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Origin of the Chemical Compositions of Some Springs and Lakes

Origin of the Chemical Compositions of Some Springs and Lakes. Authors: Robert M. Garrels Fred T. Mackenzie 1/20/10. Introduction. This paper explores the origin and composition of springs in the Sierras.

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Origin of the Chemical Compositions of Some Springs and Lakes

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  1. Origin of the Chemical Compositions of Some Springs and Lakes Authors: Robert M. Garrels Fred T. Mackenzie 1/20/10

  2. Introduction This paper explores the origin and composition of springs in the Sierras. -Sierra Nevada setting was chosen b/c of availability of careful water and soil analyses by Feth, et al. -Granitic rocks high in Feldspars, and quartz • Two parts to this paper: -Weathering process/analysis , and reconstruction of original rock from observed dissolved solids -Evaporative concentrations of Sierra waters

  3. -Sierra Nevada setting was chosen b/c of availability of careful water and soil analyses by Feth, et al. -Granitic rocks high in Feldspars, and quartz

  4. Igneous rocks + soil waters This weather system is assumed to be closed, so there is little loss or gain of C02 or H20. Soil minerals + spring water

  5. Composition of Ephemeral and Perennial Springs

  6. Reconstruction of original materials • Back react spring water w/ kaolinite (chief weathering product) and see if original rock minerals can be formed • Cations and anions in snow water are removed from spring H2O solutes. • ALL Naand Ca, and some HCO3 and Si02 react w/ kaolinite to form Plag. • ALL Mg, and some K, HCO3, and Si02 react with kaolinite to form biotite, and k-spar.

  7. Qualitative Aspects of weathering process -More silica in H20 than from only the breakdown of plag. Comes also from quartz, and the silica represents CO2 changed into HCO3 from weathering. -SiO2 and Aluminum concentrations and pH are reasonable for an alumino-silicate of the appx. stability of kaolinite. -80% of the dissolved solids in the ephemeral springs can be accounted for by the breakdown of plag.

  8. “Pick up” of constituents by continued, deeper circulation • A better idea of weathering occurring after initial “strong attack”, can be derived from subtracting out the ephemeral analyses from the perennial

  9. - Next, Mg2+ and K+ are “used up” with kaolonite to from biotite mica and K-spar. - The remaining SiO2 was apportioned b/t reconstruction of both kaolinite and montmorillonite to plagioclase. -Considerable amounts of Ca2+ and HCO3- remain. It is postulated that during deep circulation the waters have lost their original aggressiveness and start to pick up Ca2+

  10. Paths of water evolution

  11. This was a great a analyses, but Garrels et al. want to take into account what occurs when springs are isolated from the parent rock and allowed to evaporate isothermally in equil. w/ the CO2 of the present atmosphere.

  12. Evaporation changes water from a neutral Na-Ca-HCO3 to highly alkaline Na-HCO3-CO3

  13. -Dissolved silica derived from incongruent solution of silicate minerals -Waters in equil. w/ atmosphere will eventually become highly alkaline if concentrated greatly

  14. References • Feth, J. H., Roberson, C. E., Polzer, W. L., U.S. Geol. Surv. Water Supply Paper 1535-1, 170 (1964) • Garrels, R. M., Mackenzie, F. T. Origin of the Chemical Compostion of Some Springs and Lakes

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