# Boron Isotopes in the Mount Vulture Groundwaters (Southern Italy): Constraints for the Assessment of Natural and Anthropogenic Contaminant Sources

Authors: Michele Paternoster
Journal: Geofluids
 (2019)

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## Abstract

The Mt. Vulture volcanic aquifer is an important reservoir of effervescent mineral water in southern Italy. This area is included in the Vulture Regional Park and is located on a popular tourist route for its high scenic and ecological value. Unfortunately, signs of anthropogenic impact have recently increased. Among minor elements, boron and its isotopes are considered useful environmental tracers since different anthropogenic and natural sources can be distinguished. In this study, B content and (δ11B) isotope composition were determined in the Mt. Vulture groundwaters. The groundwater chemistry is strongly influenced by input of CO2-rich volcanic gases, and two different hydrofacies are observed. The first water type (BAW) has bicarbonate alkaline and alkaline-earth composition and relatively low salinity, whereas the second one (HSW) has bicarbonate-sulphate alkaline composition and higher salinity. The HSW are enriched in boron and have low δ11B values (from –9.6‰ to –4.3‰) similar to those measured in the local magmas suggesting that B contents in these waters are influenced by prolonged interaction between local meteoric water and volcanic rocks. As to BAW, a wide variability in B concentrations (from 14 μg/l to 769 μg/l) and B isotope composition (from -16.1‰ to +23‰) has been observed. A possible anthropogenic input is thought to be responsible for such variability. The water samples with high B concentrations and negative δ11B values (−12.2‰ to −16.1‰) are probably influenced by agricultural activities; for water samples characterized by positive δ11B values (from +5.8‰ to +7.9‰) and low B contents, a slight contamination by sewage effluents cannot be excluded. These conclusions are also supported by B/Cl molar ratios (7.8E−3 to 1.6E−2) different from those of the HSW group (1.1E−2 to 7.0E−2). This study highlights that also in the Mt. Vulture volcanic area B isotopes coupled to the B/Cl ratio can be a useful tool for the assessment of natural and anthropogenic contaminant sources.


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## Body

## 1. Introduction

The Mt. Vulture basin is one of the most significant aquifer systems of southern Italy. Its groundwater is an important and valuable resource for potable and mineral water supply and agricultural and industrial uses. In the last years, to satisfy the intensifying water demand, groundwater extraction has been gradually increased. However, signs of anthropogenic impact, especially nitrogen contamination, have recently become evident [1]. Assessing the processes controlling groundwater quality is essential for an effective water resource management and protection, especially in the Mt. Vulture area which has an important role for both the local economy and water supply of the whole Basilicata Region. Usually, an isotopic approach can provide information on the type of pollution and on its possible origin. Among minor elements, boron and its isotopes are considered useful environmental tracers for this purpose. Boron is geochemically very mobile and soluble, and in the aqueous environment, it may be as both borate ion (B(OH)4−) and boric acid (H3BO3). Typically, low-saline groundwaters and rivers have low B concentrations (<100 μg/l), whereas saline groundwaters may show high concentrations up to several tens of μg/l [2]. The World Health Organization (WHO) recommends an upper limit of 500 μg/l in drinking water [3]. The drinking water standard of the European Community [4, 5] and Italian legislation [6] is two times this recommended value. The B dissolved in groundwaters is generally derived by natural sources, related to hydrothermal water sources, weathering of igneous rocks, and leaching from sedimentary deposits of marine and nonmarine origin and seawater intrusion [7, 8]. However, anthropogenic boron input due to manufacturing of glass, ceramics, leather, fertilizers, cosmetics, detergents, etc., as well as municipal wastewater and sewage effluent producing a significant environmental impact, have been demonstrated in surface water and groundwater [8–10]. At lower temperatures, boron can be removed from solution by absorption onto clay minerals and incorporation into secondary minerals [8, 9]. Boron has two natural stable isotopes, 11B and 10B, with abundances of 19.8% and 80.2%, respectively. The boron isotope geochemistry has been used in defining magmatic and hydrothermal processes [11, 12], low-temperature geochemical processes [13], paleo-ocean pH reconstruction [14, 15], and global biogeochemical cycles [16]. In addition, several studies have demonstrated the ability of boron isotopes to detect distinct anthropogenic boron sources in surface water and groundwater systems [17–20]. However, it is not always possible to distinguish agricultural signatures from urban effluents and natural sources only by means of B isotopes because values may overlap. Consequently, other chemical elements or elemental ratios have to be considered along with the B isotope to assess the contaminant sources carefully [20–22]. The B/Cl molar ratio is thought to be a geochemical tracer significantly useful for this purpose. For example, B/Cl values tend to be rather homogeneous for fluids derived from a common reservoir [20, 22], denoting it as a useful tool to discriminate B’s origin. With this in mind, the paper focuses on the occurrence and distribution of boron and of its isotopes in the Mt. Vulture groundwaters in order to evaluate B’s origin and to distinguish between natural and anthropogenic contaminant sources.


## 2. Study Area

Mt. Vulture is a Pleistocene stratovolcano composed mainly of pyroclastic deposits and lava flows ranging in composition from foidite, basanites, to phonolites with several intermediate rock types [23, 24]. These products have a strongly silica-undersaturated character with alkaline potassic to ultrapotassic affinities [25]. This volcano is located at the eastern border of the Apennine compressive front, at the western margin of the Apulia foreland (Figure 1). Volcanic activity started at 742 ± 12 ka and continued, interrupted by long-lasting quiescence periods, up to 142 ± 11 ka [26]. In the peripheral sectors of Mt. Vulture, fluviolacustrine deposits of Pliocene and lower Pleistocene age are outcropping (Fiumara di Atella Super-synthem, [27]). The volcanic products lie over Pleistocene gravel and clay, in subhorizontal layers that cover the intensely deformed flysch sediments of Miocene age. The oldest pre-Miocene bedrock units consist principally of deep-sea sediments belonging to units ranging from early Triassic to lower-middle Miocene [28].Figure 1
Geological sketch map of the Mt. Vulture area (modified from Giannandrea et al. [32]). The localization of sampling sites (grey circles), rain gauge (star), and code of the water samples are shown.The Mount Vulture aquifer, mainly hosted by volcanic rocks, consists of interconnected multilayered aquifer systems, locally confined [29]. The groundwater flow pathways are conditioned by structural hydraulic parameters, anisotropy features of the aquifer, and the existing fracture network [29, 30]. The tectonic discontinuities act as deep preferential groundwater flow and promote the mineralization processes of groundwater occurring within two independent hydrogeological basins, the Monticchio–Atella basin (southern area) and the Melfi–Barile basin (north-eastern area) [31].The volcano hosts numerous springs and wells emerging at different elevations, especially in the south-eastern and western sectors (Figure1). The western area of Mt. Vulture is mainly characterized by forests, pastures, and uncultivated fields. Beech forests are widespread at higher altitudes while oak and chestnut trees are found at lower altitudes. Some industrial plants for mineral water extraction are present in a limited area near the Monticchio Bagni site. In the south-eastern area, there are two small villages as well as industrial plants and the agricultural activities are principally related to farming [1].


## 3. Sampling and Analysis

Waters from 34 springs and operating wells, owned by both companies and private individuals and used for irrigation and drinking water supply, were collected during a single field trip between January and March 2009. The wells have depths between 30 and 250 meters and were drilled into volcanic products (pyroclastic and effusive rocks). The sampling sites were selected based on a previous hydrogeological, hydrogeochemical study [31, 33] and tectonic setting of the investigated area. Waters are mainly located in the Monticchio–Atella basin, fourteen sampling sites in the western sector (Monticchio Bagni area), nineteen in the south-eastern area (near the Atella and Rionero in Vulture village), and only one sample in the north-eastern area of the Melfi–Barile basin. In addition, a single sample of meteoric water, collected at the Monticchio Bagni area in a rain gauge for one month of observation (March 2009), was analyzed.B concentrations were measured by inductively coupled plasma-mass spectrometry (Element Finnigan) at the Activation Laboratory of Actlabs (Ontario, Canada). The determination of B was performed using the external standard calibration method, employing NIST and SLRS standard reference materials for calibration. The precision of the analytical results, estimated by running triplicate analyses every 10 samples, was 2-5%. The accuracy of the results (±5%) was obtained by analyzing certified reference materials from the Merck Calibration Laboratory. Samples for B isotopic composition were filtered in the field using 0.45 mm filters. Boron isotopic composition was determined by negative-ion thermal-ionization mass spectrometry (N-TIMS), direct loading procedure at the Isotope Science Laboratory of University of Calgary (ISL-UC), following the methods described by Gaillardet [34]. The external reproducibility of ±1δ (2σmean corresponding to 2s/n0.5, where n=3 replicate analyses) for the B isotope measurements was determined by replicate analyses of the NIST-SRM 951 standard. The B isotopic composition was measured with an average analytical precision (2σ) of ±1δ‰. The boron isotopic compositions are expressed as per mil differences relative to mean 11B/10B ratios of 4.00125 determined for the NIST-SRM 951 boric acid standard.


## 4. Results and Discussion

### 4.1. Groundwater Chemistry

According to previous papers [1, 33, 35], in the Mt. Vulture area two different types of groundwater are distinguished on the basis of physical-chemical parameters and concentrations of dissolved chemical components. Most of the investigated waters have a bicarbonate alkaline-earth and alkaline composition (Figure 2). These waters show temperatures ranging from 8.8°C to 19.9°C and electrical conductivity (EC) from 140 to 1940 μS/cm. Bicarbonate is generally the main anion in solution with concentrations ranging between 66 and 1763 mg/l with an average value of about 508 mg/l [33, 35]. During basalt weathering, CO2 is converted into bicarbonate, showing a positive correlation with the concentration of major and minor elements. The composition of bicarbonate alkaline-earth and alkaline waters (hereafter BAW) is principally due to low-temperature leaching of the host volcanic rocks [35]. The other investigated waters are bicarbonate–sulphate–alkaline (hereafter HSW) in composition and show high salinity [31, 33, 35] and 1183 to 16,400 μs/cm EC values that are higher than in the previous group. The temperature values range from 15.1 to 17.4°C. Such high-salinity springs are enriched in sodium and sulphate with respect to average groundwater composition and have the highest concentration of dissolved CO2 that are likely due to the ongoing active, magmatic-mantle outgassing [36–38]. The HSW group is related to prolonged water circulation in alkali and feldspathoid-rich pyroclastic layers interbedded with clay deposits [33]. Previous studies [1] revealed an inhomogeneous distribution of NO3 concentrations in the Mt. Vulture area. In the western portion, N input has been associated with an organic soil component only. In the south-eastern part, a possible anthropogenic contamination (mainly derived from use of inorganic fertilizer) has been documented. Relative to groundwater origin, stable isotopic data of δ18O and δD highlight that these groundwaters are meteoric in origin and they are not significantly affected by seasonal variations [39]. According to the local vertical isotopic gradient (0.17‰ for δ18O/100 m) defined by Paternoster et al. [39], the meteoric recharge altitude of Mt. Vulture groundwaters is between 800 and 1200 meters above sea level.Figure 2
Piper diagram. Circles are for bicarbonate alkaline-earth and alkaline water (BAW) while squares represent the bicarbonate-sulphate-alkaline waters (high-salinity water, HSW). Data are from Parisi et al. [31, 33].


### 4.2. B Concentrations and Isotopes

Boron contents andδ11B isotopic values of the studied waters show a wide variability (Table 1). Boron concentrations range from 14 to 5130 μg/l, with an average value of 556 μg/l (Figure 3(a)). BAW samples of the western area range from 14 to 652 μg/l, with the only V30 sample having B contents higher than those in the WHO guideline (500 μg/l). BAW samples from the south-eastern area have B concentrations between 45 and 769 μg/l. V01 and V20 samples record B concentrations higher than those in the WHO guideline. The highest B concentrations (from 746 to 5130 μg/l) were observed in the HSW samples. Among these, V6, V14, and V17 samples have B contents higher than the maximum admissible concentration (1000 μg/l) by drinking water Directive 98/83/EC [5] and the Italian legislation [6]. Generally, the Mt. Vulture groundwaters have higher B concentrations compared with those in other volcanic systems in Italy [40, 41]. The δ11B values range from −16.1‰ to +23‰ (Figure 3(b)). BAW samples from the western area have δ11B values from –6.6‰ to –1.6‰. Among these, only two samples (V31 and V32) show both a significant enrichment in heavier boron isotopes (+17.8‰ and +23‰, respectively) and the lowest B concentrations (18 μg/l and 14 μg/l, respectively), similar to the local meteoric water (RW, Table 1) analyzed in this study (δ11B=+26‰ and B=12μg/l). In the south-eastern area, based on B contents and δ11B values, two BAW subsets can be identified: (a) groundwaters having B contents lower than 70 μg/l and positive δ11B values (from +5.8‰ to +7.9‰), (b) groundwaters with B concentrations higher than 70 μg/l and negative δ11B values (from −16.1‰ to −1.9‰) (Figures 3(a) and 3(b)). Finally, HSW have the highest B contents (from 746 to 5130 μg/l) and low δ11B values (from −9.6‰ to −4.3‰).Table 1
B concentration and isotopic composition of the Mt. Vulture groundwaters.
Sample no.
Long. (north)
Lat. (east)
Elevat.
Type
CH
T∗ (°C)
EC∗ (μS/cm)
pH∗
δ11B (‰)
B (μg/L)
B/Cl1
V1
4531210
555380
545
W
BAW
13
350
6.6
−5.1
504
4.8E−02
V2
4528490
554302
544
W
BAW
15.2
237
6.8
−2.7
261
6.6E−02
V3
4528250
554060
600
W
BAW
17.2
233
7.5
+7.9
59
1.4E−02
V4
4528240
555360
545
W
BAW
15
304
6.1
+5.8
67
1.6E−02
V5
4535826
556800
686
W
BAW
13.3
202
5.8
+7.6
54
1.0E−02
V6
4536694
557091
365
S
HSW
17
2910
6.5
−4.3
2210
3.4E−02
V7
4526900
556320
490
W
BAW
15.3
607
6.1
−13.1
275
9.0E−03
V8
4526817
556303
490
W
BAW
14.1
478
6.9
−12.7
233
1.2E−02
V9
4526707
556342
490
W
BAW
13.8
430
6.9
−2.2
175
3.1E−02
V10
4526605
556340
483
W
BAW
16
291
7.0
+6.7
60
1.0E−02
V11
4531235
555467
555
W
BAW
13.1
234
6.2
+6.3
57
1.2E−02
V12
4534820
557160
519
W
BAW
15.7
265
6.5
+7.6
45
7.8E−03
V13
4525820
555300
474
S
BAW
15.7
763
5.9
−4.5
451
3.9E−02
V14
4525900
555260
461
S
HSW
15.1
16,4
6.9
−5.8
3470
1.1E−02
V15
4526955
556359
487
W
BAW
14.7
896
5.9
−9
463
6.3E−02
V16
4531940
548200
575
S
HSW
15.2
1183
5.9
−6.9
746
6.7E−02
V17
4526867
551828
449
S
HSW
17
4350
6.5
−9.4
5130
7.0E−02
V18
4527726
556960
550
W
BAW
15
370
5.9
−1.9
118
1.2E−02
V19
4527739
556970
552
W
BAW
16.1
362
6.3
−12.2
213
1.4E−02
V20
4537726
548012
554
W
BAW
18.5
1717
6.3
−16.1
769
1.3E−02
V21
4526870
556607
493
W
BAW
16.1
435
6.1
−7.1
144
2.3E−02
V22
4532983
547976
547
S
HSW
17.4
2149
6.2
−9.6
923
5.8E−02
V23
4533314
549828
566
W
BAW
16.8
750
5.9
−1.6
264
2.7E−02
V24
4533306
550031
565
W
BAW
18.1
1480
6.0
−5.2
470
3.9E−02
V25
4533379
549532
657
W
BAW
14
351
6.1
−3.9
91
1.3E−02
V26
4532938
551154
690
W
BAW
19.5
1147
6.0
−6.1
146
1.2E−02
V27
4530603
547666
682
W
BAW
18.9
698
5.9
−5.4
246
2.4E−02
V28
4532832
549658
650
W
BAW
19.9
1346
6.1
−6.6
498
3.4E−02
V29
4533100
549100
644
W
BAW
19.5
918
5.9
−4.9
355
2.7E−02
V30
4533672
549815
555
W
BAW
19.6
1940
6.2
−6.6
652
3.9E−02
V31
4532900
551950
885
S
BAW
9.6
206
7.6
+17.8
18
2.6E−03
V32
4533066
552194
960
S
BAW
8.8
140
7.0
+23
14
2.5E−03
V33
4532966
551315
780
W
BAW
15.8
369
5.9
−4.6
84
9.5E−03
V34
4532930
550040
735
W
BAW
16.5
448
5.7
−4.9
102
1.3E−02
R35
rainwater
+26
12
Notes: Elevat.: elevation in meters above sea level; S: spring; W: well; CH: chemical hydrofacies; BAW: bicarbonate alkaline-earth and alkaline water; HSW: bicarbonate–sulphate–alkaline water; EC: electric conductivity measured at 25°C; n.m.: not measured. The location of the sampling point is provided in UTM Zone 33 coordinates using the European Datum of 1950.∗Data from Parisi et al. [31, 33], 1Cl data from Parisi et al. [31, 33].Figure 3
Boron concentrations (a) andδ11B values (b) of the Mt. Vulture groundwaters divided into bicarbonate alkaline-earth and alkaline (BAW) and bicarbonate-sulphate-alkaline (high-salinity water (HSW)) water types.
(a)
(b)


### 4.3. Origin of Boron

In nature, different B reservoirs exist (mantle-derived volcanic rocks:−10‰<δ11B<0‰, marine carbonates: +10‰<δ11B<+30‰, altered oceanic crustal rocks: 0‰<δ11B<+25‰, old continental rocks and siliciclastic sediments: −15‰<δ11B<+5‰, seawater: δ11B~+40‰) with wide ranges of B isotope composition that sometimes overlap each other [41]. According to D’Orazio et al. [42], the Mt. Vulture volcanic rocks display negative δ11B values (ranging between −9.6‰ and −5.6‰) that are strongly consistent with values of the HSW group (−9.6‰ and −4.3‰) suggesting that the water-volcanic rock interaction is the main process controlling the chemical composition of the HSW samples. The aforementioned silica-undersaturated volcanic products are thought to represent the main source of B also in the BAW group although the B isotope composition of some BAW samples is different. However, based on hydrogeological issue, the Mt. Vulture volcanic products clearly host all the analyzed groundwaters suggesting a negligible contribution from the other rocks (such as flysch sediments and basement rocks) occurring in the studied area. In order to assess the B origin in the BAW group, a mixing model between the local meteoric water and volcanic-hosted end-member (HSW) has been taken into account. The mixing model can be quantitatively estimated by means of the following equations:
(1)Bmix=Bc×f+Bd×1−f,(2)δ11Bmix=δ11Bc×BcBmix×f+δ11Bd×BdBmix×1−f,where Bmix, Bc, and Bd, as well as δ11Bmix, δ11Bc, and δ11Bd are the B concentrations and isotopic values of mixing water, local meteoric water, and HSW group, respectively. f is the mixing ratio. For calculations, the composition of the local meteoric water sample (RW, δ11B=+26‰, B=12μg/l) and the highest and lowest δ11B values of the HSW group were used. In Figure 4, the ML1 and ML2 mixing curves are shown along with investigated water samples. BAW samples of the western area are concentrated in a narrow range within the two mixing curves, suggesting B is mainly sourced from host volcanic rocks. V31 and V32 samples, enriched in heavier B isotopes, fall close to the local meteoric water end-member. Regarding the BAW samples from the south-eastern area, because of their compositional variability, groundwaters fall in different fields suggesting more B sources. Some water samples, similar to the western area BAW samples, lie between the two mixing curves denoting a natural B source due to water-volcanic rock interaction. The V7, V8, V19, and V20 samples, depleted in heavy B isotopes (from −16.1‰ to −12.2‰) and with B concentrations between 213 and 769 μg/l, are far from both ML1 and ML2 mixing curves falling near the MLA mixing curve between local meteoric water and water-soluble anthropogenic boron end-member consisting of synthetic Ca-borate fertilizers (Figure 4). The Ca-borate and Na-borate fertilizers are the most used synthetic compounds in the agricultural activities and along with sodium perborate, municipal wastewaters, sewage effluents, and landfill leachates represent the main anthropogenic contaminants in the groundwater and shallow water [2, 9]. According to Mongelli et al. [1], in the study area agricultural activities are widely practiced suggesting for these water samples an anthropogenic B origin mainly associated with use of synthetic Ca-borate fertilizers.Figure 4
δ11B (‰) versus B content (μg/l) binary plot. ML1 and ML2 curves indicate the mixing between local meteoric end-member (rainwater = RW, δ11B=+26‰; B=12μg/l) and range of isotopic values of the HSW group (δ11B=−4.3‰ and −9.6‰; B=5130μg/l). The MLA curve represents the hypothetical mixing behavior between local meteoric water and water-soluble anthropogenic boron compounds (synthetic Ca-borate fertilizers, average δ11B value of -13.5‰ and B=30mg/l, [21]). The MSA mixing curve is drawn with following data (RW, and sewage effluents with average δ11B value of +2.6‰ and B=1000μg/l; from Barth [43], Xue et al. [44], and Vengosh et al. [9]). Ranges of δ11B of sewage effluents (−7.7‰ to +12.9‰; [9, 43, 44]), sodium perborates (–4.8‰ to +3.1‰; [2]), detergents (−2.9‰ to +3.1‰; [2]), synthetic Ca-borate fertilizers (–15‰ to –12.1‰; [21]), and manure (+6.9‰ to +42.1‰, [17, 44]) are shown for comparison.As to BAW samples with low B values (less than 70μg/l), a marked enrichment in 11B is evident (Figure 4). These water samples do not fit the ML1, ML2, and MLA mixing curves, but their path is approximated by a new mixing curve (MLS). The MLS curve is between local meteoric water and groundwater contaminated by sewage effluents (−7.7‰<δ11B<+12.9‰, Xue et al. [44], Vengosh et al. [9]). The B/Cl ratio also supported this finding. Several studies have revealed that B/Cl ratios tend to be rather homogeneous for fluids derived from a common reservoir [20, 21, 45]. In groundwater contaminated by domestic wastewater, sewage effluents, and other anthropogenic sources (mainly from Na- or Ca–borates), the B/Cl ratios coupled to a distinctive B isotopic signature allow us to distinguish different contaminant sources [20, 21]. For the studied groundwaters, the B/Cl molar ratios were calculated based on Cl data from Parisi et al. [31, 33] and range between 7.0E−2 and 2.5E−3 (Table 1). As shown in Figure 5 (δ11B vs B/Cl molar), most of the investigated water samples fall in the field of groundwater in nonmarine aquifers [21]. Two different groups of water samples deviate from the general trend of the measured B/Cl values. In detail, the water samples with low B isotopic values show B/Cl values falling in the range of groundwater contaminated by synthetic Ca-borate fertilizers; the water samples with high B isotopic values are in the field of sewage-contaminated groundwater, although other anthropogenic contaminant sources (i.e., municipal wastewater) cannot be excluded because of overlapping between ranges of their B isotopic values (+5.3‰ to +20.5‰, [2, 46]).Figure 5
Pattern of typical ranges ofδ11B and B/Cl values of potential contamination sources, after Vengosh et al. [21]. Symbols are as in Figure 4.


## 4.1. Groundwater Chemistry

According to previous papers [1, 33, 35], in the Mt. Vulture area two different types of groundwater are distinguished on the basis of physical-chemical parameters and concentrations of dissolved chemical components. Most of the investigated waters have a bicarbonate alkaline-earth and alkaline composition (Figure 2). These waters show temperatures ranging from 8.8°C to 19.9°C and electrical conductivity (EC) from 140 to 1940 μS/cm. Bicarbonate is generally the main anion in solution with concentrations ranging between 66 and 1763 mg/l with an average value of about 508 mg/l [33, 35]. During basalt weathering, CO2 is converted into bicarbonate, showing a positive correlation with the concentration of major and minor elements. The composition of bicarbonate alkaline-earth and alkaline waters (hereafter BAW) is principally due to low-temperature leaching of the host volcanic rocks [35]. The other investigated waters are bicarbonate–sulphate–alkaline (hereafter HSW) in composition and show high salinity [31, 33, 35] and 1183 to 16,400 μs/cm EC values that are higher than in the previous group. The temperature values range from 15.1 to 17.4°C. Such high-salinity springs are enriched in sodium and sulphate with respect to average groundwater composition and have the highest concentration of dissolved CO2 that are likely due to the ongoing active, magmatic-mantle outgassing [36–38]. The HSW group is related to prolonged water circulation in alkali and feldspathoid-rich pyroclastic layers interbedded with clay deposits [33]. Previous studies [1] revealed an inhomogeneous distribution of NO3 concentrations in the Mt. Vulture area. In the western portion, N input has been associated with an organic soil component only. In the south-eastern part, a possible anthropogenic contamination (mainly derived from use of inorganic fertilizer) has been documented. Relative to groundwater origin, stable isotopic data of δ18O and δD highlight that these groundwaters are meteoric in origin and they are not significantly affected by seasonal variations [39]. According to the local vertical isotopic gradient (0.17‰ for δ18O/100 m) defined by Paternoster et al. [39], the meteoric recharge altitude of Mt. Vulture groundwaters is between 800 and 1200 meters above sea level.Figure 2
Piper diagram. Circles are for bicarbonate alkaline-earth and alkaline water (BAW) while squares represent the bicarbonate-sulphate-alkaline waters (high-salinity water, HSW). Data are from Parisi et al. [31, 33].


## 4.2. B Concentrations and Isotopes

Boron contents andδ11B isotopic values of the studied waters show a wide variability (Table 1). Boron concentrations range from 14 to 5130 μg/l, with an average value of 556 μg/l (Figure 3(a)). BAW samples of the western area range from 14 to 652 μg/l, with the only V30 sample having B contents higher than those in the WHO guideline (500 μg/l). BAW samples from the south-eastern area have B concentrations between 45 and 769 μg/l. V01 and V20 samples record B concentrations higher than those in the WHO guideline. The highest B concentrations (from 746 to 5130 μg/l) were observed in the HSW samples. Among these, V6, V14, and V17 samples have B contents higher than the maximum admissible concentration (1000 μg/l) by drinking water Directive 98/83/EC [5] and the Italian legislation [6]. Generally, the Mt. Vulture groundwaters have higher B concentrations compared with those in other volcanic systems in Italy [40, 41]. The δ11B values range from −16.1‰ to +23‰ (Figure 3(b)). BAW samples from the western area have δ11B values from –6.6‰ to –1.6‰. Among these, only two samples (V31 and V32) show both a significant enrichment in heavier boron isotopes (+17.8‰ and +23‰, respectively) and the lowest B concentrations (18 μg/l and 14 μg/l, respectively), similar to the local meteoric water (RW, Table 1) analyzed in this study (δ11B=+26‰ and B=12μg/l). In the south-eastern area, based on B contents and δ11B values, two BAW subsets can be identified: (a) groundwaters having B contents lower than 70 μg/l and positive δ11B values (from +5.8‰ to +7.9‰), (b) groundwaters with B concentrations higher than 70 μg/l and negative δ11B values (from −16.1‰ to −1.9‰) (Figures 3(a) and 3(b)). Finally, HSW have the highest B contents (from 746 to 5130 μg/l) and low δ11B values (from −9.6‰ to −4.3‰).Table 1
B concentration and isotopic composition of the Mt. Vulture groundwaters.
Sample no.
Long. (north)
Lat. (east)
Elevat.
Type
CH
T∗ (°C)
EC∗ (μS/cm)
pH∗
δ11B (‰)
B (μg/L)
B/Cl1
V1
4531210
555380
545
W
BAW
13
350
6.6
−5.1
504
4.8E−02
V2
4528490
554302
544
W
BAW
15.2
237
6.8
−2.7
261
6.6E−02
V3
4528250
554060
600
W
BAW
17.2
233
7.5
+7.9
59
1.4E−02
V4
4528240
555360
545
W
BAW
15
304
6.1
+5.8
67
1.6E−02
V5
4535826
556800
686
W
BAW
13.3
202
5.8
+7.6
54
1.0E−02
V6
4536694
557091
365
S
HSW
17
2910
6.5
−4.3
2210
3.4E−02
V7
4526900
556320
490
W
BAW
15.3
607
6.1
−13.1
275
9.0E−03
V8
4526817
556303
490
W
BAW
14.1
478
6.9
−12.7
233
1.2E−02
V9
4526707
556342
490
W
BAW
13.8
430
6.9
−2.2
175
3.1E−02
V10
4526605
556340
483
W
BAW
16
291
7.0
+6.7
60
1.0E−02
V11
4531235
555467
555
W
BAW
13.1
234
6.2
+6.3
57
1.2E−02
V12
4534820
557160
519
W
BAW
15.7
265
6.5
+7.6
45
7.8E−03
V13
4525820
555300
474
S
BAW
15.7
763
5.9
−4.5
451
3.9E−02
V14
4525900
555260
461
S
HSW
15.1
16,4
6.9
−5.8
3470
1.1E−02
V15
4526955
556359
487
W
BAW
14.7
896
5.9
−9
463
6.3E−02
V16
4531940
548200
575
S
HSW
15.2
1183
5.9
−6.9
746
6.7E−02
V17
4526867
551828
449
S
HSW
17
4350
6.5
−9.4
5130
7.0E−02
V18
4527726
556960
550
W
BAW
15
370
5.9
−1.9
118
1.2E−02
V19
4527739
556970
552
W
BAW
16.1
362
6.3
−12.2
213
1.4E−02
V20
4537726
548012
554
W
BAW
18.5
1717
6.3
−16.1
769
1.3E−02
V21
4526870
556607
493
W
BAW
16.1
435
6.1
−7.1
144
2.3E−02
V22
4532983
547976
547
S
HSW
17.4
2149
6.2
−9.6
923
5.8E−02
V23
4533314
549828
566
W
BAW
16.8
750
5.9
−1.6
264
2.7E−02
V24
4533306
550031
565
W
BAW
18.1
1480
6.0
−5.2
470
3.9E−02
V25
4533379
549532
657
W
BAW
14
351
6.1
−3.9
91
1.3E−02
V26
4532938
551154
690
W
BAW
19.5
1147
6.0
−6.1
146
1.2E−02
V27
4530603
547666
682
W
BAW
18.9
698
5.9
−5.4
246
2.4E−02
V28
4532832
549658
650
W
BAW
19.9
1346
6.1
−6.6
498
3.4E−02
V29
4533100
549100
644
W
BAW
19.5
918
5.9
−4.9
355
2.7E−02
V30
4533672
549815
555
W
BAW
19.6
1940
6.2
−6.6
652
3.9E−02
V31
4532900
551950
885
S
BAW
9.6
206
7.6
+17.8
18
2.6E−03
V32
4533066
552194
960
S
BAW
8.8
140
7.0
+23
14
2.5E−03
V33
4532966
551315
780
W
BAW
15.8
369
5.9
−4.6
84
9.5E−03
V34
4532930
550040
735
W
BAW
16.5
448
5.7
−4.9
102
1.3E−02
R35
rainwater
+26
12
Notes: Elevat.: elevation in meters above sea level; S: spring; W: well; CH: chemical hydrofacies; BAW: bicarbonate alkaline-earth and alkaline water; HSW: bicarbonate–sulphate–alkaline water; EC: electric conductivity measured at 25°C; n.m.: not measured. The location of the sampling point is provided in UTM Zone 33 coordinates using the European Datum of 1950.∗Data from Parisi et al. [31, 33], 1Cl data from Parisi et al. [31, 33].Figure 3
Boron concentrations (a) andδ11B values (b) of the Mt. Vulture groundwaters divided into bicarbonate alkaline-earth and alkaline (BAW) and bicarbonate-sulphate-alkaline (high-salinity water (HSW)) water types.
(a)
(b)


## 4.3. Origin of Boron

In nature, different B reservoirs exist (mantle-derived volcanic rocks:−10‰<δ11B<0‰, marine carbonates: +10‰<δ11B<+30‰, altered oceanic crustal rocks: 0‰<δ11B<+25‰, old continental rocks and siliciclastic sediments: −15‰<δ11B<+5‰, seawater: δ11B~+40‰) with wide ranges of B isotope composition that sometimes overlap each other [41]. According to D’Orazio et al. [42], the Mt. Vulture volcanic rocks display negative δ11B values (ranging between −9.6‰ and −5.6‰) that are strongly consistent with values of the HSW group (−9.6‰ and −4.3‰) suggesting that the water-volcanic rock interaction is the main process controlling the chemical composition of the HSW samples. The aforementioned silica-undersaturated volcanic products are thought to represent the main source of B also in the BAW group although the B isotope composition of some BAW samples is different. However, based on hydrogeological issue, the Mt. Vulture volcanic products clearly host all the analyzed groundwaters suggesting a negligible contribution from the other rocks (such as flysch sediments and basement rocks) occurring in the studied area. In order to assess the B origin in the BAW group, a mixing model between the local meteoric water and volcanic-hosted end-member (HSW) has been taken into account. The mixing model can be quantitatively estimated by means of the following equations:
(1)Bmix=Bc×f+Bd×1−f,(2)δ11Bmix=δ11Bc×BcBmix×f+δ11Bd×BdBmix×1−f,where Bmix, Bc, and Bd, as well as δ11Bmix, δ11Bc, and δ11Bd are the B concentrations and isotopic values of mixing water, local meteoric water, and HSW group, respectively. f is the mixing ratio. For calculations, the composition of the local meteoric water sample (RW, δ11B=+26‰, B=12μg/l) and the highest and lowest δ11B values of the HSW group were used. In Figure 4, the ML1 and ML2 mixing curves are shown along with investigated water samples. BAW samples of the western area are concentrated in a narrow range within the two mixing curves, suggesting B is mainly sourced from host volcanic rocks. V31 and V32 samples, enriched in heavier B isotopes, fall close to the local meteoric water end-member. Regarding the BAW samples from the south-eastern area, because of their compositional variability, groundwaters fall in different fields suggesting more B sources. Some water samples, similar to the western area BAW samples, lie between the two mixing curves denoting a natural B source due to water-volcanic rock interaction. The V7, V8, V19, and V20 samples, depleted in heavy B isotopes (from −16.1‰ to −12.2‰) and with B concentrations between 213 and 769 μg/l, are far from both ML1 and ML2 mixing curves falling near the MLA mixing curve between local meteoric water and water-soluble anthropogenic boron end-member consisting of synthetic Ca-borate fertilizers (Figure 4). The Ca-borate and Na-borate fertilizers are the most used synthetic compounds in the agricultural activities and along with sodium perborate, municipal wastewaters, sewage effluents, and landfill leachates represent the main anthropogenic contaminants in the groundwater and shallow water [2, 9]. According to Mongelli et al. [1], in the study area agricultural activities are widely practiced suggesting for these water samples an anthropogenic B origin mainly associated with use of synthetic Ca-borate fertilizers.Figure 4
δ11B (‰) versus B content (μg/l) binary plot. ML1 and ML2 curves indicate the mixing between local meteoric end-member (rainwater = RW, δ11B=+26‰; B=12μg/l) and range of isotopic values of the HSW group (δ11B=−4.3‰ and −9.6‰; B=5130μg/l). The MLA curve represents the hypothetical mixing behavior between local meteoric water and water-soluble anthropogenic boron compounds (synthetic Ca-borate fertilizers, average δ11B value of -13.5‰ and B=30mg/l, [21]). The MSA mixing curve is drawn with following data (RW, and sewage effluents with average δ11B value of +2.6‰ and B=1000μg/l; from Barth [43], Xue et al. [44], and Vengosh et al. [9]). Ranges of δ11B of sewage effluents (−7.7‰ to +12.9‰; [9, 43, 44]), sodium perborates (–4.8‰ to +3.1‰; [2]), detergents (−2.9‰ to +3.1‰; [2]), synthetic Ca-borate fertilizers (–15‰ to –12.1‰; [21]), and manure (+6.9‰ to +42.1‰, [17, 44]) are shown for comparison.As to BAW samples with low B values (less than 70μg/l), a marked enrichment in 11B is evident (Figure 4). These water samples do not fit the ML1, ML2, and MLA mixing curves, but their path is approximated by a new mixing curve (MLS). The MLS curve is between local meteoric water and groundwater contaminated by sewage effluents (−7.7‰<δ11B<+12.9‰, Xue et al. [44], Vengosh et al. [9]). The B/Cl ratio also supported this finding. Several studies have revealed that B/Cl ratios tend to be rather homogeneous for fluids derived from a common reservoir [20, 21, 45]. In groundwater contaminated by domestic wastewater, sewage effluents, and other anthropogenic sources (mainly from Na- or Ca–borates), the B/Cl ratios coupled to a distinctive B isotopic signature allow us to distinguish different contaminant sources [20, 21]. For the studied groundwaters, the B/Cl molar ratios were calculated based on Cl data from Parisi et al. [31, 33] and range between 7.0E−2 and 2.5E−3 (Table 1). As shown in Figure 5 (δ11B vs B/Cl molar), most of the investigated water samples fall in the field of groundwater in nonmarine aquifers [21]. Two different groups of water samples deviate from the general trend of the measured B/Cl values. In detail, the water samples with low B isotopic values show B/Cl values falling in the range of groundwater contaminated by synthetic Ca-borate fertilizers; the water samples with high B isotopic values are in the field of sewage-contaminated groundwater, although other anthropogenic contaminant sources (i.e., municipal wastewater) cannot be excluded because of overlapping between ranges of their B isotopic values (+5.3‰ to +20.5‰, [2, 46]).Figure 5
Pattern of typical ranges ofδ11B and B/Cl values of potential contamination sources, after Vengosh et al. [21]. Symbols are as in Figure 4.


## 5. Conclusions

B concentration and isotope composition of Mt. Vulture groundwaters were determined to assess B’s origin and to distinguish between natural and anthropogenic contaminant sources. In the study area, two water types have been identified. The first type, named high-salinity water (HSW), is characterized by the highest B contents (from 746 to 5130μg/l), and low δ11B values (from –9.6‰ to –4.3‰), similar to those measured by D’Orazio [42] in the local magmas (from −9.6‰ to−5.6‰), suggesting that prolonged water-volcanic rock interaction is the main B source. The second water type, named bicarbonate alkaline-earth and alkaline waters (BAW), records a wide variability in the B contents and isotopic values. The δ11B values of the BAW group suggest that boron is mainly derived from leaching of volcanic rocks, although a contribution from local anthropogenic contaminants cannot be excluded for two BAW subsets. The water samples with high B concentrations and the lowest δ11B values can be probably affected by water-soluble anthropogenic boron compounds consisting of synthetic Ca-borate fertilizers; other samples with high B isotopic values show B/Cl values, different from those of the HSW group, falling in the field of sewage-contaminated groundwater. This manuscript highlights that the B isotopes coupled to the B/Cl ratio in the Mt. Vulture volcanic area allow generally to define B’s origin and evaluate natural and anthropogenic contaminant sources.



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