Geochemical and isotopic characterization of the lakes in the Trento Province

We studied two large lowland lakes (L. Levico and L. Caldonazzo) and four small mid-altitude lakes (L. Santa Colomba, L. Lases, L. Lamar and L. Santo) characterized by different hydrological regimes. The purpose is to compare the new data carried out on samples collected in the years 2011 and 2012 with those of the literature in order to highlight possible compositional changes, that have possibly to be related to ongoing climatic variations. A preliminary observation revealed that TDS decreases with the altitude, reflecting the length of the path flows of water to the lakes. Chemical analyses invariably indicate a HCO3-Ca hydrochemical facies for all the studied lakes. L. Lamar and L. Santo have the highest concentrations of calcium and bicarbonates in agreement with the dolomitic rocks that characterize the basin. L. Levico is the richest in sulphate (>40 mg/l) and arsenic (up to 20 micrograms/l) due to the presence of sulphides in the surrounding lithologies. Nitrate levels never exceed the legislative threshold defined for drinking water (50 mg/l) and are higher during the winter season, plausibly in relation to the surface runoff. Among the other trace elements, rubidium and strontium effectively discriminate the lithologies interacting with the water: Rb/Sr ratio is higher for the lakes located on ignimbrite volcanic rocks, lower in the lakes located on carbonatic rocks (L. Lamar and L. Santo) and intermediate in L. Levico and L. Caldonazzo lying in metamorphic lithologies. Oxygen and hydrogen isotopic values have been also measured and compared with the meteoric components. The new data are in good agreement with those from the literature (Perini et al., 2009, J. Limnology; Flaim et al., 2013, Biogeochemistry). It can be observed a coherent relationship between the isotopic composition and the altitude of the drainage basins, with the exception of Lake of Santa Colomba that shows peculiar composition diverging from the meteoric signature, possibly as result of significant evaporation processes. In general, regional climatic conditions seem to affect the isotopic signature of all lakes that delineate slightly changing Local Meteoric Water Lines (LMWL) in the different years. However, at the same time, microclimatic effects and drainage basin dynamics control the specific composition of the diverse lakes. Therefore, this study contributed to a better understanding of the hydrodynamics of the studied lakes and implement the available isotopic database, that could represent an “hydro-archive” useful to monitor climatic changes