Sometimes you may specify a very high mass transfer rate (either advective, diffusive, or other) in order to "flush out" a Cell element.
The GoldSim solution algorithm cannot accommodate excessively high transport rates. In order for the solution to remain stable, solubility constraints must be turned off if the transport rates reach a certain level (1E6 times the current mass over a timestep), and doing so will likely result in errors. If the transport rates are even higher (1E8 times the current mass over a timestep), inaccurate results become even more likely. Therefore, GoldSim tests for these excessive transport rates in mass flux links, and outputs a warning message if the transport rate is exceedingly high. If you see such warnings in your Run Log, you should reduce the transport rate to a value which is not so excessive. Failure to do so may result in inaccurate results.