Procedia Earth and Planetary Science 7 ( 2013 ) 385 – 388

1878-5220 © 2013 The Authors. Published by Elsevier B.V.Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 – 2013doi: 10.1016/j.proeps.2013.03.137

Water Rock Interaction [WRI 14]

Geochemistry of iron in fresh groundwater of the Sredneobskoy basin, Russia

Ivanova I.S.a,b*, Lepokurova O.E.a,b, Pokrovsky O.S.c, Shvartsev S.L.a,b aNational Research Tomsk Polytechnic University, Lenin Avenue, 30, Tomsk 634050, Russia

bTomsk Division of Trofimuk Institute of Petroleum-Gas Geology and Geophysics of the Siberian Branch of the RAS, 3, Academichesky ave., Tomsk 634021, Russia

cGéosciences Environnement Toulouse (GET–UMR 5563 UR 154 CNRS, University Paul Sabatier, IRD), 14 Édouard Belin, Toulouse 31400, France

Abstract

In the central part of Western Siberia a study of the chemical composition of fresh groundwater in the upper 600 m of the Sredneobskoy basin was carried out. It was shown that groundwater generally contains high concentrations of iron. The features of the distribution of iron in groundwater and sources of its origin are investigated. The maximum iron content is confined to the waters of the Neogene-Quaternary and Paleogene sediments that form a favorable gleysol (Eh -100 to +50 mV), neutral pH (6,8 to 7,5), the geochemical environment with a high content of dissolved organic matter. Aluminum silicate rocks are a source of iron, namely, the primary mineral water-bearing rocks, which in these circumstances are extremely soluble and therefore not in equilibrium with respect to the release of iron, calcium, magnesium, sodium, and other elements. © 2012 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of Organizing and Scientific Committee of WRI 14 – 2013. Keywords: Western Siberia, fresh groundwater, the sources of iron, water–rock interaction.

1. Introduction

In Western Siberia, at depths of 10-30 m, groundwater is generally rich in iron, whose concentration often reaches30-40 mg/l. Such water is common in many artesian basins in the world [1-4]. In addition the Vasyuganskoe bog, the largest bog in the world, is located in the area, and the iron content of its waters is high. Meanwhile, the sources of this element are still a matter of speculation.

The study area is the lowland wetland, folded on top of Neogene-Quaternary sediments up to 350 m in thickness (Fig. 1). The following table shows underlying Paleogene sand-clay sediments reaching up to 200-400 m depth, Upper Cretaceous sediments up to 600 m in thickness occur within this developed two

* Corresponding author. Tel.: + 73822658862. E-mail address: iren1986@sibmail.com.

Available online at www.sciencedirect.com

© 2013 The Authors. Published by Elsevier B.V.Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 – 2013

386 I.S. Ivanova et al. / Procedia Earth and Planetary Science 7 ( 2013 ) 385 – 388

water-bearing complex: Eocene-Quaternary and Upper Eocene are separated over most of the powerful aquitard in the form of clays. Each aquifer system is divided into several levels, the most important of which are the Neogene-Quaternary, Paleogene and Cretaceous [5].

2. Results

206 samples of groundwater and 47 samples of bog water were collected (Fig. 2). Typical analyzes described in detail in previous studies [6] are shown in Table 1.

The maximum iron content is confined to the waters of the Neogene-Quaternary and Paleogene sediments which form a favorable gley (Eh -100 to +50 mV), neutral (pH 6,8 to 7,5), the geochemical environment with a high content of dissolved organic matter.

Fig. 1. Schematic geological section along the line I-II (see Fig. 2)

What are the sources of accumulated iron in the groundwater? It is believed that the main sources of iron in the water are the major minerals: sulfides, oxides and carbonates [7]. However, pyrite is not very common in the area, and siderite, and Fe oxides are presented here as secondary minerals as evidence below shows. However, the most common minerals and, therefore, the most likely source of iron silicates are: epidote, hornblende, pyroxene, etc. To prove this, thermodynamic calculations were performed at equilibrium water with minerals of various origins using the software system Hydro Geo.

Fig. 2. Arrangement of sampling in natural waters: 1 - border of Western Siberia, 2 – selection from the wells, 3-

city, and 4- selection of bog water, 5 - line of the geological section

3 Interpretation

Studies have shown (Fig. 3) that all groundwater are not at equilibrium with the primary minerals: feldspar, muscovite, biotite, pyroxene, hornblende, epidote, chlorite, and many others. Therefore, these minerals dissolve actively in these conditions, and thus are a source of not only iron, but also other elements. Concurrently the waters studied are in equilibrium with montmorillonite, illite, kaolinite and

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other clays, and calcite, siderite and partially rhodochrosite. Therefore, these minerals cannot be a source of iron. They are formed in the aqueous medium, and do not dissolve. Further down the section, the proportion of Fe to be associated with secondary minerals increases and the content in the water is reduced accordingly as the water becomes saturated with hydroxides, carbonates, clays and Fe. In addition, the amount of dissolved organic matter decreases with decrease in depth, and this hinders the accumulation of Fe in solution.

Table 1. Chemical analyzes of typical groundwater from the Sredneobskoy basin.

No of sample

pH Σ* HCO3– SO4

2– Cl– Ca2+ Mg2+ Na+ K+ Fe3+ Fe2+ DOC

- mg/l mgО2/l Bog water

5 4,8 18 5,9 5,2 1,6 3,4 0,1 1,7 1,0 1,5 1,1 63 17 4,1 41 0,5 5,0 4,7 2,0 2,4 1,5 0,4 1,9 2,5 113 26 4,6 66 13 7,3 10,2 11,1 3,8 0,1 0,1 8,8 3,7 46 29 5,5 122 12 0,5 28,4 20,0 48,8 2,5 0,4 3,2 0,1 46

Neogene-Quaternary aquifer 19 6,7 228 171 4,2 1,0 20,0 8,5 22,0 1,2 0,3 10,0 3,8 99 7,2 308 238 0,8 1,1 46,0 11,0 11,0 1,0 0,4 8,9 1,5

100 6,3 291 220 0,5 5,2 46,0 12,2 6,7 0,7 0,5 15,0 3,0 120 6,8 183 133 1,1 5,6 27,7 7,1 7,0 0,8 0,3 18,0 5,6 123 6,8 278 177 0,8 7,1 42,0 12,1 9,6 1,1 0,3 14,7 32,0

Paleogene aquifer 12 6,9 675 512 3,2 6,2 126,0 20,7 6,0 1,3 1,5 8,0 4,3 26 6,9 469 305 1,7 43,7 66,0 14,6 38,0 1,6 3,0 7,0 3,1 98 6,7 550 317 10,6 78,0 116,0 15,9 11,0 1,6 0,3 22,1 4,5

110 7,0 622 458 6,9 11,4 120,0 15,9 9,0 1,0 0,3 10,0 2,8 116 7,1 418 311 4,9 0,8 78,0 13,4 9,0 0,8 1,8 9,8 2,7 126 7,2 540 369 4,5 9,6 88,8 13,4 7,5 1,9 1,9 18,1 10,7

Cretaceous aquifer 1 7,2 886 634 11,3 22,4 116,0 35,4 65,0 2,2 0,3 0,8 1,5 6 7,2 1062 744 2,4 51,1 126,0 40,3 95,0 2,7 0,2 2,0 2,0

23 8,4 836 488 1,2 74,6 2,4 1,0 245,0 0,6 0,2 0,8 0,5 44 8,1 2652 402 0,1 1266,6 46,0 20,7 900,0 6,0 0 0,1 1,0

Aluminum silicate rocks are also a source of iron in the bog water of the region, but they are an

indirect source, as they undergo a concentration stage in plants prior to that. The large amount of organic material contributes to the accumulation of Fe3 + and Fe2 + in the form of organo-mineral complexes. Bog waters, due to the low salinity and pH, as well as the high content of organic matter, are not in-equilibrium with respect to calcite and siderite in addition to the primary aluminum silicate rocks. This ensures a higher capacity for concentration of this element in comparison to the underlying water.

4 Conclusion

Thus, fresh groundwater in Western Siberia, including water of Neogene-Quaternary, Paleogene and Upper Cretaceous sediments, almost universally contain high concentrations of Fe. It mostly comprises calcium bicarbonate waters rich in manganese and organic matter, in addition to iron, but is low in free O2. The maximum concentration of iron typically occurs in the first two aquifers at depths of about 120 m. A favorable geochemical environment is created here. Aluminum silicate rock is also a source of iron,

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namely, the primary minerals, which in these circumstances are extremely soluble. and therefore not in equilibrium with respect to the release of iron, calcium, magnesium, sodium and other elements.

Fig. 3. Equilibrium diagram of calcium (a), magnesium (b), sodium (c), potassium (d), calcium-sodium (e) and iron (f) minerals with: 1- bog water 2- groundwater: a- Neogene-Quaternary deposits, b- Paleogene deposits, c- Cretaceous deposits in the region.

Acknowledgements

This work has been supported by the Russian Foundation for Basic Research, Grants No 11-05-93112, 11-05-98016 and Federal Target Program No 11.519.11.6044.

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