This is a major mistake. When calculating Dispersion matrix,
we shall following the equation on page 219 of our benchmark book.
The equation reads,
D_{ij} = \alpha_T |v| \delta_{ij} + (\alpha_L - \alpha_T) \frac{v_i v_j}{|v|} + D^{d}_{ii}
Therefore, the first muliplication with the norm of velocity should be transverse dispersion, not the longitudinal one.

Now it is fixed in the FemKinReactGIA module. Other mass transport processes should be very careful about this.
1e-12 should be good enough.