Re: convergence problem

["David L Parkhurst" <dlpark@xxxxxxxx>]

> I have come across a PHREEQC convergence problem for which I couldn't
find a cure. I am not sure, maybe there is something wrong with
my input or my definitions in the database. Could you, if you have
the time, have a quick look at it ? I have attached the input file
and the database file that I used.
I had this type of problem before after I included Ferrihydrite or Goethite
as equ. mineral (allowed precipitation) in an anaerobic, pyritic aquifer in
which aerorobic water was injected.

Pretty subtle problem, but I think it is caused by fixing the pe but
letting the pH vary. You are right at the edge of water stability and the
pH must increase in this problem to produce charge balance. The result is,
in a sense, stepping over the stability of water boundary. The real effect
is that the dissolved oxygen concentration is calculated to be large. This
in turn affects the activity of water, and the numerical solution gets
unstable. Maybe the numerical solution could be better, but the real
problem is that you are calculating a higher pH than input with a fixed pe.

Several things will alleviate the problem: (1) if you do not charge balance
you should have no problem, (2) if you charge balance on anything but pH
and pe, (3) you set the pe by a redox couple, or (4) define the
concentration of O(0). The last may be the easiest, if you set O(0) to a
small number, say 1e-18 things will probably work ok.

David


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

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled