Geochemical Journal, Vol. 46, pp. 117 to 129, 2012
Chemical and isotopic characterization of the thermomineral water of Terme Sibarite springs (Northern Calabria, Italy) CARMINE A POLLARO,1 ELISSAVET DOTSIKA,2,3 LUIGI M ARINI,4* DONATELLA BARCA ,1 ANDREA BLOISE,1 ROSANNA DE R OSA,1 MARCO DOVERI,3 MATTEO LELLI3 and FRANCESCO MUTO 1 1
Department of Earth Sciences, University of Calabria, via Ponte Bucci 4, cubo 15B, 87036 Arcavacata di Rende (CS), Italy 2 National Center for Scientific Research “Demokritos”, 15310 Aghia Paraskevi, Athens, Greece 3 Institute of Geosciences and Georesources, CNR, Area della Ricerca, Via G. Moruzzi 1, I-56124 Pisa, Italy 4 Consultant in Applied Geochemistry, Via A. Fratti 253, 55049 Viareggio (LU), Italy (Received September 11, 2010; Accepted December 12, 2011) The Terme Sibarite and nearby thermomineral water discharges have Ca–SO4 to Ca–SO4–HCO3 composition, near neutral pH value of 6.9 to 7.6, outlet temperatures of 16° to 25.5°C, variable redox potentials (–0.23 to 0.2 V) and total dissolved solids from 20 to 35 meq L –1. The total flow rate of the Terme Sibarite springs is about 130 L s –1. Solubility of chalcedony and the K–Mg geothermometer show a full equilibrium reservoir temperature of 33°C. The δ34S values of dissolved sulfate are constrained by mixing of thermomineral waters with cold waters, both interacting with Upper Triassic carbonate-evaporite rocks, as well as occurrence of bacterial sulfate reduction. In the light of the geological and hydrogeological framework of the study area, this chemical and isotopic evidence suggests that the thermal circuit develops entirely into the Upper Triassic sedimentary sequence, without any interaction with the Messinian evaporites which are possibly present below the Upper Triassic sedimentary sequence. The thermomineral waters are meteoric precipitations infiltrating in the Pollino Massif, at average altitudes of 950–1090 m a.s.l, as indicated by the δ18O values of water. These waters descend to maximum depths of 600 m below the Sibari Plain, where the geothermal reservoir is situated. Circulating into it, waters extract heat from reservoir rocks, attaining thermo-chemical equilibrium at the temperature suggested by chemical geothermometers. Then, the thermomineral waters locally rise relatively quickly to the surface, along subvertical faults and fractures, preserving part of their physical and chemical characteristics. The methodological approach utilized in this research may be applied to other fault-controlled, low-temperature geothermal systems in other zones of Italy and other nations. Keywords: thermomineral water, water geochemistry, geothermometry, sulfur isotopes, geothermal system
both dissolved sulfate and sulfide have been included among the used geochemical tools to investigate the provenance of dissolved sulfur species and the active processes controlling their concentrations and isotopic ratios. The thermomineral waters discharged by the Terme Sibarite have been known and used therapeutically since Roman times, owing to the large flow rate, in the order of 130 L s–1 as whole (Arena, 1978). Up to now, they were the subject of regional geochemical studies, chiefly aimed at the surveillance of seismic activity (Gurrieri et al., 1984; Duchi et al., 1991; Calcara and Quattrocchi, 1993; Italiano et al., 2010). To gain more insights on this thermal circuit, we carried out a detailed geochemical survey in July to September 2009, comprising the collection of water samples from 26 springs and 6 domestic wells and their analysis for major and trace species, as well as for sulfur isotopes as already recalled above. Although this work is focused on a local low-temperature geothermal circuit, there is no doubt on the general validity of the used approach, which is based on the inte-
INTRODUCTION In the investigation of natural environments, including geothermal systems of high, medium and low temperature, geochemical data are of utmost importance for the reconstruction of the conceptual geochemical model. This task, representing the main objective of geothermal investigations, is achieved through the synthesis of results provided by different geochemical techniques, which have to be suitably selected and properly applied, in the light of the hydrogeological-geological framework of the system of interest. The study of the thermal area of Cassano Ionio (Terme Sibarite), which is the subject of this communication, has been undertaken following these general guidelines. In particular, owing to the presence of two different evaporite sequences, sulfur isotopes of *Corresponding author (e-mail:
[email protected]) Copyright © 2012 by The Geochemical Society of Japan.
117
Fig. 1. (A) Geological sketch map of the study area and (B) location map of thermomineral and cold water samples, also showing the simplified hydrographic network and the boundary of the Cassano Ionio catchment.
gration of different methodological tools. It can be applied to other similar geological-hydrogeological frameworks in other areas of Italy and other countries. GEOLOGICAL-HYDROGEOLOGICAL F RAMEWORK The Meso–Cenozoic successions outcropping in the Pollino Massif and in the Coastal Chain belong to two distinct carbonate tectonic units (Iannace et al., 2005): the lower one is named Pollino–Ciagola Unit (Norian– Langhian), whereas the upper one is known as Lungro– Verbicaro Unit (Anisian–Lower Burdigalian). Evaporite levels of Upper Triassic age are present in both units. Neogene–Quaternary deposits locally cover the two carbonate tectonic units. To the south of the studied area, the Neogene–Quaternary sequence comprises: (i) Miocene siliciclastic-carbonate deposits including Messinian evaporites (predominantly halite, gypsum and gypsumarenites) and (ii) a terrigenous Pliocene–Pleistocene succession (Fig. 1A). In the fault-bounded structural highs, such as the Coastal Chain, the Messinian evaporites are overthrusted by the carbonate tectonic units, along transpressive faults (Cotecchia et al., 1983, 1988; Van Dijk et al., 2000). In the Sibari plain, close to the study area, Messinian 118 C. Apollaro et al.
evaporites were encountered by exploration wells, but the few surface and subsurface data could not clarify the tectonic and geometric relationships between the Messinian deposits and the Mesozoic carbonates of the Pollino– Ciagola Unit. The hydrogeological complex constituted by the Meso–Cenozoic carbonate successions represents the main aquifers of the study area, characterized by very high permeabilities (possibly up to 10 –3 m s –1; Freeze and Cherry, 1979) due to well-developed fracturing and karstification. Cold springs fed by these aquifers generally show average discharges from a few tens L s–1 to over 100 L s–1 (Fabbrocino and Perrone, 2009). The upper range of these flow rate values compares with the total flow rate of the Terme Sibarite springs, 130 L s –1 (Arena, 1978). Pumping tests data obtained by Fabbrocino and Perrone (2009) in a small zone of the carbonate aquifer adjacent to the investigated area highlight a high variability of the transmissivity values, between 1.4 × 10–2 m 2 s –1 and 2.3 × 10 –5 m 2 s –1 . Such a large range of transmissivity indicates a strong heterogeneity of the carbonate hydrogeological complex, locally increased by tectonic effects. Faults and fractures significantly influence the groundwater flow pattern, both the downward
infiltration of meteoric waters and the uprise of thermal waters, as already recognized by previous studies (Italiano et al., 2010 and references therein). FIELD SAMPLING AND LABORATORY A NALYSES As mentioned in the introduction, water samples from 26 springs and 6 shallow wells (drilled for domestic use at depths of 50–70 m, codes A12, A16, A18, A24, A25 and A28) were collected in July to September 2009. The location of all the sampling points is shown in Fig. 1. Temperature, pH, Eh and electrical conductivity (EC) were measured in the field by means of portable instruments. Two pH buffers, with nominal pH values of 4.01 and 7.01 at 25°C, were used for pH calibration at each sampling site. Total alkalinity was also determined in the field by acidimetric titration using HCl 0.1 N as titrating agent and methylorange as indicator. Waters were filtered in situ through a 0.45 µm poresize membrane filter. Samples for the determination of anions were stored without further treatments. Samples for the determination of cations, SiO 2, and traces were acidified, by addition of suprapure acid (1% HNO3) after filtration, and stored. New polyethylene bottles were used for all the samples; they were left overnight in dilute HNO3 and rinsed with Milli-Q demineralised water in the laboratory; in the field they were rinsed three times with small amounts of the aqueous solution to be stored. Blank solutions were prepared in the field using demineralized water and following the same procedure as for water samples. In the laboratory, the concentrations of F–, Cl–, NO 3– and SO42– were determined by HPLC (DIONEX DX 120), whereas the concentrations of Na + , K + , Mg 2+ , Ca 2+ , SiO 2(aq) and some trace elements, were analysed by a quadrupole ICP-MS (Perkin Elmer/SCIEX, Elan DRCe) with collision reaction cell capable to reduce or avoid the formation of polyatomic spectral interferences. Data quality for major components was evaluated by charge balance. Deviation between the sum of concentrations of cations and the sum of concentrations of anions varies between –5 and +5%. Data quality for minor and trace elements was checked running the NIST1643e standard reference solution. Deviations from the certified concentrations were found to be lower than 5%. Results of laboratory analyses for major and minor components are given in Table 1 (together with field data and ancillary information), whereas those for trace constituents are reported in Table 2. The 34S/32S isotopic ratios of dissolved sulfate and sulfide were determined in 6 selected samples (A1, A18, A19, A28, A29, A31). To minimize the fractionation of sulfur isotopes, the approach of Truesdell and Hulston (1980) was followed for sample preparation. In the field,
1 kg of water was treated with 100 mL of ZnCl2 1 M, to precipitate sulfide as ZnS. In the laboratory, solid ZnS was separated through filtration from the aqueous solution and the latter was heated and treated with BaCl2, to precipitate sulfate as BaSO4. The isotopic composition of ZnS and BaSO4 was then measured in the Stable Isotope Unit, Institute of Material Science, National Center for Scientific Research “Demokritos” (Athens, Greece) on a Finnigan DELTA V plus (Thermo Electron Corporation, Bremen, Germany) stable isotope mass spectrometer. Results are reported in Table 3 in the standard delta notation as per-mil deviation from the standard V-CDT. Measurement precision, based on the repeated analysis of inhouse standards was found to be ±0.2‰ (1σ). WATER CHEMISTRY Water classification and mixing processes Water chemistry is initially discussed by means of the triangular plots among major anionic and cationic constituents (Fig. 2). Inspection of these diagrams shows that thermomineral waters have Ca–Mg–SO4 to Ca–Mg–SO4– HCO3 composition, whereas cold waters belong to the Ca–HCO3 to Ca–Mg–HCO3 facies. Another useful parameter for water classification is Ionic Salinity or Total Ionic Salinity (TIS), representing the sum of the concentrations of major anions and cations, expressed in meq L–1 (Chiodini et al., 1991). IsoTIS lines are shown in the correlation graph of SO42– vs. HCO3– + Cl– (Fig. 3), in which most thermomineral waters are found between the iso-TIS lines of 20 and 35 meq L–1, whereas most cold waters are characterized by lower TIS, from 5 to 20 meq L–1. However, the two cold samples A10 and A12 have TIS close to 40 meq L–1. This anomalously high TIS value is due to addition of a Na– Cl component, probably of anthropogenic origin (see below). All in all, these inferences suggest that: (i) Ca–HCO3 to Ca–Mg–HCO3 cold waters of low salinity derive their main chemical characteristics from dissolution of calcite and dolomite, which are present as clastic constituents in the Pliocene–Pleistocene deposits of the Crati Valley and Sibari Plain, and (ii) Ca–SO 4 to Ca–SO 4 –HCO 3 thermomineral waters of intermediate salinity interact with carbonate-evaporite rocks. At this stage of the discussion, it is not clear if the thermomineral water circuit is confined into the Upper Triassic sequence or if it extends into the underlying Messinian deposits (Cotecchia et al., 1983, 1988). Sulfur isotopes were used to reply this question (see later discussion). Further evaluations are provided by the correlation plots of dissolved constituents against SO4, in which a mixing trend between a SO4-rich thermomineral component and a SO4-poor cold end-member is generally recGeochemistry of Terme Sibarite springs 119
120 C. Apollaro et al.
4405379
4404472 4404945 4405075 4404438
4404440 4404942 4405807 4405059 4407073 4402682 4403308 4403497 4404257 4403721 4405951 4405507 4404610 4408182 4408284 4401702 4404428 4404108 4404886 4404597 4404631 4405214 4405945 4405918 4405395 4404461 4404468
m
m
613264 612343 610874 612769 611902 612324 611701 610429 610025 611543 612052 612802 615062 611266 611380 613446 615432 611683 613160 613263 613315 613982 613575 617483 613168 613108 613129 613159 613164 612590 612996 613145
Northings
Eastings
9-Nov-07 9-Nov-07 9-Nov-07
3-Oct-07 3-Oct-07 3-Oct-07 3-Oct-07 3-Oct-07 3-Oct-07 3-Oct-07 3-Oct-07 9-Oct-07 9-Oct-07 9-Oct-07 9-Oct-07 9-Oct-07 16-Oct-07 16-Oct-07 16-Oct-07 16-Oct-07 28-Oct-07 28-Oct-07 28-Oct-07 28-Oct-07 28-Oct-07 28-Oct-07 28-Oct-07 28-Oct-07 31-Oct-07 31-Oct-07 31-Oct-07 31-Oct-07
Date 9:17 AM 11:00 AM 11:50 AM 1:15 PM 2:21 PM 5:27 PM 6:17 PM 6:47 PM 9:34 AM 10:25 AM 12:30 PM 2:52 PM 6:15 PM 10:35 AM 12:02 PM 2:49 PM 5:40 PM 9:12 AM 10:42 AM 11:42 AM 12:25 PM 3:26 PM 4:00 PM 4:45 PM 5:39 PM 8:54 AM 10:04 AM 10:43 AM 1:08 PM 9:15 AM 10:12 AM 11:21 AM
Time
16
16.6 22.3
25.5 24.6
24.3 15.8 17.9 17.8 17.5 18.5 20.3 20.9 17 19.2 17.2 18 18.5 15.7 19.7 18.7 17.9 22.4 20 23.8 23.8 18.4 19 17.3 18.6 24.1 24.5
°C
T (w)
7.42
7.3 7.11
7.43 7.01
7.22 7.69 7.55 7.3 8.07 6.97 6.9 7.2 7.43 8.01 7.99 7.13 7.11 7.26 7.03 6.98 7.08 7.19 7.26 7.62 7.5 7.29 7.23 7.22 7.43 7.07 7.6
pH
147
154 −188
123 167 200 199 198 206 215 199 116 183 158 214 187 0.71 181 198 197 0.31 174 180 192 138 198 204 202 −206 −100 −174 −233
mV
Eh
1515
883 1093
98.9
40.6 48.3
45.8 3.1 12.0 24.5 29.1 52.3 55.2 46.9 24.6 140.0 17.6 112.6 39.1 24.9 19.5 45.8 35.1 44.1 37.6 43.8 43.6 66.5 16.8 28.4 18.8 43.5 50.1 44.2 63.2
ppm
µS/cm 1019 122 474 656 672 1039 867 913 747 1963 677 1609 764 597 691 978 790 1038 1024 1040 1042 838 647 754 421 1035 1036 1020 1334
Na +
Cond.
2.52
3.4 0.84 0.83 25.2 1.15 3.9 1.49 1.48 1.12 0.34 0.84 2.46 1.49 0.81 2.59 3.4 1.59 3.01 13.3 3.87 3.79 0.66 1.65 1.83 0.39 3.46 3.53 3.33 4.94 5.7 3.81
K+ ppm
70.4
43.8 13.4 8.4 27.0 30.5 35.3 31.8 36.1 15.0 68.7 28.7 109 26.2 20.7 25.0 43.8 20.6 47.8 66.1 52.7 52.2 25.6 15.2 38.7 14.0 52.6 53.2 52.7 62.7 52.1 54.8
Mg2+ ppm
172
80.3 145
106 42.2 85.2 57.8 69.8 119 94.6 108 110 139 76.8 92.3 79.9 74.3 88.9 107 106 110 85.6 133 130 76.9 89.8 82.0 54.7 103 125 101 164
Ca 2+ ppm
84.1
39.5 4.34 12.3 16.7 36.3 72.6 58.9 35.3 25.7 273 36.7 125 24.6 22.2 34.0 43.6 24.4 29.9 34.7 36.3 36.5 98.0 13.5 55.0 20.4 36.2 36.6 35.8 56.2 55.0 40.2
Cl− ppm
523
80.0 368
383 3.36 22.7 59.8 45.6 45.2 61.6 75.1 62.7 228 30.2 191 26.2 26.9 17.9 241 28.9 195 237 344 342 48.0 12.5 51.7 16.3 324 344 324 500
SO 42− ppm
293
170 178 261 323 336 445 396 490 354 479 335 692 372 320 374 351 438 264 364 235 235 310 327 334 225 258 288 264 229 414 246
HCO3− ppm
