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## Craig M. Bethke

Print publication date: 1996

Print ISBN-13: 9780195094756

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195094756.001.0001

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# Equilibrium Models of Natural Waters

Chapter:
6 Equilibrium Models of Natural Waters
Source:
Geochemical Reaction Modeling
Publisher:
Oxford University Press
DOI:10.1093/oso/9780195094756.003.0010

Having derived a set of equations describing the equilibrium state of a multicomponent system and devised a scheme for solving them, we can begin to model the chemistries of natural waters. In this chapter we construct four models, each posing special challenges, and look in detail at the meaning of the calculation results. In each case, we use program REACT and employ an extended form of the Debye-Hückel equation for calculating species' activity coefficients, as discussed in Chapter 7. In running REACT, you work interactively following the general procedure: • Swap into the basis any needed species, minerals, or gases. Table 6.1 shows the basis in its original configuration (as it exists when you start the program). You might want to change the basis by replacing SiO2(aq) with quartz so that equilibrium with this mineral can be used to constrain the model. Or to set a fugacity buffer you might swap CO2(g) for either H+ or HCO-3. • Set a constraint for each basis member that you want to include in the calculation. For instance, the constraint might be the total concentration of sodium in the fluid, the free mass of a mineral, or the fugacity of a gas. You may also set temperature (25°C, by default) or special program options. • Run the program by typing go. • Revise the basis or constraints and reexecute the program as often as you wish. In this book, input scripts for running the various programs are set in a "typewriter" typeface. Unless a script is marked as a continuation of the previous script, you should start the program anew or type reset to clear your previous configuration. For a first chemical model, we calculate the distribution of species in surface seawater, a problem first undertaken by Garrels and Thompson (1962; see also Thompson, 1992). We base our calculation on the major element composition of seawater (Table 6.2), as determined by chemical analysis. To set pH, we assume equilibrium with CO2 in the atmosphere (Table 6.3).

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