2019 | 72/2 | 145–162 | 10 Figs. | 4 Tabs. |  www.geologia-croatica.hr
 Journal of the Croatian Geological Survey 
and the Croatian Geological Society 

1. INTRODUCTION
In Serbia to date, encrustations have frequently been noted on the 
technical elements of mineral water wells (wellheads, screens, 
pumps, discharge pipes, and the like) (LAZIĆ, 2004), as well as 
within zones of emergence (PROTIĆ, 1995; ŠARABA et al., 
2017).

Encrustations are typical only of the upper part of the well 
casing, caused by a higher flow velocity in that region (HOUBEN 
& WEIHE, 2010).

Encrustations are precipitates of salts and alkalis, which most 
frequently takes place in mineral waters from aquifers with dis­
solved calcium, magnesium and iron. Precipitation is the result of 
a change in ambient conditions (pH, temperature, pressure, solute 
concentration) due to mineral water pumping, which causes super­

 
Macroscopic, optical and diffraction assessment  
of encrustations and SEM analyses of phototrophic  
microbial mats from wellheads and select zones of emergence  
of mineral water in Serbia

Vladimir Šaraba1*, Slađana Popović2, Vesna Obradović3, Jana Štrbački1, Violeta Gajić4,  
Predrag Vulić4, Gordana Subakov Simić5 and Olivera Krunić1

1  University of Belgrade, Faculty of Mining and Geology, Department of Hydrogeology, Đušina 7, 11000 Belgrade, Serbia;  
(*corresponding author: vladimirsarabaa@gmail.com)

2  University of Belgrade, Institute of Chemistry, Technology and Metallurgy, National Institute, Department of Ecology and Technoeconomics, Njegoševa 12,  
11000 Belgrade, Serbia

3  Jaroslav Černi Water Institute, Jaroslava Černog 80, 11000 Belgrade, Serbia
4  University of Belgrade, Faculty of Mining and Geology, Department of Mineralogy, Crystallography, Petrology and Geochemistry, Đušina 7,  

11000 Belgrade, Serbia
5 University of Belgrade, Faculty of Biology, Department of Algology, Mycology and Lichology, Studentski trg 16, 11000 Belgrade, Serbia

doi: 10.4154/gc.2019.09 

Abstract
Investigations focusing on wellhead encrustations within select zones of emergence of mineral 
water, in different hydrogeological provinces, were conducted from 2014 to 2017 in Serbia. They 
included: well BB-1 in Bogatić (Inner Dinaric Alps of western Serbia), wells LB-4 and LB-5 in 
Lukovska Banja (Šumadija-Kopaonik-Kosovo Province), and wells VG-2 and VG-3 in Vranjska 
Banja (Serbian Crystalline Core). The studied occurrences belong to the group of hot mineral 
waters, with temperatures ranging from 62°C to 93.9°C, and total dissolved solids (TDS) from 
600 to 1267 mg/L. They are mildly acidic to mildly alkaline (pH 6.6 – 8.0) and their genetic types 
are HCO3

--Na++K+ (Bogatić and Lukovska Banja) and SO4
2-, HCO3

--Na++K+ (Vranjska Banja). 
Macroscopic, optical and diffraction assessment revealed that the encrustations comprised cal-
cite and aragonite minerals, along with some local quartz, muscovite, albite and clinochlore. The 
conclusion was that there was a predominant presence of calcium carbonate in all the occur-
rences, represented by crystalline calcite grains, characteristically birefractive, and dark, semi-
translucent grains in the form of microcrystalline calcite highly stained by iron hydroxides 
(Bogatić), as well as crystalline calcite with radially developed carbonate - aragonite (Lukovska 
Banja) and rod-like and filamentous aggregates of aragonite and calcite, highly translucent in 
places (Vranjska Banja).The texture of the samples exhibits alternating horizontal or wavy car-
bonate laminae. In Vranjska Banja, they include thin micritic to medium crystalline laminae and 
laminae composed of aggregates of aragonite needles in the form of bunch. In Bogatić and Lu-
kovska Banja, there are thick micritic and highly porous laminae, 0.025 to 1 mm thick, or inter-
layers of a coarser crystalline structure. Apart from encrustations, all the occurrences exhibit 
phototrophic microbial mats, which were analyzed by scanning electron microscopy. The onset 
and progress of encrustation are primarily influenced by the physical and chemical composition 
of the mineral water (T, pH, TDS, HCO3

-, Ca2+, CO2, etc.), as corroborated by hydrogeochemi-
cal modeling software PHREEQC 3.4.0. In addition, phototrophic biofilms might be contributing 
to the creation and accumulation of minerals in the encrustations.

saturation of the solution and the formation of encrustations (PAPIĆ, 
1999; HOUBEN, 2000; HOUBEN, 2001; GARCIA et al., 2005; 
de ZWART, 2007; DRAGIŠIĆ & POLOMČIĆ, 2009). Encrustation 
of the technical components of wells is non-uniform (HOUBEN, 
2006), with more intensive precipitation on smooth than rough 
surfaces (DRISCOLL, 1986; HOUBEN & TRESKATIS, 2007).

The main drivers of encrustation formation (PAPIĆ, 1999; 
VAN BEEK, 2011): (i) dissolution, creating suspended substances, 
(ii) transport of dissolved substances, and (iii) precipitation of 
dissolved substances. 

The hydrodynamic parameters of mineral water, which faci­
li tate encrustation, include changes from laminar to turbulent 
flow and discharge via an overflow or groove (PAPIĆ, 1999; 
HOUBEN & TRESKATIS, 2007).

Article history:

Manuscript received October 26, 2018 
Revised manuscript accepted February 15, 2019 
Available online June 4, 2019

 

 
 
 
 
 
 
 
 
 

 
 
 
 

 
Keywords: mineral water, encrustations, 
phototrophic microbial mats, macroscopic and 
optical assessment, SEM analyses, X-ray powder 
diffraction, hydrogeochemical modeling



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Encrustations often comprise chemical compounds of cal­
cium, silicon, manganese, iron and phosphorus (HOUBEN, 2000; 
HOUBEN, 2001; HOUBEN & TRESKATIS, 2007; WOYESSA, 
2012). Calcium carbonate encrustations are the most frequent 
type encountered in mineral water extraction (BROWN, 2013). 
PROTIĆ (1995) reported similar findings, pointing out that cal­

cium carbonate encrustations in the form of calcite veins, traver­
tine, and calcite interactions in the soil and sedimentary rocks are 
often found in places of emergence of mineral waters in Serbia, 
corroborated by examples of select mineral water occurrences in 
Serbia where calcium carbonate encrustations have been de­
tected.

Figure 1. Schematic representation of the studied mineral water sites on the hydrogeological map of Serbia, including geologic columns (FILIPOVIĆ et al., 2005; 
modified).
Geological legend: 1 – Medium-grain gravel, gravelly clays, sand, sandstones, conglomerates, debris, tuffs, marls and Neogene limestones; 2 – Tertiary limestones; 
3 – Compact and fractured granodiorites; 4 – Sandstones and marls, alternating marls, marly limestones and sandstones; 5 – Sandstones, limestones and marls; 
6 – Carbonate rocks with chert, pyrite and quartz; 7 – Limestones, limestones with quartz; 8 – Diabases and spilites; 9 – Carbonate rocks with chert and pyrite; 
10 – Serpentinites; 11 – Hydrothermally altered rocks; 12 – Heterogeneous agglomeratic material; 13 – Agglomeratic, unconsolidated material; 14 – Peridotbreccias; 
15 – Chloritic rocks; 16 – Marls, occasional limestones; 17 – Schistose metamorphic rocks; 18 – Muscovite-biotite gneisses; 19 – Silicate-amphibole schists, with 
quartzite interbeds; 20  – Fault.
Hydrogeological legend: Hydrogeological provinces: I – Pannonian Plain; II – Inner Dinarides of western Serbia; III – Šumadija-Kopaonik-Kosovo province; IV – Ser-
bian Crystalline Core; V – Carpathian-Balkanide province; VI – Dacian Basin; 1 – Intergranular porosity (high conductivity); 2 – Intergranular porosity (medium con-
ductivity); 3 – Intergranular porosity (low conductivity); 4 – Hydrogeologic complex: subartesian and artesian aquifers of the Neogene Pannonian Basin; 5 – Hydro-
geologic complex: subartesian and artesian aquifers of smaller Neogene basins; 6  – Karst and karst-fractured porosity of high conductivity; 7 – Fractured porosity 
of low and medium conductivity (including parts of the terrain conditionally devoid of water); 8 – Occurences of mineral water; 9 – Hydrogeological boundary.



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The main feature of calcium carbonate is that in the presence 
of carbon dioxide it becomes a readily soluble compound. The 
solubility of calcium hydrogencarbonate is about 160 g/L. In the 
absence of carbon dioxide, the solubility of calcium carbonate 
decreases to only about 13 mg/L. In addition, calcium carbonate 
exhibits retrograde solubility – as the temperature increases, the 
solubility decreases and vice-versa (PLUMMER & BUSEN­
BERG, 1982; OGINO et al., 1987; APELLO & POSTMA, 2004).

Encrustations are almost always accompanied by various 
microbial processes (BUIK & WIILEMSEN, 2006; DEED & 
PREENE, 2015), represented by prototrophic biofilms in the se­
lected occurrences of mineral water in Serbia. This was the main 
reason why they were analyzed.

Microbial activity in phototrophic mats can intensify encrus­
tation, in which case the initial stage is small­scale encrustation, 
dominated by the biological component. Over time, minerals take 
over and become most of the encrusted mass (SCHNIEDERS, 
2003; ŠARABA et al., 2017). 

Phototrophic biofilms are considered to be the the earliest 
communities on planet Earth (TICE & LOWE, 2006; NOFFKE 
et al., 2006). The life of microorganisms in phototrophic biofilms 
is limited by numerous ecological factors. Light is the basic fac­
tor for their growth (JEMCEV & ĐUKIĆ, 2000), but also, among 
others, pH value, ionic activity and temperature are unavoidable 
(DONLAN, 2002; LI & LOGAN, 2004). In most cases, micro­
organisms survive as long as they have sufficient resources for 
development and reproduction (LAWRENCE & CALDWELL, 
1987; SZEWZYK & SCHINK, 1988; ŠARABA et al., 2017).

The study of encrustations at the selected mineral water oc­
currences in Serbia began with analysis of mineral deposits on 
the wellhead of well BB-1 at Bogatić, where the scale of encrus­
tation was such that the discharge capacity of the well decreased 
from 60 L/s, measured in 1986 when the well was installed, to 
only 37.5 L/s, determined by pumping tests in 2014. Conse­
quently, the well needed to be rehabilitated. The study was then 
extended to include a number of other wells. The objective was 
to detect and analyze encrustations and show that well BB-1 at 
Bogatić was not a solitary Serbian case. Additional sites included 
Lukovska Banja and Vranjska Banja which, like the mineral wa­
ter occurrence at Bogatić, featured extremely high temperatures 
(above 60°C) and HCO3

­ – Na+ + K+ as the basic ionic composi­
tion, along with some SO4

2- at Vranjska Banja. Apart from en­
crustations, phototrophic biofilms were detected in all the occur­
rences.

To date, encrustations of wellheads and zones of emergence 
of mineral water have not been the object of hydrogeological re­

search in Serbia. The study focused on determining the mineral 
composition of encrustations by microscopic and optical assess­
ment and X-ray powder diffraction. Another objective was to es­
tablish a correlation between the chemical composition of the 
mineral waters and the mineral composition of the encrustations 
through hydrogeochemical modeling, and to assess the propen­
sity for encrustation by prognostic analyses.

2. STUDY AREAS
The studied occurrences are characterized by different structural-
geologic, historic­geologic, petrographic­mineralogic, hydrogeo­
logical and other properties. They are located in the hydrogeo­
logical provinces of the Inner Dinarides of western Serbia (the 
occurrence in Bogatić: well BB-1), the Šumadija-Kopaonik-
Kosovo Zone (the occurrence in Lukovska Banja: wells LB-4 and 
LB-5), and the Serbian Crystalline Core (the occurrence in Vran­
jska Banja: wells VG-2 and VG-3) (Fig. 1). 

The hydrogeological provinces of Serbia have been deline­
ated based on tectonic, lithofacies, geomorphological, hydrogeo­
logical, physico-geographic and other characteristics. They coin­
cide with geotectonic units, albeit with differing boundaries, to 
greater or lesser extents (FILIPOVIĆ et al., 2005; KRUNIĆ, 
2012) (Fig. 1).

The characteristics of the studied mineral waters are shown 
in Table 1.

3. MATERIALS AND METHODS 
Investigations that focused on the encrustation of wellheads and 
selected zones of occurrence of mineral water were conducted from 
2014 to 2017 in Serbia, at Bogatić (well BB-1), Lukovska Banja 
(wells LB-4 and LB-5) and Vranjska Banja (wells VG-2 and VG-3). 
SEM analyses of phototrophic biofilms were also performed.

Consistent with the set requirements and objectives of the 
study, the materials used and methods applied corresponded to the 
scope and sequence of the investigations as shown in Figure 2.

3.1. Sampling of mineral waters
Samples of mineral water for physical and chemical analyses 
were collected in plastic bottles. The bottles were sterile and the 
caps complied with international standards ISO 9001, ISO 14001 
and OHSAS 18001. Prior to use, the bottles were rinsed and 
cleaned with a 1% HCl solution, then flushed with distilled water. 
Upon sampling of mineral water, the bottles were capped to pre­
vent leakage and then sealed with a nut. The bottles were labeled, 
indicating serial numbers and other important information, in­

Table 1. Characteristics of the studied mineral water wells (MILIVOJEVIĆ, 1989; MARTINOVIĆ & MILIVOJEVIĆ, 1998; KRUNIĆ, 1999; FILIPOVIĆ, 2003; FILIPOVIĆ et al., 
2005; KRUNIĆ, 2012; MARTINOVIĆ, 2012).

Locality
Well 

designation
Type 

of well
Depth 

of well (m)
Function Lithology

Type of aquifer and 
hydrodynamic characteristic

Yield
(L/s)

Bogatić BB-1

Drilled

470
Heating of residential 

buildings and hot houses
Triassic limestone Karst, confined 37.5

Lukovska 
Banja

LB-4 401
Health-and-recreation, 
heating of buildings;

Cretaceous limestone Karst-fractured, confined 4.7

LB-5 870
Health-and-recreation, 
heating of buildings;

Cretaceous limestone Karst-fractured, confined 4.6

Vranjska 
Banja

VG-2 1064
Health-and-recreation, 
heating of buildings;

Petrified granodiorite and gneiss- 
-granite, amphibole-biotite schist

Fractured, confined 26

VG-3 1470
Health-and-recreation, 
heating of buildings;

Petrified quartz-diorite, grandiorite, 
with schist intercalations

Fractured, confined 21



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cluding the name of the sampling site, water temperature (T), pH 
and conductivity (EC). 

3.2. Sampling of encrustations
Well ageing and the sampling points within the studied zones of 
emergence of mineral water are illustrated in Figure 3. Encrusta­
tions (Fig. 3) were sampled at Bogatić (the well BB-1), Lukovska 
Banja (wells LB-4 and LB-5) and Vranjska Banja (wells VG-2 
and VG-3), and labeled after wells: BB-1, VG-2, VG-2* and VG-
3. Sample VG-2* was part of the sample VG-2. Samples from 
wells LB-4 and LB-5 at Lukovska Banja were pulverized at the 
same time for the X-ray powder diffraction assay and labeled LB. 
Hence, a total of five samples were tested (Fig. 4).

3.3. Sampling of phototrophic microbial mats
Mat samples were collected near the points of emergence of 
mine ral water and at wellheads in Bogatić, Lukovska Banja and 

Vranj ska Banja (Fig. 3). Sampling sites with notable highly-de­
veloped and variously coloured biofilm were chosen for analysis 
by scanning electron microscopy. The samples were collected 
using a sterilized scalpel, placed in sterile plastic bottles and fixed 
with a 1.5% formaldehyde solution.

3.4. Physical and chemical analyses of mineral waters
The mineral water samples were delivered to the laboratory at the 
Public Health Institute of Belgrade, within the prescribed time­
frame, for all the required physical and chemical analyses (Tab. 2). 

The temperature (T), pH and conductivity (EC) of the mine-
ral waters were determined in situ. CyberScan CON 510 pH me­
ter was used to determine pH levels and temperatures. This in­
strument could also be used to measure total dissolved solids 
(TDS). Conductometer HI98192 was used to measure conducti-
vity. This instrument is equipped with a four-ring HI763133 con­
ductivity electrode. 

Figure 2. Algorithm of the applied methodology.



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The colour of the mineral water samples was determined 
colorimetrically (Pt-Co scale), gravimetry was used to determine 
TDS and volumetry to test the KMnO4 demand. With regard to 
macrocomponents, Na+, K+,Ca2+, Mg2+ were determined by AAS 
(Atomic Adsorption Spectrometry), HCO3

­, Cl­ by the volumetric 
method, and SO4

2- by the turbidimetric method. The concentra­
tions of NH4

+, NO3
­ and NO2

­ were established by means of UV-
VIS spectrophotometry, and silica concentrations by ICPOES 
spectrometry. The concentrations of metals and metalloids (Pb 
total, Al total, Fe3+, Ni total, Ag total, Cd total, Hg total, As total, 
Zn total) were determined by AAS. Gas analyses were performed 
to determine the concentrations of free CO2, free H2S and dis­
solved O2. The volumetric method was used for dissolved O2 and 
free CO2, while H2S was determined by AAS. 

3.5.  Petrographic-mineralogic testing of  
the encrustations

Petrographic­mineralogic testing was undertaken at the labora­
tory of the University of Belgrade/Faculty of Mining and Geo-

logy, to define the structural and textural properties of the encrus­
tations, by means of macroscopic, optical and diffraction 
techniques.

3.5.1. Macroscopic assessment of the encrustations
The samples were tested with diluted hydrochloric acid (HCl) in 
a glass, by adding 10 mL of the acid. The samples were observed 
by the naked eye and through a 20x magnifying glass.

3.5.2. Optical assessment of the encrustations
Three thin sections of the encrustations were prepared and 
viewed on a polarization microscope. A complex configuration 
of several software­interfaced devices was used, which included 
a Leica polarization microscope (model DMLSP) and Leica dig­
ital camera (model DC300).

3.5.3. X-ray powder diffraction of the encrustations
The encrustation samples (Fig. 4) were analyzed on an automatic 
powder diffractometer, Rigaku Smart Lab, under the following 
conditions: radiation from a copper anticathode – wavelength 

Figure 3. Condition of wells and points of sampling of phototrophic microbial mats and encrustations in the studied zones of occurrence of mineral water (photo: 
M. LAZIĆ, 2014-2017; V. ŠARABA, 2017). Legend: a, b. Bogatić; c, d. Lukovska Banja; e, f. Vranjska Banja.



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CuKα = 1.54178 Å, tube voltage U = 40 kV, and current I = 30 
mA. The samples were analyzed in the 3 – 70º 2Q range, with a 
0.01º degree step and a data acquisition rate of 5.0º/min. The re­
sulting diffraction maxima 2Q(º) and corresponding intensities 
(I) of each identified mineral were graphically represented. The 
crystalline phases (minerals) were identified based on intensities 
(I/Imax) and interplanar spacing (di), which were compared with 
data from the literature and ICDD PDF-2 (2016) standards.

3.6. Hydrogeochemical modeling
Hydrogeochemical modeling, using PHREEQC 3.4.0. (U.S. Geo­
logical Survey) and database phreeqc.dat (PARKHURST & AP­
PELO, 1999), and the results of chemical analyses of five mineral 
water samples, was undertaken to determine the saturation index 
(SI) of the primary minerals from the carbonate group (calcite, 
dolomite, aragonite), the oxide and hydroxide group (gibbsite, 
goethite, haematite), and the silicate group (quartz, clay mine rals, 
micas, etc.).

3.7. Prognostic analyses
Prognostic analyses included mathematical calculations of the 
Langelier Saturation Index (SILang) and Ryznar Stability Index 
(SIRyz), based on the chemical parameters of the mineral waters, 
which were used to approximate the propensity of the mineral 
waters for encrustation. 

3.8. SEM analyses of phototrophic microbial mats
Phototrophic microbial mats and microorganisms were analyzed 
in detail using a JEOL JSM-6610LV Scanning Electron Micro­
scope in the Laboratory of the Faculty of Mining and Geology at 
the University of Belgrade. The samples were coated with gold 
using a BALTECSCD-005 SPUTTER coating device and re­
corded under high vacuum conditions, after which a high­reso­
lution scanning electron image was obtained. 

4. RESULTS AND DISCUSSION
The chemical composition of mineral water is a key driver of the 
onset and progress of encrustation, given that the process is pri­
marily steered by dissolved salts.

The results of physical and chemical analyses showed that 
the mineral waters at Bogatić and Lukovska Banja are of the 
HCO3

­-Na++K+ type, and at Vranjska Banja of the SO4
2-, HCO3

­

-Na++K+ type.

Elevated concentrations of HCO3
­ and Ca2+ ions are a mine­

ral water parameter that causes carbonate precipitation (WOY­
ESSA, 2012), as was found to be the case with the studied mineral 
water occurrences, at least with respect to the HCO3

­ ion (HCO3
­ 

: BB-1 – 361.6 mg/L; LB-4: 961.0 mg/L; LB-5: 961.0 mg/L;  VG-2: 
589.0 mg/L; VG-3: 598.0 mg/L) (Tab. 2), given that the proportion 
of the Ca2+ ion is smaller than of Na+ + K+ (Ca2+ : BB-1 – 33.5 
mg/L; LB-4 – 41.6 mg/L; LB-5 – 83.2 mg/L; VG-2 – 12.8 mg/L; 
VG-3 – 16.0 mg/L) (Tab. 2).

Waters with a pH of 7.5 and above and a HCO3
­ concentra­

tion of 120 mg/L and above are more likely to precipitate carbona-
tes and form encrustations, if such waters also contain the Ca2+ 
ion (SCHWANKL et al., 2008), as in the case of the studied mine-
ral waters.

The chemical composition of mineral waters is a result of the 
geologic framework. The geologic columns at Bogatić and Lu­
kovska Banja include limestones, whereas those at Vranjska 
Banja comprise rocks of igneous/metamorphic origin (Fig. 1). 
The geologic framework has an indirect effect on encrustation. 
According to MAJKIĆ (2013), carbonate encrustations can be 
formed not only in limestone environments, but also at wells that 
tap mineral waters from other lithologic settings, with carbonate 
concentrations greater than several mass percent, as in the case 
of the wells at Vranjska Banja, where all the occurrences exhib­
ited encrustations of carbonate origin. VAN BEEK et al. (2010), 
report similar findings, pointing out the dominant presence of 
carbonate encrustations in limestone environments. Encrusta­
tions of various origins, including carbonate, can be found in 
metamorphic settings and those with intercalations of igneous 
rocks.

All the tested occurrences belong to the group of hot mineral 
waters, with a temperature range of 62 – 93.9°C and a pH range 
of 6.6 – 8.0: mildly acidic (well LB-5) to mildly alkaline (wells 
BB-1, LB-4, VG-2 and VG-3) mineral waters. According to 
JONES & RENAUT (2010), high temperatures and elevated pH 
of mineral waters are conducive to, inter alia, carbonate encrus­
tation.

WOYESSA (2012) and DEMIR et al. (2014) also point out 
that carbonate encrustations on mineral water wells, especially 
where mineral waters are hot (>60°C). RICHARDS (2016) dis­
cusses the importance of temperature from the carbonate encrus­
tations perspective and shows that waters of moderate hardness 
and alkalinity tend to be devoid of carbonate encrustations at 

Figure 4. Samples of encrustations: a. Bogatić (sample BB-1); b. Lukovska Banja (sample LB); and c. Vranjska Banja (samples VG-2, VG-2* and VG-3) (photo: O. KRUNIĆ 
& V. ŠARABA, 2017).



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lower temperatures. A temperature increase leads to encrusta­
tion, if all other conditions remain constant.

The following types of encrustations are mostly observed in 
mineral waters with elevated temperature: (i) carbonates, such as 
calcium and strontium carbonates, (ii) silicates, (iii) heavy metal 
sulfides, and (iv) various types of chloride compounds (JU­
RÁNEK et al., 1987; ARNÓRSSON, 1989; GILL, 1998; PO­
TAPOV et al., 2001; GALLUP, 2002; DEMIR et al., 2014). In ad­
dition, waters of high temperature are saturated with silicon 
dioxide and nearly saturated with calcite, calcium sulfate, calcium 
fluoride, magnesium silicates, aluminum, iron, and metal sulfides 
(ÖLÇENOĞLU, 1986; GUNNLAUGSSON & EINARSSON, 
1989; KRISTMANNSDÓTTIR, 1989; HONEGGER et al., 1989; 
PÁTZAY et al., 2003; DEMIR et al., 2014). 

WOYESSA (2012) reports data on carbonate encrustation 
from mineral waters with pH levels lower than 8.3, which is the 
case in the present study (pH: BB-1– 6.9; LB-4 – 7.5; LB-5 – 6.6; 
VG-2 – 7.9; VG-3 – 8.0) (Tab. 2), and APPELO & POSTMA 
(2004) state that carbonate encrustations are dominant at pH˂8.3.

Conductivity, a property of mineral water determined in situ, 
which depends on the concentration of dissolved salts, measured 
higher values in Lukovska Banja and Vranjska Banja (wells: LB-4 

– 1513 μS/cm; LB-5 – 1588 μS/cm; VG-2 – 1584 μS/cm; VG-3 -  
1707 μS/cm). This was an indicator of a higher TDS level com­
pared to the mineral water at Bogatić (well BB-1 - 800 μS/cm). 
Following accurate determination of TDS, the occurrences in 
Lukovska Banja and Vranjska Banja were classified as mineral 
waters where total dissolved salts were greater than 1 g/L (wells: 
LB-4 – 1267 mg/L; LB-5 – 1058 mg/L; VG-2 – 1213 mg/L; VG-3 
- 1145 mg/L). The occurrence at Bogatić belongs to the group of 
mineral waters with concentrations of dissolved salts less than 
1 g/L (well BB-1 - 600 mg/L) (Tab. 2).

TDS directly affects encrustation; the higher the TDS level, 
the more probable the creation of encrustations (PAPIĆ, 1999).

Gas analyses included carbon dioxide (CO2), oxygen (O2) 
and hydrogen sulfide (H2S). The concentration of CO2 was sig­
nificant from a carbonate encrustations perspective. According 
to HOUBEN (2001) and DEED & PREENE (2015), carbonate 
encrustations, contrary to iron encrustations, occur when the par­
tial pressure of CO2 is lower than the atmospheric pressure, es­
pecially under high water temperature conditions as in the study 
cases. CO2 is released from the water and disturbs the carbonate 
equilibrium of the solution, leading to carbonate encrustation. 

Table 2. Results of physical and chemical analyses of the studied occurrences of mineral water (ŠARABA et al., 2017).

Parameter
Bogatić Lukovska Banja Vranjska Banja

BB-1 LB-4 LB-5 VG-2 VG-3

T (°C) 65.9 68.1 62.0 93.9 91.2

TDS (mg/l) 600 1267 1058 1213 1145

pH 6.9 7.5 6.6 7.9 8.0

T (NTU) / / 14.9 0.6 0.8

KMnO4 demand (mg/L) 6.9 2.2 3.2 5.5 4.2

Color (PT-Co scale) / / 25 <5 <5

EC (µS/cm) 800 1513 1588 1584 1707

MACROCOMPONENTS (mg/L)

Na+ 144.0 309.6 166 358.0 346.4

K+ 13.0 23.6 18.6 19.4 18.4

Ca2+ 33.5 41.6 83.2 12.8 16.0

Mg2+ 6.6 17.1 32 <1.0 <1.0

HCO3
- 361.6 961.0 961.0 589.0 598.0

SO4
2- 7.5 91.3 50.3 548.7 487.0

Cl- 103.4 82.0 46.0 64.0 60.0

MICROCOMPONENTS (mg/L)

Pb <0.003 / 0.04 <0.01 <0.01

Fe3+ 0.17 0.23 0.07 0.20 <0.10

Al <0.04 <0.04 <0.04 5.04 5.04

Ni / / 0.11 <0.01 <0.01

Ag / / <0.013 / /

Cd / / / <0.02 <0.02

Hg / / / <0.001 <0.001

As 0.002 / 0.689 <0.01 <0.01

Zn 0.002 / <0.014 <0.02 <0.02

NO3
- 0.3 0.7 0.5 2.3 5.3

NO2
- 0.002 0.005 <0.005 <0.02 <0.02

NH4
+ 1.80 1.05 0.75 0.31 0.32

SiO2 64 50 50 90 90

GASES (mg/L)

Free CO2 50.5 140.0 150.0 184.6 194.2

Dissolved O2 0.5 / / 4.5 3.7

Free H2S 2.5 / <0.06 0.008 0.006

Legend: Forward slash (/) = not analyzed.



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CO2 release from the water increases the pH level, which 
contributes to carbonate precipitation (APPELO & POSTMA, 
2004).

In addition, gas solubility decreases with increasing water 
temperature, which leads to the release of CO2. The solubility of 
carbonates in water depends on the amount of dissolved CO2. As 
this amount decreases, so does the solubility of carbonate mine­
rals, resulting in a higher rate of carbonate encrustation of the 
well. This reaction is rapid, such that carbonate encrustations are 
formed almost instantly (APPELO & POSTMA, 2004; HOUBEN 
& TRESKATIS, 2007; BROWN, 2013).

In such a setting, carbonate encrustation can be expressed 
by a reverse reaction (1), where there is an equilibrium between 
calcium hydrogencarbonates, calcium and hydrogencarbonate 
ions, solid calcium carbonate, and carbon dioxide in the mineral 
water solution (APPELO & POSTMA, 2004):

Ca (HCO3)2 ↔ Ca2+ + 2HCO3
– ↔ CaCO3↓ + CO2 + H2O, (1)

The highest concentrations of this gas were recorded at Vranj-
ska Banja (well VG-2 – 184.6 mg/L free CO2; well VG-3 – 194.2 
mg/L free CO2) and Lukovska Banja (wellLB-4 – 140.0 mg/L 
free CO2; wellLB-5 – 150.0 mg/L free CO2). Those at Bogatić 
were lower (well BB-1 – 50.5 mg/L free CO2) (Tab. 2).

The chemical compositions of the examined occurrences 
were illustrated in hydrochemical diagrams, which clearly indi­
cate the hydrochemical types of mineral water at each location.

Figure 5 shows the basic ionic compositions, which are in­
directly indicative of the TDS levels. It is apparent that the HCO3

­ 
and Na+ + K+ ions were dominant in the mineral waters at Bogatić 
and Lukovska Banja, while at Vranjska Banja there were also ele-
vated concentrations of the SO4

2- ion. Figure 5 also shows that at 
Bogatić the highest proportion is of the HCO3

­ion, followed by 
the Cl­ ion. The waters at Lukovska Banja and Vranjska Banja 
measured the lowest Cl­ ion concentrations. With regard to all the 
mineral water occurrences, the decreasing order was Na+ + K+, 
Ca2+ and Mg2+. It is apparent that the mineral water from Bogatić 
measured the lowest concentration of dissolved salts and that 
TDS was less than 1 g/L, compared to the other occurrences 
where TDS was greater than 1 g/L.

Figure 6 is a hydrochemical Cl– – SO4
2– – HCO3

– ternary 
graph, which shows that all the tested mineral water samples were 
in the high bicarbonate ion concentration range and that the mine­
ral waters could be classified as peripheral waters that may have 
mixed with cold groundwater. The mineral water at well BB-1 
(Bogatić) was near the Cl­ range but still in the HCO3

­ range, 
which is indicative of Cl­ being the second prevalent ion, after 
bicarbonate. The position on the graph also suggests very low 
concentrations of the SO4

2- ion. Closely positioned mineral wa­
ters from wells LB-4 and LB-5 (Lukovska Banja), and wells VG-2 
and VG-3 (Vranjska Banja), are a result of the same geologic set­
ting from which the mineral waters are extracted. This has led to 
similar concentrations of the HCO3

­, SO4
2- and Cl­ anions. In the 

case of the mineral waters from wells LB-4 and LB-5 (Lukovska 

Figure 5. Diagram of radial coordinates of the tested mineral waters.
Legend: BB-1 – well in Bogatić; LB-4, LB-5 – wells in Lukovska Banja; VG-2, VG-3 – wells in Vranjska Banja.



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Banja), the points are close to the corner of HCO3
­, such that these 

waters have high bicarbonate concentrations. The mineral waters 
from wells VG-2 and VG-3 are near the steam heated waters, but 
still in the peripheral range. In such cases bicarbonate and sulfate 
ions are predominant.

According to DOLGORJAV (2009), closely positioned mine-
ral waters on the Cl­ – SO4

2- – HCO3
­ graph (Fig. 6), such as those 

from wells VG-2 and VG-3 (Vranjska Banja), originate from 
high­temperature systems and can be associated with deep geo­
logic structures.

Encrustation samples (Fig. 4) were tested by various me-
thods, in order to conduct a detailed analysis. 

Macroscopic assessment of the samples (Fig. 4) revealed 
their colour, structure and texture: grain size, type and shape; 
lamination; and traces of damage/abrasion. Optical assessment 
and the increased contrast offered by this type of microscopy, 
provided a sample image with a high level of sensitivity for quan­
titative and qualitative determinations (Fig. 4). The X-ray powder 
diffraction technique was used to determine the phase composi­
tion of the samples (Fig. 4), or, in other words, to conduct quali­
tative analyses. The advantage compared to other methods is that 
it is possible to determine which mineral is present in the samples 
even where the total chemical composition has been determined.

Macroscopic tests of all samples (wells BB-1, LB-4, LB-5, 
VG-2, VG-3) with diluted hydrochloric acid (HCl) resulted in 
strong reactions, which indicated a significant presence of calcite 
(CaCO3). The colour of the post-test solutions was dark, a poten­
tial indicator of iron oxides and hydroxides in the samples.

GUNNLAUGSSON et al. (2014) report data on encrustations 
of carbonate origin (calcite and aragonite) in hot mineral water 
wells, which are exceptionally white in color and devoid of im­
purities. This was not the case in the present study.

The carbonate encrustations, along with iron and manganese 
oxides and hydroxides, were of the standard type found in and 
around wells, as corroborated by the examples of select occur­
rences of mineral waters, where as carbonate encrustations in 
combination with sulfates, sulfides and aluminum hydroxides are 
much less frequent (HOUBEN, 2000; HOUBEN, 2001).

Macroscopic assessment of the samples from wells BB-1 
(Bogatić), LB-4 and LB-5 (Lukovska Banja) revealed a micro/
coarse crystalline structure and a relatively well­preserved 
texture. Multiphasic development was noted, which lent a certain 
laminarity to the samples. The colours of the laminae varied, 

from brown/ochre at well BB-1 (Bogatić) to light brown, dark 
brown or dark gray at wells LB-4 and LB-5 (Lukovska Banja). 
The samples also exhibited thin and irregular lamination. 
Structurally, there were thick micritic and highly porous laminae 
or coarse crystalline interlayers.

The two alternating types of laminae could be a result of cal­
cite precipitation from warm and cold water, and the growth of 
Cyanobacteria driven by seasonal changes (TIŠLJAR, 2001). 

PEDLEY (1992), PERRY (1999) and RIDING (2000) point 
out frequent examples of the influence of phototrophic microbial 
mats on the formation of sediment textures, such as laminations 
and stromatolites.

The samples were relatively hard and could not be crushed 
between fingers when dry.

Optical assessment corroborated a distinct laminar texture 
of the samples from wells BB-1 (Bogatić), LB-4 and LB-5 
(Lukovska Banja), where the laminae were very thin, from 0.025 
to 1 mm. Their colour differed. All the laminae, regardless of 
thickness, ware more or less wavy, at times leading to a deformed 
texture – microconvolutions.

Optical assessment of the sample from well BB-1 (Bogatić) 
led to the conclusion that the predominant component was calcite. 
It was present in crystalline grains of various sizes – from mi­
crons (greater than 10 microns) to parts of a millimetre. The sizes 
of the largest monocrystals or monocrystal aggregates were up 
to 0.5 mm. The crystalline calcite grains were clear and birefrac­
tive. Such calcite was associated with light, predominantly calcite 
laminae (Fig. 7a). 

de ZWART (2007) reports similar calcite crystal sizes in car­
bonate encrustations around wells, indicative of irregular calcite 
crystal shapes in the size range of 3–5 μm.

Dark, semi-transparent grains were the second largest con­
stituent of samples from well BB-1 (Bogatić). These grains were 
aggregates of microcrystalline calcite, heavily stained by iron 
hydroxides (probably amorphous). They originated from thin, 
dark brown laminae (Fig. 7a). 

The non-carbonate part of a sample from well BB-1 (Bogatić) 
was a terrigenous component of pelitic size (clay minerals), inti­
mately mixed with microcrystalline calcite, including a very 
small quantity of quartz grains and phyllosilicate flakes (sericite/
fine flaky muscovite) of silt size. Organic matter was finely dis­
persed in the sample. According to optical indicators, its concen­
tration was less than 2%. The sample from well BB-1 (Bogatić) 
was identified as a wavy laminated calcite deposit.

X-ray powder diffraction on a sample from well BB-1 
(Bogatić) corroborated that the primary constituent was calcite, 
accompanied by a much smaller quantity of quartz (Fig. 8a).

Optical assessment of samples from wells LB-4 and LB-5 
(Lukovska Banja) led to the conclusion that the predominant con­
stituent was crystalline calcite. There were also aggregates of 
microcrystalline calcite, intimately mixed with iron hydroxides. 
The sample contained distinct grains with radially developed car­
bonate (Fig. 7b). This carbonate – aragonite generally crystallizes 
in high-temperature waters. At Lukovska Banja (wells LB-4 and 
LB-5), measured water temperatures ranged from 62°C to 68°C 
(Tab. 2). 

In addition, aragonite aggregate needles can host microor­
ganisms which, inter alia, cause aragonite precipitation (GUO & 
RIDING, 1992). As already mentioned, microbial growth in pho­
totrophic mats was noted in the studied occurrences. PLUMMER 
et al. (1978), BUHMANN & DREYBRODT (1987), CHAFETZ 

Figure 6. Ternary Cl- - SO4
2- – HCO3

- graph of the tested mineral waters.



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(1994) and FLÜGEL (2004) state that carbonates are the most 
frequent types of encrustations and those microorganisms are 
mediators of their creation, while FLÜGEL (2013) claims that 
carbonates are very often formed by freshwater microorganisms. 
The studied occurrences of mineral water belong to this category.

Samples from wells LB-4 and LB-5 (Lukovska Banja) also 
included terrigenous components (fine-grained sand fraction), but 
their concentrations were low. It is possible that fine quartz grains 
were a result of contamination during sampling or that they be­
came attached during carbonate incrustation. Based on optical 
indicators, samples from wells LB-4 and LB-5 (Lukovska Banja) 
were identified as a laminar calcite deposit.

The predominant calcite mineral in samples from wells LB-4 
and LB-5 (Lukovska Banja) was corroborated by X-ray powder 

diffraction. Aragonite was next, but its concentration was much 
lower (Fig. 8b).

Macroscopic assessment of samples VG-2, VG-2*and VG-3 
from wells VG-2 and VG-3 (Vranjska Banja) led to the conclu­
sion that their texture was disturbed during sampling. The sam­
ples were loose, comprised of particles, grains and fragments of 
sub-millimetre to centimetre size. The coarse, centimetre frag­
ments were in shades of light gray and light brown. Based on 
sample VG-2* from well VG-2 (Vranjska Banja), it was assumed 
that the texture of the other two samples (VG-2 and VG-3) from 
wells VG-2 and VG-3 (Vranjska Banja) was the same. They ex­
hibited irregular lamination, with thin micritic to semi­crystal­
line laminae and laminae comprised of aggregates of clustered 
aragonite needles (Fig. 7c).

Figure 7. Microscopic view of the samples collected from: a. well BB-1 (Bogatić), b. wells LB-4 and LB-5 (Lukovska Banja), c. well VG-2 (Vranjska Banja).
Legend: a. Fine crystalline calcite base. Dark lamellae comprised of grains, aggregates of micritic calcite stained by Fe hydroxides; b. Fine crystalline calcite base. 
Dark grains/lamellae are micritic calcite stained by Fe hydroxides; c. Fine crystalline calcite base, interspersed with aragonite filaments. Darker grains are aggregates 
of calcite and aragonite stained by Fe hydroxides.

Figure 8. Results of X-ray powder diffraction of the samples: a. BB-1 from well BB-1 (Bogatić); b. LB from wells LB-4 and LB-5 (Lukovska Banja). 



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Sample VG-2 from well VG-2 (Vranjska Banja) was tested 
by optical microscopy and samples VG-2, VG-2* and VG-3 from 
wells VG-2 and VG-3 (Vranjska Banja) were analyzed in detail 
by X-ray powder diffraction.

Optical assessment of sample VG-2 from well VG-2 (Vranj-
ska Banja) led to the conclusion that the sample comprised calcite 
and aragonite aggregate rods and filaments. Breakage of the fila­
ments was not readily visible. The structure was micro to coarse 
crystalline. The coarse calcite monocrystals were highly trans-
lucent. The other parts of the sample were difficult to determine 
optically but they included clay minerals and Fe oxides and 
 hydroxides, which had stained the sample in shades of brown 
(Fig. 7c). Based on optical indicators, sample VG-2 from well 
VG-2 (Vranjska Banja) was identified as a calcite-aragonite 
 deposit. 

X-ray powder diffraction revealed that in sample VG-2 from 
well VG-2 (Vranjska Banja) the prevalent mineral was calcite, fol­
lowed by aragonite. There were no other mineral phases (Fig. 9a). 
Sample VG-2* from well VG-2 (Vranjska Banja) contained only 
calcite (Fig. 9b). The primary constituent of sample VG-3 from 
well VG-3 (Vranjska Banja) was quartz, followed by calcite and a 
mineral from the mica group (muscovite). Minerals from the pla­
gioclase and chlorite groups were less represented (Fig. 9c).

The results of petrographic­mineralic testing showed that the 
mineral and chemical compositions of the encrustations were 
practically the same. Their common features included the pre­
dominance of calcium carbonate, a thickness of only several cen­
timetres, and a high porosity of the samples.The texture was 
chara cterized by alternating horizontal or wavy carbonate lami­
nae, comprised of micrites in shades of brown, stained by iron 

Figure 9. Results of X-ray powder diffraction of the samples: a. VG-2 from well VG-2 (Vranjska Banja); b. VG-2* from well VG-2 (Vranjska Banja); c. VG-3 from well  VG-3 
(Vranjska Banja). 



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oxides and hydroxides or organic substances, and those comprised 
of radial or medium-to-coarse crystalline calcite and aragonite.

de ZWART (2007) describes encrustations around wells and 
points out the carbonate composition and iron oxides in the min­
eral matrix, along with organic components originating from the 
aquifer. Sedimentation is governed by changes in hydrochemical 
and flow conditions, with other potential additional factors. 

Apart from calcite, the precipitation of minerals such as chlo­
rite, mica, etc. is also possible (de ZWART, 2007). This is true of 
the mineral water in Vranjska Banja, where quartz, albite, mus­
covite and clinochlore were detected.

DEMIR et al. (2014) also provide data on carbonate encrus­
tations in hot mineral water wells and on water pipes, represented 
by calcite and aragonite, along with lead sulfide. Given that arago-
nite is not thermodynamically stable under ambient conditions, 
but stable at elevated temperatures and pressures, there was a 
gradual transition from calcite to aragonite at increasing depths 
in the well, as the temperature and pressure gradually increased 
with depth (DEMIR et al., 2014). MITIĆ et al. (2014)  report com­
parable findings, indicative of the carbonate composition of en­
crustations in HCO3

­ – Na+ + K+ mineral waters, at a temperature 
of 75°C,  primarily composed of aragonite, along with calcite and 
vaterite, as well as small amounts of anhydrite, in a 1232 m-deep 
well.  Similar temperatures characterized the selected occur­
rences of mineral waters in Serbia, as well as an identical basic 
ionic composition. The presence of aragonite, detected in Luko­
vska Banja (wells LB-4 and LB-5) and Vranjska Banja (well 
 VG-2), keeping in mind the reports by DEMIR et al. (2014) and 
MITIĆ et al. (2014), might be indicative of a temperature gradi­
ent in the zone of discharge and at the wellheads, as well as that 
calcite, dominant in the composition of the encrustations, could 
be a result of aragonite inversion, due to decreasing temperatures 
and pressures in discharge zones and on wellheads, relative to the 
increase in these parameters with depth. In addition, PERDIK­
OURI et al. (2011) claim that the creation of calcite need not be a 
result of aragonite inversion. 

Such encrustations are created through precipitation from 
mineral waters and can be interpreted as deposition of terrestrial 
freshwater carbonates (calc tufa, travertine, etc.) (GUO & RI-
DING, 1992; CHAFETZ & FOLK, 1984; PERDIKOURI et al., 
2011). 

DRISCOLL (1986) and HOUBEN & TRESKATIS (2007) 
state that the main types of encrustations are chemical com­
pounds generated through the precipitation of calcium and mag­
nesium carbonates and sulfates, and iron and manganese com­
pounds – primarily hydroxides and oxides, but that there are also 
encrustations created by mechanical or chemical action of vari­
ous microbial species, which are the most frequently encountered 
encrustations in Serbia (ŠARABA et al., 2017).

Carbonate encrustations favour other conditions as well, 
such as (SCHWANKL et al., 2008): (i) groundwater extraction 
leading to changes in pressure at the point of entry into the well, 
which promotes carbonate encrustation, (ii) mixing of waters 
from different geological formations, increasing the probability 
of carbonate encrustation given that the studied mineral water 
occurrences are characterized by a complex geological frame­
work (Fig. 1), (iii) evaporation of drops of mineral water, which 
increases the concentration of dissolved salts in the groundwater, 
thus contributing to precipitation.

Based on the mechanical characteristics of the macroscopi­
cally assessed encrustations, they can be classified keeping in 
mind GAVRILKO & ALEKSEEV (1985), who identified four 

types, disregarding the mineral composition of the encrustations: 
(i) very hard conglomeratic type, (ii) loosely stratified type, rep­
resented by cemented fine-grained material, (iii) unconsolidated 
porous type, characterized by high porosity of the deposited 
mass, and (iv) soft, pasty or jelly like deposits that contain a sig­
nificant amount of organic matter. In this regard, the encrusta­
tions from Bogatić and Lukovska Banja belong to the first type 
and those from Vranjska Banja to the third type of encrustations.

In addition, very hard carbonate encrustations are generally 
associated with rhombohedral calcite, whereas soft and soluble 
carbonate encrustations are mostly built up of orthorhombic cal­
cite and are easier to remove from different surfaces 
(STOJILJKOVIĆ et al., 2015).

The hydrogeochemical modeling program PHREEQC 3.4.0. 
(U.S. Geological Survey) and database phreeqc.dat (PARKHURST 
& APPELO, 1999) were used to process the set of hydrochemical 
data, shown in Table 2, and to calculate the saturation index (SI) 
of all minerals that can hypothetically be formed from the mine­
ral water of a given chemical composition. The focus was on the 
primary minerals of the carbonate group (calcite, dolomite and 
aragonite), the oxide and hydroxide group (gibbsite, goethite and 
haematite), and the silicate group (quartz, clay minerals, micas, 
etc.), as the most probable constituents of the studied encrusta­
tions. Only positive values of the saturation index (SI>0) were 
interpreted, given that they suggested supersaturation of the water 
with a given mineral and, consequently, the possibility of its pre­
cipitation (APPELO & POSTMA, 1994). The results of modeling 
are shown in Table 3.

Given that the presence or absence of a certain mineral in 
encrustations depends, inter alia, on the precipitation rate under 
given conditions (i.e. SI>0 does not necessarily mean that the 
mineral is in fact present in the encrustations), the results of hy­
drogeochemical modeling of SI were interpreted in accordance 
with those of the macroscopic and optical assessments and X­ray 
powder diffraction.

The saturation index of the sample of mineral water from 
Bogatić (well BB-1) was indicative of only a slight calcite super­
saturation, which suggested the possibility of small carbonate 
deposits through encrustation. This was corroborated by miner­
alogic analyses of the sample from that location. Mild supersatu­
ration of the mineral water with quartz (SI = 0.15) was insufficient 
to create silicate encrustations (BROWN, 2013). The other posi­
tive SI values were associated with iron oxides and hydroxides, 
as well as muscovite and kaolinite, which comprised the non­
carbonate part of the examined encrustations. A comparison of 
the hydrogeochemical model of the mineral water sample from 
well BB-1 with the results of optical and X-ray assessments of the 
encrustations from well BB-1 revealed that all of the minerals 
identified in the encrustations were indeed precipitated from the 
mineral water, with the exception of quartz grains, which were 
likely a result of contamination during sampling.

Hydrogeochemical modeling of the mineral water samples 
from Lukovska Banja (wells LB-4 and LB-5) indicated possible 
precipitation of calcite and aragonite, as well as iron oxides and 
hydroxides. The presence of these minerals was corroborated by 
mineralogic and X-ray analyses of the encrustation samples. The 
model also suggested the possibility of precipitation of magne­
sium and potassium silicates, as well as kaolinite and dolomite, 
but they were not detected in the sample. Quartz was virtually in 
a state of chemical equilibrium with water (positive SI, but close 
to zero), so the assumption was that its presence in the encrusta­
tions was a result of contamination during sampling.



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Positive SI were calculated for the following minerals from 
the mineral water samples from Vranjska Banja (wells VG-2 and 
VG-3): calcite and aragonite; iron oxides and hydroxides; and 
kaolinite. These minerals were also identified by optical assess­
ment of the encrustation samples. The hydrogeochemical model 
suggested the possibility of precipitation of muscovite and chlo­
rite from the water, the presence of which in the encrustations 

was corroborated by X-ray testing of the sample from well VG-3. 
However, it should be noted that the petrographic-mineralogic 
analysis revealed that this sample also contained part of the pa­
rent rock. Mild supersaturation with dolomite and gibbsite was 
not confirmed. Quartz was virtually in a state of equilibrium with 
water in this case as well (negative SI, but close to zero).

Based on macroscopic and optical assessments of the encrus­
tations from the threelocations, the conclusion was that the en­
crustations originated from the water solution, through precipita­
tion of calcite, laminar development, and varying degrees of 
staining with iron hydroxides. The silicate minerals could have 
been attached in the development process, particularly phyllo­
silicates (sericite and clay minerals), but the possibility of con­
tamination during sampling should not be excluded. However, 
the positive SI of the silicate minerals in the mineral water sam­
ples from all three locations tend to suggest that they were at­
tached during the precipitation of encrustations.

According to MULLIN (2001), BREZONIK & ARNOLD 
(2011) and RICHARDS (2016), carbonate precipitation begins 
with nucleation in the mineral water solution. Colliding ions, ori­
ented in a certain way, initiate the creation of a crystalline struc­
ture, forming carbonate encrustations that gradually become a 
continuous layer. The nucleation process can be primary or sec­
ondary. Primary nucleation refers to original carbonate precipi­
tates, which did not pre-exist in the solution. Secondary nuclea­
tion is the growth of existing carbonate encrustations.

The Langelier Saturation Index (SILang) (2) and Ryznar 
 Stability Index (SIRyz) (4) were used to test the stability of the 
mine ral waters at Bogatić, Lukovska Banja and Vranjska Banja 
 (Tab. 4).

 SILang = pH – pHs, (2)

 pHs = 9.92 – t
40

 – log[Ca2+] – log[HCO3
–] + 0.2log[S.O.], (3)

 SIRyz = 2pHs – pH, (4)

where:
SILang – Langelier Stability Index, 
SIRyz – Ryznar Stability Index,
pH – measure of hydrogen ion (H+) activity in solution,
pHs –  pH at which the water is saturated with calcium 

carbonate,
t – groundwater temperature (°C),
Ca2+ – calcium concentration (mg/L),
HCO3

–  – bicarbonate concentration (mg/L),
S.O.  – dry residue (mg/L).

These two indices consider water solely from the standpoint 
of the balance between concentrations of the following compo­
nents: carbonates and bicarbonates, on one hand, and carbonic 
acid on the other (DEBERRY, 1982).

Table 4. Langelier Saturation Index (SILang) and Ryznar Stability Index (SIRyz) 
(ŠARABA et al., 2017).

Well pH pHs SILang SIRyz

BB-1 6.9 6.2 0.7 5.5

LB-4 7.5 6.6 0.9 5.7

LB-5 6.6 6.4 0.2 6.2

VG-2 7.9 6.2 1.7 4.5

VG-3 8.0 6.1 1.9 4.2

Legend: BB-1 – well in Bogatić; LB-4, LB-5 – wells in Lukovska Banja; VG-2, VG-3 – wells 
in Vranjska Banja.

Table 3. Saturation indices (SI) of the primary minerals from the carbonate group 
(calcite, dolomite and aragonite), the oxide and hydroxide group (gibbsite, goe-
thite and haematite), and the silicate group (quartz, clay minerals, micas, etc.).

Locality/Well Mineral phase SI Formula

Bogatić
(BB-1)

Calcite 0.08 CaCO3

Goethite 7.03 FeOOH

Haematite 16.24 Fe2O3

K-mica 3.67 KAl3Si3O10(OH)2

Kaolinite 0.86 Al2Si2O5(OH)4

Quartz 0.15 SiO2

Lukovska Banja
(LB-4)

Aragonite 0.93 CaCO3

Calcite 1.04 CaCO3

Chlorite(14A) 2.92 Mg5Al2Si3O10(OH)8

Chrysotile 0.05 Mg3Si2O5(OH)4

Dolomite 2.19 CaMg(CO3)2

Goethite 7.14 FeOOH

Haematite 16.46 Fe2O3

K-mica 2.09 KAl3Si3O10(OH)2

Quartz 0.01 SiO2

Lukovska Banja
(LB-5)

Aragonite 0.31 CaCO3

Calcite 0.43 CaCO3

Dolomite 0.96 CaMg(CO3)2

Goethite 6.63 FeOOH

Haematite 15.43 Fe2O3

K-mica 4.49 KAl3Si3O10(OH)2

Kaolinite 1.54 Al2Si2O5(OH)4

Quartz 0.09 SiO2

Vranjska Banja
(VG-2)

Anorthite 0.12 CaAl2Si2O8

Aragonite 0.72 CaCO3

Ca-Montmorillonite 0.00 Ca0.165Al2.33Si3.67O10(OH)2

Calcite 0.83 CaCO3

Chlorite(14A) 5.55 Mg5Al2Si3O10(OH)8

Dolomite 0.34 CaMg(CO3)2

Gibbsite 0.30 Al(OH)3

Goethite 6.44 FeOOH

Haematite 15.14 Fe2O3

Illite 0.00 K0.6Mg0.25Al2.3Si3.5O10(OH)2

K-mica 6.25 KAl3Si3O10(OH)2

Kaolinite 1.61 Al2Si2O5(OH)4

Quartz -0.02 SiO2

Vranjska Banja
(VG-3)

Anorthite 0.26 CaAl2Si2O8

Aragonite 0.89 CaCO3

Ca-Montmorillonite 0.06 Ca0.165Al2.33Si3.67O10(OH)2

Calcite 0.99 CaCO3

Chlorite(14A) 6.19 Mg5Al2Si3O10(OH)8

Chrysotile 0.14 Mg3Si2O5(OH)4

Dolomite 0.65 CaMg(CO3)2

Gibbsite 0.28 Al(OH)3

Goethite 5.85 FeOOH

Haematite 13.96 Fe2O3

Illite 0.11 K0.6Mg0.25Al2.3Si3.5O10(OH)2

K-mica 6.29 KAl3Si3O10(OH)2

Kaolinite 1.60 Al2Si2O5(OH)4

Quartz 0.00 SiO2



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SILang can only be used to predict the ability of water to 
precipitate dissolved calcium carbonate, not calcium sulfate or 
calcium phosphate (MAJKIĆ-DURSUN et al., 2016). SIRyz is 
suitable for moderately hard to hard waters, but not soft or brack­
ish waters. Even though the use of these indices is widespread, 
they can only approximate the stability of mineral waters (DE­
BERRY, 1982).

SIRyz˂7 indicates precipitation of encrustations, SIRyz>9 
that the predominant process is corrosion, and 7˂SIRyz˂9 that 
both, precipitation of encrustations and corrosion, are possible 
(RYZNAR, 1994). All of the studied occurrences of mineral wa­
ter are characterized by SIRyz˂7 (Tab. 4), which indicates that 
the precipitation of encrustations is the predominant process on 
wellheads and within the zones of emergence.

SILang values greater than zero in the case of the three stu­
died mineral water occurrences (Tab. 4) indicate calcium carbo-
nate precipitation on all the wells. Based on SIRyz values (Tab. 4), 
the mineral waters of wells BB-1, LB-4 and LB-5 tend to precip­
itate calcium carbonate because SIRyz≥5.5, while SIRyz˂5.5 
(wells VG-2 and VG-3) suggests that the mineral water at Vranj-
ska Banja has an exceptional propensity for calcium carbonate 
precipitation. 

SILang can also be used to determine the rate of encrusta­
tion. If SILang ˂ 0.5, then the rate of encrustation is up to 0.2 g/
m2 h; if 0.5˂SILang˂1, then 0.2 to 0.5 g/m2 h; and if SILang>1, 
then it is greater than 0.5 g/m2 h (LANGELIER, 1936; VUKOVIĆ 
& SORO, 1992).

According to the values of SILang in Tab. 3, the mineral wa­
ter from well LB-5 belongs to the first category (encrustation rate 
0.2 g/m2h, SILang˂0.5), the mineral water from wells BB-1 and 
LB-4 to the second category (rate from 0.2 to 0.5 g/m2 h, 0.5˂SI­
Lang˂1), and the mineral water from wells VG-2 and VG-3 to the 
third category (rate higher than 0.5 g/m2h, SILang˂0.5).

According to MOROZOV & MERSHCHII (1979), mineral 
waters characterized by SIRyz˂9, SILang>0 and Fe3+˂0.3 mg/L 
will likely precipitate calcium and magnesium compounds, with 
CaCO3 being predominant. All the studied occurrences measured 
Fe3+ less than 0.3 mg/L (wells: BB-1 – 0.17 mg/L; LB-4 – 0.23 
mg/L; LB-5 – 0.07 mg/L; VG-2 – 0.20 mg/L; VG-3 - ˂ 0.10 mg/L) 
(Tab. 2), and SILang>0 and SIRyz˂9 (Tab. 4), which corroborated 
the predominant carbonate composition of the analyzed encrus­
tations.

4.1. Phototrophic microbial mats
It is well known that many microorganisms, including phototro­
phic ones, can contribute to the precipitation of minerals on some 
surfaces. Thus, the encrustations can be affected by the formation 
of phototrophic biofilms, which can lead to the creation and ac­
cumulation of minerals in several ways. Cyanobacteria, for ex­
ample, can incorporate different particles into their sheaths, 
among which carbonate minerals are the most common (WHIT­
TON, 2012). Sheaths are mostly composed of extracellular poly­
meric substances (EPS), highly organized polymers that accu­
mulate outside the cell forming a matrix with multiple roles 
(SECKBACH & OREN, 2010). The most common role is the pro­
tection of the microorganism’s cell from various harsh conditions. 
Harsh conditions are mostly high or low temperatures, UV ra­
diation, changes in environmental chemistry, desiccation, preda­
tors, etc. According to many researchers, the most important role 
of EPS is its water retention capability and protection from de­
siccation. 

The binding ability of EPS is certainly very useful and an in­
teresting feature (WHITTON, 2012). EPS is capable of binding 
different particles, but also nutrients that can be of importance for 
biofilm functioning. Mineral precipitation often occurs within the 
EPS, rather than on the surface of cyanobacterial sheaths (SECK­
BACH & OREN, 2010). Uronic acids play the main role in EPS. 
These charged groups are capable of interacting with sediment 
particles and it is believed that they can be involved in the regula­
tion of the calcification process. Extracellular layers are particu­
larly favourable sites for nucleation, and cyanobacterial species 
that produce sheaths or EPS generally precipitate more calcium 
carbonate than species without sheaths (WHITTON, 2012).

The analysis of phototrophic biofilms in this study was per­
formed using scanning electron microscopy. Light microscopy 
in ŠARABA et al. (2017) provided an insight into the diversity of 
phototrophic microorganisms but scanning electron microscopy 
enabled more detailed analysis and allowed that the sample sur­
face and morphology, the shape and size of different biofilm con­
stituents, and their orientation, distribution and the way of attach­
ment to be explored. By using this technique, we wanted to 
highlight the samples where EPS was present or dominant or 
where some mineral particles were visible and attached to EPS.

The presence of the three dominant algal divisions in the 
phototrophic biofilms was recorded, including Chlorophyta, Cya-
nobacteria and Bacillariophyta (ŠARABA et al., 2017) (Fig. 10), 
where Cyanobacteria, especially non-heterocytous trichal forms, 
were dominant in biofilm samples from all the localities. 

The SEM micrographsof phototrophic biofilms (Figs. 10a, b, 
d, e below) show the developed layer of extracellular polymeric 
substances (EPS). In general, mineral deposits attached to EPS 
are visible in the form of small particles, but also large particles 
(e.g. Fig. 10a). SEM analysis of biofilm samples from Bogatić 
mainly revealed the domination of trichal Cyanobacteria that were 
densely entangled and embedded in the extracellular matrix cove­
red with different mineral particles (Figs. 10c and 10d). Trichal 
Cyanobacteria were also visible in the sample from Lukovska 
Banja (Fig. 10c), but representatives of Bacillariophyta were also 
present. In this sample, EPS was not as developed as in the sam­
ples from Bogatić. Particles of different origin were noted in EPS 
from Vranjska Banja (Fig. 10e), along with Bacillariophyta repre­
sentatives. According to the micrographs, biofilm was highly de­
veloped and Cyanobacteria abundant in samples from Bogatic and 
Lukovska Banja (less abundant in Vranjska Banja). 

Cyanobacteria (when phototrophic microorganisms are con­
sidered) can play an important role in the formation of encrustra­
tions. For example, according to PEDLEY (1992), PERRY (1999) 
and RIDING (2000), laminar carbonate encrustations, generally 
wavy, in combination with phototrophic biofilm, are a result of 
carbonate silt and fine sediment trapped by the microbial popula­
tions. Recent encrustations, such as those sampled in this re­
search, generally contained alternating organic and inorganic 
laminae. While the organic laminae were composed of different 
species of Cyanobacteria, inorganic laminae comprised carbo-
nate silt, which Cyanobacteria had captured from the water.

Besides the capability of EPS to attach different particles, the 
microorganisms themselves can induce mineral precipitation. 
Phototrophic microbial matsare characterized by temporal and 
spatial dynamics and highly diverse metabolic pathways in the 
creation of minerals. During the day, photosynthesis and its or­
ganic products in the surface layer of the biofilm are the main 
factors of the origin of mineral precursors (metabolic fixation of 
inorganic carbon tends to increase the pH of the solution and lead 



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to a state of carbonate supersaturation). On the other hand, at 
night, during respiration, totally different micro conditions are 
created (lower pH, O2, higher CO2, and the like), where products 
of respiration can dissolve minerals. In addition, the stratification 
of phototrophic microbial mats that is common in biofilms near 
thermal springs and the coexistence and cooperative metabolism 
of a large number of microorganisms can lead to the creation of 
diverse minerals in a small space, depending on the hydrochemi­
cal characteristics of the mineral water and environmental con­
ditions. Due to their anoxygenic photosynthetic ability in the 
presence of H2S, Cyanobacteria can survive in extreme environ­
ments where many algae cannot live. The product of this process, 
elemental sulphur, can be deposited in the form of fine granules 
on the filament surface, which is no longer oxidized except in the 
presence sulfur oxidizing bacteria. Therefore, if calcium is pre-
sent in high concentrations, they can facilitate the formation of 
various morphotypes of gypsum minerals (EDINGER, 1973; 
TANG et al., 2014; KAŹMIERCZAK et al., 2015).

4.2. Implications
The studied occurrences of mineral water definitely deserve spe­
cial attention in the future, including the establishment of perma­
nent monitoring, which would, inter alia, track the condition of 
steel wellheads and zones of emergence of mineral water, which 
are ideal substrates for encrustation and biofouling. The life mani-
festations of phototrophic mats come with different well ageing 
symptoms and they contribute to encrustation, among other 
things. Preemptive action indicated by monitoring would help 
avoid the need for well rehabilitation, which is costly. The wells 
would thus be maintained in a state of maximum productivity. 
Spending would be minimized, mineral water would be used for 
heating and health­and­recreation purposes without interruption, 
and the quality of the mineral water would be sustained, with no 
adverse effect on its organoleptic properties.

In addition, many of the identified phototrophic microorgan­
isms are able to generate medicinal and economically-significant 
products and can be used by the biotechnological industry as a 

Figure 10. SEM micrographs of Cyanobacteria and phototrophic biofilm algae.
Legend: a, b. Bogatić (trichal Cyanobacteria embedded in EPS covered with particles of different origin); c, d. Lukovska Banja (trichal Cyanobacteria and Bacillari-
ophyta); e, f. Vranjska Banja (Bacillariophyta representatives visible on both micrographs).



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source of vitamins, enzymes, lactic acid and other substances of 
relevance to agriculture and medicine (ŠARABA et al., 2017).

The present study shows that the examined mineral waters, 
regardless of their extremely high temperatures and specific 
physical and chemical compositions, support the development of 
various microbial communities and constitute a starting point for 
studying thermal habitats, which to the present day remain unin­
vestigated biotopes, at least in Serbia. There is also the need to 
protect the environment and the genetic diversity of such habitats.

Moreover, encrustations are significant because they have a 
serious impact on the life cycle of a well, as it reduces the well’s 
capacity, threatens the integrity of its structural components 
(Fig. 3), and changes the organoleptic properties of the mineral 
water, causing foul odour, unpleasant taste, turbidity etc., (HOUBEN 
& TRESKATIS, 2007). The research conducted in Lukovska Banja 
and Vranjska Banja corroborated that the well in Bogatić is not a 
solitary case of encrustation coupled with biofouling.

A large number of wells have encrustation issues. The water 
industry has been aware of the problem for more than 100 years 
(HOWSAM et al., 1995; de ZWART, 2007; STERRETT et al., 
2008; DEED & PREENE, 2015). Preventative measures require 
an in­depth understanding of the processes, which the present 
research attempted to explain using examples of carbonate en­
crustations.

5. CONCLUSION
The study conducted from 2014 to 2017 revealed that in Serbia 
there are sources of mineral water that exhibit numerous specific 
features with regard to their basic ionic composition, tempera­
ture, total dissolved solids, pH, and the like.

Encrustations were noted at all study locations (Bogatić, Lu­
kovska Banja and Vranjska Banja), on wellheads and within 
zones of mineral water emergence. The results of macroscopic 
and optical assessments, as well as X-ray powder diffraction, in­
dicated a predominantly calcium carbonate composition of the 
encrustations, with calcite and aragonite detected at all the loca­
tions. Quartz, albite, muscovite and clinochlore were also de­
tected at some of the locations.

The study concluded that the physico­chemical composition 
of the mineral waters was a key driver of the onset and progress 
of encrustation. High water temperatures (> 60°C), pH levels 
from 6.6 to 8.0, elevated TDS – from 600 to 1267 mg/L, and the 
dominance of the HCO3

­ ion (361.6 – 598.0 mg/L), along with a 
certain proportion of the Ca2+ ion (12.8 – 83.2 mg/L), favour the 
formation of carbonate encrustations.  In addition, the concentra­
tion of CO2 in the mineral waters (50.5 – 194.2 mg/L free CO2) 
is also important, as it contributes to carbonate precipitation by 
disturbing the carbonate balance in the mineral water solution.

In addition, hydrogeochemical modeling confirmed that 
there was a correlation between the chemical composition of the 
mineral waters and the mineral composition of the encrustations 
or, in other words, that the minerals detected in the encrustations 
likely originated from the mineral waters.

Prognostic analyses corroborated a propensity for encrusta­
tion at all locations, given that all the studied occurrences of mi­
neral water were characterized by SIRyz˂7 and SILang>0, indi­
cating carbonate encrustations.

Apart from encrustations, phototrophic microbial mats were 
found at all the sites, represented by Cyanobacteria, Chlorophyta 
and Bacillariophyta, which, inter alia, can facilitate the creation, 
accumulation of minerals in encrustations, indirectly affecting 
the rate of carbonate precipitation.

ACKNOWLEDGEMENT
The Serbian Ministry of Science has financially supported this 
research under contracts TR33053, OI176020 and OI176018.

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