Geological Milieu of Potentially Toxic Elements in Groundwater: Case Study of Dubravica (Braničevo District, Serbia)

 

Miloš B. Rajković1, Mirjana Stojanović2, Slađana Milojković3, Lazar Kaluđerović4, Melina Vukadinović2, Jelena Popović–Đorđević1

 

 

1 University of Belgrade, Faculty of Agriculture, Department of Chemistry and Biochemistry, Nemanjina 6, 11080 Belgrade, Serbia
2 Institute for Technology of Nuclear and Other Mineral Raw Materials (ITNMS), Franše d'Eperea 86, 11000 Belgrade, Serbia
3 Agricultural School with boarding school "Sonja Marinković", Ilije Birčanina 70, 12000 Požarevac, Serbia
4 University of Belgrade, Faculty of Agriculture, Department of Pedology and Geology, Nemanjina 6, 11080 Belgrade

 

Abstract

Toxic and potentially toxic elements may be present in water due to the natural processes and/or human activities. In the present work, selected physicochemical parameters and twenty-one major and trace elements in well water used for human consumption were studied in the Braničevo District (Northern–Eastern Serbia). The content of elements was determined using an indirect analysis method based on scale tests. The results obtained for Cr, Fe and Pb indicated their presence above the allowable values. Additionally, U was detected in the studied samples. The presence of these elements can be associated with mining activities and the mineral composition of the study area. The present measurements create a database and contribute to a better understanding of the overall distribution of elements and their behavior in the given site as well as in the surrounding area.

Keywords: groundwater, drinking water, potentially toxic elements, geological background.

Introduction

 

Water is one of the most treasured natural resources. In recent decades, the downward trend in the quality of water and pollution with toxic elements have caused serious threats to human health as well as to the environment. Potentially toxic elements (PTE), which are mostly metals or metalloids, represent the most important persistent inorganic pollutants (Heltai et al., 2018). These elements can be very harmful even at low concentrations when ingested over a long period. Trace elements are defined as elements that are present in low concentrations (mg kg-1 or less) in most land plants and living organisms. They are present in natural waters and their sources are related to natural processes and/or human activities. Chemical weathering of rocks and soil leaching are the main natural processes that contribute to the presence of trace elements in waters. Additionally, anthropogenic sources of trace elements in waters include mining of coal and mineral ores and manufacturing and municipal waste waters. Most trace metals do not remain in soluble forms in water for longer periods. They are present mainly as suspended colloids or are fixed with organic and mineral substances (Alloway, 2012). Copper (Cu), zinc (Zn), manganese (Mn), iron (Fe), molybdenum (Mo) and boron (B) are trace elements that are essential for plant growth. Besides B, these elements are heavy metals, and are toxic to plants in high concentrations. Other trace elements such as cobalt (Co) and selenium (Se), are not essential for plants, but are of physiological importance for animals and humans. Cadmium (Cd), lead (Pb), chromium (Cr), nickel (Ni), mercury (Hg) and arsenic (As) have toxic effects on living organisms and are considered contaminants (He et al., 2005). Uranium is naturally present in various concentrations. In drinking water most of U comes from the erosion of natural deposits (Alloway, 2012). However, because of the problematic environmental pollution in all the countries in the world, some elements enter the environment more than they should and cause severe damage (He et al, 2016).

In Braničevo District, the register of all existing and potential groundwater sources for public water supply exists. Most interesting are the potential groundwater sources in alluvial plain sediments such as those present in the Kostolac area (Figure 1) (Anonymous, 2001). Beside deposits of natural gas, which are located at the contact between schists and younger sediments, all other useful minerals are geologically related to Neogene and Quaternary sediments. Coal deposits are located in the Kostolac Basin and Kovin depression. They are geologically related to kaspibrakish sediments of the Upper Pontian. In the Kostolac Basin, coal covers a 130 km2 surface area with an average thickness of approximately 150 m. Water from sandy-pebble sediments of Quaternary age are mostly sulfate-hydrocarbonate, with NaCa and MgCa-sulfate-hydrocarbonate type. Total mineralization varies from 450 to 750 mg L-1. Fe content (and Mn in some locations) is elevated in a significant number of locations including the area around Kostolac Island (Anonymous, 2001). As indicated in our previous study, the quality of well water in Dubravica village, which was part of the study area, was disturbed due to heavy rainfall (Milojkovic et al., 2016).

The aim of this preliminary study was to: 1) analyse the selected physicochemical parameters in well water, and 2) determine the content of major elements (K, Na, Mg, Ca and Al) and trace elements (As, Cd, Co, Cr, Cu, F, Fe, I, Mo, Mn, Ni, Pb, Se, Si, U and Zn) using an indirect analysis method based on scale tests. The results were compared to the maximum allowable concentrations (MAC) proposed by the Regulation on the hygienic quality of drinking water of the Republic of Serbia (Official Gazette, 1999), mandatory values (I) set by the European Union Environmental Protection Agency (EU EPA) (EPA, 2005) and World Health Organization (WHO) guideline values for some physicochemical parameters in drinking water (WHO, 2008). The presence of potentially toxic elements in respect to the geology of terrain, mining activities and mineral composition was considered.

 

Materials and Methods

Individual wells in settlements located in the Municipality of Požarevac, represent some of the main water sources supplying the local population (Anonymous, 2012). The present study was conducted in Dubravica (Braničevo District, Northern-Eastern Serbia). The village is located north-west of the city of Požarevac, with coordinates 44° 41' N /21° 4' E, at 75 m above sea level in the wider Danube coastal zone and belongs to the Municipality of Požarevac, Figure 1.

 

fig01
Figure 1: Geographical position of Dubravica within the Braničevo District.

 

Water samples were collected from wells, which serve as the main drinking water supply in the village. Waste waters are mainly channeled through septic tanks (Anonymous, 2012). Sampling was conducted in compliance with the Regulations on hygienic safety for drinking water (Official Gazette, 1999). Sample I (S-I) was taken from the location in the center of a densely populated village, while sample II (S-II) was taken from the periphery of the village, at a mutual distance of about 1 km. Samples I and II are located approximately 2.5 km and 3.5 km from the Danube, respectively. Water samples from both wells were taken at a depth of ~12 m.

 

Determination of Physicochemical Properties of Water and Scale Analysis

The pH value, the total organic matter content, and conductivity were determined as described in our previous work (Kostić et al., 2016). Briefly, the content of total organic matter was determined by titration with a potassium permanganate in acid solution (Kubel-Tiemann method). Conductivity was measured using a conductometer (WTW inoLab COND Level 1, WTW GmbH & Co. KG, Weilheim, Germany). The pH of each sample was measured by inserting the probe into the water immediately after collection (potentiometric, ion-selective electrode (ISE). Nitrate, nitrite, ammonia and chlorides were determined following the procedures given in the Laboratory manual (2012). Content of nitrates and nitrites were determined by ion chromatography (IC), while for the content of chlorides, an ion-selective electrode (ISE) was used. The concentration of ammonia was determined by spectrophotometry (VARIAN Cary 1 UV-visible spectrophotometer, Varian Australia Pty Lt.).

The analysis of major elements (K, Na, Mg, Ca and Al) and trace elements (As, Cd, Co, Cr, Cu, F, Fe, I, Mo, Mn, Ni, Pb, Se, Si, U and Zn) in water was performed using an indirect analysis method based on a scale test (Rajkovic et al., 2008). For the examination of element content, the scale from a home water heater, formed by deposition of inorganic nonvolatile substances in drinking water over a long period of time, was used. Detailed analysis of scale by X-ray diffraction method is described in our previous work (Rajković et al., 2017).

 

Results and Discussion

Physicochemical Analysis

The knowledge of the physical, chemical and biological parameters of water are very important for determining water quality and type. WHO guidelines state that drinking water should be clear with no color, odor and taste, and without the presence of pathogens and other toxic chemicals (Pantelić et al., 2017).

The results of typical physicochemical parameters of well water samples (S-I and S-II) and maximum allowable concentrations (MAC) are shown in Figure 2(a-b). pH value is a very important water parameter due to its influence on the chemical and biological properties of water. For the analysed samples pH values were 7.29 (S–I) and 7.21 (S–II), which is within the values recommended by national and EU EPA regulations (Official Gazette, 1999, EPA, 2005). The results indicated that the water samples were neutral to weakly alkaline.

The threshold limit value for ammonium in drinking water is 0.1 mg L-1 according to the national regulation (Official Gazette, 1999). Concentration of ammonia in both water samples was below the limit of detection, Figure 2a. WHO has not established a guideline value for ammonium, for the reason that its occurrence in drinking-water is at low concentrations (well below those at which toxic effects may occur) (WHO, 2008).

According to WHO guidelines for drinking water quality, nitrate and nitrite belong to chemicals that are of health significance in drinking water. Nitrate (NO3) is naturally present in the environment and is of great importance for plants as a nutrient. Its concentration in all plants varies and is a part of the nitrogen cycle. On the other hand, nitrite (NO2) is not usually present in significant concentrations except in a reducing environment and can be formed by the microbial reduction of nitrate. Nitrate is one of the chemicals of greatest health concern in some natural waters. It can reach both surface water and groundwater as a consequence of agricultural activity (including excess application of inorganic nitrogenous fertilizers and manures), from the disposal of wastewater and from the oxidation of nitrogenous products in human and animal excrements, including septic tanks. The contamination of some ground waters with nitrate may also be a consequence of leaching from natural vegetation. In our study, both NO3 and NO2 were within permissible values set by the national regulations, Figure 2a (Official Gazette, 1999). The obtained results indicate no agricultural activity in the vicinity of the wells.

 

fig02
Figure 2: The results of physicochemical analysis of well water (S-I and S-II) and maximum allowable concentrations (MAC); (a) nitrate, nitrite, ammonia and total organic matter (mg L-1), (b) chlorides (mg L-1) and conductivity (μS cm-1).

 

The organic substances present in the water may be naturally present in water due to the geology of the terrain. The content of total organic matter in both samples was below MAC, Figure 2a (Official Gazette, 1999). Chlorides in drinking water come from natural sources, sewage and industrial wastewater, or urban runoff containing de-icing salt. No health-based guideline value is proposed for chloride in drinking-water. However, high concentrations (>250 mg L-1) of chloride give a salty taste to water (WHO, 2008). In analyzed samples the content of Cl- was within the permissible value, Figure 2b (Official Gazette, 1999).

The conductivity in natural unpolluted waters usually ranges from 10 to1000 μS cm-1 (Kostić et al., 2016). The most common ions that contribute significantly to conductivity are H+, NH4+, Na+, Ca2+, Mg2+, K+, Al3+, Fe2+ F-, Cl-, HSO4-, SO42-, HCO3-, CO32- and NO3- (Blaine McCleskey et al., 2011). As it could be observed, values for conductivity in the studied water samples (644/703 μS cm-1, S–I and S–II, respectively) were below MAC in drinking water (Figure 2c) which indicates no presence of polluting inorganic substances in ionized form (Official Gazette, 1999). Nevertheless, the content of Ca and elevated concentrations of Fe probably contributed to the resulting conductivity values.

The National Recommended Water Quality Criteria (US EPA) which deals with toxic chemicals and specific pollutants in water, categorizes Cd, Cr, Cu, Ni, Pb, Se and Zn as priority pollutants, whereas Al, As, B, and Fe as non–priority pollutants (USEPA, 2016).

The results of the analysis of a scale developed from the well water (S–I and S–II), are shown in Figure 3. The limit values (I and MAC) according to EU EPA, regulations, and WHO guideline values (Gl) in drinking water for the detected elements are given in Figure 4. Among the twenty-one examined elements, As, Co, F, I, Mo, Ni, Se and Si were not detected. Aluminium, copper, cadmium, manganese and sodium were measured in concentrations that were within MAC, EU EPA mandatory values (I) and WHO guideline values (Gl) (Official Gazette, 1999; EPA, 2005; WHO, 2008).

 

fig03
Figure 3: The results of element analysis (μg L-1) in water (S–I and S–II):  (a) Al, Cu, Cd, Cr, Mn, Pb and U; (b) Mg, Ge, K, Na, Zn and Ca.

 

Mercury, cadmium and their compounds are on List I (so-called the 'black list'), whilst Zn, Cu, Ni, Cr, Pb, Se, As, Sb, Mo, Ti, Sn, Ba, Be, B, U, V, Co, Tl, Te and Ag are on List II (so-called the 'gray list') of the Dangerous Substances Directive (EPA, 2005). Calcium, potassium, magnesium and zinc were below the maximum allowable concentration in drinking water (Official Gazette, 1999). Concentrations of Cr, Fe and Pb exceeded the limit and guideline values established by national and EPA regulations.

WHO health-based guidelines are not established for Al, Na, Fe and Zn; for Al it is due to the restrictions in the animal data as a model for humans. However, some recent studies indicate a connection between some neurological disorders in humans and their exposure to aluminium (Shaw and Tomljenovic, 2003; Ma et al., 2016). In spite of that, the manageable levels based on optimization of the coagulation process in drinking-water, plants using aluminium-based coagulants are specified to ≤0.1 mg L-1 and ≤0.2 mg L-1, in large and small water treatment facilities respectively. The reason for not inlcuding Na, Fe, and Zn in WHO guidelines is that these elements are not of health concern at concentrations normally observed in drinking-water. Nevertheless, Na and Zn can affect the acceptability of drinking water (WHO, 2008).

Iron is a fundamental nutrient for cell growth and metabolism, and plays a key role in the active sites of certain enzymes. On the other hand, the recent studies reveal that iron could be associated to incidence of colorectal, liver, breast and lung cancers. Considering the dual nature of iron, maintaining an optimal iron level in the body is important (Davoodi et al., 2016). The concentrations of Fe in S–I and S–II were 1830 and 870 μg L-1, respectively. These concentrations exceeded MAC and I values 3 to 9.3 times (Official Gazette, 1999; EPA, 2005).

Chromium and lead are known as elements with a high-level of toxicity and even low levels of exposure to these elements may cause damage to different organs. According to the United States Environmental Protection Agency (U.S.EPA), and the International Agency for Research on Cancer (IARC), these metals are also classified as either "known" or "probable" human carcinogens based on epidemiological and experimental studies showing an association between exposure and cancer incidence in humans and animals (Tchounwou et al., 2002).

Accordingly, they have high priority when public health is discussed. The limit values of Cr and Pb in drinking water are 50 and 10 μg L-1, Figure 4c (Official Gazette, 1999; EPA, 2005; WHO, 2008). In well water S–I and S–II concentrations of Cr were 54.50 and 4.04 μg L-1, respectively, which indicated the elevated content of this element in the well located in the center of the village. On the other hand, a significanty elevated concentration of Pb (78.33 μg L-1) was detected in the well (S–I) located on the periphery of the village, Figure 3a.

The presence of U in drinking water and food consequently results in human ingestion of a few micrograms (mg) of this element daily. Raised concentrations of U are mostly found in ground waters accompanying uranium-rich rocks (Alloway, 2012). Although U mostly originates from food (77%), some studies point out that the presence of U in drinking water can contribute to its daily intake 31–98%. However, only a small portion of U intake (10.3–13.8%) is bio-available and will deposit in the human body, predominantly in bones (Larivière et al., 2012). Health effects of uranium are of a carcinogenic and non-carcinogenic nature. Uranium has been identified as a nephrotoxic metal in humans and animals, but still an inferior nephrotoxin when compared to cadmium (Cd), lead (Pb), and mercury (Hg), described as conventional neurotoxic metals (USEPA, 2000; Yadav, 2014). The US Environmental Protection Agency has proclaimed a drinking water Maximum Contaminant Level (MCL) 30 μg L-1 in 2003, as the value that may be protective against kidney toxicity (WHO, 2008).

According to Regulation on hygienic quality of drinking water of the Republic of Serbia, MAC for U is not defined in drinking water from public water supply systems, while in bottled natural drinking water MAC is 50 μg L-1 (Official Gazette, 1999). The EU EPA imperative value for U in drinking water is not defined, while the Directive on ground water puts U on the list of dangerous substances which have detrimental effects on the aquatic environment (EPA, 2005). The WHO guideline value for U is 15 μg L-1, taking into consideration only the chemical aspects of uranium. Nevertheless, concentrations of U measured in wells S–I/S–II (3.71 and 1.56 μg L-1, respectively) should be taken into account when the health of the people who drink the well water is considered.

 

fig04
Figure 4: The limit values for elements: (a) Na, K, Mg and Ca (μg L-1); (b) Al, Cu, Fe and Zn (μg L-1); (c) Cr, Cd, Mn, Pb and U (μg L-1).

I – Imperative values according to the EU EPA Drinking Water Directive;
Gl – Guidelines for drinking-water quality WHO;
MAC – Maximum allowable concentration in drinking water by Regulation on hygienic quality of drinking water of the Republic of Serbia.

 

Geological Background and Mineral Resources

The Bela Crkva Sheet is located between 44 °40'–45 °00' N latitude and 21 °00'–21 °30' E longitude. Most of the area belongs to South Banat, and smaller part belongs to Požarevac and the Republic of Romania (Rakić, 1984). The location where samples were collected lies on alluvial terrace sediments which were deposited in the Quaternary (Figure 5). These sediments of alluvial terrace with 7–12 m relative heights were found on the left bank of the Danube, between Kovin and Gaj and on right banks of the Great Morava and Mlava, downstream of Dubravica and Batovac.

 

fig05
Figure 5: Generalized geological map of the Bela Crkva Sheet; Qa – Alluvium and proluvium; Qp – Bolic sand; Qt – River terrace; Ql – Loess formations. Qob – Organogenous march sediments; Qd – Piedmont deluvial-proluvial curtain. PI – Pliocene. M3 – pper miocene. Sco – Crystalline  schist of low metamorphism.

 

The vertical profile of the terrace consist of two units: lower - with gravels and sands, and upper - with silts made by flooding. The lower unit belongs to riverbed facies, and the upper unit to flood facies. The lithology of riverbed facies was represented by coarse and medium sized gravels and sands with 4–8 m thickness. Silts could rarely be found in this facies. Psefites have been deposited in one or two cycles. They are made of medium sized, fine to medium rounded pebbles of quartz, chert, green schist, sandstone, serpentinite, limestone, and volcanic rocks. On the other hand, psammites were found in the form of lenses, with a sand component of up to 85.5–100.0%. Coarse and medium grain size sands are dominant. Occurrences of alevritic sands and silts, which lie over gravels, probably represent equivalents of former oxbow facies.

Flood facies are made of alevritic sands and sands, with a 40.0–96.0% sand component, a 4.0–54.5% silt component and a 5.5–15.0% clay component. Coarse psefites are very rare. Sediments are poor in CaCO3 (0.0–5.82%), and extremely badly sorted (So=2.06–4.47). The lower part of the facies was characterized by lenses and intercalations with Planorbis planorbis, Planorbis carinatus, Planorbis corneus, Limnaea palustris, Galba truncatula, Clausillia dubia and the upper part with Discus ruderatus, Euconulus fulvus, Punctum pygmaeum, Carychium tridentatum, Cochlicopa lubrica, Perferatella bidentata, etc. Biofacial analysis of alluvial plains of the Danube, Great Morava and Mlava rivers, show a slow transformation from a gravelly river bottom, over a periodically flooded accumulative plain, to a fully dried river terrace. Subsequently hydro-chemical processes occurred.

Alluvial sediments deposited in the Holocene were represented by riverbed facies, flood facies and oxbow facies. Oxbow facies consist of alevritic sands, alevrites, alevritic clays, and peat with fauna that indicates a steady water environment. It was formed in abandoned river beds of the Danube and Great Morava. Marsh sediments, as well as other alluvial sediments, contain a significant amount of garnets (24.1–40.0%), but they differ in epidote content (20.8–26.8%). Light fractions contain a medium content of quartz (21.6%). The sediment thickness was 3–4 m. Flooding facies cover the entire alluvial plane. These facies are characterized by finer sediments (0.11–0.21mm), laminations, horizontal stratification, marsh fauna and a high content of CaCO3 (11.82–24.66%). These facies contains alevritic sands and sandy alevrites with small lenses of fine grained gravel. Sediments contain a high content of micas (more than 23.4%) and altered minerals (16.8–61.4%). The thickness of the alevrites is between 4–12 m in alluvial deposits of the Danube and 2–5 m and in alluvial deposits of the Great Morava and Mlava rivers (2–5 m).

The riverbed facies is developed on the entire river profile. It was made of gravels and sands with 5–20 m thickness. Sand component is dominant (85.5–100.0%), mostly made of coarse sands. Psammites occur in form of poorly sorted lenses, with low CaCO3 content (0.0–6.2%). Sediments are rich in garnets (9.0–61.8%) and amphiboles (13.4–28.5%), while epidote (6.5–29.2%) and metallic minerals (9.7–27.1 %) occur in smaller amounts. Quartz is dominant in light fraction (49.0–69.9%). Psefites are medium to coarse grained, made of pebbles of quartz, chert, quartzite, metamorphic rocks, red sandstones, volcanic rocks, etc., originating from the Carpathian Mountains and the Great Morava drainage basin (Rakić, 1984).

Coal deposits of the Kostolac Basin and Kovin depression are located between the lower Pontian sands and Quaternary sediments. The deposit is slightly inclined towards the north-west. Coal is classified as lignite. Lignite is mined in the Kostolac area in the form of outcrop. This basin is made up of several outcrop fields which are used following exploitation for successive deposition of tailings and drainage water of newly formed fields. This practice corresponds to most of the ecological requirments for the minimization of negative impacts to the environment. However, further development of lignite exploitation leads to serious modification of the surrounding groundwater regime. Water quality of artesian and sub artesian aquifers in Pliocene sediments, which are used as a water supply in South Banat are almost always high in Fe content and humic matter, up to 1 mg L-1, so the water exhibits a yellow color (so-called yellow water). Total mineralization of water is high in South Banat (2000 mg L-1) (Anonymous, 2001).

Chromium is a polyvalent, litophile element by geochemical character. In the environment without oxygen it demonstrates a chalcophile character. In rocks and ore deposits it is closely associated with Al, Fe, Ni and Mg. Cr3+ exhibits characteristics similar to the Fe3+ ion, so isomorphic substitutions can occur. Chromium is abundant in ultramafic igneous rocks such as dunites and peridotites. In exogenic conditions, in contact with water and oxygen, it can undergo a physicochemical transformation which can release it from primary minerals. Primary minerals are resistant to chemical transformation so they concentrate in deposits. Chromium is in the form of about 30 minerals. Chromspinels like chromite, magnochromite and alumochromite are the most important industrial sources of this element (Jelenković, 1999).

From a geochemical aspect, Fe is a siderophile, lithophile and chalcophile element, but also a biophile element. In nature it occurs as Fe2+ and Fe3+. Usually Fe2+ occurs in endogenous conditions while Fe3+ is more related to exogenous conditions. In lithosphere, the average amount of Fe is around 5%. In igneous rocks it is more abundant in ultramafic rocks and its content decreases as acidity increases. Usually it is associated with Cr, V, Ti, Ni and Co. These elements have similar ionic radiuses, so they can isomorphically substitute one another. In nature, Fe2+ is more connected with Mg, Ni, Co, Mn and Fe3+ with V, Ti, Cr and Al. In metamorphic environments in which the Eh value is low, Fe in Fe2+ forms in minerals such as sulfides, while in environments with high Eh values it is in Fe3+ form in mineral hematite. Magnetite and ilvaite could be formed when Eh conditions are somewhere in between. In exogenous conditions it can be dissolved, transported and precipitated in the appropriate environment, depending on pH and Eh conditions, CO2 and bacteria presence in water, etc. In an acidic environment and reduction conditions, Fe can shift into solution and precipitate as hydroxide in oxidation conditions and in alkaline solutions. Only when pH is lower than 3, Fe3+ can remain in solution. Otherwise it precipitates as hydroxide. On the other hand, Fe2+ can remain in solution in a pH greater than 7. More than 450 minerals of Fe exist in nature, the most important of which are magnetite, hematite, limonite, siderite, chamosite, pyrite and pyrrhotite (Jelenković, 1999).

Uranium is a heavy metal, found in nature and its isotopes 238U, 235U and 234U have a relative abundance of 99.28%, 0.72% and 0.0055%, respectively. It has both chemotoxic and radiotoxic properties, but the fact that all isotopes are unstable with half–lives (T1/2) between 2.48×105 and 4.51×109 years, depicts U as weakly radioactive. Nevertheless, it is potentially hazardous to ecosystems and human health. As a α particle emitter, 238U decomposes over an 18 member 'uranium' or 'radium' series of elements which ends with the stable 206Pb. Uranium is naturally present, in various concentrations, in deposits such as black shales, coal, phosphorites, certain sandstones and some limestone formations. Use of phosphate fertilizers, combustion of coal and other fuels also contribute to its presence in the environment (Alloway, 2012; Yadav, 2014; Sharma and Singh, 2016). Its primary minerals are zircon, apatite, uraninite, and secondary carbonates, phosphates and vanadates. Uraninite is a mineral usually found in pegmatite, but hydrothermal veins with sulfide and arsenide minerals usually contain massive pitchblende. Its most important property is high concentration in residual solutions, high concentration in sedimentary rocks together with vanadium and in coals (Jelenković, 1999). U6+ and U4+, are the major forms of U in oxic surface waters and in anoxic waters, respectively (Waseem, 2015).

Lead is relatively abundant metallic chemical element in the Earth's crust, and is contained in more than a hundred minerals; mostly sulfides and sulfosalt minerals, carbonates and sulfates. Lead usually forms minerals in hydrothermal veins in association with Zn. Lead is not a very mobile element, due to co-precipitation with limonite, forming secondary minerals such as jarosite and anglesite, as well as other carbonates, phosphates and oxides. It can be adsorbed in clays. Maximum amounts of lead can be found in groundwater with pH<5.5 (Jelenković, 1999).

 

Conclusions

The results of the scale analysis developed from the well water supply centers showed a substantial increase in the content of priority pollutants Cr (54.50 μg L-1) in the center village well, and Pb (78.33 μg L-1) in the periphery village well. Concentration of Fe as a non–priority pollutant was elevated in both wells (1830 and 870 μg L-1). In addition, these wells contained U in concentrations of 3.71 and 1.56 μg L-1. Due to the fact that these elements are of high toxicity in addition to being carcinogenic as well as non-carcinogenic in nature, the attention should be paid to well water used for water supply. The coal deposits of the Kostolac Basin depression and continuous development of lignite exploitation may lead to a serious modification of the surrounding groundwater regime.

 

Acknowledgements

This work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Grants Nos. TR31003, 43007 and 46009). The authors thank D. Popović-Beogračić for the design and preparation of figures.

 

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