A stable isotope study of the Garda lake, northern Italy: Its hydrological balance
Introduction
Lake Garda is the largest Italian lake with the following major morphometric parameters: altitude 65 m a.s.l.; surface 370 km2; maximum depth of about 346 m; water volume estimated at 50 × 109 m3; catchment basin 2360 km2; maximum elevation of the catchment basin 3556 m a.s.l.; main input the Sarca river; main output the Mincio river. The main tributary flows southward from the Alps entering the Garda lake at its northernmost section, near the town of Riva. A few minor streams, along both the western and the eastern side of the lake, are also tributaries but their contribution is fairly small, although difficult to calculate precisely. The only effluent, the Mincio river, is located at the southernmost section of the lake (Fig. 1). Systematic limnological studies have been carried out since the Seventies, mainly concentrating on biological, chemical and environmental problems (e.g. Gerletti, 1974, Chiaudani and Premazzi, 1990, Gelmini, 1991, Provincia Autonoma di Trento, 1989-1990, Provincia Autonoma di Trento, 1992–1993, Provincia Autonoma di Trento, 1994–1995, Provincia Autonoma di Trento and Agenzia Provinciale per la Protezione dell’Ambiente, 1996–1998, Provincia Autonoma di Trento and Agenzia Provinciale per la Protezione dell’Ambiente, 1999–2000, Provincia Autonoma di Trento and Agenzia Provinciale per la Protezione dell’Ambiente, 2001–2004, Salmaso and Naselli-Flores, 1999, Crema et al., 2002, Pellegrini et al., 2003, Monauni et al., 2005). However, stable isotope studies, tritium measurements and hydrological mass balance have not been carried out so far. During 1998 and 1999, in view of a possible international project for the isotopic study of large lakes in central and southern Europe (never initiated), surface water samples of the Garda lake were collected and measured for their stable isotope composition (oxygen and hydrogen) to obtain a first set of data on the lake values. With the only exception of two samples from the northernmost section of the lake, not far from the mouth of the inflowing Sarca river, the isotopic values were quite homogeneous throughout the whole lake surface. In deep lakes, the horizontal mixing is generally rather fast, and it is mainly due to turbulent mass transport caused by wind stirring. However, these isotopic values are practically constant over nine years (1998–2006), regardless of season and collection point. These data suggested the timeliness of a detailed stable isotope study, whose main purpose would be to understand the results obtained so far and to check the vertical distribution of the isotopic values.
Lakes are often well suited to environmental isotope based study methods. The main reference for the application of stable isotope techniques to limnological studies is the fundamental paper by Craig and Gordon (1965) who calculated the isotopic fractionation determined by evaporation processes providing a quantitative explanation of the isotopic behaviour of water bodies evaporating under natural conditions. Their model explains the deviation of lake water isotopic composition from the Local Meteoric Water Line in a δ18O–δD diagram, a deviation that may change according to local environmental conditions. The application of stable isotope techniques to limnological studies has been repeatedly reviewed, beginning with a dedicated IAEA meeting (1977) and proceedings (1979) followed by further reviews, by Gat, 1981, Gonfiantini, 1986 and Gat (1995). In particular, the isotopes incorporated into the water molecules, 16O, 18O, H, D, and 3H, are almost ideal tracers to investigate certain lake parameters.
To this purpose, monthly water samples were collected along vertical profiles from the surface to the bottom at a location in the northern section of the lake (45° 50′ 44″ N and 10° 50′ 56″ E) where the depth is about 280 m (Fig. 2). This location was particularly suitable for detecting the effect of the inflowing water from the Sarca river whose isotopic composition should be rather negative, because of the elevation of its Alpine catchment area, with elevations well above 3000 m. As previously said, the overall inflow from the small streams on both the eastern and western sides of the lake should be of minor importance, considering their very small catchment area, and the karstic morphological features of the limestone formations. The northern 2/3 of the lake are surrounded by these formations while the southern section of the lake is mainly bordered by sedimentary formations. These are related to the Late Pleistocene glacial drift and to the last glacial terminal moraine of the huge glacier that excavated what is now the lake basin (Cremaschi, 1987).
As regards the thermal properties of lake Garda, it should be pointed out that the superficial winter cooling causes a vertical mixing and a homogenization of the water properties only at the end of years with particular climatic conditions. During the last two decades this happened in 1991, 1999, 2000, 2004, 2005, and 2006. The bottom temperature tends to increase very slowly and this may favour episodes of complete vertical mixing. From 7.65 °C in March 1991 at 250 m, during January 1999 it reached 8.41 °C. After minor oscillations, in January 2004 it again reached 8.4 °C. While the isotopic composition of the water should be fairly homogeneous along a vertical profile after a mixing episode, vertical isotopic gradients should be found, as well as vertical chemical gradients, in the years when the vertical mixing does not take place, particularly during summer taking into account the rather elevated summer temperature of surface layers that should favour a marked evaporation and, hence, a detectable fractionation effect. The results of the detailed stable isotope study carried out during 2006 and 2007 are reported here along with the suggested interpretation of the relevant data.
Section snippets
Materials and methods
The water samples of the vertical profiles were collected by means of Niskin type bottles, introduced into polyethylene bottles with stoppers and screw caps and stored in a refrigerator at about 4 °C until measurement. The oxygen isotopic composition was measured using the Epstein and Mayeda (1953) water–CO2 equilibration technique. The samples collected from 1998 to 2003 underwent manual preparation and the CO2 measurements were carried out by means of a Finnigan Delta S mass spectrometer. The
Stable isotopes and hydrological data
The δ18O and the δD values obtained from surface water nearshore samples collected from 1998 to 2007 in different sections of the lake (Fig. 2) and in different periods of the year are reported in Table 1, along with the d-excess. Even though we have no isotopic values for the warmest summer months (July and August), both the oxygen and the deuterium isotopic data, as well as the d-excess show quite constant values throughout time and space. As previously said, this can be, at least partially,
Conclusions
The stable isotope study of the Garda lake is of interest because of the peculiar features exhibited by the results obtained. These features may be summarized as follows:
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despite the marked summer warming of surface water, no fractionation effect can be detected, related to evaporation processes. This is an anomalous behaviour, restricted basins typically show a surface isotopic enrichment during summer;
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the isotopic composition of lake water is constant through time (ten years) and homogeneous
Acknowledgements
The authors wish to thank the Garda Community and particularly Dr. B. Frazzini, the Provincia Autonoma di Trento and its Ufficio Dighe, Incarico Speciale Sicurezza del Sistema Idraulico, for kindly supplying the data on the Mincio outflow and the Sarca inflow, for their help in the collection of the samples, and for their kind authorization to publish some of the physico-chemical data on the lake vertical sampling during 2006. The suggestions of T.W.D. Edwards and of an anonymous reviewer are
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