Lake Cristallina

Comparison of water composition of four lakes influenced by different bedrocks in their catchments. Drainage areas of Lake Zota and Lake Cristallina contain only gneiss and granitic gneiss that of Lake Piccolo Naret contains small amounts of calcareous schist that of Lake Val Sabbia exhibits a higher proportion of schist. 

The composition of Lakes Cristallina and Zota, situated within a drainage area characterized by the preponderance of gneissic rocks and the absence of calcite and dolomite, and Lake Val Sabbia, the catchment area of which contains dolomite, are markedly different. The waters of Lake Cristallina and Lake Zota exhibit mineral acidity (i.e., caused by mineral acids and HN03), their calcium concentrations are 10 - 15 pmol/liter and their pH is < 5.3. On the other hand, the water of Lake Val Sabbia is characterized by an alkalinity of 130 pmol/liter and a calcium concentration of 85 pmol/liter. The water of Lake Piccolo Naret is intermediate its alkalinity is < 50 jimol/liter and appears to have been influenced by the presence of some calcite or dolomite in its catchment area. 

According to Table 5.3, in the drainage area of Lake Cristallina, about 4 pmol of plagioclase, 2 pmole of epidote, 1 pmole of biotite, and 1 pmol of K-feldspar per liter of runoff water are weathered per year. This amounts to 19 meq of cations per m2-year. 

Table 5.4 Weathering of Cristalline Rocks in the Catchment area of Lake Cristallina... 

Lake Cristallina - Weathering reactions 1 timol calcite -2 2 ... 

As discussed in previous subchapters, the rate of the photochemical reductive dissolution of iron(III)(hydr)oxides depends on the concentration and type of surface complexes present and on the light intensity and its energy. Because the light intensity varies diurnally, also a diurnal variation in the iron(II) concentration can be expected in surface waters. This has been observed in acidic waters (McKnight and Bencala, 1988 Sulzberger et al., 1990). Fig. 10.17 shows such a diurnal variation in the concentration of dissolved Fe(II) in a slightly acidic alpine lake (Lake Cristallina) of Switzerland. 

Diurnal variation of the concentration of dissolved Fe(II) and of the incident light intensity O in Lake Cristallina. (The maximal measured light intensity is arbitrarily set to one). 

Schnoor, J. L., R. Giovanoli, L. Sigg, W. Stumm, B. Sulzberger, and J. Zobrist (1989), Fate of Iron and Aluminum in Lake Cristallina (Switzerland), EAWAG News, No. 26/27. 

Figure 3. SEM/EDX images of (a) ferric hydroxide coating on a feldspar grain, (b) EDX spectra of ferric hydroxide on a quartz grain, (c) chemical weathering of biotite platelets, and (d) etch pits on a plagioclase mineral grain. All grains were taken from surface sediments of Lake Cristallina, Switzerland. Photographs are courtesy of Professor Rudolf Giovanoli, Laboratory of Electron Microscopy, University of Bern.

Figure 5.14. Water composition of four lakes in southern Alps of Switzerland. The difference in composition is caused by the geology of the bedrocks in the catchment areas. Lakes Zota and Cristallina are situated within a drainage area of gneissic rocks. The other two lakes are in catchment areas that contain calcite and dolomite.

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