Isotopic and Chemical Survey of Two Thermal Manifestations in The

IAEA-SM-361/60P XA9950152 Isotopic and chemical survey dfttao thermal manifestations in the Serchio River vail 5y, Northern Tuscany, Italy. E. Calvi, A. Capraiii, G. Leone* and M. Mussi International Institute for Qeotherma,Researches, CNR, Pisa, Italy (*also Earth Sciences Department, Pisa University, Italy). Tuscany is a region rich in thermal manifestations, the most famous being the well-known Larderello geothermal field located so rth of the Amo River The thermal manifestations examined in this paper are spaced over an area of about 2000 km2 bounded to the west by the Ugurian Sea, to the east by the Apennine chain and cut by the Serchio River valley. It is a mountainous region with maximum eevations of about 2000 m a.s.l. which includes the Apuane Alps, a mountain group world-wide known for the famous Carrara marble quarries, but where many other mineral deposit: have been mined in the past. High mean annual rainfall values, 900-3200 mm, charactt rise this area with strong gradients related to the altitude and location of the two mountair chains lying roughly parallel to the sea coast (fig.1). Two main tectonic and stratigraphic un ts have been defined in the area: the metamorphic sequence of the Apuane Alps that outciops in a tectonic window and the corresponding not metamorphic members of the Tuscan nappe. An intense faulting, due to the extensive tectonics occurred from Pliocene to Quaternary but carbonatic formations represents the main regional water circulation system (Haldacci etal., 1993). Some thermal springs are placed o 1 the northern border of the Alpi Apuane but those located in the Serchio River valley, which runs along the eastern border of the authoctonus units of the Apuane Alps, were firstly e tamined. Among these, of particular importance are the thermal manifestations located at Torrtte and Pieve Fosciana with maximum temperatures of 34 + 39°C and TDS o - 6 -5-7 mg/l. They are at a few km of distance but placed on the opposite sides of Serchio River, probably fed by two different circulation system, linked to the Apuane and Ap snnine relieves. The same deep thermal reservoir, namely the evaporitic glide horizon of the Norian Cavernoso cellular dolomitic limestone responsible of the detachment of the Ti scan nappe, interested all the thermal manifestation of the area. Being now available detailed data of the isotopic composition of rain and surface waters of the area {Mussi et a!., 19$ 9), a detailed study of the isotopic (18O, 2H, 3H) composition of these springs was started from 1992, also with the aim to bring to light the existence of local and/or temporal fluctuations of their isotopic composition. Some determinations of the chemical composi ion of the associated gas were also made. Torrite spring is located on the Apu ane side of Serchio River outflowing from Massiccio limestone formation that directly overlie Calcare cavernoso dolomitic limestone of evaporitic 221 character. This manifestation is very hard to sample because now emerges deep inside and under the water level in the tunnel of water discharge of a hydroelectric power plant High Radon content like 38.1 nCi/l, 3 +• :«3 times higher than measured in the Larderello geothermal field, and an Helium conteit in the associated gas of 0.1% agree with a deep origin and a shorter rising time Stable sotopes, d"O -7.5 %o and tritium content, 6 + 7 TU, agree with a meteoric recharge at the top of Apuane Alps and short times of circulation. Pieve Fosciana thermal waters ccme out through Pliocene silt and clay lacustrine sediments on the Apennine side of Serchio River feeding a small pond of about 800 m2. Stable isotopes and tritium content has been continuously monitored from 1992. These waters show very stable isotope contents with values of d18O -7.8 ± 0.15V £*H -50 ± 2%o and 0 -5-2.5 TU, the same values were found in an ephemeral spring which appeared when in march 1996 the pond suddenly disappeared and re-established within two weeks. These values, in very good agreement with the local meteoric water line, suggest extended residence times and mean infiltration altitudes of more than 1000 m as.I Radon content of 1.3 nCi/l and Helium abundance of 0.4% characterise these waters in agreement with the longer residence time of Pieve Fosciana waters than Tom'te ones. mo analysed Mora aofiiGve their nMMion and temperature probaDly trauon tm same reservoir but the different ways aid times they reach the surface are reflected in their isotopic and chemical features. When considering the bordering fracture system of the Apuane Alps, longer rising times of waters appear to associate with the Apennine side circulation system and the sedimentary cover of Pieve Fosciana type nnanifestation. IM A Mar' HSA o io 20 30, Fig.1 - Map showing the location of (he siud-ed area. References Baldacci F., Cecchini S., Lopane G.j, Raggi G. (1993). Le risorse idriche del bacino del fiume Serchio ed il loro contributo all'alimentazione dei bacini idrografici adiacenti. Mem. Soc. Geol It, 49, 365-391. | Mussi M , Leone G. & Nardi I (1999)! Isotopic geochemistry of natural waters from the Alpi Apuane-Garfagnana area, Northern Tuscany, Italy. Mm. Petr. Acts, {in press) 222.

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