There are a number of stability criteria that MT3D considers when determining if the MODFLOW time step is small enough to use for transport.

If the MODFLOW step is NOT small enough, MT3D will use the minimum of the following step sizes.

If this does not evenly divide the MODFLOW time step, one shorter step will be simulated to complete the MODFLOW step.

Time step length limit based on user specified Courant number to prevent packets from advecting too far relative to the size of grid cells during one time step.

R = retardation coefficient

V = velocity

Time step length limit based on prevention of injecting so much mass into a cell that the cell concentration would exceed the source concentration before having a chance to migrate away from the source.

= effective porosity

q = volumetric source rate.

Time step length limit based on keeping the time step approximately less than 1.5 half lives.

= decay constant for dissolved constituent

= decay constant for sorbed constituent

Time step length limit based on the period of time allowed for dispersion at prevailing concentration versus the grid size.