Sierra Nevada spring waters

Fig. 24.1. Volumes of minerals (amorphous silica, calcite, and sepiolite) precipitated during a reaction model simulating at 25 °C the evaporation of Sierra Nevada spring water in equilibrium with atmospheric C02, plotted against the concentration factor. For example, a concentration factor of x 100 means that of the original 1 kg of water, 10 grams remain.

Fig. 24.2. Calculated effects of evaporation at 25 °C on the chemistry of Sierra Nevada spring water. Top figures show how pH and ionic strength vary over the reaction path in Figure 24.1 bottom figure shows variation in the fluid s bulk composition.

Feth et al. (3) carefully studied the Sierra Nevada spring waters. We will summarize their results before extending some of their interpretations. 

Figure 2. pK for equilibrium between Ca-beidellite and kaolinite plotted as a function of Na content of Sierra Nevada spring waters. Arrow ABC is the path of water evolution calculated for the reaction from Na0 62Ca038 plagioclase to kaolinite in a closed system with an initial dissolved C02 of 0.0006 moles per liter. Arrow ABD is the expected path if evolution is also controlled by the two-phase equilibrium kaolinite-montmorillonite... 

Figure 4. Calculated results of evaporation of typical Sierra Nevada spring water at constant temperature in equilibrium with atmospheric...

For fresh waters there is a further restraint on pH rise the CO2 reservoir of the atmosphere. For a given pco2 the pH is a function of alkalinity. In order to raise the pH of a water in equilibrium with the atmosphere from 8 to 9, alkalinity must increase by nearly 5 meq liter (either by base addition or by evaporation). Hence only soda lakes, that is, lakes containing substantial amounts of soluble carbonates and bicarbonates, can attain high pH values for example, Sierra Nevada spring waters discharged to the east of the Sierra and evaporated in a plaza of the California desert. 

Figure 1.4. Phase diagram for the system K O-AfCVSiCVF O at 25 °C and 1 atmosphere. Open circles are analytical data for water from springs or seeps in a granitic terrain in the Sierra Nevada (from Feth and others) (%) (from Birkeland, 1984, after Feth and others, 1964. Reprinted from Soils and Geomorphology, pill, Copyright (1984), with permission from Oxford University Press, New York, Oxford)...

We choose as a first example the evaporation of spring water from the Sierra Nevada mountains of California and Nevada, USA, as modeled by Garrels and Mackenzie (1967). Their hand calculation, the first reaction path traced in geochemistry (see Chapter 1), provided the inspiration for Helgeson s (1968 and later) development of computerized methods for reaction modeling. 

Table 24.1. Mean composition of spring water from the Sierra Nevada, California and Nevada, USA (Garrels and Mackenzie, 1967)...

The spring waters of the Sierra Nevada result from the attack of high C02 soil waters on typical igneous rocks and hence can be regarded as nearly ideal samples of a major water type. Their compositions are consistent with a model in which the primary rock-forming silicates are altered in a closed system to soil minerals plus a solution in steady-state equilibrium with these minerals. Isolation of Sierra waters from the solid alteration products followed by isothermal evaporation in equilibrium with the eartKs atmosphere should produce a highly alkaline Na-HCO.rCOA water a soda lake with calcium carbonate, magnesium hydroxy-silicate, and amorphous silica as precipitates. 

We have chosen to study the genesis of the spring waters of the Sierra Nevada because of the availability of a careful set of analyses of the waters plus determinations of the primary igneous rock minerals and of the soil minerals derived from them (3). As we will demonstrate, the Sierra system emerges as one in which a few primary igneous rock... 

In the earlier studies, the idealized composition of the minerals involved in the weathering reactions was used. This generally gave satisfactory balances. However, where measured compositions of primary minerals and their reaction products were used, the balances were improved and provided a closer approximation to reality (Bowser and Jones, 2002). Differences between the results obtained by using idealized versus acmal mineral compositions in mass-balance studies of Sierra Nevada perennial spring-waters are illustrated in Table 2 (Bowser and Jones, 2002). 

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