Notice Concerning Copyright Restrictions
Total Page:16
File Type:pdf, Size:1020Kb
NOTICE CONCERNING COPYRIGHT RESTRICTIONS This document may contain copyrighted materials. These materials have been made available for use in research, teaching, and private study, but may not be used for any commercial purpose. Users may not otherwise copy, reproduce, retransmit, distribute, publish, commercially exploit or otherwise transfer any material. The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material. Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specific conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement. This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. Geothermal Resources Council TRANSACTIONS, Vol. 14, Part II, August 1990 THE WATERS OF THE SIMPLON TUNNEL (SWISS-ITALIAN ALPS) AND OF THE ADJACENT OSSOLA DISTRICT (ITALY): GEOTHERMAL CONSIDERATIONS. J.C. Hunziker(l1 G. Martinotti(2) L. Marix~i(~)and C. Prin~ipe(~) (1) Institut de Minkralogie et Petrografie, Universitk de Lausanne (2) Dipartimento di Scienze della Terra, Universitd di Torino (3) Geotermica Italiana srl, Lungarno Mediceo 16, Pisa (4) Istituto di Geocronologia e Geochimica Isotopica, CNR, Pisa and Gruppo Nazionale per la Vulcanologia, CNR, Roma ABS TRACT is given in fig. 1. A warm water flow and a cold one, both THE SIMPLON TUNNEL of large flow rate, are present inside the Simplon tunnel. Their physico-chemical The Simplon tunnel was completed in features, the temperatures measured during 1905. Its total length is 19,729 m. Its the construction of the tunnel and the elevation increases from 686 m a.s.1. geological observations made at that time (south entrance) up to 705 m a.s.1. in its intermediate portion and then it decreases are of upmost interest to establish the ' rock-water heat transfer mechanism. again until 634 m a.s.1. (north portal). This information together with that The morphology of the overlying massifs is provided by the Brigerbad and Bagni di very rough due to the effect of glacial Craveggia thermal springs indicates that erosion. The highest peak is Monte Leone the geothermal possibilities of the Ossola (3553 m a.s.1.). district are linked either to water The geological cross-sections along the circuits within Mesozoic metasedimentary tunnel, proposed by different authors rocks, if suitable structural and since the time of its excavation, show hydrogeological conditions are fulfilled, remarkable changes, which reflect the or to rapid upflow, along deep reaching different theories on the geology of the fault zones, of deep circulating thermal entire Alpine edifice. Nevertheless, the waters. original observations made along the Simplon tunnel mantain their great value INTRODUCTION for the understanding of the geology of a large Alps sector. According to the most This paper analyzes the geochemical recent interpretations (e.g. Milnes, 1973) features of the cold and warm water flows the tunnel crosses the Lower Pennidic in the Simplon tunnel and of some relevant springs located in the adjoining Ossola district. Such data are considered in the light of the geological-hydrogeological framework of the region, for the two following purposes: (1) to understand the mechanism of rock- water heat transfer into the Simplon tunnel ; (2) to provide a contribution to the assessment of the geothermal possibilities of the Ossola district. In order to accomplish these objectives existing geological documents were taken into account, aerial photographs and satellite imageries were studied to detect lineaments associated with active tectonic structures and field controls were performed. Besides a total of 27 water samples were collected and analyzed, both chemically and isotopically (table 1). Six samples were taken from different flows inside the Simplon tunnel, while the other I. I 21 come from springs of the Ossola Figure 1. Location map of the sampled district. The location of sampling points water points. 1477 Hunziker et al. Figure 2. Geological cross-section along the Simplon tunnel (modified from Milnes, nappes (mainly made up of granitic gneiss, CHEMICAL CLASSIFICATION OF WATERS schistose gneiss and schist) and the intercalated Mesozoic metasedimentary Water samples have been classified by units, constituted by impure marbles and means of the square plots of Langelier and calcareous schists, with thin but locally Ludwig (19421, e.g. fig. 4, and of the important associations of dolomitic, triangular cross-sectlons of the related gypsum-bearing, and quartzitic rocks (fig. compositional pyramids (e.g. fig. 5) . 2) - Most cold springs of the region exhibit These metasedimentary rocks constitute calcium bicarbonate composition and low the main aquifers, owing to both TDS (1 to 3 meq/l). Their temperature, fracturing and dissolution. In fact the which is negatively correlated with their main water flows occurring inside the altitude and close to the average annual tunnel, both cold and warm, are through value of the site, is generally within the these rocks. A region with thermal waters 6 to 14 OC interval. All these waters are is located at 9.5-10.5 km from the north related to quick circulation through portal; temperature and total flow rate fractured gneiss. were 30 to 49OC and 300 l/s, respectively, One of the Alpe Veglia cold springs when the tunnel was excavated; an (sample 19) and two of the Bognanco cold important cold flow is found at 4.4 km springs (samples 11 and 12) are relatively from the south entrance; its temperature similar to previous waters: they have in and flow rate were 10 to 14 OC and 1000 fact calcium bicarbonate and magnesium l/s, respectively, when the tunnel was bicarbonate composition, respectively, and opened (Clark and Nibblet, 1956). temperature close to the average annual rnl- - IU~Lemperature measured along the value of the site. On the other hand they tunnel during its construction reaches a show much higher TDS (27 to 99 meq/l) and maximum of 55 OC at about 8 km from the north entrance (Clark and Nibblet, 1956). The temperature profile (fig. 3) exhibits a remarkable relative minimum, caused by the cold water flow, and a less marked negative inflection in the section of the warm waters, the effect of which was "to cool the tunnel rather than to heat it1! (Clark and Niblett, 1956). Hence it seems likely that also the thermal waters are downward percolating meteoric waters, rather than uprising waters of deep provenance. A geothermal gradient of 36 OC/km and a geothermal heat flow of 2.2 pcal/cm* s were calculated for the Simplon area on the basis of the temperature measured during the construction of the tunnel wttl 2 3 L 5 6 7 8 9 109 8 7 6 5 4 3 2 18J (Clark and Nibblet, 1956). Thus this area appears thermally anomalous, though slightly, with respect to the average terrestrial geothermal gradient and heat Figure 3. Temperature measured along the flow. Simplon tunnel during its construction (modified from Clark and Nibblet, 1956). 1478 Hunziker et al. C02 partial pressure (0.9 to 1.7 atm) than waters aggressive and is consequently most cold springs. The latter features are responsible of their high TDS, which is linked to input of deep-originated C02- not due to long deep circulation. The rich gases, which uprise along vertically composition of these waters is controlled permeable tectonic structures: the Alpe instead by the prevailing aquifer Veglia Fault Zone and the Insubric Line. lithotype: gneiss and calcareous schist at The high C02 partial pressure makes these Alpe Veglia, amphibolite and prasinite at Bognanco . The waters which circulate within gypsum-bearing Mesozoic metasedimentary So,+C/% rocks for a sufficiently long time acquire 50 - a calcium sulfate composition and high TDS (35 to 60 meq/l). These features are shown by the warm waters of the Simplon tunnel (samples 1 to 5) and by two cold springs 1 located at Crodo (sample 24) and Baceno (sample 25). The thermal waters of the x Simplon tunnel have low C02 partial k pressure (0.0014 to 0,0032 atm), similar + to that of the cold water flow (0.0015 ;r" atm). This feature agrees with the fvshallowffgenetic mechanism of these warm waters presented above. Nevertheless the ffdeepffhypothesis can not be completely 5 e ruled out, even though the occurrence of C02 loss should be admitted in that case. Other conspicuous characteristics of these warm waters are the varying contents of aqueous species and the different outlet temperature. Such differences indicate that the tunnel does not cross a well- 0 mixed geothermal reservoir, but rather a zone still dominated by meteoric recharge. The cold waters of the Simplon tunnel rn3%7 (sample 6) have calcium sulfate bicarbonate composition and intermediate Figure 4. Langelier-Ludwig square plot TDS (7 to 9 meq/l). Similar chemical with the bicarbonate as isolated anion. features are shown by two cold spring located at Crodo (sample 23) and San Michele (sample 28) and by the slightly 0.04 warm spring of Piedilago (sample 30, temperature 17 cC); their origin is related either to quick circulation within Mesozoic metasedimentary rocks or to mixing of calcium sulfate waters and calcium bicarbonate waters. Only two surface thermal manifestations are known in the studied area and both are found beyond the proper limits of the Ossola district. One is located at Brigerbad (Valais, Switzerland), 9 km west of the north entrance of the Simplon tunnel; sampling of this thermal site was not authorized, but its hydrogeological, geochemical and geothermal characteristics are given by Vuataz (1982): e.g.