Ribeirão Preto. Brasil

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1 300 km 400 km Abstract In this report, some results of the evaluation of hydrochemical data from the southern part of the Guarani aquifer in Paraguay are described. Calculations with the modelling code PhreeqC-2 could reconstruct water evolution in the area of Ciudad del Este, where old, saline water ascends from deeper layers to the surface. High fluorine concentrations are explained by dissolution of fluorine-containing minerals like fluorite or apatite. In the western part of the study area, elevated Asconcentrations were found. Field data suggest, that As is released in consequence of reduction of goethite, to which As was sorbed. This process could be calculated with PhreeqC, too. Analogies to investigation results from Brazil were found, where similar conclusions for water evolution in the state São Paulo were drawn. Hydrogeology The Guarani-Aquifer system is situated in the eastern and south central part of South America and underlies parts of Argentina, Brazil, Paraguay and Uruguay (Fig. 1). Bolivia Ribeirão Preto Paraguay 400 km Brasil Encarnación Caaguazú Ciudad del Este Argentina Rivera-Santana Concordia-Salto Uruguay Fig. 1: Extension of the Guarani-Aquifer in South America In each country, the formations of the Guarani-Aquifer have different names (Wendland et al. 2004). In Paraguay, this system comprises layers of Permian and Triassic (Misiones Formation) sandstone and Cretaceous basalt (Alto Paraná Formation). Groundwater flow is directed towards the Paraná River. In a field 1

2 campaign in 2004 (Vassolo 2004*), water samples in the southern part of the Guarani-Aquifer in Paraguay were taken and analysed for major cations and anions and trace elements. Sampling points were mostly wells constructed for water supply. At places, river water was also sampled. In total 80 water samples were analysed and evaluated. General Features Hydrochemistry of the groundwater in the southern part of the Guarani-Aquifer in Paraguay is characterised by mostly small to moderate dissolved solid contents, shown by small EC-values, which only in places exceed 400 µs/cm (Fig. 2). Especially waters in the SW are comparatively young (~380 years, 14 C-dating) and immature with small EC-values. Calculated saturation indices (SI) for various minerals, like e.g. calcite, siderite, albite etc. were mostly strongly negative in this area, indicating that water-rock-interactions have not yet proceeded much and waters still tend to dissolve minerals and ions from the aquifer. Apart from some few exceptions, ph-values in general vary between 5 and 8 (Fig. 3). Most waters of this aquifer system belong to the HCO 3 -Ca-Mg-type or HCO 3 -Na-type (see Schmidt 2005). Exception from this is the NE of the investigation area, the region around Ciudad de Este, where EC-values are in the range of µs/cm. Fig. 2: EC values in the Guarani-Aquifer 2

3 Fig. 3: ph-values in the Guarani-Aquifer Region Ciudad del Este Groundwater sampled in this region usually belongs to the SO 4 -Na-Cl-type and frequently acts artesian. Mostly, it originates from the Misiones Sandstone underlying the Cretaceous basalt in large depths (> 450 m). Obviously, old groundwater ascends here from deeper layers to the surface. An age-dating with 14 C revealed groundwater ages of ca. 30,000 years. The ph-values of these waters were slightly basic in a range between 8 and 9.5. Apart from high Na-, Cl- and SO 4 - concentrations, these waters also show elevated fluorine-contents between 3.66 and 11.1 mg/l. The origin of fluorine, which is locally contaminating water throughout the whole Guarani-Aquifer, is still a matter of discussion (Kittl 2003) Possible sources of fluorine are the minerals (Fluor)Apatite (Ca 5 [(F,Cl)(PO 4 ) 3 ] and Fluorite (CaF 2 ), which are frequently found in igneous rocks and sediments (Mattheß, 1994). In Fig. 4 and 5, fluorine-concentrations are plottet against calculated SI for Fluorite and Fluorapatite, showing that waters with high F-contents are nearly in equilibrium with those minerals. As PO 4 -concentrations are comparatively small in waters with high F-content (< 0.1 mg/l), it is assumed that Fluorite and not Apatite is the source, although it cannot be excluded, that PO 4 is sorbed to clays and ironhydroxides. 3

4 Fig. 4 Correlations between fluorineconcentrations and SI for fluorite Fig. 5 Correlations between fluorineconcentrations and SI for fluorapatite The spatial distribution of the calculated SI for Fluorite is shown in Fig. 6. Again, it is striking, that in the region of Ciudad de Este, where elevated concentrations of fluorine were found, the calculated SI are near equilibrium. Fig 6: Spatial distribution of calculated saturation indices for fluorite With the modelling code PhreeqC-2 (USGS, authors: David L. Parkhurst, C.A.J. Appelo) a scenario was calculated to reconstruct the water-evolution. 4

5 Starting point of the calculation was an arbitrary water-composition upstream of the region CDE, in this case water-sample No. 66 (Juan E. O'Leary). The water was equilibrated with calcite (CaCO 3 ), fluorite (CaF 2 ) and 0,012 mol mirabilite (Na 2 SO 4 * 10 H 2 O). This increased Alkalinity, ph, F-, SO 4 - and Na- content. Also, some halite was dissolved in 10 steps, which increased the Cl-concentration and also attributed some additional Na to the solution. Table 1: Changes in water composition during dissolution of calcite, fluorite, mirabilite and halite Na HCO 3 Ca F Cl SO 4 ID (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) ph , step step step step step step step step step step Step 10 of the model in general reasonably matches with the composition of ID 56 (CDE), only the Ca-concentration differs considerably. Obviously, one important process is not yet included in the model. This scenario was tried to reconstruct with an inverse-modelling likewise. Again, I tried to generate the water composition of the sample 56 from sample 66 by solving the minerals halite, fluorite, calcite and mirabilite. The calculated amounts, which had to be dissolved therefore, are given in table 2: Table 2: Calculated mole transfers Mineral formula dissolved mole Halite NaCl 1.147*10-2 Fluorite CaF *10-4 Calcite CaCO *10-4 Mirabilite Na 2 SO 4 *10 H 2 O 9.87*10-3 Table 3: Resulting amounts from the inverse-modelling (mg/l) Parameter Na HCO 3 Ca F Cl SO 4 resulting concentration (mg/l) Keeping in mind that there are still other processes as dilution, mixing, sorption and precipitation and dissolution of other minerals taking place in this aquifer, this statement again gives reasonable results, with the only exception Ca. 5

6 Ca-cations can be removed from groundwater through cation-exchange processes for Na-ions, as divalent ions are generally sorbed stronger and more likely to clays than monovalent ions (Mattheß, 1994). So, a typical evolution of a groundwater is a change in the cation composition from Ca- and Mg-rich waters towards Na-waters. The generalised reaction is: Ca 2+ + Na 2 -X 2Na + + Ca-X This process acts as a sink for Ca 2+ and a source for Na +, which would explain partly the small discrepancy of missing Na in our calculation as well as the disappearance of Ca 2+ from solution. Furthermore, it would drive calcite and fluorite dissolution to continue, leading also to higher HCO 3 - and F-concentrations. Sorption of 28.6 mg Ca 2+ would release 33 mg Na +, resulting in a total amount of 772 mg Na/l in our system. Taking a rather small Cation Exchange Capacity of 0.6 meq/100g for Quartz (major component of sandstone), we come up with an amount of exchange sites of moles. A water composition corresponding to that given in sample 56 would lead to a sorbed amount of 247 mg Ca and 545 mg Na, if equilibrated with the exchanger, so the calculated numbers are fairly reasonable. Sracek & Hirata (2002) came to similar results for the Guarani-Aquifer in the state of São Paulo, Brazil. In that region, the Aquifer is divided into Piramboia Formation (aeolian and fluvial silty-clayish sandstones) and well-sorted aeolian sandstone of the Botucatu Formation. Loss of calcium is explained through cation exchange for sodium. Part of the sodium probably also enters the aquifer from the Piramboia Formation, together with chloride and sulphate from evaporites like halite and mirabilite or gypsum. High concentrations of fluorine (up to 13.3 mg/l) are explained by dissolution of fluorite, although mineralogical evidence for fluorite is lacking so far. Arsenic At some places, elevated As-concentrations ( mg/l) were found. Fig. 7 shows the distribution of samples with higher As-amounts. The samples are widespread especially over the western part of the study-area, were the Triassic and Permian sandstones crop out. 6

7 Fig. 7: Distribution of As in the southern part of the Guarani-Aquifer in Paraguay As-contents above 0.02 mg/l correspond with ph values above 8 and HCO 3 - concentrations above 230 mg/l (see Fig. 8 and 9). Fig. 8: As-concentration versus ph Fig. 9: As-concentration versus HCO 3 - concentration Frequently, arsenic is sorbed either to sulfides, like e.g. pyrite, or in oxidised form as As(V) to iron-hydroxides (Appelo & Postma, 2005, Grossl et al. 2004). Oxidation of sulfides or reduction of iron-hydroxides may thus release As into groundwater. Oxidation of sulfides would generally be accompanied by an increase of SO 4 - concentration or decrease of ph. 7

8 FeS 2 -As + 7 / 2 O 2 + H 2 O Fe SO H + + As However, elevated As-concentrations in our system usually correspond with low sulfate-concentrations and high ph, therefore this possibility is not regarded to be very likely in our case. On the other hand, reduction of ironhydroxides increases ph through consumption of H +. Fe(OH) 3 -As + 3H + + e - Fe H 2 O + As This scenario fits to our field data. The process of reduction of goethite, which was equilibrated with an As-containing water (in this case sample ID 64, Eugenio A. Garay) was calculated with PhreeqC. Organic matter (CH 2 O) served as electron-donor for reduction of goethite and was added to the groundwater in several steps. In addition to that, water was equilibrated with calcite and siderite. The resulting water evolution is presented in table 4: Table 4: Water composition changes during reduction of goethite mole CH 2 O added HCO 3 (mg/l) As (mg/l)*10 Fe (mg/l) SO 4 ph ID In line 1 the starting concentration of water sample No. 64 is given. Organic matter is added in 9 steps, leading to a significant increase in HCO 3 - and ph: CH 2 O + 4Fe(OH) 3 -As + 7 H+ HCO Fe H 2 O + 4As Reduction of goethite releases As, but does not increase Fe 2+ in solution, because Fe 2+ precipitates immediately as siderite: Fe 2+ + HCO 3 - FeCO 3 + H + Reduction processes are kept at a low level, so that sulphate is not yet reduced. Conclusions The results described above illustrate some important processes going on in the Guarani-aquifer. A more embracing picture of those processes could be obtained by 8

9 evaluation of data from the northern part of the aquifer in Paraguay, which will be raised during the next months. Additional information through analysis of further parameters might give deeper insights into the nature of the processes. Measurement of DOC for example could clarify if organic matter is the electron-donor responsible for the goethite reduction and analysis of the solid phase could restrict possible sources of fluorine. 9

10 References Appelo, C.A.J., Postma, D. (2005): Geochemistry, Groundwater and Pollution, 2.nd edition, A.A. Balkema, The Netherlands, 649 p. Kittl, S. T. (2003): Stratigraphy and hydrochemistry of the Guaraní aquifer system, South America. In: Festschrift zum 70. Geburtstag von Prof. Dr. Hans-Jürgen Behr. Göttinger Arbeiten zur Geologie und Paläontologie. SB 5. Geowissenschaftliches Zentrum der Universität Göttingen., p Mattheß, G (1994): Die Beschaffenheit des Grundwassers. Lehrbuch der Hydrogeologie, Band 2. Gebr. Borntraeger, Berlin, Stuttgart, 499 p. Schmidt, G. (2005): TZ-Projekt "Systematische Abdeckung von Know-how Defiziten auf dem Geo-Sektor in Entwicklungsländern" - Nachhaltige Nutzung der grenzüberschreitenden Grundwasservorkommen des Guarani-Aquifer-Systems Status-Bericht 05/ /2005, 17 p. Sracek, O., Hirata, R. (2002): Geochemical and stable isotopic evolution of the Guarani Aquifer System in the state of São Paulo, Brazil. Hydrogeology Journal 10: Vassolo, S. (2004)*: TZ-Überregional - Teilprojekt "Nachhaltige Nutzung der grenzüberschreitenden Grundwasser-Vorkommen des Guaraní-Aquifer-Systems" - Bericht über eine Dienstreise nach Asunción vom Wendland, E., Rabelo, J., Roehrig, J. (2004): Guarani Aquifer System The Strategical Water Source In South America. In: Technology Resource Management and Development, Vol. 3, Fachhochschule Köln, p