Answer to Question 1
The chemical tracer was diluted by spreading out over a large area. Each time the water containing the tracer encountered a sand grain, it had to flow around the grain. The flow of water over many curving paths caused the chemical tracer to spread out over a larger and larger area, diluting its concentration.
Answer to Question 2
In ground water, most of the water is in tiny pores and is in contact with nearby solid surfaces. Friction along these surfaces slows the water down. In a stream, only the thin layer of water along the perimeter of the channel is slowed down by friction.
