Pyrite Oxidation Phreeqc Question

["Ross, Cheryl" <CRoss@xxxxxxxxxx>]

Dr. Parkhurst,

I was hoping you might be able to provide some insight for a warning message
I am getting running a pyrite oxidation simulation.  I am modeling injecting
oxygenated water into a basalt aquifer containing 0.1% pyrite to simulate
what may occur during aquifer storage and recovery operations.  For my
initial simulation I have not included possible precipitation of
ferrihydrite.  There is no iron in my initial solution so the only source of
iron is what is released due to the oxidation of pyrite.  At the point in
the simulation where iron concentrations plateau as oxygen is depleted I get
the following warning:

WARNING: Element Fe has negative moles in solution, -2.943717e-005. 
	Erroneous mole balance occurs as moles are added to produce zero
moles.
	Caused by KINETICS or REACTION.
	May be due to large time steps in early part of KINETICS simulation.
Using solution 2.	Richland Water (9/25/00)
Using kinetics 2.	Kinetics defined in simulation 2.

The final iron concentration in solution however is equal to the total
amount of pyrite that has dissolved during the simulation as would be
expected.  Since I have no reactions that would remove iron, I'm not sure
how I could have negative moles in solution.  I have tried decreasing my
time step but this does not seem to help.  Any help would be greatly
appreciated.

Cheryl Ross

_____________________________

Cheryl Ross, M.Sc.
Hydrogeochemist
Golder Associates Inc.
18300 NE Union Hill Rd., Suite 200
Redmond, WA 98052
Office: 425-883-0777
Fax:     425-882-5498