VELOCITY INTERPOLATION is required for particle tracking and MT3D uses piecewise linear interpolation, which assumes the Darcy velocity varies linearly from one cell face to the other


In the following plan view for this location, qx is known from the flow model and qy is interpolated by equally weighting velocity at 1, 2, 3, and 4.


In the following cross sectional view for this location, qx is known and qz is interpolated by equally weighting velocity at 5, 6, 7, and 8.


In the following plan view, qy at the red dot is calculated first by computing
v1 as (75% of 3 + 25% of 1)
v2 as (75% of 4 + 25% of 2)
then calculating V as (75% of v1 +25% of v2)