Limitations with a low mass of water?

[Charlie]

We are trying to simulate evaporation with precipitation of mineral phases. We are aware that the ion association model (we are using llnl.dat) might not hold up for high ionic strengths, but we have tracked this output with our study and it remains low (0.013 at 90% evaporation) so that should be OK. However, if we evaporate from 1L down to 0.1 L would this pose any issues for the calculations? In other words, is there a lower limit to the mass of water that can be simulated accurately with PHREEQC?
Thanks

[dlparkhurst]

Removing 90 percent of the water should not cause a problem. Concentration factors higher than 99.9 could cause numerical issues, either because of high concentrations or low amounts of water.

If you have issues, try starting with a larger volume of water (MIX 1; 1 10; END). You can also remove water by setting the activity of water with EQUILIBRIUM_PHASES; this tends to work better at very small H2O concentrations. You have to iterate to get a specific evaporation factor, but the calculation can be more stable.

Code: [Select]

PHASES
H2O
H2O = H2O
log_k 0
SOLUTION
Na 1
Cl 1
EQUILIBRIUM_PHASES
H2O -0.2 10  # activity of water 0.63