X1T - moving a low pH fluid through aquifer with ferrihydrite

[CShort]

Hi GWB Folks,

I'm working on an X1T model that should move a contaminated, pH 1 fluid into a clean aquifer (pH 6.8) for 22 years. Then, clean water (pH 6.8) flushes the system back out. Ferrihydrite is distributed through the system in the initial conditions and should allow some sorption of contaminants.

I've proved to myself that the model runs without ferrihydrite swapped for Fe+++ in the initial pane. When ferrihydrite is swapped into the model, the model only runs through a few iterations before the fig newton fails to converge - it seems like the solution is becoming unstable in the initial nodes, but no amount of tweaking the stepping parameters seems to make a serious difference. Increasing the pH of the contaminated fluid allows the model to run further; increasing pH to 4 allows the model to run to completion.

Any advice or observations are welcome, I've been banging my head against this all night.

I'll attach the current version of the model. Unswapping ferrihydrite or removing Fe from the basis will produce a model that runs fine. Removing the surface database doesn't change anything, suggesting to me it's something to do with ferrihydrite stability (?)... or maybe user error.

Thanks,

Clark

SorbModel02.x1t

FeOH_minteq_plusU.sdat