Transcript 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.
-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
-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
Igneous rocks + soil waters
Soil minerals + spring water
This weather system is assumed to be closed, so there is little loss or gain
of C02 or H20.
Composition of Ephemeral and Perennial Springs
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 Na and 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.
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.
“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
- 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+
Paths of water evolution
 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.
Evaporation changes water from a neutral Na-Ca-HCO3 to highly
alkaline Na-HCO3-CO3
-Dissolved silica derived from incongruent solution of
silicate minerals
-Waters in equil. w/ atmosphere will eventually become
highly alkaline if concentrated greatly
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