GEOCHEMICAL
FEATURES
OF Scienze
THERMAL
WATERS
AT BENETUTTI
Rendiconti Seminario
Facoltà
Università
Cagliari
Vol. 73 Fasc. (SARDINIA)
1 (2003)
Geochemical features of thermal waters
at Benetutti (Sardinia)
ROSA CIDU(*), ANTONIO DAMIANO MULAS(*)
Riassunto. Questo lavoro riporta le caratteristiche chimiche delle acque termali e
fredde nella zona di Benetutti (Sardegna Centrale). In 24 campioni d’acqua sono stati
determinati numerosi componenti maggiori ed in tracce utilizzando diverse tecniche
analitiche. Le acque termali hanno una composizione cloruro sodica dominante e
salinità di 0.5 g/L; sono caratterizzate da pH alcalino (9.5-9.7), concentrazioni molto
basse in Mg (0.01-0.1 mg/L) e concentrazioni di F, B, Li, Rb, Cs e Mo molto più elevate
rispetto a quelle osservate nelle acque fredde dell’area. Per i campioni di cui si
dispongono dati bibliografici, non sono state osservate variazioni significative nei
valori di temperatura, portata e composizione chimica negli ultimi 20 anni. Gli
acquiferi profondi associati alle acque termali sono stimati avere bassa entalpia (la
temperatura calcolata in profondità è di circa 60°C). Oltre l’uso terapeutico, le acque
termali possono essere utilizzate per il riscaldamento di edifici e serre.
Parole chiave: acque termali, elementi in tracce, Sardegna.
Abstract. This paper reports the geochemistry of the thermal and cold waters at
Benetutti in central Sardinia. Both major and trace components in 24 water samples
were analysed using various techniques. The thermal waters show a sodium chloride
composition and a salinity of 0.5 g/L; they are characterised by a pH of 9.5-9.7, very
low magnesium (0.01-0.1 mg/L), and much higher F, B, Li, Rb, Cs, and Mo concentrations
than in the cold waters of the area. No variations in temperature, flow, or chemical
composition were observed at the thermal springs for which records dating back 20
years are available. Subsurface reservoirs associated with the thermal springs are of
a low enthalpy (calculated temperature at depth is about 60°C), and are therefore
candidates for direct uses, such as balneology, space heating, and horticulture.
Key words: thermal waters, trace elements, Sardinia.
(*) Dipartimento Scienze della Terra Università di Cagliari, via Trentino 51, 09127 Cagliari, Italy.
Presentato il 27/03/03
39
40
R. CIDU, A.D. MULAS
INTRODUCTION
In the Tirso valley of Central Sardinia, thermal waters occur from north to south at
Benetutti, Oddini, and Fordongianus. They show variable emergent temperatures (33-55°C)
but all are characterised by high pH (> 9), low salinity (< 1 g/L), sodium-chloride
composition, very low (< 20 µg/L) magnesium, and high (up to 9 mg/L) fluoride
concentrations [1] [2]. The thermal waters in the Tirso valley are related to important tectonic
structures trending NE-SW and E-W [3]. Isotopic studies based on deuterium and oxygen18 indicate that the thermal waters from the Tirso valley are meteoric waters that infiltrate
at depth and warm up [4].
At Benetutti, a village of about 2,000 inhabitants, thermal waters have long been known,
as testified by the Roman bridge on the river Tirso nearby and by the ancient church of San
Saturnino. They represent an important resource in an area in which the main economic
activities rely on poorly developed farming. Thermal waters have been used therapeutically,
especially at the Terme Aurora spa. The local authority is now planning to build a resort
area for the development of tourist activities near the thermal springs. This research is aimed
at studying the hydrogeochemical features at Benetutti and at highlighting the peculiar
characteristics of the thermal waters compared to the cold waters of the area.
STUDY AREA
The study area of Benetutti in Central Sardinia extends for about 60 km2, with a
maximum elevation of 417 m a.s.l (fig. 1). The climate is characterised by rainy seasons,
usually extending from October to April, and long periods of dry weather. At the Bultei
meteorological station (the closest to the study area) at 510 m a.s.l., the long-term (19301980 period) mean rainfall is 800 mm/y with a mean of 70 rainy days; the highest mean
monthly precipitations occur in December and January, while the lowest are in July and
August; the mean annual temperature is 15°C [5].
The geology of the investigated area is shown schematically in Figure 1. Late-tectonic
and post-tectonic Hercynian granite are the dominant rocks, and are mainly represented
by medium-grained, equigranular granodiorite [6] [7] [8]. In Tertiary times, the Palaeozoic
basement of Sardinia was affected by important tectonic activity, and the island was
characterised by massive volcanic activity [9] [10]. In the Benetutti area, volcanism
developed between the Upper Oligocene and the Lower Miocene, with deposition of lava
and ignimbrite tuffaceous sequences [11]. Recent deposits consist of alluvial sediments
and outcrop close to the river Tirso (Fiume Tirso in fig. 1).
The hydrology is characterised by the river Tirso and its main tributary Rio Mannu; lowflow streamlets occur temporarily over the rainy season. Both the surface and ground waters
of the area mainly drain granite rocks; sample no. 16 only partially drains volcanic rocks.
The thermal springs are located few kilometres south of Benetutti in a limited area close to
the river Tirso (fig. 1).
GEOCHEMICAL FEATURES OF THERMAL WATERS AT BENETUTTI (SARDINIA)
41
Figure 1. Schematic geological map of the Benetutti area and location of the water samples.
METHODS
Water sampling was carried out in May 2001 after two months of drought and a lowrain winter season. The sampling period therefore represents low-flow conditions. At the
sampling site, temperature, pH, redox potential, conductivity, hydrogen sulphide and
May 2001
2000
May 2001
May 2001
1985
1988
Apr. 1989
July 1989
Oct. 1989
1990
May 2001
May 2001
1982
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
May 2001
1
1
2
3
3
3
3
3
3
3
4
5
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
0.05
0.1
0.03
0.05
0.05
0.05
0.03
50
0.02
0.05
0.3
0.1
0.1
0.3
0.3
0.05
100
0.2
0.03
22
nr
22
24
nr
nr
nr
nr
nr
nr
24
29
nr
29
29
28
27
26
22
25
20
21
22
26
33
29
29
33
33
30
32
32
40
40
36
42
41
41
42
42
42
42
27
41
43
42
33
33
39
42
19
26
21
24
18
18
35
18
22
24
15
18
17
27
9.6
9.4
9.5
9.6
9.3
9.4
9.2
9.2
9.3
9.0
9.5
9.5
9.0
9.6
9.6
9.6
9.7
9.6
9.6
7.5
9.5
9.5
6.6
8.0
9.5
7.0
6.8
7.4
6.6
7.4
6.2
7.7
82
nr
120
86
nr
nr
90
110
100
-20
290
170
nr
290
85
100
73
56
185
410
150
360
420
330
220
450
450
430
480
450
490
460
1.27
0.76
1.10
1.08
nr
nr
nr
nr
nr
0.91
0.98
1.03
nr
1.05
0.99
1.00
0.95
1.00
1.01
0.59
1.07
1.10
1.07
1.01
0.85
1.14
1.11
1.01
0.62
0.89
0.75
0.30
520
502
545
525
477
528
509
502
508
489
519
507
409
512
498
503
492
492
486
274
525
545
531
709
512
836
897
615
476
574
686
197
Flow T°C T°C pH Eh Cond TDS
L/s air water
mV mS/cm mg/L
TDS: total dissolved solids; [17]: reference; nr: not reported.
Terme Aurora A
Terme Aurora [17]
Terme Aurora B
Banzu sos beccos
Banzu sos beccos [15]
Banzu sos beccos [1]
Banzu sos beccos [16]
Banzu sos beccos [16]
Banzu sos beccos [16]
Banzu sos beccos [2]
Sorgente Sos Beccos
Banzu Mazzore
Banzu Mazzore [3]
Sorgente Casa Tanda
San Saturnino chiesa
San Saturnino
Su Giudice
Banzu sos nervios
Abba Putida
Fiume Tirso
Pozzo Agriturismo Lai
Fontana Lai Maurizio
Sorgente degli occhi
Pozzo N.ghe Luzzanas
Sorgente Ziu Paulu
Sorgente Urchi
Sorgente F.lli Lai
Sorgente Linia
Sorgente Su Cantaru
Sorgente Boloe
Sorgente Pauleddu
Rio Mannu
Date of
sampling
No. Name
12
11
14
12
11
12
11
11
14
13
11
11
10
11
9.4
9.5
8.6
8.7
8.7
19
14
14
13
18
12
45
77
16
32
39
37
14
0.045
0.02
0.042
0.029
0.1
0.01
0.12
0.12
0.14
<0.1
0.019
0.063
<1
0.023
0.012
0.012
0.011
0.022
0.043
11.4
0.045
0.067
0.064
11.2
0.10
28.6
38.2
11.9
19.7
26.9
29.6
9.3
161
154
167
163
165
177
169
162
167
156
161
155
146
158
153
152
152
151
160
53
165
170
167
184
161
164
89
147
78
98
101
39
2.8
3.3
2.8
3.0
3.0
3.1
3.1
3.1
3.0
2.8
3.2
3.1
3.0
2.7
2.6
2.6
2.2
2.3
2.0
2.9
2.8
2.9
2.7
2.6
2.6
6.1
18.4
1.7
5.7
3.3
1.0
1.2
37
32
29
31
42
31
34
34
35
38
25
24
nr
25
27
30
32
24
26
82
25
28
40
218
24
287
163
207
175
173
102
54
4.5
nr
6.0
7.2
nr
nr
nr
nr
nr
nr
6.6
8.4
6.0
9.1
9.4
10.8
9.6
10.5
7.0
5.9
5.5
8.0
233
213
250
231
203
219
221
219
215
208
235
228
211
229
224
225
220
219
217
85
235
244
242
204
229
245
204
150
121
157
142
64
Ca
Mg
Na
K HCO3 CO3
Cl
mg/L mg/L mg/L mg/L mg/L mg/L mg/L
41
42
45
42
33
40
40
42
40
40
42
42
33
41
39
40
39
40
37
17
46
49
42
58
42
49
44
26
25
25
51
15
SO4
mg/L
Table 1. Chemical composition of the waters at Benetutti. Previous records of the thermal waters at Benetutti are reported in italics for comparison.
>1
5.0
1
0.6
nr
nr
nr
nr
0.6
nr
0.1
0.2
nr
0.05
0.9
1
>1
>1
0.4
<0.05
0.4
0.05
<0.05
<0.05
0.7
nd
nd
nd
nd
nd
nd
nd
HSmg/L
<0.1
nr
<0.1
<0.1
nr
nr
nr
nr
nr
nr
<0.1
<0.1
nr
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
0.53
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
16.7
<0.1
<0.1
0.2
<0.1
<0.1
NO2
mg/L
<0.1
0.62
<0.1
<0.1
nr
nr
nr
nr
nr
nr
0.22
<0.1
nr
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
4.35
0.22
0.23
249
46
2.9
37
210
<0.1
NO3
mg/L
<0.1
0.01
<0.1
<0.1
nr
nr
nr
nr
nr
nr
<0.1
<0.1
nr
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
0.37
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
0.71
<0.1
1.19
0.63
<0.1
<0.1
PO4
mg/L
42
R. CIDU, A.D. MULAS
0.74
4.85
0.79
0.75
nr
nr
nr
nr
nr
nr
0.71
0.76
nr
0.71
0.67
0.76
0.69
0.74
0.68
0.20
0.70
0.79
0.80
0.63
0.71
0.78
0.45
0.44
0.23
0.44
0.48
0.18
1
1
2
3
3
3
3
3
3
3
4
5
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
9.37
20.6
9.43
9.83
8.9
8.8
8.7
9.1
9.7
9.1
9.81
8.79
nr
8.88
8.07
8.22
7.06
6.82
7.49
0.16
8.82
8.81
8.84
1.54
9.22
0.42
0.23
0.38
0.19
0.25
0.22
0.13
Br
F
mg/L mg/L
No.
Table 1. Continued.
38.4
37.7
35.5
37.5
41.8
42.1
39.1
38.3
42.0
40.7
37.8
39.1
41.0
39.3
37.9
38.9
38.8
38.3
31.4
6.3
34.0
34.6
30.3
14.9
35.7
22.2
26.7
18.9
22.2
29.8
23.1
6.7
SiO2
mg/L
<6
40
<6
15
nr
nr
nr
nr
nr
nr
<6
44
nr
12
<6
<6
11
16
13
69
<6
19
25
11
14
<6
650
<6
6
33
8
41
Al
µg/L
2.2
nr
1.9
2.1
nr
nr
nr
nr
nr
3.0
2.2
2.4
nr
2.5
2.5
2.7
2.7
2.2
1.2
0.16
1.3
1.6
0.37
<0.1
2.2
0.41
1.9
<0.1
0.56
0.17
0.19
0.11
Ga
µg/L
108
290
105
108
100
105
111
111
121
109
112
116
nr
117
109
112
112
115
67
66
102
101
105
77
109
61
43
42
209
43
28
14
B
µg/L
81
60
83
86
84
93
86
87
94
79
87
88
nr
89
78
81
74
74
73
2.9
89
89
85
19
85
64
19
35
5.4
9.5
8.9
2.5
Li
µg/L
27.5
nr
27.6
25.3
26
20
10
15
24
29
26.0
24.3
nr
25.1
22.9
23.9
23.0
18.1
15.9
1.8
26.7
28.9
25.6
6.9
24.8
1.2
4.4
<0.1
0.13
0.48
0.87
0.71
Rb
µg/L
4.5
nr
4.4
4.1
<5
nr
10
13
17
4.7
4.2
3.8
nr
4.1
3.7
3.9
3.7
2.8
1.5
<0.1
3.6
4.1
2.8
1.5
3.8
0.32
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
Cs
µg/L
169
180
204
173
167
175
167
161
168
nr
169
146
nr
152
131
136
116
114
116
98
215
223
190
125
183
315
488
114
190
220
260
90
0.38
0.08
0.37
0.46
nr
nr
nr
nr
nr
nr
0.18
0.22
nr
0.78
0.14
0.3
0.4
0.46
1.01
11
0.33
1.06
0.79
3
0.34
32
110
4.9
36
13.4
16.1
9.1
Sr Ba
µg/L µg/L
6
10
10
19
nr
nr
nr
nr
nr
nr
<4
13
nr
5
12
<4
15
8
18
280
<4
24
83
21
12
<4
1150
6
23
214
16
260
Fe
µg/L
0.21
7
0.77
0.79
nr
nr
nr
nr
nr
nr
0.14
1.76
nr
0.66
0.22
0.22
1.17
1.24
5.07
30
0.51
1.92
8.5
22
0.73
470
257
3.3
4.3
100
4.3
16
Mn
µg/L
0.6
nr
7.5
25
nr
nr
nr
nr
nr
nr
1.4
3.6
nr
10
2.3
3.5
5
9.5
4.3
6.5
4.5
4.9
7.3
29
5.2
8.5
17
2.5
6.2
6.2
11
7.7
Zn
µg/L
0.05
2
0.06
0.06
nr
nr
nr
nr
nr
nr
0.06
0.05
nr
0.06
0.05
0.06
0.05
<0.05
0.07
<0.05
0.06
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
Cd
µg/L
<0.05
5
<0.05
0.08
nr
nr
nr
nr
nr
nr
<0.05
0.15
nr
<0.05
<0.05
<0.05
0.24
0.35
0.12
0.34
<0.05
<0.05
<0.05
0.24
0.29
0.07
0.77
<0.05
<0.1
0.2
<0.05
0.09
Pb
µg/L
<0.2
3.0
1.2
2.3
nr
nr
nr
nr
nr
nr
<0.2
1.7
nr
0.8
0.5
0.4
0.4
0.6
1.4
1.3
0.6
0.8
0.4
1.6
0.6
0.3
17.0
0.3
2.1
0.7
0.3
0.6
Cu
µg/L
0.6
8.0
0.3
0.3
nr
nr
nr
nr
nr
nr
0.2
1.2
nr
0.5
<0.2
<0.2
<0.2
0.2
0.4
0.4
<0.2
<0.2
<0.2
1.1
<0.2
0.5
0.4
<0.2
<0.2
<0.2
<0.2
0.2
As
µg/L
<0.3
0.3
<0.3
<0.3
nr
nr
nr
nr
nr
nr
<0.3
<0.3
nr
<0.3
<0.3
0.5
<0.3
<0.3
0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
Hg
µg/L
11.9
nr
13.1
12.2
nr
nr
nr
nr
nr
15.0
12.0
10.8
nr
10.9
9.8
10.0
8.7
8.1
9.5
0.1
12.7
13.5
10.9
5.7
12.3
1.8
0.2
0.3
0.2
<0.1
<0.1
<0.1
Mo
µg/L
<0.1
nr
<0.1
<0.1
nr
nr
nr
nr
nr
nr
<0.1
<0.1
nr
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
0.19
<0.1
<0.1
<0.1
13.3
<0.1
1.82
<0.1
11.2
1.26
1.08
0.23
<0.1
U
µg/L
GEOCHEMICAL FEATURES OF THERMAL WATERS AT BENETUTTI (SARDINIA)
43
44
R. CIDU, A.D. MULAS
alkalinity were measured, and the waters were filtered (0.4 µm, Nuclepore 111130),
acidified, and stored for metal analyses. The redox potential (Eh) measured by a platinum
electrode was corrected against the Zobell’s solution [12].
Hydrogen sulphide was estimated by the Visicolor Macherey-Nagel-Duren test kit,
anions were determined by ion chromatography, metals by inductively coupled plasma
optical emission spectroscopy (ICP-OES, ARL3520®) and inductively coupled plasma
mass spectrometry (ICP-MS, ELAN5000®), while Hg, As and Sb were determined by
ICP-MS after flow injection Hg-vapour or hydride generation [13].
The detection limit for chemical components was calculated at five times the standard
deviation of blank solutions. The accuracy was evaluated using the NIST1643d standard
reference solution. Both precision and accuracy were estimated at ± 6% and ± 11%, or
better, at the mg/L and µg/L concentrations, respectively. The ionic balance calculated
by the PHREEQC [14] computer program was always less than ± 5% suggesting that the
analyses are of reasonable good accuracy. In this paper, the terms «dissolved» and «in
solution» refer to components present in the fraction below 0.4 µm. Speciation and
equilibrium calculations were carried out using the PHREEQC program.
RESULTS AND DISCUSSION
Analytical results of the 24 waters considered in this study are reported in table 1. The
following elements are not reported because were found below the detection limit: Ag (<
0.1 µg/L), Be (< 0.5 µg/L), Co (< 0.2 µg/L), Cr (< 0.4 µg/L), Ni (< 0.8 µg/L), Sb (< 0.6
µg/L), Tl (< 0.2 µg/L), and Bi (< 0.2 µg/L). Previous records on thermal waters at
Benetutti, and related references, are also shown in table 1. It can be observed that most
of trace components were not reported prior to this study. A comparison of data on thermal
waters collected at Benetutti over 20 years with data from this study shows a remarkably
stable chemical composition, as well as similar temperature, pH and concentrations of
minor components.
The water samples show temperatures of 15-42°C. The air temperature over the
sampling period reached peaks of 33°C affecting the temperature of the surface and lowflow spring waters (e.g., samples No. 12, 24, 20); for this reason, when the water
temperature is below 30°C, it does not appear to be a reliable parameter in distinguishing
the thermal waters.
Total dissolved solids (TDS) range from 0.2 to 0.9 g/L with the lowest TDS values
observed in the Tirso and Rio Mannu surface waters (samples No. 12 and 24, respectively).
The pH ranges from 6.2 to 9.7. The waters with a high pH (9.5-9.7) generally show higher
temperatures and lower Eh values; they are characterised by a strong H2S smell at
emergence, and often have a detectable amount of HS- (see table 1). Gas emission is
observed in most of the high-pH waters. The gas composition was not analysed in this
study, but previous records show dominant N2 (98.6 vol. %), with minor Ar (1.25 vol. %)
and CH4 (0.084 vol. %), and H2S <0.005 vol. % [18].
GEOCHEMICAL FEATURES OF THERMAL WATERS AT BENETUTTI (SARDINIA)
45
Nitrogen species (NO3 and NO2) and PO4 are usually below the detection limit of 0.1
mg/L in the high-pH waters. Nitrate concentrations higher than the drinking water limit
(50 mg/L) established by Italian regulations [19] occur in two water samples (No. 19 and
23). The occurrence of NO3 and PO4 in sample No. 19 is associated with the highest K,
Al, Sr, Ba, Fe, Mn, Zn, and Cu concentrations (see table 1) and may reflect anthropogenic
inputs, such as fertilisers and uncontrolled discharge of untreated urban wastes. Sample
No. 19 will be omitted when discussing water-rock interaction processes.
The concentrations of toxic components, such as heavy metals (Cd, Pb, Hg) and As,
are very low, much lower than the drinking water limits, and often below the detection
limits of the used methods. The different redox environments affect the concentration of
dissolved uranium: the reduced waters do not show any detectable U, while at oxidising
conditions U occurs in the range of 0.2 to 13 µg/L.
The Piper diagram (fig. 2) shows that all the waters have a dominant Na-Cl
composition with a subordinate Ca, Mg, and SO4 contribution. A group of waters (sample
Figure 2. Piper diagram showing the main chemical composition of the waters at Benetutti.
46
R. CIDU, A.D. MULAS
250
200
R2 = 0.81
Na (mg/L)
150
Na/Cl in seawater
100
50
low-Mg waters
0
0
50
100
150
200
250
300
Cl (mg/L)
Figure 3. Sodium versus
chloride concentrations
in the Benetutti waters.
1,0
Br/Cl in seawater
R2 = 0.93
0,8
Br (mg/L)
0,6
0,4
0,2
low-Mg waters
0,0
0
50
100
150
200
250
Cl (mg/L)
Figure 4. Bromide versus chloride concentrations in the Benetutti waters.
300
GEOCHEMICAL FEATURES OF THERMAL WATERS AT BENETUTTI (SARDINIA)
47
Nos. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,13, 14, 15, 17) is confined at the Cl and Na corners, and
appears well distinguished by the low Mg concentration (0.01-0.1 mg/L, see table 1);
hereafter, this group of waters will be indicated as low-Mg waters. The other samples
show relatively higher HCO3, Ca, and Mg.
In all the waters, the dissolved Na is correlated with Cl (R2 = 0.81, fig. 3). The samples
showing a higher Na/Cl ratio than observed in seawater probably reflect an increased
interaction of the water with Na-plagioclase. The concentration of Br increases with
increasing Cl. The observed high correlation (R2 = 0.93, fig. 4) probably indicates a
common source for these elements. The Br/Cl ratio is slightly lower than observed in
seawater.
Figure 5 shows that Sr concentrations increase with increasing Ca. Both Ca and Sr
derive mostly from plagioclase and feldspar dissolution. The low-Mg waters have a Sr/
Ca ratio higher than observed in the other waters. Equilibrium or slight supersaturation
with respect to calcite occurs in the low-Mg waters, and the possible precipitation of
calcite might control the concentration of Ca, while Sr appears to be uncontrolled
indicating that it is only partially incorporated in the calcite lattice.
Figure 6 shows the comparison between element concentrations in spring n. 23
representing the shallow groundwater and spring n. 10 representing the low-Mg waters.
Figure 6 and table 1 show that the low-Mg waters have significantly higher F, B, Li, Rb,
Cs, Mo, and Ga concentrations than observed in the shallow groundwater and the surface
water of the area. Since all the waters, except sample no. 16, apparently circulate in granite
rocks of similar composition, this behaviour might indicate a higher mobility of these
elements when the waters warm up and/or deep circuits with longer residence time. The
increased mobility of these elements with increasing temperature has been reported in
several studies (eg [2] [20] [21]). The concentration of fluoride seems to be controlled by
the fluorite mineral: the low-Mg waters are at approximate equilibrium (SIfluorite ranges
from –0.09 to +0.15) while the other waters are undersaturated (SIfluorite ranges from –1.28
to -3.12). In figures 7 to 10 the concentrations of F, Li, Rb, and Mo are plotted versus the
concentration of Cl. Chloride is assumed to behave conservatively, i.e. it remains in
solution, since it is unaffected by processes such as precipitation of solid phases, ionic
exchange, or sorption. When considering all samples, none of these components appear
correlated with the concentration of Cl, while a rough correlation is observed among the
low-Mg waters.
Silica concentration in the surface waters is low (6-7 mg/L SiO2) and increases in the
low-Mg waters (30-39 mg/L SiO2). Silica concentration can be used to estimate the
temperature of waters in the deep reservoir, provided that mixing does not occur during
uprise to the surface [22]. At Benetutti, the low Mg concentrations observed in the highpH waters indicate that mixing between the water at depth and the cold waters is unlikely
to occur because the Mg concentrations of the cold waters are about 2 orders of magnitude
higher (9 mg/L in the less saline water, sample No. 24). H4SiO4 activity derived from
speciation is nearly constant (0.46±0.04 10–3 mole/L) in the low-Mg waters. These waters
48
R. CIDU, A.D. MULAS
500
450
Sr/Ca in seawater
400
350
Sr (µg/L)
300
250
200
150
100
50
low-Mg waters
0
0
5
10
15
20
25
30
35
40
45
50
Ca (mg/L)
Figure 5. Strontium versus calcium concentrations in the Benetutti waters.
1000000
Alk
Figure 6. Comparison of element
concentrations in
water sample No.
23 representing the
shallow groundwater, and in water
sample No. 10
representing the
low-Mg water.
Concentrations in the spring No. 23 (µg/L)
100000
SO4
Mg
Cl
Na
Ca
SiO2
10000
K
1000
Sr
Br
F
100
Ba
Fe
B
Zn
10
Mn
Li
Al
1
Rb
Ga
U
0,1
Mo
Cs
0,01
0,01
0,1
1
10
100
1000
10000
100000
Concentrations in the spring No. 10 (µg/L)
1000000
49
GEOCHEMICAL FEATURES OF THERMAL WATERS AT BENETUTTI (SARDINIA)
12
low-Mg waters
10
F (mg/L)
8
6
4
2
0
0
50
100
150
200
250
300
Cl (mg/L)
Figure 7. Fluoride versus chloride concentrations in the Benetutti waters.
100
low-Mg waters
80
18
Li (µg/L)
60
40
20
0
0
50
100
150
200
250
Cl (mg/L)
Figure 8. Lithium versus chloride concentrations in the Benetutti waters.
300
50
R. CIDU, A.D. MULAS
35
low-Mg waters
30
Rb (µg/L)
25
20
15
10
20
5
0
0
50
100
150
200
250
300
Cl (mg/L)
Figure 9. Rubidium versus chloride concentrations in the Benetutti waters.
16
low-Mg waters
14
12
Mo (µg/L)
10
8
6
4
2
18
0
0
50
100
150
200
250
Cl (mg/L)
Figure 10. Molybdenum versus chloride concentrations in the Benetutti waters.
300
GEOCHEMICAL FEATURES OF THERMAL WATERS AT BENETUTTI (SARDINIA)
51
are slightly supersaturated with respect to quartz, and at equilibrium with respect to
chalcedony. Assuming that the deep water is at equilibrium with respect to chalcedony,
the calculated temperature at depth is estimated at 60±5°C in the low-Mg waters, i.e. all
these waters are thermal. If this is the case, and assuming a normal geothermal gradient,
it can be inferred that the thermal waters reach maximum depths of about 1,500 m. The
calculated temperature at depth is not much higher than the temperature at emergence
(maximum measured temperature: 42°C) and suggests a relatively fast rise of the water
from the deep aquifer to the surface.
Occurrence of low-magnesium, alkaline waters that have a high pH, dominant
sodium, and low salinity, is well documented in the granite areas of France, Spain,
Bulgaria, Italy, and Sweden [21] [23]. Similar characteristics are also observed in the
high-temperature (150-160°C) waters in the Himalayan granite [24]. Alkaline waters can
be considered as the equilibrium composition of a water reacting with granite minerals
[21] [25]. Primary minerals, such as Na-Ca-feldspars, are transformed into newly formed
minerals that are more stable at the interaction temperature [21]. This interpretation
appears reliable for the thermal waters at Benetutti. The dissolution of aluminosilicate
minerals consumes protons and contributes to an increase in pH. The pH is also controlled
by the equilibrium with carbonate minerals. In fact, the thermal waters at Benetutti are
slightly supersaturated with respect to calcite. The precipitation of calcite will lead to low
concentrations of calcium and carbonate species. Therefore, silicate hydrolysis combined
with carbonate geochemistry might play an important role in producing the observed
water composition of the alkaline waters at Benetutti: sodium will be the dominant cation,
and chloride the dominant anion. The observed low salinity might be due to the lack of
deep origin CO2 inputs, which limit the supply at depth of carbonic acid (i.e. the main
source of protons necessary for silicate hydrolysis). The source of main and trace
components, including that of the rare alkaline elements, F and Mo, is compatible with
the weathering of granite minerals. Fluid inclusions in the granite rocks of north Sardinia
have been documented [26], and may represent an additional source of Na, Cl, and Br in
solution.
CONCLUSION
At Benetutti in central Sardinia, both the surface waters and the groundwaters show
a dominant sodium-chloride composition and relatively low salinity (0.2-0.9 g/l). The
thermal waters are characterised by gas emission, detectable amounts of HS-, a high pH,
very low magnesium, and much higher concentrations of SiO2, F, B, Li, Rb, Cs, and Mo
than in the cold waters of the area. The overall characteristic of the thermal waters can be
considered the result of a water reacting with granite minerals where silicate hydrolysis
combined with carbonate geochemistry play an important role. No variation in temperature at emergence, discharge, or chemical composition were observed at the thermal
springs for which records dating back 20 years are available. The deep reservoirs
52
R. CIDU, A.D. MULAS
associated with thermal springs are of low enthalpy (calculated temperature at depth is
about 60°C), and are suitable for direct uses, such as balneology, space heating, and
horticulture.
Acknowledgements. This study was supported by funds from the MURST (ex 60%;
Scientific Co-ordinator: R. Cidu). The manuscript benefited from the review of L.
Fanfani.
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