1515 e CP 178, Bela Vista CP 178, CEP 13506-900 Rio Claro, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2013 Received in revised form 8 January 2014 Accepted 10 January 2014 Available online 14 February 2014
This paper describes the natural radioactivity due to 222Rn and 220Rn in mineral waters occurring at São Paulo and Minas Gerais states, Brazil, that are extensively used for drinking in public places, bottling and bathing purposes, among other. The measurements of these alpha-emitting radionuclides were also accompanied by the monitoring of temperature and some dissolved gases (O2, CO2 and H2S) in 75 water sources located in 14 municipalities of those states. Eight water sources yielded 220Rn activity concentration values below the detection limit of 4 mBq/L. On other hand, 222Rn activity concentration values exceeding the WHO guidance level of 100 Bq/L in drinking-water for public water supplies were found in two springs, named Villela and Dona Beja, whose discharge occurs in areas characterized by the presence of enhanced levels of natural radioelements in rocks. The obtained results were compared with the guidelines of the Brazilian Code of Mineral Waters (BCMW) that was established in 1945 and is still in force in the country. The 222Rn and 220Rn activity concentration data allowed perform dose radiation calculations based on the potential alpha energy concentration (PAEC), whose implications for health risk have been also considered in this paper. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Radon and thoron Mineral waters Dissolved gases Radiation dose
1. Introduction In many countries, spring waters have been extensively used for consumption purposes as an option to tap water, as many people believe they are healthy and/or can be utilized for health cures. Additionally, economic reasons have also favored their use as bottled waters so that the commercialization of mineral waters has widely increased. The thermal and mineral waters use in Brazil is not recent due to arrival of European immigrants, mainly from Portugal. For instance, Gonsalves (1936) reported that the thermal waters occurrence in Caldas Velhas (Goiás State) was realized in 1722. The thermal spas were gradationally constructed in Brazil for therapeutic and leisure purposes, corresponding the period elapsed between the 1930s and 1950s to the most auspicious hydrothermal period in the country (Mourão, 1992). The Brazilian Code of Mineral Waters (BCMW) was established in this time, under French influence, by Register 7841 published on 8 August 1945 (DFPM, 1966). This rule is still in force without any actualization, focusing the mineral waters for spas and bottling uses, as well the potable waters for bottling (Serra, 2009). * Tel.: þ55 19 35269244; fax: þ55 19 35249644. E-mail addresses: [email protected], danielmarcosbonotto@gmail. com. 0265-931X/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2014.01.005
In Brazil, the production and commercialization of mineral waters is managed by the National Department of Mineral Production (DNPM). Traditionally, the mineral waters had been used or directly consumed in the springs, where touristic centers developed around. However, in the present days, the mineral water for consumption is distributed in vessels for ingestion distant from the springs, whereas hydrothermal spas for therapeutic baths and leisure exhibit infrastructure with hotels and many facilities for users. The first available information about the production of bottled mineral waters in Brazil dates back 1911 (CPRM, 2012). Only Minas Gerais and Rio de Janeiro States had industries by that time, whose annual production was 1,420,000 L (Minas Gerais- 1,220,000 L; Rio de Janeiro- 200,000 L) (CPRM, 2012). However, an accentuated expansion of the national production occurred in the period between 1996 and 2007, i.e. North region (þ386%), West Central region (þ287%), South region (þ207%), Northeastern region (þ130%) and Southeastern region (þ127%) (CPRM, 2012). The Southeastern region in 2007 was responsible by about 48% of the national production of mineral and potable water, the highest in the country (2.08 billion liters) (CPRM, 2012). São Paulo State presented a production superior to 1.5 billion liters in 2007, corresponding to 34% of the total in the country, followed by the states of Rio de Janeiro (7%) and Minas Gerais (6%) (CPRM, 2012). In general terms, circa 20 million consumers are involved (SEBRAE, 2012).
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D.M. Bonotto / Journal of Environmental Radioactivity 132 (2014) 21e30
Many spring waters in Brazil do not contain high concentrations of dissolved constituents, but the waters are considered mineral due to the radioactivity in them, chiefly due to the presence of dissolved radon (222Rn) and thoron (220Rn) (DFPM, 1966). 222Rn (half-life 3.82 days) and 220Rn (half-life 55.6 s) are noble gas isotopes belonging to the 238U (4n þ 2) and 232Th (4n) decay series, respectively. 222Rn is produced by a-decaying 226Ra and reaches the stable 206Pb, whereas 220Rn is generated by a-emitting 224Ra, forming the stable 208Pb (Clayton, 1983; Chu et al., 1999). Some 222 Rn and 220Rn fraction escapes from the rocks and minerals to the surrounding fluid phase, such as groundwater and air, whose proportion depends on factors such as total surface area of solids and the concentration (and distribution) of 238U (226Ra) and 232Th (224Ra) in them (Flügge and Zimens, 1939). Potential health hazards from natural radionuclides in consuming water have been considered worldwide, with many countries adopting the guideline activity concentration for drinking water quality recommended by WHO (2011). In general, the recommendations apply to routine operational conditions of watersupply systems, however, special attention must be also given to mineral waters frequently ingested by population. Thus, the approach for radioactivity in the BCMW (DFPM, 1966) is contradictory to that focused by WHO (2011). Another relevant aspect in this situation is the lack of confident data on the 222Rn and 220Rn activity concentration in Brazilian spring waters, mainly for 220Rn. The available 222Rn data in some cases is partially a consequence of the timescale of its half-life that is longer than that of the 220Rn. This paper describes a comparative study of well-known Brazilian mineral waters occurring at São Paulo and Minas Gerais states that was held for evaluating the presence of the following dissolved gases: 222Rn, 220Rn, O2, CO2 and H2S. With exception of oxygen, all other gases are taken into account in the BCMW (DFPM, 1966) that has been utilized on the data evaluation. The radiation dose due to the presence of dissolved 222 Rn and 220Rn has been calculated as both radionuclides are health threat when ingested in water in activity concentrations exceeding the guideline reference values. 2. Study area The groundwater samples (75) for dissolved gases analyses were taken from springs and pumped tubular wells drilled at different aquifer systems that are inserted in Paraná and Southeastern Shield hydrogeological provinces (Fig. 1 and Table 1). Águas de São Pedro, Águas de Santa Bárbara and Termas de Ibirá cities are inserted in the intercratonic Paraná sedimentary basin, in which the sedimentary sequence covers since the SilurianDevonian up to the Cretaceous periods (IPT, 1981). The sampling point at Águas de Santa Bárbara city corresponded to a 120 m-deep tubular well that cut the Serra Geral and Botucatu formations of the Paraná sedimentary basin. The waters of Águas de São Pedro city provided from tubular wells drilled at the Tubarão Group (Paraná basin) in 1936 by DNPM (National Department of Mineral Production) for petroleum exploration (Kimmelmann et al., 1987). However, such water sources have been popularly referred to Gioconda (Tubarão Group depth ¼ 275e625 m), Juventude (Tubarão Group depth ¼ 240e469 m) and Almeida Salles (Tubarão Group depth ¼ 207e329 m) “springs” (Kimmelmann et al., 1987). Águas de Lindóia, Serra Negra and Lindóia cities are located in a region characterized by the occurrence of several phases and cycles, involving different aspects of metamorphism, deformation and magmatism that acted from the Archean to the Upper Proterozoic times and affected rocks characterized by high metamorphic grade, generally of granulite and amphibolite facies (Ebert, 1955; Almeida and Hasui, 1984). The water circulation at Águas de Lindóia city
Fig. 1. Sketch map of the research region in Brazil and location of the groundwater sampling points in the following cities of São Paulo and Minas Gerais states: ASP ¼ Águas de São Pedro, ADL ¼ Águas de Lindóia, SEN ¼ Serra Negra, LIN ¼ Lindóia, TEI ¼ Termas de Ibirá, ASB ¼ Águas de Santa Bárbara, ADP ¼ Águas da Prata, PDC ¼ Poços de Caldas, PRV ¼ Pocinhos do Rio Verde, LAM ¼ Lambari, SLO ¼ São Lourenço, CAM ¼ Cambuquira, CAX ¼ Caxambu, AXA ¼ Araxá.
generally realizes through fractures and the flow is from the higher to lower altitudes (del Rey, 1989), leaching migmatite (Lindália and Santa Isabel), quartzite (Comexim, Curie, Filomena and Beleza) and milonite/quartzite (São Roque). Águas da Prata, Poços de Caldas and Pocinhos do Rio Verde cities are geologically situated in the Poços de Caldas alkaline complex that is a ring structure of Mesozoic age, comprising a suite of alkaline volcanic and plutonic rocks, mainly phonolites and nepheline syenites (Schorscher and Shea, 1992). Seven springs were sampled at Águas da Prata city: Villela (discharges into a well silicified and lightly folded sandstone), Vitória (discharges through fissures in diabase), Platina (discharges in outcropping phonolites), Prata (discharges in diabase), Boi (discharges in a silicified and recrystallized sandstone), Paiol and Padre (discharges through volcanic tuffs, phonolites and eudialite-bearing nepheline syenites) (Szikszay, 1981). Groundwater at Poços de Caldas area has been mainly exploited in crystalline fractured rocks due to the better yielding of the wells, but it also occurs in diffuse and punctual thermal and non-thermal springs that discharge at the depression in Poços de Caldas city (Cruz, 1987; Cruz and Peixoto, 1989). The geological substrate of Lambari, São Lourenço, Cambuquira and Caxambu cities in south of Minas Gerais State comprises rocks of the Proterozoic times like ophthalmic biotite gneisses, migmatized granitoids, protomilonites and milonite gneisses, migmatized garnet-biotite-gneisses, metabasites intercalations secondarily cut by pegmatoids veins and schists. There is also the presence of very weathered quartzites and Quaternary alluvial deposits (CPRM, 1999). The hydrogeological model for mineral waters occurring in São Lourenço, Lambari and Caxambu cities involves the rainwater infiltration in weathered horizons of gneissic rocks at the more elevated topographic areas close to the springs (CPRM, 1999). Then, this is succeeded by percolation through milonitized zones (São Lourenço, Caxambu, and Lambari) and fractures partially filled by pegmatoids
D.M. Bonotto / Journal of Environmental Radioactivity 132 (2014) 21e30
23
Table 1 Description of the groundwater samples analyzed in this paper. City (Statea)
Spring (well) name/Sample code
Hydrogeological Provinceb
Dominant flow
Major rock types
Geological context/age
Águas de São Pedro (SP)
Almeida Salles/ALS Gioconda/GIO Juventude/JUV
Paraná
Porous
Sandstones
Botucatu Fm. (Jurassic) Pirambóia Fm. (Triassic) Itararé Fm. (Permian) Irati Fm. (Permian)
Águas da Prata (SP)
Platina/PLA Paiol/POL
Paraná
Fractures
Diabases Phonolites
Botucatu Fm. (Jurassic) Serra Geral Fm. (Jurassic-Cretaceous) Poços de Caldas intrusive complex (Cretaceous)
Águas de Lindóia (SP)
Serra Negra (SP)
Lindóia (SP) Termas de Ibirá (SP)
Vitória/VIT
Alkaline rocks
Boi/BOI Prata/PTA Villela/VIL Padre/PDE
Silicified sandstones
Santa Isabel/SIL
Yes
Yes
Paraná
Fractures
Filomena/FIL Beleza/BEL São Roque/SRE Comexim/COM Lindália/LIN Curie/CUR São Jorge/SJO São Carlos/SCA Italianos/ITA Santa Luzia/SLU Santo Agostinho/SAT Brunhara/BRU Laudo Natel/LAN Sant’Anna/SAA São Benedito/SBE Bioleve/BIO Jorrante/JOR Ademar de Barros/ADB
Commercializationc
Granites
Amparo Gp. (Lower Proterozoic)
Gneisses Migmatites Schists Quartzites Limestones Dolomites
Yes
Yes Yes
Yes Yes Paraná
Porous and Fractures
Sandstones Basalts
Bauru Gp. (Cretaceous) Serra Geral Fm. (Jurassic-Cretaceous)
Bauru Gp. (Cretaceous) Serra Geral Fm. (Jurassic-Cretaceous) Paraíba do Sul Gp. (Proterozoic) Barbacena Gp. (Proterozoic) São João dél Rei Gp. (Proterozoic) Andrelândia Gp. (Proterozoic) magmatic plutonic series (Brasiliano)
Carlos Gomes/CGO Saracura/SRC Seixas/SEI Águas de Santa Bárbara (SP)
Balneário Municipal/BMU
Paraná
Porous and Fractures
Sandstones Basalts
Lambari (MG)
No. 1/LA1
Southeastern shield
Porous and fractures
Ortogneisses
No. 2/LA2
Granulites
No. 3/LA3
Migmatites
No. 4/LA4
Metassedimentary seq. Metavulcanossedimentary seq.
No. 5/LA5
São Lourenço (MG)
Cambuquira (MG)
Caxambu (MG)
No. 6/LA6 No. 7-Bis/SL7 No. 5-Alcalina/SL5 No. 6-Sulfurosa/SL6 No. 3-Vichy/SL3 No. 4-Ferruginosa/SL4 No. 1-Oriente/SL1 No. 10-Primavera/SL10 No. 9-Carbogasosa/SL9 Roxo Rodrigues/ROR Regina Werneck/REW Com. Augusto Ferreira/CAF Fernandes Pinheiro/FEP Marimbeiro/MAR Souza Lima/SLI Geiser Floriano de Lemos/GFL Venâncio/VEN Mayrink/MAY (continued on next page)
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D.M. Bonotto / Journal of Environmental Radioactivity 132 (2014) 21e30
Table 1 (continued ) City (Statea)
Spring (well) name/Sample code
Hydrogeological Provinceb
Dominant flow
Major rock types
Geological context/age
Southeastern shield
Fractures
Alkaline rocks
Poços de Caldas intrusive complex (Cretaceous)
Commercializationc
Ernestina Guedes/EGU Viotti/VIO D. Pedro II/DPE Beleza/BZA Duque de Saxe/DXE Da. Leopoldina/LEO Da. Isabel/Conde dÉu/ISA Poços de Caldas (MG)
Pocinhos do Rio Verde (MG)
Araxá (MG) a b c
Quisisana/QUI XV de Novembro/NOV Macacos/MAC Sinhazinha/SIN Frayha/FRA Pedro Botelho/PEB Rio Verde/RIV Samaritana/SMA São José/SJO Amorosa/AMO Dona Beja/DBJ Andrade Júnior/AJU
Nepheline syenites Phonolites Pyroclastics Volcanic tuffs
Southeastern shield
Porous and fractures
Quartzites and schists Alkaline-carbonatitic rocks
Cretaceous PreCambrian
SP ¼ São Paulo, MG ¼ Minas Gerais. According to Mente (2008). Water commercialized by private company in addition to consumption in taps accessible to population in public areas.
dykes or alkaline breccias (Caxambu) (CPRM, 1999). The local conditions favor the periodical eruption (up to 5-m height) of a nongeothermal geyser at the Caxambu waterpark due to the build-up of pressure from dissolved carbon dioxide in the water. Araxá city is geologically located at Alto Paranaíba Igneous Province that includes the renowned carbonatite intrusion of Araxá, which covers approximately 16 km2 and is in general related to a NW-trending linear structure bordering the São Francisco cratonic area that is thought to be in evidence since late Precambrian times (Traversa et al., 2001). Two springs were sampled at Araxá city: 1) Dona Beja, associated to an aquifer system classified as granular, free and semi-confined, mainly in the intrusive body domain (Beato et al., 2000); 2) Andrade Júnior, related to a deep fractured aquifer, unconfined to semi-confined, mainly occurring in rocks surrounding the carbonatite complex (Beato et al., 2000). 3. Sampling and analytical methods The sampling points (75) were chosen according to the hydrochemical information from previous studies and to the easy access for collection. The groundwater chemical composition is attained due to processes occurring at the liquidesolid interface when different rock matrices are extensively leached. The sampling sites were (Fig. 1 and Table 1): Águas de São Pedro (3 water sources), Águas da Prata (7 water sources), Águas de Lindóia (7 water sources), Serra Negra (8 water sources), Lindóia (2 water sources), Termas de Ibirá (5 water sources), Águas de Santa Bárbara (1 water source), Lambari (6 water sources), São Lourenço (8 water sources), Cambuquira (6 water sources), Caxambu (10 water sources), Poços de Caldas (6 water sources), Pocinhos do Rio Verde (4 water sources) and Araxá (2 water sources). All water sources (except geyser “Floriano de Lemos”, Caxambu city) have been used for drinking purposes in thermal and non-thermal spas and some of them are commercialized by private companies (Table 1). The sampling campaign occurred between March and June 2012 (autumn), when the local air temperature was often lower than that of the summer months (average in January ¼ 30e32 C). The rainfall was lower than that of the wet season (summer, DecembereJanuary) but higher than that of the dry season (winter, JulyeAugust).
About 500 mL of groundwater sample was directly taken from taps/pipes installed in each spring/well, which was stored in glass/ plastic flasks of variable volume as required by each gas (222Rn, 220 Rn, O2, CO2 and H2S) analysis. Temperature was also measured by the use of a digital portable meter in addition to the dissolved gases readings. All data acquisition was in situ performed for avoiding gases losses and modification of other physical and chemical parameters of the waters. The dissolved oxygen (DO) concentration was determined in a digital portable meter recording the potential values generated by an O2 sensible electrode consisting on a metallic wire covered by a thin gold layer (detection limit ¼ 0.1 mg/L). The electrode was inserted into a 60-mL Winkler bottle filled with the water sample, then, the final value was recorded after stabilization of the reading. All DO data were corrected due to differences in altitude of the sampling points, using as reference the value corresponding to the sea level. The carbon dioxide was evaluated by the buret titration method on using water volumes of 25 or 50 mL (Hach, 1992). This amount was inserted in a clean erlenmeyer flask, then, one powder pillow of a phenolphthalein indicator solution (5 g/L) (Hach, 1992) was added and the solution mixed gently. A 25-mL buret was filled to the zero mark with 0.0227 N NaOH solution and the prepared sample was titrated while gently swirling the flask until a light pink color formed and persisted for 30 s. The reaction of NaOH with CO2 (as carbonic acid) occurs in two steps, first a reaction from carbonic acid to bicarbonate and then to carbonate. Because the conversion of CO2 to bicarbonate is complete at pH 8.3, phenolphthalein was used as a color indicator for the titration (Hach, 1992). The dissolved CO2 (in mg/L) was calculated on multiplying the volume (in mL) of the titrant used by the factor 20 (sample volume ¼ 50 mL; detection limit ¼ 2 mg/L) or 40 (sample volume ¼ 25 mL; detection limit ¼ 4 mg/L) (Hach, 1992). The pink color in some samples was immediately formed with the addition of the phenolphthalein indicator. The dissolved CO2 corresponded to 0 mg/L in these cases as it was not necessary the addition of the titrant (0.0227 N NaOH solution) for the color change. The dissolved sulfide gas was determined by colorimetry (wavelength 665 nm) using a program stored in the Hach DR/2000 spectrophotometer (Hach, 1992). The methylene blue method was adopted in which the hydrogen sulfide and acid-soluble metal
D.M. Bonotto / Journal of Environmental Radioactivity 132 (2014) 21e30
sulfides react with N,N-dimethyl-p-phenylenediamine oxalate to form methylene blue (Hach, 1992). One glass cell (the prepared sample) was filled with 25 mL of sample, whereas another (the blank) was filled with 25 mL of deionized water. Then, 1 mL of Sulfide 1 Reagent (N,N-dimethyl-p-phenylenediamine oxalate) was added to each cell and, after mixing, 1 mL of Sulfide 2 Reagent (potassium dichromate) was put into both cells. After mixing and a 5-minute reaction time, the blank was inserted into the cell holder, the spectrophotometer was set to zero reading, the prepared sample was placed into the cell holder and the equipment displayed the result in mg/L sulfide (S2) (Hach, 1992). The intensity of the blue color was proportional to the sulfide concentration and proper dilution was used to determine high sulfide levels is some samples. The detection limit of the method corresponded to 1 mg/L sulfide. The radon and thoron dissolved in water were analyzed on site using RAD7 alpha particles detector coupled to accessory RADH2O from Durridge Co. The RAD7 utilizes a solid state alpha detector, comprising a Si semiconductor material that converts the energy of the alpha particles into an electrical signal. The accumulation of many signals results in a spectrum that is presented on a scale of alpha energy in the range of 0e10 MeV. Such energy interval is suitable for 222Rn and 220Rn readings as data acquisition takes place between 6 and 9 MeV (Durridge, 2009a). The spectrum is displayed in a set of 200 channels grouped into eight windows of different energy range. A, B, C and D are the main windows, whereas E, F, G and H are the diagnostic of main windows. Windows A and C provide 222Rn activity concentration data from 218Po and 214Po decays, representing “new” (218Po) and “old” (214Po) 222Rn, respectively (Durridge, 2009a). Windows B and D provide 220Rn activity concentration data from 216Po and 212Po decays, representing “new” (216Po) and “old” (212Po) 220Rn, respectively (Durridge, 2009a). Thus, the RAD7 separates the radon and thoron signals by energy of the alpha particles from their progeny, becoming possible to measure both Rn isotopes simultaneously (Durridge, 2009a). The equipment was factory calibrated by way of inter-comparison with radon chambers run by the U.S. EPA and the U.S. Department of Energy (Durridge, 2009a). The groundwater samples were collected in 100-mL flasks, avoiding as much as possible the exposure to atmospheric air. The RADH2O accessory unit employed an aeration system in a closed circuit connecting the sample, RAD7 and a desiccant tube containing Drierite to humidity absorption (Durridge, 2009b). Then, the air circulated through the sample during about 10 min, extracting radon and thoron until reaching an equilibrium state (for readings in windows A and B). The radon and thoron extracted were pumped into the RAD7, where their progeny were detected. The instrument display recorded the dissolved 222Rn and 220Rn activity concentration data in pCi/L that were converted to the SI unit (Bq/L for 222Rn and mBq/L for 220Rn). The Detection Limit (DL) corresponded to 4 mBq/L (Durridge, 2009a), whereas the uncertainties given of the results were one standard deviations, resulting from propagation of all statistical uncertainties in the entire measurement process. The low activity concentration in some samples implied on high analytical uncertainty as reported elsewhere (for instance, Jia et al., 2002). 4. Results and discussion 4.1. Temperature and dissolved oxygen, carbon dioxide and sulfide in the waters The groundwater temperature ranged from 20.4 to 35.7 C (mean ¼ 24.8 C) (Table 2). According to the BCMW guidelines for temperature (DFPM, 1966), three categories can be defined for the
25
analyzed waters: cold (values lower than 25 C ¼ 41 samples), hypothermal (values ranging from 25 to 33 C ¼ 33 samples) and mesothermal (values ranging from 33 to 36 C ¼ 1 sample). The mean dissolved oxygen (DO) concentration was 3.5 mg/L (Table 2). The lowest DO content was 0.8 mg/L (Macacos spring, Poços de Caldas city, Minas Gerais State), whereas the highest DO concentration was 9 mg/L (São Benedito water source, Lindóia city, São Paulo State) that corresponds to the solubility of oxygen at 100% air saturation at 21 C and 760 mmHg mount air barometric pressure (EIFAC, 1986). The gas content (in solution) of a mass of water can be evaluated considering the Henry’s law and Dalton’s law of partial pressure. From both gas laws, it has been widely recognized that the solubility of oxygen in water varies with temperature, the DO content reducing sharply with increase in temperature (Cole, 1983; APHA, 1985; EIFAC, 1986). Treusdale et al. (1955) reported the solubility value for oxygen at various temperature, which was re-estimated using improved Henry coefficients (Benson and Krause, 1980) as discussed by Cole (1983). The values of solubility of oxygen in water at various temperatures, from moist air at 760 mmHg at 0.0 ppt salinity, taken from EIFAC (1986) is 9.1 mg/L at 21 C and 6.9 mg/L at 35 C. There is a significant inverse relationship between temperature and dissolved O2 in the analyzed groundwater (Fig. 2a), confirming that the increase in temperature causes a decrease in dissolved O2. The two-tailed P value estimated by GraphPad software from the Pearson correlation coefficient (r ¼ 0.41, n ¼ 75; Fig. 2a) equals 0.0003 by conventional criteria, suggesting this difference is extremely statistically significant. The mean dissolved CO2 concentration was 590 mg/L (Table 2). The dissolved CO2 was absent in nine samples, from which five were collected at Termas de Ibirá city in São Paulo State. Degassing favored by cracks, faults and fissures in basalts of the Serra Geral Formation may be a possible reason for the null CO2 values at Termas de Ibirá city. The dissolved CO2 ranged from 52 to 1840 mg/ L in the remaining samples (Table 2) and its presence in waters is mainly related to the carbonates dissolution in the rock matrices. The groundwater temperature range is from 25 to 35.7 C in the hypothermal and mesothermal waters (Table 2) and such large difference (10.7 C) favors the significant inverse relationship between temperature and dissolved carbon dioxide (Fig. 2b), indicating that its solubility decreases accompanying the temperature raising as also verified for oxygen. The two-tailed P value estimated by GraphPad software from the Pearson correlation coefficient (r ¼ 0.49, n ¼ 34; Fig. 2b) equals 0.0033 by conventional criteria, suggesting this difference is very statistically significant. According to the BCMW guidelines, the carbogaseous waters contain dissolved carbon dioxide gas corresponding to a minimum of 200 mL per liter of water (DFPM, 1966). The CO2 concentration data in Table 2 can be used for estimating the CO2 volume (in mL) per liter of water from the ideal gas law that is the state equation of a hypothetical (ideal) gas. Although it has limitations, it is a rough approximation to the behavior of some gases like CO2, under some conditions. It is often written as PV ¼ nRT where P is the gas pressure, V is the gas volume, n is the gas number of moles, T is the gas temperature and R is the ideal, or universal, gas constant. If T is expressed in Kelvin, then, R ¼ 0.0821 L.atm/K.mol. Remembering that 1 mol CO2 corresponds to approximatelly 44 g, then, it is possible estimate V values (in mL) varying from 0 to 1025 mL (Table 2). The comparison of the V values (in mL) for CO2 in Table 2 with the BCMW guideline of 200 mL per liter of water (DFPM, 1966) indicates that 33 water sources are carbogaseous. However, some of them are popularly recognized by other chemical attributes rather than dissolved CO2, for instance: No. 2 (Lambari city) and No. 5 (São Lourenço city) are said alkaline waters; No. 3 (Lambari city) and Com. Augusto Ferreira (Cambuquira city) are said magnesian
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D.M. Bonotto / Journal of Environmental Radioactivity 132 (2014) 21e30
Table 2 Temperature, dissolved gases, radon and thoron data in groundwater samples analyzed in this paper. Sample codea
Temp. ( C)
O2 (mg/L)
CO2 (mg/L)
CO2 (mL)b
H2S (mg/L)
222
220
ALS GIO JUV PLA POL VIT BOI PTA VIL PDE SIL FIL BEL SRE COM LIN CUR SJO SCA ITA SLU SAT BRU LAN SAA SBE BIO JOR ADB CGO SRC SEI BMU LA1 LA2 LA3 LA4 LA5 LA6 SL7 SL5 SL6 SL3 SL4 SL1 SL10 SL9 ROR REW CAF FEP MAR SLI GFL VEN MAY EGU VIO DPE BZA DXE LEO ISA QUI NOV MAC SIN FRA PEB RIV SMA SJO AMO DBJ
27.1 27.4 26.7 25.4 25.8 24.5 24.2 25.1 24.3 23.7 22.5 26.9 26.4 27.3 23.6 24.6 26.2 22.0 21.7 20.4 21.5 21.8 22.1 23.1 21.9 24.9 23.6 26.8 27.2 28.4 28.7 26.4 24.6 23.0 23.0 23.1 24.0 25.4 24.0 24.7 23.6 25.2 26.7 26.6 25.6 25.2 23.9 24.0 24.0 26.3 25.0 25.5 23.2 23.8 25.0 25.5 25.8 25.3 24.7 23.9 25.4 24.5 24.2 22.4 25.7 32.1 25.7 23.2 35.7 23.3 23.9 22.0 21.7 22.1
3.13 5.08 2.81 1.33 3.06 1.33 3.77 7.14 5.71 5.10 5.35 4.65 3.84 4.44 6.06 5.66 4.65 6.16 5.66 5.66 2.83 3.23 4.85 3.64 4.65 9.05 6.45 2.05 2.59 1.30 0.97 2.38 5.99 5.00 4.69 4.49 4.69 6.73 5.51 1.53 1.73 1.22 3.67 1.94 1.33 3.06 1.02 3.74 3.03 4.34 2.63 3.33 3.13 4.69 2.86 4.18 1.73 1.84 1.94 1.84 1.84 1.02 1.53 2.25 1.37 0.78 2.94 3.82 1.18 1.30 1.40 1.30 5.80 5.00
120 100 88 400 800 1032 128 180 140 256 96 108 88 164 100 136 128 132 236 212 188 236 160 212 200 180 132 0 0 0 0 0 252 1840 1600 1480 600 1840 1480 800 1560 1720 1320 920 1420 1200 1480 800 1380 1000 1760 1200 1080 1100 1680 800 1600 800 880 1440 720 680 920 104 0 0 140 52 0 180 80 60 112 252
67.2 56.1 49.2 222.8 446.2 573.2 71.0 100.2 77.7 141.8 53.0 60.5 49.2 91.4 55.4 75.6 71.5 72.7 129.8 116.1 103.4 129.9 88.1 117.2 110.1 100.1 73.1 0 0 0 0 0 140 1016.8 884.1 818.1 332.7 1025.0 820.6 444.6 863.8 957.5 738.5 514.6 791.6 668.0 820.3 443.6 765.1 558.7 979.1 668.7 597.2 609.5 934.6 445.8 892.5 445.5 489.1 798.1 401.1 377.7 510.4 57.4 0 0 78.1 28.8 0 99.6 44.3 33.0 61.6 138.8
0.003 0.006 3.064 0.005 0.012 0.002 0.004 0.004 0.003 0.004 0.001 0.002 0.002 0.002 0.001 0.004 0.005 0.002
