Ecology & Safety Journal of International Scientific Publications ISSN 1314-7234, Volume 13, 2019 www.scientific-publications.net

ARSENIC IN THE THERMAL WATERS OF SOTHERN Simeon Valtchev1, Diana Rabadjieva2, Vladimir Hristov1, Mila Trayanova1, Aleksey Benderev1 1Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev str. bl.24, 1113 Bulgaria 2Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., Bl. 11, 1113 Sofia, Bulgaria

Abstract The research is based on water samples from thermal mineral waters and individual water sources associated with confined fractured aquifers in Southern Bulgaria. The analysis indicated that more than 65% of them contain arsenic below the drinking water standard of 0.01 mg/l, as defined in Regulation No. 9 from 2001 for the drinking water quality. In 3.5% of the samples (11 water sources), the arsenic concentration exceeds 0.05mg/l, which was the quality standard before the adoption of Regulation No. 9. Some of the thermal waters with highest arsenic concentration come from Triassic limestones and dolomites (Kazichene, Starozagorski Bani, Slivesnki Bani, Ovcha Kupel), individual sources are associated to gneiss (Kozhuh, Gotsedelchevski Bani), granites (), andesites (Merichleri). In some cases, thermal water of elevated arsenic concentration from fractured hydrothermal systems discharge in younger unconsolidated deposits, where they come into contact with freshwater (Ravno Pole, , Merichleri). In such environments, if drinking water abstractions are overexploited, it is possible to compromise them by drawing in mineral water of naturally elevated arsenic concentration. Such risk depends on the hydrodynamic conditions, the physicochemical conditions, and the forms of arsenic in the water. Key words: thermal waters, arsenic, geothermal contaminant, hydrochemistry, South Bulgaria

1. INTRODUCTION Arsenic is one of the toxic elements, known to cause oncologic diseases. For this reason, following the recommendations of the Wold Health Organisation [1], in 2001 the maximum allowable concentration of arsenic in Bulgarian drinking water was reduced from 0.05mg/l to 0.001 mg/l [2]. In this regard, it is an essential problem that in certain groundwater sources, the exceeding concentrations of arsenic are not caused by anthropogenic contamination but the result of natural processes, namely groundwater- rock interaction. The thermal waters can be largely affected by such processes due to the specific conditions of formation of their chemical composition. There are numerous studies on contaminant distribution, on the causes and processes that produce elevated concentrations of arsenic in various sources of thermal water in the world [3, 4, 5, 6]. However, such generalising studies have not been undertaken in Bulgaria so far, despite the fact that thermal waters are abundant in the country. Thermal waters are widely available and characterised by various source conditions, chemical composition, temperatures and possible uses [7, 8, 9, 10]. In general, two groups of sources of thermal water can be distinguished. The first group comprises the sources from a large artesian reservoir in Northern Bulgaria and associated with deep aquifers occupying of large areas. The second group, which is the subject of the present study, includes fractured hydrothermal systems, associated with tectonic dislocations in Southern Bulgaria. A large amount of chemical data is available, collected with different analytical methods over many years. The aims of this study was to systematically summarise the arsenic concentration data for thermal waters from fractured hydrothermal systems in Southern Bulgaria, as well as to establish the processes that produces the relatively high arsenic concentrations in some of the well-known and widely used sources of thermal water.

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2. MATERIALS AND METHODS The study is based on the two latest datasets of systematically summarised chemical analyses of thermal water. The first dataset includes results of field testing and laboratory analyses of a wide range of physicochemical and chemical parameters of 143 groundwater sources reported by Pentcheva et al. in 1997 [12]. It includes in-situ test results for pH, electrical conductivity, redox potential and dissolved oxygen. The laboratory data includes analytical results for approximately 70 parameters describing the chemical composition of the water samples. The other dataset consists of unpublished analytical results compiled by the Scientific Centre for Physical Treatment and Rehabilitation (SCFTR). It includes results for 35 hydrochemical parameters, excluding redox potential, from 203 sources of thermal waters. The two datasets together contain information about arsenic concentrations in 315 thermal water samples from 258 sources thermal waters from 137 locations. The analytical results for arsenic concentration were arranged in groups according to the regulations that were in place before and after 2001 [2, 13]. Based on the arsenic concentration in the water, the following groups were distinguished: 1 – below the current drinking quality standard of 0.01 mg/l, including the reported concentrations below the detection limits of the corresponding analytical method; 2 - between 0.01 mg/l and 0.05 mg/l; and 3 – above 0.05 mg/l. The locations of the sources from the different groups were plotted on a map. The information was used for analysing the possible factors determining the presence of arsenic in the groundwater, such as the nature of rock formations and thermodynamic conditions. The distribution of arsenic concentrations in thermal waters was considered in terms of rock formations in the bedrock. The data set was also analysed by comparing the arsenic concentrations to other parameters, however no co-relations were revealed. The values for arsenic concentrations above detection limits from the dataset of Pentcheva et. al. [12] were plotted on pH-Eh a diagram, which was generated on the basis of thermo.dat database by the Lawrence Livermore National Lab (LLNL) [13]. The distribution of dissolved chemical species in the studied solutions under was calculated using the classical ion-association model [15] using the software Visual MINTEQ ver. 3 [16]. Considering the large number of analyses, models were set up for selected samples representative for thermal waters associated with different thermodynamic conditions, as well as samples with higher arsenic concentrations. The total concentrations of 22 elements, forming the base composition of the waters, temperature and pH were used as input data. In cases when the analytic results for a given element were below its detection limit, the latter was used for the modelling. The following additional assumptions were made: (i) the activity coefficients of all possible simple and complex species were calculated using the extended Debye-Hückel theory and thermodynamic equilibrium was assumed only for the complex formation processes; (ii) Mn6+ and Mn7+ ions were not considered in the calculations, since they are not stable in natural waters [17]; (iii) redox processes were evaluated with the inclusion of all possible oxy- redox pairs, namely Cu+/Cu2+; Fe2+/Fe3+ и As5+/As3+.

3. RESULTS The analysis of the datasets indicates that in most hydrothermal systems of fractured rock, the arsenic concentrations are not high (Fig.1). The arsenic concentrations are below the drinking water standard [2] of 0.01 mg/l in most thermal water sources, or 60% of all water sources. Out of these, 13.5% are samples in which the arsenic concentrations are below the detection limits of the corresponding analytical method. In 36% of the water samples, the arsenic concentrations are between 0.01 mg/l and 0.05 mg/l, which means that these waters were complied with the quality regulations for drinking water before 2001. Only 4% of the samples have arsenic concentrations exceeding 0.05 mg/l (Table 1).

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Figure 1. Thermal water sources in fractured hydrothermal systems in Southern Bulgaria and ranges of arsenic concentration (the indexed locations correspond to the data listed in Tables 1 and 2).

Table 1. Host rock type and hydrochemical parameters of thermal waters sources, where arsenic concentrations exceed 0.05 mg/l, including several important sources where the concentrations are lower (the numbers correspond to the indexes in Fig. 1). No Locality Hostrock Temp., ᵒC pH TDS, mg/l As, mg/l 1 Starozagorski Bani limestones 41.5 7.90 514 0.069 2 Rupite gneiss 71 6.81 1837 0.070 3 Gotze Delchev Bani gneiss 43.0 8.45 270 0.084 4 Sandanski sand 68.0 8.59 1380 0.092 5 Topolniysa sandstones 0.100 6 Merichleri latite, limestones 42.0 7.10 4799 0.120 7 Yakoruda granites 34.0 8.29 626 0.125 8 Ovcha Koupel limestones 30.5 7.16 1720 0.179 9 Slivenski Bani limestones 40.5 6.70 1923 0.184 10 Kazichane limestones 79.5 6.92 1850 0.260 11 Mihalkovo marble 23.0 6.04 2526 0.045 12 Ravno Pole sands, clays 51.0 7.69 719 0.12 13 Dolna Bania gneiss 62.5 9.10 606 0.084 14 Haskovski Bani rhiolites 54.0 7.50 0.009 15 Katarino gneiss 20.0 8.90 494 0.044 16 gneiss, granites 98 9.3 650 0.037

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The type of host rock is one of the important factors influencing arsenic concentrations in the water. Most of the thermal water sources in Bulgaria form in granites however, elevated concentrations of arsenic are not typical (Figure 2). Relatively higher concentrations of arsenic are typical for thermal water sources in limestone and marble in the first place, followed by sources in sandstone and sand. To some extent, elevated arsenic concentrations are associated with thermal waters in gneiss.

Figure 2. Distribution of thermal water sources in terms of arsenic concentrations and host rock type.

The availability of arsenic in the water may be influenced by other factors, namely pH, redox potential (Fig. 3) and temperature (Fig. 4). The data do not show unequivocal correlation between the values of the above parameters and arsenic content in the water.

Figure 3. Distribution of thermal water sources from the dataset of Pentcheva et. al. [12] on a pH-Eh diagram [13]

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Figure 4. Scatter plot of arsenic concentration and thermal water temperature.

Correlations could not be established between arsenic concentrations and other hydrochemical parameters, including Hg, Sb, Se,Tl, B, Li, F and H2S, which together with As form the so called ‘geothermal suite’ of contaminants (Webster-Brown 2000 in [3]). The only apparent correlation is observed between the concentrations of As and Sb, in the range where As concentrations exceed 0.012 mg/l (Fig. 5).

Figure 5. Correlation between arsenic and antimony concentrations in the thermal waters of Southern Bulgaria.

4. DISCUSSION The results of the analysis indicate to the absence of regional hydrochemical regularities in the distribution of arsenic in the thermal waters of the fractured hydrothermal systems in Southern Bulgaria. This confirms that the presence of arsenic depends mostly on the particular local conditions, most importantly the type and composition of host rock, with which the water is in contact. It was established that elevated concentrations of arsenic are relatively most often associated with carbonate rocks, which in Southern Bulgaria can be primary as well as secondary reservoirs of thermal water. According to research by Kim et al., 2000 [19], the leaching of arsenic sulphide minerals is facilitated by the presence

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Ecology & Safety Journal of International Scientific Publications ISSN 1314-7234, Volume 13, 2019 www.scientific-publications.net of hydrogencarbonate ion in the solution and the arsenic carbonate complexes that form in the process - - - + As (CO3)2 , As(CO3)(OH)2 and AsCO3 - are stable in the groundwater. Similar processes can also occur in graben-like depressions, filled with diverse unconsolidated or loosely cemented deposits with zones of elevated carbonate content, where thermal water flows upward from the bedrock. In such regions, clay deposits would have the opposite effect of absorbing not only heavy metals, but arsenic as well. According to Kraynov et al., 2004 [20], Na type water, which is the groundwater in granite, gneiss and some sandstones, is more beneficial for leaching of arsenic as compared to Ca-Mg type water. This is expected from the considerably higher solubility the arsenates and arsenates of sodium, compared to those of calcium and magnesium. Such a correspondence could not be established in the available data from Southern Bulgaria. The reason for this could be the absence of arsenic minerals in the zones of thermal water formation, except in the case of a few particular sources. Arsenic in groundwater is generally observed in areas, where the bedrock is characterised by elevated arsenic concentrations on a regional scale, in As metallogenic provinces, e.g. the Urals, Donbas, Lesser Caucasus, etc. In such hydrochemical provinces, relatively elevated concentrations of arsenic are generally observed in groundwater of HCO3-Na and HCO3-Na-Cl type. Vertical zones of hydrochemical distribution regional hydrogeological structures have also been reported, where the arsenic concentration increases at the transition from NCO3-Ca type groundwater of low mineralisation to HCO3-Na and HCO3-Na-Cl types of comparatively higher mineralisation [20]. However, such hydrochemical zones have not been observed in Southern Bulgaria. Mc Grory et al. 2018 [21], performed statistical analysis on a regional groundwater dataset for Ireland and reported a relationship between arsenic and lithology, the elevated arsenic concentrations associated with sandstone derived bedrock. Similar relationship is confirmed in this study with one of the important lithological factors being the presence of arsenic minerals in the zones of thermal water formation and flow. Arsenic is also present in geothermal water associated with volcanic gases (Hem, 1989 [22]). There is no volcanic activity on the territory of Bulgaria, hence this problem is not considered. The most recent and single event of Neogene orogenic volcanism is in the area of Rupite [23] and there is one of the interesting thermal water sources in the country (No. 2 in Fig.1 and Table 1). The thermal waters from Rupite have high dissolved CO2 and arsenic concentrations. An interesting example for the effect of lithology on arsenic concentration is the well in Kazichene (No. 10 in Fig. 1 and Table 2), where the highest arsenic concentration has been recorded – 0.26 mg/l. The borehole is located in Sofia lowland, a complex graben structure, filled with Pliocene and Quaternary lacustrine alluvium, mostly clay and sand. The bedrock is a fault-block structure of Mesozoic rocks, some with thermal water. The borehole is in a block of limestone and dolomite. Water from this block infiltrates the overlying sediments, where there are a series of groundwater abstractions. As a result from the change in the filtration properties in the upper layers, the concentrations of arsenic notably lower - between 0.007 mg/l and 0.019 mg/l in the groundwater from the alluvium. The valence of arsenic and its forms of migration in the thermal waters are very important properties, related to potential adverse health risks. Two oxidation states are typical for arsenic, Аs(V) and As(III), and have different toxicity. It is considered that As(III) is more toxic than Аs(V) and in addition, inorganic species of arsenic are more toxic than organic [24]. In this study we applied thermodynamic equilibrium modelling of oxidation-reduction and inorganic complexation processes to evaluate the content of Аs(V) and As(III) forms and to calculate their inorganic species distribution. The calculations were based on the data for eleven thermal water samples from Southern Bulgaria of temperature, pH and composition. The calculation of As(V):As(III) ratio (Fig. 6) show that As(V) prevail in all studied waters in different degree, depending on pH and temperature of the waters.

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6 000 260.4 5

5 000 941.6

4 000 3 3 000

As(V):As(III) 2 000

034.4

1

959.3 879.1

1 000 740.4

295.2

83.4

3.5 2.0 0 0.6 15 7 11 10 12 13 3 14 16 2 6 sample points

Figure 6. Calculated ratio As(V) : As(III) in studied waters

The combinations of low temperature with high pH and high temperature with low pH shift the equilibrium to As(V). The highest content of the latter is calculated for sample point 15 (t = 20oC, pH 8.9), point 7 (t = 34oC, pH 8.29) and point 10 (t = 79.5oC, pH 6.91). The lowest concentration of As(V), respectively the highest concentration of As(III) is calculated for sample points 16 and 11 where high temperature is combined with high pH (point 16, t = 98oC, pH 9.3) and low temperature is combined with low pH (point 11, t = 23.0oC, pH 6.04). The distribution of As(V) and As(III) species is presented on Fig. 7. It is established that the species 2- 3- distribution depends only on pH. As can be expected, the share of the basic forms (HAsO4 , AsO4 and - - 0 3- H2AsO3 ) increase by pH at the expense of the more acidic forms (H2AsO4 and H3AsO3 ). The AsO4 species appear at highest pH value, above 8.9.

As(V) 100 90 80 70 60 50 40 30 20 10 species distribution, % distribution, species 0 6.04 6.81 6.92 7.10 7.50 7.69 8.29 8.45 8.90 9.10 9.30

2- - 3- HAsO4-2HAsO H2AsO4-H2AsO4 AsO4-3AsO 4 4 As(III)

100

80

60

40

20

species distribution, % distribution, species 0 6.04 6.81 6.92 7.10 7.50 7.69 8.29 8.45 8.90 9.10 9.30

H3AsO3H AsO 0 H2AsO3-H AsO - 3 3 2 3 Figure 7. Species distribution of As (V) and As(III) depending on pH of the studied waters.

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5. CONCLUSION This is the first regional-scale study of arsenic in the thermal waters from fractured hydrothermal systems in Southern Bulgaria. The analysis of available chemical data indicates that in most cases, i.e. in 60% of the sources of thermal water, the concentrations arsenic are low and in compliance with the current drinking water regulations. Approximately 96% of the considered water sources complied with the drinking water standard that was in place before 2001, when the maximum allowable concentration of arsenic in the water was reduced from 0.05 mg/l to 0.01 mg/l. Arsenic concentration exceeding 0.05 mg/l in 3.5 % of the thermal water sources and half these sources are associated with carbonate rocks. In most cases, the presence of arsenic and its concentration in the water depend on local conditions in the zones of thermal water formation and flow paths to the surface. No correlation could be established between the total arsenic content and other chemical parameters of the thermal waters. A relationship was derived for the distribution between the two oxidation states Аs(V) and As(III) depending on pH and the temperature of the water. The thermal waters in Southern Bulgaria are traditionally used for balneotherapy, heating of buildings and green houses, swimming pools, etc., as well as for bottling of mineral water in the recent 50 years. The present study shows that due to the elevated arsenic concentrations, approximately 40% of the studied water sources are not appropriate for bottling of mineral water.

ACKNOWLEDGMENT This work has been carried out in the framework of the National Science Program "Environmental Protection and Reduction of Risks of Adverse Events and Natural Disasters", approved by the Resolution of the Council of Ministers № 577/17.08.2018 and supported by the Ministry of Education and Science (MES) of Bulgaria(Agreement № ДО-230/06-12-2018).

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