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Re: question about PHREEQC

> I'd like to make a question about the User's Guide of PHREEQC2.0,
specifically about the Example 11. This example is, acording the User's
Guide, derived from a sample calculation of the program PHREEQM (Appelo and
Postma, example 10.13, p. 431-434).
In page 239, line 9, the user's guide said "the number of exchange sites in
each cell is 1.1 mmol", which is according with the keyword data block
intructions given at page 82. Nevertheless, the referred PHREEQM example
states a CEC of 1.1 meq/L of pore water.

> My question is: the nunmber of exchange sites must be given in mol or in
meq/L as in PHREEQM? and if the first is correct: it is claire how to
transform the measured CEC in meq/100 g of soil to meq/L of solution
(Appelo and Postma, 1993, Example 5.1. p.149), but, how can I make the
recalculation from meq/100 g soil to mol as requested in PHREEQC?

In PHREEQM, every solution contained 1.0 kilograms of water by definition.
Within a few percent, every solution had 1 liter of water, so the exchange
capacity could be defined as meq/L (or mmol/L). PHREEQC is normally the
same. Most solutions have about 1 kilogram of water, so in a batch reaction
or transport calculation, you normally define the number of sites that are
in contact with a kilogram (liter) of water.

The reason that the units are actually mol (or equivalents) is because the
concentration is not really determined until you have defined a solution
that is in contact with the exchange sites. It is possible to define a
solution to have 0.5 kilogram of water. So if you define 1 equivalent of
sites that you put into a beaker and then add the solution, the
concentration of sites is ~2 eq/L. Note that the 2 eq/L is not specified
explicitly, only the number of sites and the kilograms of water.

You should take CEC and multiply by the number of grams of soil that are in
contact with the solution in your system, and define the solution to have a
number of kilograms of water equal to the volume of water in your system,
in liters.


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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